This file documents the GNU C library.

This is Edition 0.05, last updated 30 August 1993, of `The GNU C Library Reference Manual', for Version 1.07 Beta.

Copyright (C) 1993 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the section entitled "GNU Library General Public License" is included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the text of the translations of the section entitled "GNU Library General Public License" must be approved for accuracy by the Foundation.


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This is the reference manual for version 1.07 Beta of the GNU C Library.

* Menu:

Introduction	Purpose of the GNU C Library.

Error Reporting	How the GNU Library functions report error conditions.

Memory Allocation	Your program can allocate memory dynamically and manipulate it via pointers.

Character Handling	Character testing and conversion functions.

String and Array Utilities	Utilities for copying and comparing strings and arrays.

Extended Characters	Support for extended character sets.

Locales	The country and language can affect the behavior of library functions.

Searching and Sorting	General searching and sorting functions.

Pattern Matching	Matching wildcards and regular expressions, and shell-style "word expansion".

I/O Overview	Introduction to the I/O facilities.

Streams	High-level, portable I/O facilities.

Low-Level I/O	Low-level, less portable I/O.

File System Interface	Functions for manipulating files.

Pipes and FIFOs	A simple interprocess communication mechanism.

Sockets	A more complicated interprocess communication mechanism, with support for networking.

Low-Level Terminal Interface	How to change the characteristics of a terminal device.

Mathematics	Math functions (transcendental functions, random numbers, absolute value, etc.).

Arithmetic	Low-level arithmetic functions.

Date and Time	Functions for getting the date and time, and for conversion between formats.

Non-Local Exits	The setjmp and longjmp facilities.

Signal Handling	All about signals; how to send them, block them, and handle them.

Process Startup	Writing the beginning and end of your program.

Child Processes	How to create processes and run other programs.

Job Control	All about process groups and sessions.

Users and Groups	How users are identified and classified.

System Information	Getting information about the hardware and software configuration of the machine a program runs on.

System Configuration	Parameters describing operating system limits.

  Appendices

Language Features	C language features provided by the library.

Library Summary	A summary showing the syntax, header file, and derivation of each library feature.

Maintenance	How to install and maintain the GNU C Library.

Copying	The GNU Library General Public License says how you can copy and share the GNU C Library.

  Indices

Concept Index	Index of concepts and names.

Type Index	Index of types and type qualifiers.

Function Index	Index of functions and function-like macros.

Variable Index	Index of variables and variable-like macros.

File Index	Index of programs and files.

  -- The Detailed Node Listing --

  Introduction

Getting Started	Getting Started

Standards and Portability	Standards and Portability

Using the Library	Using the Library

Roadmap to the Manual	Roadmap to the Manual

  Standards and Portability

ANSI C	The American National Standard for the C programming language.

POSIX	The IEEE 1003 standards for operating systems.

Berkeley Unix	BSD and SunOS.

SVID	The System V Interface Description.

  Using the Library

Header Files	How to use the header files in your programs.

Macro Definitions	Some functions in the library may really be implemented as macros.

Reserved Names	The C standard reserves some names for the library, and some for users.

Feature Test Macros	How to control what names are defined.

  Error Reporting

Checking for Errors	How errors are reported by library functions.

Error Codes	What all the error codes are.

Error Messages	Mapping error codes onto error messages.

  Memory Allocation

Memory Concepts	An introduction to concepts and terminology.

Dynamic Allocation and C	How to get different kinds of allocation in C.

Unconstrained Allocation	The malloc facility allows fully general dynamic allocation.

Obstacks	Obstacks are less general than malloc but more efficient and convenient.

Variable Size Automatic	Allocation of variable-sized blocks of automatic storage that are freed when the calling function returns.

Relocating Allocator	Waste less memory, if you can tolerate automatic relocation of the blocks you get.

Memory Warnings	Getting warnings when memory is nearly full.

  Unconstrained Allocation

Basic Allocation	Simple use of malloc.

Malloc Examples	Examples of malloc.  xmalloc.

Freeing after Malloc	Use free to free a block you got with malloc.

Changing Block Size	Use realloc to make a block bigger or smaller.

Allocating Cleared Space	Use calloc to allocate a block and clear it.

Efficiency and Malloc	Efficiency considerations in use of these functions.

Aligned Memory Blocks	Allocating specially aligned memory: memalign and valloc.

Heap Consistency Checking	Automatic checking for errors.

Hooks for Malloc	You can use these hooks for debugging programs that use malloc.

Statistics of Malloc	Getting information about how much memory your program is using.

Summary of Malloc	Summary of malloc and related functions.

  Obstacks

Creating Obstacks	How to declare an obstack in your program.

Preparing for Obstacks	Preparations needed before you can use obstacks.

Allocation in an Obstack	Allocating objects in an obstack.

Freeing Obstack Objects	Freeing objects in an obstack.

Obstack Functions	The obstack functions are both functions and macros.

Growing Objects	Making an object bigger by stages.

Extra Fast Growing	Extra-high-efficiency (though more complicated) growing objects.

Status of an Obstack	Inquiries about the status of an obstack.

Obstacks Data Alignment	Controlling alignment of objects in obstacks.

Obstack Chunks	How obstacks obtain and release chunks. Efficiency considerations.

Summary of Obstacks

  Automatic Storage with Variable Size

Alloca Example	Example of using alloca.

Advantages of Alloca	Reasons to use alloca.

Disadvantages of Alloca	Reasons to avoid alloca.

GNU C Variable-Size Arrays	Only in GNU C, here is an alternative method of allocating dynamically and freeing automatically. Relocating Allocator

Relocator Concepts	How to understand relocating allocation.

Using Relocator	Functions for relocating allocation.

  Character Handling

Classification of Characters	Testing whether characters are letters, digits, punctuation, etc.

Case Conversion	Case mapping, and the like.

  String and Array Utilities

Representation of Strings	Introduction to basic concepts.

String/Array Conventions	Whether to use a string function or an arbitrary array function.

String Length	Determining the length of a string.

Copying and Concatenation	Functions to copy the contents of strings and arrays.

String/Array Comparison	Functions for byte-wise and character-wise comparison.

Collation Functions	Functions for collating strings.

Search Functions	Searching for a specific element or substring.

Finding Tokens in a String	Splitting a string into tokens by looking for delimiters.

  Extended Characters

Extended Char Intro	Multibyte codes versus wide characters.

Locales and Extended Chars	The locale selects the character codes.

Multibyte Char Intro	How multibyte codes are represented.

Wide Char Intro	How wide characters are represented.

Wide String Conversion	Converting wide strings to multibyte code and vice versa.

Length of Char	how many bytes make up one multibyte char.

Converting One Char	Converting a string character by character.

Example of Conversion	Example showing why converting one character at a time may be useful.

Shift State	Multibyte codes with "shift characters".

  Locales and Internationalization

Effects of Locale	Actions affected by the choice of locale.

Choosing Locale	How the user specifies a locale.

Locale Categories	Different purposes for which you can select a locale.

Setting the Locale	How a program specifies the locale.

Standard Locales	Locale names available on all systems.

Numeric Formatting	How to format numbers for the chosen locale.

  Searching and Sorting

Comparison Functions	Defining how to compare two objects. Since the sort and search facilities are general, you have to specify the ordering.

Array Search Function	The bsearch function.

Array Sort Function	The qsort function.

Search/Sort Example	An example program.

  Pattern Matching

Wildcard Matching	Matching a wildcard pattern against a single string.

Globbing	Finding the files that match a wildcard pattern.

Regular Expressions	Matching regular expressions against strings.

Word Expansion	Expanding shell variables, nested commands, arithmetic, and wildcards. This is what the shell does with shell commands.

  I/O Overview

I/O Concepts	Some basic information and terminology.

File Names	How to refer to a file.

  I/O Concepts

Streams and File Descriptors	The GNU Library provides two ways to access the contents of files.

File Position	The number of bytes from the beginning of the file.

  File Names

Directories	Directories contain entries for files.

File Name Resolution	A file name specifies how to look up a file.

File Name Errors	Error conditions relating to file names.

File Name Portability	File name portability and syntax issues.

  I/O on Streams

Streams	About the data type representing a stream.

Standard Streams	Streams to the standard input and output devices are created for you.

Opening Streams	How to create a stream to talk to a file.

Closing Streams	Close a stream when you are finished with it.

Simple Output	Unformatted output by characters and lines.

Character Input	Unformatted input by characters and words.

Line Input	Reading a line or a record from a stream.

Unreading	Peeking ahead/pushing back input just read.

Formatted Output	printf and related functions.

Customizing Printf	You can define new conversion specifiers for printf and friends.

Formatted Input	scanf and related functions.

Block Input/Output	Input and output operations on blocks of data.

EOF and Errors	How you can tell if an I/O error happens.

Binary Streams	Some systems distinguish between text files and binary files.

File Positioning	About random-access streams.

Portable Positioning	Random access on peculiar ANSI C systems.

Stream Buffering	How to control buffering of streams.

Temporary Files	How to open a temporary file.

Other Kinds of Streams	Other Kinds of Streams

  Unreading

Unreading Idea	An explanation of unreading with pictures.

How Unread	How to call ungetc to do unreading.

  Formatted Output

Formatted Output Basics	Some examples to get you started.

Output Conversion Syntax	General syntax of conversion specifications.

Table of Output Conversions	Summary of output conversions, what they do.

Integer Conversions	Details of formatting integers.

Floating-Point Conversions	Details of formatting floating-point numbers.

Other Output Conversions	Details about formatting of strings, characters, pointers, and the like.

Formatted Output Functions	Descriptions of the actual functions.

Variable Arguments Output	vprintf and friends.

Parsing a Template String	What kinds of arguments does a given template call for?

  Customizing Printf

Registering New Conversions

Conversion Specifier Options

Defining the Output Handler

Printf Extension Example

  Formatted Input

Formatted Input Basics	Some basics to get you started.

Input Conversion Syntax	Syntax of conversion specifications.

Table of Input Conversions	Summary of input conversions and what they do.

Numeric Input Conversions	Details of conversions for reading numbers.

String Input Conversions	Details of conversions for reading strings.

Other Input Conversions	Details of miscellaneous other conversions.

Formatted Input Functions	Descriptions of the actual functions.

Variable Arguments Input	vscanf and friends.

  Stream Buffering

Buffering Concepts	Terminology is defined here.

Flushing Buffers	How to ensure that output buffers are flushed.

Controlling Buffering	How to specify what kind of buffering to use.

  Other Kinds of Streams

String Streams

Custom Streams

  Programming Your Own Custom Streams

Streams and Cookies

Hook Functions

  Low-Level I/O

Opening and Closing Files	How to open and close file descriptors.

I/O Primitives	Reading and writing data.

File Position Primitive	Setting a descriptor's file position.

Descriptors and Streams	Converting descriptor to stream or vice-versa.

Stream/Descriptor Precautions	Precautions needed if you use both descriptors and streams.

Waiting for I/O	How to check for input or output on multiple file descriptors.

Control Operations	Various other operations on file descriptors.

Duplicating Descriptors	Fcntl commands for duplicating descriptors.

Descriptor Flags	Fcntl commands for manipulating flags associated with file descriptors.

File Status Flags	Fcntl commands for manipulating flags associated with open files.

File Locks	Fcntl commands for implementing file locking.

Interrupt Input	Getting a signal when input arrives.

  File System Interface

Working Directory	This is used to resolve relative file names.

Accessing Directories	Finding out what files a directory contains.

Hard Links	Adding alternate names to a file.

Symbolic Links	A file that "points to" a file name.

Deleting Files	How to delete a file, and what that means.

Renaming Files	Changing a file's name.

Creating Directories	A system call just for creating a directory.

File Attributes	Attributes of individual files.

Making Special Files	How to create special files.

  Accessing Directories

Directory Entries	Format of one directory entry.

Opening a Directory	How to open a directory stream.

Reading/Closing Directory	How to read directory entries from the stream.

Simple Directory Lister	A very simple directory listing program.

Random Access Directory	Rereading part of the directory already read with the same stream.

  File Attributes

Attribute Meanings	The names of the file attributes, and what their values mean.

Reading Attributes	How to read the attributes of a file.

Testing File Type	Distinguishing ordinary files, directories, links...

File Owner	How ownership for new files is determined, and how to change it.

Permission Bits	How information about a file's access mode is stored.

Access Permission	How the system decides who can access a file.

Setting Permissions	How permissions for new files are assigned, and how to change them.

Testing File Access	How to find out if your process can access a file.

File Times	About the time attributes of a file.

  Pipes and FIFOs

Creating a Pipe	Making a pipe with the pipe function.

Pipe to a Subprocess	Using a pipe to communicate with a child.

FIFO Special Files	Making a FIFO special file.

  Sockets

Socket Concepts	Basic concepts you need to know about.

Communication Styles	Stream communication, datagrams, and others.

Socket Addresses	How socket names ("addresses") work.

File Namespace	Details about the file namespace.

Internet Namespace	Details about the Internet namespace.

Open/Close Sockets	Creating sockets and destroying them.

Connections	Operations on sockets with connection state.

Datagrams	Operations on datagram sockets.

Socket Options	Miscellaneous low-level socket options.

Networks Database	Accessing the database of network names.

  Socket Addresses

Address Formats	About struct sockaddr.

Setting Address	Binding an address to a socket.

Reading Address	Reading the address of a socket.

  Internet Domain

Internet Address Format	How socket addresses are specified in the Internet namespace.

Host Addresses	All about host addresses of Internet hosts.

Protocols Database	Referring to protocols by name.

Services Database	Ports may have symbolic names.

Byte Order	Different hosts may use different byte ordering conventions; you need to canonicalize host address and port number.

Inet Example	Putting it all together.

  Host Addresses

Abstract Host Addresses	What a host number consists of.

Data type	Data type for a host number.

Functions	Functions to operate on them.

Names	Translating host names to host numbers.

  Open/Close Sockets

Creating a Socket	How to open a socket.

Closing a Socket	How to close a socket.

Socket Pairs	These are created like pipes.

  Connections

Connecting	What the client program must do.

Listening	How a server program waits for requests.

Accepting Connections	What the server does when it gets a request.

Who is Connected	Getting the address of the other side of a connection.

Transferring Data	How to send and receive data.

Byte Stream Example	An example client for communicating over a byte stream socket in the Internet namespace.

Server Example	A corresponding server program.

Out-of-Band Data	This is an advanced feature.

  Transferring Data

Sending Data	Sending data with write.

Receiving Data	Reading data with read.

Socket Data Options	Using send and recv.

  Datagrams

Sending Datagrams	Sending packets on a datagram socket.

Receiving Datagrams	Receiving packets on a datagram socket.

Datagram Example	An example program: packets sent over a datagram stream in the file namespace.

Example Receiver	Another program, that receives those packets.

  Socket Options

Socket Option Functions	The basic functions for setting and getting socket options.

Socket-Level Options	Details of the options at the socket level.

  Low-Level Terminal Interface

Is It a Terminal	How to determine if a file is a terminal device, and what its name is.

I/O Queues	About flow control and typeahead.

Canonical or Not	Two basic styles of input processing.

Terminal Modes	How to examine and modify flags controlling terminal I/O: echoing, signals, editing.

Line Control	Sending break sequences, clearing  buffers...

Noncanon Example	How to read single characters without echo.

  Terminal Modes

Mode Data Types	The data type struct termios and related types.

Mode Functions	Functions to read and set terminal attributes.

Setting Modes	The right way to set attributes reliably.

Input Modes	Flags controlling low-level input handling.

Output Modes	Flags controlling low-level output handling.

Control Modes	Flags controlling serial port behavior.

Local Modes	Flags controlling high-level input handling.

Line Speed	How to read and set the terminal line speed.

Special Characters	Characters that have special effects, and how to change them.

Noncanonical Input	Controlling how long to wait for input.

  Special Characters

Editing Characters

Signal Characters

Start/Stop Characters

  Mathematics

Domain and Range Errors	How overflow conditions and the like are reported.

Not a Number	Making NANs and testing for NANs.

Trig Functions	Sine, cosine, and tangent.

Inverse Trig Functions	Arc sine, arc cosine, and arc tangent.

Exponents and Logarithms	Also includes square root.

Hyperbolic Functions	Hyperbolic sine and friends.

Pseudo-Random Numbers	Functions for generating pseudo-random numbers.

Absolute Value	Absolute value functions.

  Pseudo-Random Numbers

ANSI Random	rand and friends.

BSD Random	random and friends.

  Low-Level Arithmetic Functions

Normalization Functions	Hacks for radix-2 representations.

Rounding and Remainders	Determinining the integer and fractional parts of a float.

Integer Division	Functions for performing integer division.

Parsing of Numbers	Functions for "reading" numbers from strings.

Predicates on Floats	Some miscellaneous test functions.

  Parsing of Numbers

Parsing of Integers	Functions for conversion of integer values.

Parsing of Floats	Functions for conversion of floating-point.

  Date and Time

Processor Time	Measures processor time used by a program.

Calendar Time	Manipulation of "real" dates and times.

Setting an Alarm	Sending a signal after a specified time.

Sleeping	Waiting for a period of time.

  Processor Time

Basic CPU Time	The clock function.

Detailed CPU Time	The times function.

  Calendar Time

Simple Calendar Time	Facilities for manipulating calendar time.

High-Resolution Calendar	A time representation with greater precision.

Broken-down Time	Facilities for manipulating local time.

Formatting Date and Time	Converting times to strings.

TZ Variable	How users specify the time zone.

Time Zone Functions	Functions to examine or specify the time zone.

Time Functions Example	An example program showing use of some of the time functions.

  Signal Handling

Concepts of Signals	Introduction to the signal facilities.

Standard Signals	Particular kinds of signals with standard names and meanings.

Signal Actions	Specifying what happens when a particular signal is delivered.

Defining Handlers	How to write a signal handler function.

Generating Signals	How to send a signal to a process.

Blocking Signals	Making the system hold signals temporarily.

Waiting for a Signal	Suspending your program until a signal arrives.

BSD Signal Handling	Additional functions for backward compatibility with BSD.

BSD Handler	BSD Function to Establish a Handler

  Basic Concepts of Signals

Kinds of Signals	Some examples of what can cause a signal.

Signal Generation	Concepts of why and how signals occur.

Delivery of Signal	Concepts of what a signal does to the process.

  Standard Signals

Program Error Signals	Used to report serious program errors.

Termination Signals	Used to interrupt and/or terminate the program.

Alarm Signals	Used to indicate expiration of timers.

Asynchronous I/O Signals	Used to indicate input is available.

Job Control Signals	Signals used to support job control.

Miscellaneous Signals	Miscellaneous Signals

Nonstandard Signals	Implementations can support other signals.

Signal Messages	Printing a message describing a signal.

  Specifying Signal Actions

Basic Signal Handling	The simple signal function.

Advanced Signal Handling	The more powerful sigaction function.

Signal and Sigaction	How those two functions interact.

Sigaction Function Example	An example of using the sigaction function.

Flags for Sigaction	Specifying options for signal handling.

Initial Signal Actions	How programs inherit signal actions.

  Defining Signal Handlers

Handler Returns

Termination in Handler

Longjmp in Handler

Signals in Handler

Nonreentrancy

Atomic Data Access

  Generating Signals

Signaling Yourself	Signaling Yourself

Signaling Another Process	Send a signal to another process.

Permission for kill	Permission for using kill

Kill Example	Using kill for Communication

  Blocking Signals

Why Block	The purpose of blocking signals.

Signal Sets	How to specify which signals to block.

Process Signal Mask	Blocking delivery of signals to your process during normal execution.

Testing for Delivery	Blocking to Test for Delivery of a Signal

Blocking for Handler	Blocking additional signals while a handler is being run.

Checking for Pending Signals	Checking for Pending Signals

Remembering a Signal	How you can get almost the same effect as blocking a signal, by handling it and setting a flag to be tested later.

  Waiting for a Signal

Using Pause	The simple way, using pause.

Pause Problems	Why the simple way is often not very good.

Sigsuspend	Reliably waiting for a specific signal.

  BSD Signal Handling

POSIX vs BSD	Comparison of BSD and POSIX signal functions.

  BSD Function to Establish a Handler

Blocking in BSD	BSD Functions for Blocking Signals

Signal Stack	Using a Separate Signal Stack

  Process Startup and Termination

Program Arguments	Parsing your program's command-line arguments.

Environment Variables	How to access parameters inherited from a parent process.

Program Termination	How to cause a process to terminate and return status information to its parent.

  Program Arguments

Argument Syntax	By convention, options start with a hyphen.

Parsing Options	The getopt function.

Example of Getopt	An example of parsing options with getopt.

Long Options	GNU utilities should accept long-named options. Here is how to do that.

Long Option Example	An example of using getopt_long.

  Environment Variables

Environment Access	How to get and set the values of environment variables.

Standard Environment	These environment variables have standard interpretations.

  Program Termination

Normal Termination	If a program calls exit, a process terminates normally.

Exit Status	The exit status provides information about why the process terminated.

Cleanups on Exit	A process can run its own cleanup functions upon normal termination.

Aborting a Program	The abort function causes abnormal program termination.

Termination Internals	What happens when a process terminates.

  Child Processes

Running a Command	The easy way to run another program.

Process Creation Concepts	An overview of the hard way to do it.

Process Identification	How to get the process ID of a process.

Creating a Process	How to fork a child process.

Executing a File	How to make a child execute another program.

Process Completion	How to tell when a child process has completed.

Process Completion Status	How to interpret the status value returned from a child process.

BSD Wait Functions	More functions, for backward compatibility.

Process Creation Example	A complete example program.

  Job Control

Concepts of Job Control	Concepts of Job Control

Job Control is Optional	Not all POSIX systems support job control.

Controlling Terminal	How a process gets its controlling terminal.

Access to the Terminal	How processes share the controlling terminal.

Orphaned Process Groups	Jobs left after the user logs out.

Implementing a Shell	What a shell must do to implement job control.

Functions for Job Control	Functions to control process groups.

  Implementing a Job Control Shell

Data Structures	Introduction to the sample shell.

Initializing the Shell	What the shell must do to take responsibility for job control.

Launching Jobs	Creating jobs to execute commands.

Foreground and Background	Putting a job in foreground of background.

Stopped and Terminated Jobs	Reporting job status.

Continuing Stopped Jobs	How to continue a stopped job in the foreground or background.

Missing Pieces	Other parts of the shell.

  Functions for Job Control

Identifying the Terminal	Determining the controlling terminal's name.

Process Group Functions	Functions for manipulating process groups.

Terminal Access Functions	Functions for controlling terminal access.

  Users and Groups

User and Group IDs	Each user and group has a unique numeric ID.

Process Persona	The user IDs and group IDs of a process.

Why Change Persona	Why a program might need to change its user and/or group IDs.

How Change Persona	Restrictions on changing user and group IDs.

Reading Persona	Examining the process's user and group IDs.

Setting User ID

Setting Groups

Enable/Disable Setuid

Setuid Program Example	Setuid Program Example

Tips for Setuid

Who Logged In	Getting the name of the user who logged in, or of the real user ID of the current process.

User Database	Functions and data structures for accessing the user database.

Group Database	Functions and data structures for accessing the group database.

Database Example	Example program showing use of database inquiry functions.

  User Database

User Data Structure

Lookup User

Scanning All Users	Scanning the List of All Users

Writing a User Entry

  Group Database

Group Data Structure

Lookup Group

Scanning All Groups	Scanning the List of All Groups

  System Information

Host Identification	Determining the name of the machine.

Hardware/Software Type ID	Determining the hardware type and operating system type.

  System Configuration Limits

General Limits	Constants and functions that describe various process-related limits that have one uniform value for any given machine.

System Options	Optional POSIX features.

Version Supported	Version numbers of POSIX.1 and POSIX.2.

Sysconf	Getting specific configuration values of general limits and system options.

Minimums	Minimum values for general limits.

Limits for Files	Size limitations on individual files. These can vary between file systems or even from file to file.

Options for Files	Optional features that some files may support.

File Minimums	Minimum values for file limits.

Pathconf	Getting the limit values for a particular file.

Utility Limits	Capacity limits of POSIX.2 utility programs.

Utility Minimums	Minimum allowable values of those limits.

String Parameters	Getting the default search path.

  Library Facilities that are Part of the C Language

Consistency Checking	Using assert to abort if something "impossible" happens.

Variadic Functions	Defining functions with varying numbers of arguments.

Null Pointer Constant	The macro NULL.

Important Data Types	Data types for object sizes.

Data Type Measurements	Parameters of data type representations.

  Variadic Functions

Why Variadic	Reasons for making functions take variable arguments.

How Variadic	How to define and call variadic functions.

Argument Macros	Detailed specification of the macros for accessing variable arguments.

Variadic Example	A complete example.

  How Variadic Functions are Defined and Used

Variadic Prototypes	How to make a prototype for a function with variable arguments.

Receiving Arguments	Steps you must follow to access the optional argument values.

How Many Arguments	How to decide whether there are more arguments.

Calling Variadics	Things you need to know about calling variable arguments functions.

  Data Type Measurements

Width of Type	How many bits does an integer type hold?

Range of Type	What are the largest and smallest values that an integer type can hold?

Floating Type Macros	Parameters that measure floating-point types.

Structure Measurement	Getting measurements on structure types.

  Floating Type Macros

Floating Point Concepts	Definitions of terminology.

Floating Point Parameters	Dimensions, limits of floating point types.

IEEE Floating Point	How one common representation is described.

  Library Maintenance

Installation	How to configure, compile and install the GNU C library.

Reporting Bugs	How to report bugs (if you want to get them fixed) and other troubles you may have with the GNU C library.

Porting	How to port the GNU C library to a new machine or operating system. * Contributors::	Who wrote what parts of the GNU C Library.

  Porting the GNU C Library

Hierarchy Conventions	How the 'sysdeps' hierarchy is layed out.

Porting to Unix	Porting the library to an average Unix-like system.


Introduction
Node: Introduction, Next: Error Reporting, Prev: Top, Up: Top

Introduction

The C language provides no built-in facilities for performing such common operations as input/output, memory management, string manipulation, and the like.  Instead, these facilities are defined in a standard "library", which you compile and link with your programs.  KThe GNU C library, described in this document, defines all of the library functions that are specified by the ANSI C standard, as well as additional features specific to POSIX and other derivatives of the Unix operating system, and extensions specific to the GNU system.

The purpose of this manual is to tell you how to use the facilities of the GNU library.  We have mentioned which features belong to which standards to help you identify things that are potentially nonportable to other systems.  But the emphasis in this manual is not on strict portability.

* Menu:

Getting Started	What this manual is for and how to use it.

Standards and Portability	Standards and sources upon which the GNU C library is based.

Using the Library	Some practical uses for the library.

Roadmap to the Manual	Overview of the remaining chapters in this manual.


Getting Started
Node: Getting Started, Next: Standards and Portability, Prev: , Up: Introduction

Getting Started

This manual is written with the assumption that you are at least somewhat familiar with the C programming language and basic programming concepts.  Specifically, familiarity with ANSI standard C (see ANSI C), rather than "traditional" pre-ANSI C dialects, is assumed.

The GNU C library includes several "header files", each of which provides definitions and declarations for a group of related facilities; this information is used by the C compiler when processing your program.  For example, the header file 'stdio.h' declares facilities for performing input and output, and the header file 'string.h' declares string processing utilities.  The organization of this manual generally follows the same division as the header files.

If you are reading this manual for the first time, you should read all of the introductory material and skim the remaining chapters.  There are a lot of functions in the GNU C library and it's not realistic to expect that you will be able to remember exactly how to use each and every one of them.  It's more important to become generally familiar with the kinds of facilities that the library provides, so that when you are writing your programs you can recognize when to make use of library functions, and where in this manual you can find more specific information about them.


Standards and Portability
Node: Standards and Portability, Next: Using the Library, Prev: Getting Started, Up: Introduction

Standards and Portability

This section discusses the various standards and other sources that the GNU C library is based upon.  These sources include the ANSI C and POSIX standards, and the System V and Berkeley Unix implementations.

The primary focus of this manual is to tell you how to make effective use of the GNU library facilities.  But if you are concerned about making your programs compatible with these standards, or portable to operating systems other than GNU, this can affect how you use the library.  This section gives you an overview of these standards, so that you will know what they are when they are mentioned in other parts of the manual.

See Library Summary, for an alphabetical list of the functions and other symbols provided by the library.  This list also states which standards each function or symbol comes from.

* Menu:

ANSI C	The American National Standard for the C programming language.

POSIX	The IEEE 1003 standards for operating systems.

Berkeley Unix	BSD and SunOS.

SVID	The System V Interface Description.


ANSI C
Node: ANSI C, Next: POSIX, Prev: , Up: Standards and Portability

ANSI C

The GNU C library is compatible with the C standard adopted by the American National Standards Institute (ANSI): `American National Standard X3.159-1989--"ANSI C"'.  The header files and library facilities that make up the GNU library are a superset of those specified by the ANSI C standard.

If you are concerned about strict adherence to the ANSI C standard, you should use the '-ansi' option when you compile your programs with the GNU C compiler.  This tells the compiler to define only ANSI standard features from the library header files, unless you explicitly ask for additional features.  See Feature Test Macros, for information on how to do this.

Being able to restrict the library to include only ANSI C features is important because ANSI C puts limitations on what names can be defined by the library implementation, and the GNU extensions don't fit these limitations.  See Reserved Names, for more information about these restrictions.

This manual does not attempt to give you complete details on the differences between ANSI C and older dialects.  It gives advice on how to write programs to work portably under multiple C dialects, but does not aim for completeness.


POSIX (The Portable Operating System Interface)
Node: POSIX, Next: Berkeley Unix, Prev: ANSI C, Up: Standards and Portability

POSIX (The Portable Operating System Interface)

The GNU library is also compatible with the IEEE "POSIX" family of standards, known more formally as the "Portable Operating System Interface for Computer Environments".  POSIX is derived mostly from various versions of the Unix operating system.

The library facilities specified by the POSIX standards are a superset of those required by ANSI C; POSIX specifies additional features for ANSI C functions, as well as specifying new additional functions.  In general, the additional requirements and functionality defined by the POSIX standards are aimed at providing lower-level support for a particular kind of operating system environment, rather than general programming language support which can run in many diverse operating system environments.

The GNU C library implements all of the functions specified in `IEEE Std 1003.1-1990, the POSIX System Application Program Interface', commonly referred to as POSIX.1.  The primary extensions to the ANSI C facilities specified by this standard include file system interface primitives (see File System Interface), device-specific terminal control functions (see Low-Level Terminal Interface), and process control functions (see Child Processes).

Some facilities from `IEEE Std 1003.2-1992, the POSIX Shell and Utilities standard' (POSIX.2) are also implemented in the GNU library.  These include utilities for dealing with regular expressions and other pattern matching facilities (see Pattern Matching).


Berkeley Unix
Node: Berkeley Unix, Next: SVID, Prev: POSIX, Up: Standards and Portability

Berkeley Unix

The GNU C library defines facilities from some versions of Unix which are not formally standardized, specifically from the 4.2 BSD, 4.3 BSD, and 4.4 BSD Unix systems (also known as "Berkeley Unix") and from "SunOS" (a popular 4.2 BSD derivative that includes some Unix System V functionality).  These systems support most of the ANSI and POSIX facilities, and 4.4 BSD and newer releases of SunOS in fact support them all.

The BSD facilities include symbolic links (see Symbolic Links), the select function (see Waiting for I/O), the BSD signal functions (see BSD Signal Handling), and sockets (see Sockets).


SVID (The System V Interface Description)
Node: SVID, Next: , Prev: Berkeley Unix, Up: Standards and Portability

SVID (The System V Interface Description)

The "System V Interface Description" (SVID) is a document describing the AT&T Unix System V operating system.  It is to some extent a superset of the POSIX standard (see POSIX).

The GNU C library defines some of the facilities required by the SVID that are not also required by the ANSI or POSIX standards, for compatibility with  System V Unix and other Unix systems (such as SunOS) which include these facilities.  However, many of the more obscure and less generally useful facilities required by the SVID are not included.  (In fact, Unix System V itself does not provide them all.)


Using the Library
Node: Using the Library, Next: Roadmap to the Manual, Prev: Standards and Portability, Up: Introduction

Using the Library

This section describes some of the practical issues involved in using the GNU C library.

* Menu:

Header Files	How to include the header files in your programs.

Macro Definitions	Some functions in the library may really be implemented as macros.

Reserved Names	The C standard reserves some names for the library, and some for users.

Feature Test Macros	How to control what names are defined.


Header Files
Node: Header Files, Next: Macro Definitions, Prev: , Up: Using the Library

Header Files

Libraries for use by C programs really consist of two parts: "header files" that define types and macros and declare variables and functions; and the actual library or "archive" that contains the definitions of the variables and functions.

(Recall that in C, a "declaration" merely provides information that a function or variable exists and gives its type.  For a function declaration, information about the types of its arguments might be provided as well.  The purpose of declarations is to allow the compiler to correctly process references to the declared variables and functions.  A "definition", on the other hand, actually allocates storage for a variable or says what a function does.) KIn order to use the facilities in the GNU C library, you should be sure that your program source files include the appropriate header files.  This is so that the compiler has declarations of these facilities available and can correctly process references to them.  Once your program has been compiled, the linker resolves these references to the actual definitions provided in the archive file.

Header files are included into a program source file by the '#include' preprocessor directive.  The C language supports two forms of this directive; the first,

#include "header"

is typically used to include a header file header that you write yourself; this would contain definitions and declarations describing the interfaces between the different parts of your particular application.  By contrast,

#include <file.h>

is typically used to include a header file 'file.h' that contains definitions and declarations for a standard library.  This file would normally be installed in a standard place by your system administrator.  You should use this second form for the C library header files.

Typically, '#include' directives are placed at the top of the C source file, before any other code.  If you begin your source files with some comments explaining what the code in the file does (a good idea), put the '#include' directives immediately afterwards, following the feature test macro definition (see Feature Test Macros).

For more information about the use of header files and '#include' directives, see Header Files.

The GNU C library provides several header files, each of which contains the type and macro definitions and variable and function declarations for a group of related facilities.  This means that your programs may need to include several header files, depending on exactly which facilities you are using.

Some library header files include other library header files automatically.  However, as a matter of programming style, you should not rely on this; it is better to explicitly include all the header files required for the library facilities you are using.  The GNU C library header files have been written in such a way that it doesn't matter if a header file is accidentally included more than once; including a header file a second time has no effect.  Likewise, if your program needs to include multiple header files, the order in which they are included doesn't matter.

Compatibility Note: Inclusion of standard header files in any order and any number of times works in any ANSI C implementation.  However, this has traditionally not been the case in many older C implementations.

Strictly speaking, you don't have to include a header file to use a function it declares; you could declare the function explicitly yourself, according to the specifications in this manual.  But it is usually better to include the header file because it may define types and macros that are not otherwise available and because it may define more efficient macro replacements for some functions.  It is also a sure way to have the correct declaration.


Macro Definitions of Functions
Node: Macro Definitions, Next: Reserved Names, Prev: Header Files, Up: Using the Library

Macro Definitions of Functions

If we describe something as a function in this manual, it may have a macro definition as well.  This normally has no effect on how your program runs--the macro definition does the same thing as the function would.  In particular, macro equivalents for library functions evaluate arguments exactly once, in the same way that a function call would.  The main reason for these macro definitions is that sometimes they can produce an inline expansion that is considerably faster than an actual function call.

Taking the address of a library function works even if it is also defined as a macro.  This is because, in this context, the name of the function isn't followed by the left parenthesis that is syntactically necessary to recognize a macro call.

You might occasionally want to avoid using the macro definition of a function--perhaps to make your program easier to debug.  There are two ways you can do this:

•	You can avoid a macro definition in a specific use by enclosing the name of the function in parentheses.  This works because the name of the function doesn't appear in a syntactic context where it is recognizable as a macro call.

•	You can suppress any macro definition for a whole source file by using the '#undef' preprocessor directive, unless otherwise stated explicitly in the description of that facility.

For example, suppose the header file 'stdlib.h' declares a function named abs with

extern int abs (int);

and also provides a macro definition for abs.  Then, in:

#include <stdlib.h>
int f (int *i) { return (abs (++*i)); }

the reference to abs might refer to either a macro or a function.  On the other hand, in each of the following examples the reference is to a function and not a macro.

#include <stdlib.h>
int g (int *i) { return ((abs)(++*i)); }

#undef abs
int h (int *i) { return (abs (++*i)); }

Since macro definitions that double for a function behave in exactly the same way as the actual function version, there is usually no need for any of these methods.  In fact, removing macro definitions usually just makes your program slower.


Reserved Names
Node: Reserved Names, Next: Feature Test Macros, Prev: Macro Definitions, Up: Using the Library

Reserved Names

The names of all library types, macros, variables and functions that come from the ANSI C standard are reserved unconditionally; your program may not redefine these names.  All other library names are reserved if your program explicitly includes the header file that defines or declares them.  There are several reasons for these restrictions:

•	Other people reading your code could get very confused if you were using a function named exit to do something completely different from what the standard exit function does, for example.  Preventing this situation helps to make your programs easier to understand and contributes to modularity and maintainability.

•	It avoids the possibility of a user accidentally redefining a library function that is called by other library functions.  If redefinition were allowed, those other functions would not work properly.

•	It allows the compiler to do whatever special optimizations it pleases on calls to these functions, without the possibility that they may have been redefined by the user.  Some library facilities, such as those for dealing with variadic arguments (see Variadic Functions) and non-local exits (see Non-Local Exits), actually require a considerable amount of cooperation on the part of the C compiler, and implementationally it might be easier for the compiler to treat these as built-in parts of the language.

In addition to the names documented in this manual, reserved names include all external identifiers (global functions and variables) that begin with an underscore ('_') and all identifiers regardless of use that begin with either two underscores or an underscore followed by a capital letter are reserved names.  This is so that the library and header files can define functions, variables, and macros for internal purposes without risk of conflict with names in user programs.

Some additional classes of identifier names are reserved for future extensions to the C language.  While using these names for your own purposes right now might not cause a problem, they do raise the possibility of conflict with future versions of the C standard, so you should avoid these names.

•	Names beginning with a capital 'E' followed a digit or uppercase letter may be used for additional error code names.  See Error Reporting.

•	Names that begin with either 'is' or 'to' followed by a lowercase letter may be used for additional character testing and conversion functions.  See Character Handling.

•	Names that begin with 'LC_' followed by an uppercase letter may be used for additional macros specifying locale attributes.  See Locales.

•	Names of all existing mathematics functions (see Mathematics) suffixed with 'f' or 'l' are reserved for corresponding functions that operate on float and long double arguments, respectively.

•	Names that begin with 'SIG' followed by an uppercase letter are reserved for additional signal names.  See Standard Signals.

•	Names that begin with 'SIG_' followed by an uppercase letter are reserved for additional signal actions.  See Basic Signal Handling.

•	Names beginning with 'str', 'mem', or 'wcs' followed by a lowercase letter are reserved for additional string and array functions.  See String and Array Utilities.

•	Names that end with '_t' are reserved for additional type names.

In addition, some individual header files reserve names beyond those that they actually define.  You only need to worry about these restrictions if your program includes that particular header file.

•	The header file 'dirent.h' reserves names prefixed with 'd_'.  K

•	The header file 'fcntl.h' reserves names prefixed with 'l_', 'F_', 'O_', and 'S_'.  K

•	The header file 'grp.h' reserves names prefixed with 'gr_'.  K

•	The header file 'limits.h' reserves names suffixed with '_MAX'.  K

•	The header file 'pwd.h' reserves names prefixed with 'pw_'.  K

•	The header file 'signal.h' reserves names prefixed with 'sa_' and 'SA_'.  K

•	The header file 'sys/stat.h' reserves names prefixed with 'st_' and 'S_'.  K

•	The header file 'sys/times.h' reserves names prefixed with 'tms_'.  K

•	The header file 'termios.h' reserves names prefixed with 'c_', 'V', 'I', 'O', and 'TC'; and names prefixed with 'B' followed by a digit.  K


Feature Test Macros
Node: Feature Test Macros, Next: , Prev: Reserved Names, Up: Using the Library

Feature Test Macros

The exact set of features available when you compile a source file is controlled by which "feature test macros" you define.

If you compile your programs using 'gcc -ansi', you get only the ANSI C library features, unless you explicitly request additional features by defining one or more of the feature macros.  See GNU CC Command Options, for more information about GCC options.

You should define these macros by using '#define' preprocessor directives at the top of your source code files.  These directives must come before any #include of a system header file.  It is best to make them the very first thing in the file, preceded only by comments.  You could also use the '-D' option to GCC, but it's better if you make the source files indicate their own meaning in a self-contained way.

 - Macro: _POSIX_SOURCE

If you define this macro, then the functionality from the POSIX.1 standard (IEEE Standard 1003.1) is available, as well as all of the ANSI C facilities.

 - Macro: _POSIX_C_SOURCE

If you define this macro with a value of 1, then the functionality from the POSIX.1 standard (IEEE Standard 1003.1) is made available.  If you define this macro with a value of 2, then both the functionality from the POSIX.1 standard and the functionality from the POSIX.2 standard (IEEE Standard 1003.2) are made available.  This is in addition to the ANSI C facilities.

 - Macro: _BSD_SOURCE

If you define this macro, functionality derived from 4.3 BSD Unix is included as well as the ANSI C, POSIX.1, and POSIX.2 material.

Some of the features derived from 4.3 BSD Unix conflict with the corresponding features specified by the POSIX.1 standard.  If this macro is defined, the 4.3 BSD definitions take precedence over the POSIX definitions.

Due to the nature of some of the conflicts between 4.3 BSD and POSIX.1, you need to use a special "BSD compatibility library" when linking programs compiled for BSD compatibility.  This is because some functions must be defined in two different ways, one of them in the normal C library, and one of them in the compatibility library.  If your program defines _BSD_SOURCE, you must give the option '-lbsd-compat' to the compiler or linker when linking the program, to tell it to find functions in this special compatibility library before looking for them in the normal C library.  K

- Macro: _SVID_SOURCE

If you define this macro, functionality derived from SVID is included as well as the ANSI C, POSIX.1, and POSIX.2 material.

 - Macro: _GNU_SOURCE

If you define this macro, everything is included: ANSI C, POSIX.1, POSIX.2, BSD, SVID, and GNU extensions.  In the cases where POSIX.1 conflicts with BSD, the POSIX definitions take precedence.

If you want to get the full effect of _GNU_SOURCE but make the BSD definitions take precedence over the POSIX definitions, use this sequence of definitions:

#define _GNU_SOURCE
#define _BSD_SOURCE
#define _SVID_SOURCE

Note that if you do this, you must link your program with the BSD compatibility library by passing the '-lbsd-compat' option to the compiler or linker.  Note: If you forget to do this, you may get very strange errors at run time.

We recommend you use _GNU_SOURCE in new programs.  If you don't specify the '-ansi' option to GCC and don't define any of these macros explicitly, the effect is the same as defining _GNU_SOURCE.

When you define a feature test macro to request a larger class of features, it is harmless to define in addition a feature test macro for a subset of those features.  For example, if you define _POSIX_C_SOURCE, then defining _POSIX_SOURCE as well has no effect.  Likewise, if you define _GNU_SOURCE, then defining either _POSIX_SOURCE or _POSIX_C_SOURCE or _SVID_SOURCE as well has no effect.

Note, however, that the features of _BSD_SOURCE are not a subset of any of the other feature test macros supported.  This is because it defines BSD features that take precedence over the POSIX features that are requested by the other macros.  For this reason, defining _BSD_SOURCE in addition to the other feature test macros does have an effect: it causes the BSD features to take priority over the conflicting POSIX features.


Roadmap to the Manual
Node: Roadmap to the Manual, Next: , Prev: Using the Library, Up: Introduction

Roadmap to the Manual

Here is an overview of the contents of the remaining chapters of this manual.

•	See Error Reporting, describes how errors detected by the library are reported.

•	See Language Features, contains information about library support for standard parts of the C language, including things like the sizeof operator and the symbolic constant NULL, how to write functions accepting variable numbers of arguments, and constants describing the ranges and other properties of the numerical types.  There is also a simple debugging mechanism which allows you to put assertions in your code, and have diagnostic messages printed if the tests fail.

•	See Memory Allocation, describes the GNU library's facilities for dynamic allocation of storage.  If you do not know in advance how much storage your program needs, you can allocate it dynamically instead, and manipulate it via pointers.

•	See Character Handling, contains information about character classification functions (such as isspace) and functions for performing case conversion.

•	See String and Array Utilities, has descriptions of functions for manipulating strings (null-terminated character arrays) and general byte arrays, including operations such as copying and comparison.

•	See I/O Overview, gives an overall look at the input and output facilities in the library, and contains information about basic concepts such as file names.

•	See I/O on Streams, describes I/O operations involving streams (or FILE * objects).  These are the normal C library functions from 'stdio.h'.

•	See Low-Level I/O, contains information about I/O operations on file descriptors.  File descriptors are a lower-level mechanism specific to the Unix family of operating systems.

•	See File System Interface, has descriptions of operations on entire files, such as functions for deleting and renaming them and for creating new directories.  This chapter also contains information about how you can access the attributes of a file, such as its owner and file protection modes.

•	See Pipes and FIFOs, contains information about simple interprocess communication mechanisms.  Pipes allow communication between two related processes (such as between a parent and child), while FIFOs allow communication between processes sharing a common file system.

•	See Sockets, describes a more complicated interprocess communication mechanism that allows processes running on different machines to communicate over a network.  This chapter also contains information about Internet host addressing and how to use the system network databases.

•	See Low-Level Terminal Interface, describes how you can change the attributes of a terminal device.  If you want to disable echo of characters typed by the user, for example, read this chapter.

•	See Mathematics, contains information about the math library functions.  These include things like random-number generators and remainder functions on integers as well as the usual trigonometric and exponential functions on floating-point numbers.

•	See Low-Level Arithmetic Functions, describes functions for simple arithmetic, analysis of floating-point values, and reading numbers from strings.

•	See Searching and Sorting, contains information about functions for searching and sorting arrays.  You can use these functions on any kind of array by providing an appropriate comparison function.

•	See Pattern Matching, presents functions for matching regular expressions and shell file name patterns, and for expanding words as the shell does.

•	See Date and Time, describes functions for measuring both calendar time and CPU time, as well as functions for setting alarms and timers.

•	See Extended Characters, contains information about manipulating characters and strings using character sets larger than will fit in the usual char data type.

•	See Locales, describes how selecting a particular country or language affects the behavior of the library.  For example, the locale affects collation sequences for strings and how monetary values are formatted.

•	See Non-Local Exits, contains descriptions of the setjmp and longjmp functions.  These functions provide a facility for goto-like jumps which can jump from one function to another.

•	See Signal Handling, tells you all about signals--what they are, how to establish a handler that is called when a particular kind of signal is delivered, and how to prevent signals from arriving during critical sections of your program.

•	See Process Startup, tells how your programs can access their command-line arguments and environment variables.

•	See Child Processes, contains information about how to start new processes and run programs.

•	See Job Control, describes functions for manipulating process groups.  This material is probably only of interest if you are writing a shell.

•	See User Database, and See Group Database, tell you how to access the system user and group databases.

•	See System Information, describes functions for getting information about the hardware and software configuration your program is executing under.

•	See System Configuration, tells you how you can get information about various operating system limits.  Most of these parameters are provided for compatibility with POSIX.

•	See Library Summary, gives a summary of all the functions, variables, and macros in the library, with complete data types and function prototypes, and says what standard or system each is derived from.

•	See Maintenance, explains how to build and install the GNU C library on your system, how to report any bugs you might find, and how to add new functions or port the library to a new system.

If you already know the name of the facility you are interested in, you can look it up in See Library Summary.  This gives you a summary of its syntax and a pointer to where you can find a more detailed description.  This appendix is particularly useful if you just want to verify the order and type of arguments to a function, for example.  It also tells you what standard or system each function, variable, or macro is derived from.


Error Reporting
Node: Error Reporting, Next: Memory Allocation, Prev: Introduction, Up: Top

Error Reporting

Many functions in the GNU C library detect and report error conditions, and sometimes your programs need to check for these error conditions.  For example, when you open an input file, you should verify that the file was actually opened correctly, and print an error message or take other appropriate action if the call to the library function failed.

This chapter describes how the error reporting facility works.  Your program should include the header file 'errno.h' to use this facility.  K

* Menu:

Checking for Errors	How errors are reported by library functions.

Error Codes	Error code macros; all of these expand into integer constant values.

Error Messages	Mapping error codes onto error messages.


Checking for Errors
Node: Checking for Errors, Next: Error Codes, Prev: , Up: Error Reporting

Checking for Errors

Most library functions return a special value to indicate that they have failed.  The special value is typically -1, a null pointer, or a constant such as EOF that is defined for that purpose.  But this return value tells you only that an error has occurred.  To find out what kind of error it was, you need to look at the error code stored in the variable errno.  This variable is declared in the header file 'errno.h'.  K

 - Variable: volatile int errno

The variable errno contains the system error number.  You can change the value of errno.

Since errno is declared volatile, it might be changed asynchronously by a signal handler; see See Defining Handlers.  However, a properly written signal handler saves and restores the value of errno, so you generally do not need to worry about this possibility except when writing signal handlers.

The initial value of errno at program startup is zero.  Many library functions are guaranteed to set it to certain nonzero values when they encounter certain kinds of errors.  These error conditions are listed for each function.  These functions do not change errno when they succeed; thus, the value of errno after a successful call is not necessarily zero, and you should not use errno to determine whether a call failed.  The proper way to do that is documented for each function.  If the call the failed, you can examine errno.

Many library functions can set errno to a nonzero value as a result of calling other library functions which might fail.  You should assume that any library function might alter errno when the function returns an error.

Portability Note: ANSI C specifies errno as a "modifiable lvalue" rather than as a variable, permitting it to be implemented as a macro.  For example, its expansion might involve a function call, like *_errno ().  In fact, that is what it is on the GNU system itself.  The GNU library, on non-GNU systems, does whatever is right for the particular system.

There are a few library functions, like sqrt and atan, that return a perfectly legitimate value in case of an error, but also set errno.  For these functions, if you want to check to see whether an error occurred, the recommended method is to set errno to zero before calling the function, and then check its value afterward.

All the error codes have symbolic names; they are macros defined in 'errno.h'.  The names start with 'E' and an upper-case letter or digit; you should consider names of this form to be reserved names.  See Reserved Names.

The error code values are all positive integers and are all distinct, with one exception: EWOULDBLOCK and EAGAIN are the same.  Since the values are distinct, you can use them as labels in a switch statement; just don't use both EWOULDBLOCK and EAGAIN.  Your program should not make any other assumptions about the specific values of these symbolic constants.

The value of errno doesn't necessarily have to correspond to any of these macros, since some library functions might return other error codes of their own for other situations.  The only values that are guaranteed to be meaningful for a particular library function are the ones that this manual lists for that function.

On non-GNU systems, almost any system call can return EFAULT if it is given an invalid pointer as an argument.  Since this could only happen as a result of a bug in your program, and since it will not happen on the GNU system, we have saved space by not mentioning EFAULT in the descriptions of individual functions.


Error Codes
Node: Error Codes, Next: Error Messages, Prev: Checking for Errors, Up: Error Reporting

Error Codes

The error code macros are defined in the header file 'errno.h'.  All of them expand into integer constant values.  Some of these error codes can't occur on the GNU system, but they can occur using the GNU library on other systems.

 - Macro: int EPERM

Operation not permitted; only the owner of the file (or other resource) or processes with special privileges can perform the operation.

 - Macro: int ENOENT

No such file or directory.  This is a "file doesn't exist" error for ordinary files that are referenced in contexts where they are expected to already exist.

 - Macro: int ESRCH

No process matches the specified process ID.

 - Macro: int EINTR

Interrupted function call; an asynchronous signal occured and prevented completion of the call.  When this happens, you should try the call again.

You can choose to have functions resume after a signal that is handled, rather than failing with EINTR; see See Interrupted Primitives.

 - Macro: int EIO

Input/output error; usually used for physical read or write errors.

 - Macro: int ENXIO

No such device or address.  The system tried to use the device represented by a file you specified, and it couldn't find the device.  This can mean that the device file was installed incorrectly, or that the physical device is missing or not correctly attached to the computer.

 - Macro: int E2BIG

Argument list too long; used when the arguments passed to a new program being executed with one of the exec functions (see Executing a File) occupy too much memory space.  This condition never arises in the GNU system.

 - Macro: int ENOEXEC

Invalid executable file format.  This condition is detected by the exec functions; see See Executing a File.

 - Macro: int EBADF

Bad file descriptor; for example, I/O on a descriptor that has been closed or reading from a descriptor open only for writing (or vice versa).

 - Macro: int ECHILD

There are no child processes.  This error happens on operations that are supposed to manipulate child processes, when there aren't any processes to manipulate.

 - Macro: int EDEADLK

Deadlock avoided; allocating a system resource would have resulted in a deadlock situation.  The system does not guarantee that it will notice all such situations.  This error means you got lucky and the system noticed; it might just hang.  See File Locks, for an example.

 - Macro: int ENOMEM

No memory available.  The system cannot allocate more virtual memory because its capacity is full.

 - Macro: int EACCES

Permission denied; the file permissions do not allow the attempted operation.

 - Macro: int EFAULT

Bad address; an invalid pointer was detected.

 - Macro: int ENOTBLK

A file that isn't a block special file was given in a situation that requires one.  For example, trying to mount an ordinary file as a file system in Unix gives this error.

 - Macro: int EBUSY

Resource busy; a system resource that can't be shared is already in use.  For example, if you try to delete a file that is the root of a currently mounted filesystem, you get this error.

 - Macro: int EEXIST

File exists; an existing file was specified in a context where it only makes sense to specify a new file.

 - Macro: int EXDEV

An attempt to make an improper link across file systems was detected.  This happens not only when you use link (see Hard Links) but also when you rename a file with rename (see Renaming Files).

 - Macro: int ENODEV

The wrong type of device was given to a function that expects a particular sort of device.

 - Macro: int ENOTDIR

A file that isn't a directory was specified when a directory is required.

 - Macro: int EISDIR

File is a directory; attempting to open a directory for writing gives this error.

 - Macro: int EINVAL

Invalid argument.  This is used to indicate various kinds of problems with passing the wrong argument to a library function.

 - Macro: int EMFILE

The current process has too many files open and can't open any more.  Duplicate descriptors do count toward this limit.

 - Macro: int ENFILE

There are too many distinct file openings in the entire system.  Note that any number of linked channels count as just one file opening; see See Linked Channels.  This error never occurs in the GNU system.

 - Macro: int ENOTTY

Inappropriate I/O control operation, such as trying to set terminal modes on an ordinary file.

 - Macro: int ETXTBSY

An attempt to execute a file that is currently open for writing, or write to a file that is currently being executed.  (The name stands for "text file busy".)  This is not an error in the GNU system; the text is copied as necessary.

 - Macro: int EFBIG

File too big; the size of a file would be larger than allowed by the system.

 - Macro: int ENOSPC

No space left on device; write operation on a file failed because the disk is full.

 - Macro: int ESPIPE

Invalid seek operation (such as on a pipe).

 - Macro: int EROFS

An attempt was made to modify something on a read-only file system.

 - Macro: int EMLINK

Too many links; the link count of a single file is too large.  rename can cause this error if the file being renamed already has as many links as it can take (see Renaming Files).

 - Macro: int EPIPE

Broken pipe; there is no process reading from the other end of a pipe.  Every library function that returns this error code also generates a SIGPIPE signal; this signal terminates the program if not handled or blocked.  Thus, your program will never actually see EPIPE unless it has handled or blocked SIGPIPE.

 - Macro: int EDOM

Domain error; used by mathematical functions when an argument value does not fall into the domain over which the function is defined.

 - Macro: int ERANGE

Range error; used by mathematical functions when the result value is not representable because of overflow or underflow.

 - Macro: int EAGAIN

Resource temporarily unavailable; the call might work if you try again later.  Only fork returns error code EAGAIN for such a reason.

 - Macro: int EWOULDBLOCK

An operation that would block was attempted on an object that has non-blocking mode selected.

Portability Note: In 4.4BSD and GNU, EWOULDBLOCK and EAGAIN are the same.  Earlier versions of BSD (see Berkeley Unix) have two distinct codes, and use EWOULDBLOCK to indicate an I/O operation that would block on an object with non-blocking mode set, and EAGAIN for other kinds of errors.

 - Macro: int EINPROGRESS

An operation that cannot complete immediately was initiated on an object that has non-blocking mode selected.  Some functions that must always block (such as connect; see Connecting) never return EWOULDBLOCK.  Instead, they return EINPROGRESS to indicate that the operation has begun and will take some time.  Attempts to manipulate the object before the call completes return EALREADY.

 - Macro: int EALREADY

An operation is already in progress on an object that has non-blocking mode selected.

 - Macro: int ENOTSOCK

A file that isn't a socket was specified when a socket is required.

 - Macro: int EDESTADDRREQ

No destination address was supplied on a socket operation that needed one.

 - Macro: int EMSGSIZE

The size of a message sent on a socket was larger than the supported maximum size.

 - Macro: int EPROTOTYPE

The socket type does not support the requested communications protocol.

 - Macro: int ENOPROTOOPT

You specified a socket option that doesn't make sense for the particular protocol being used by the socket.  See Socket Options.

 - Macro: int EPROTONOSUPPORT

The socket domain does not support the requested communications protocol (perhaps because the requested protocol is completely invalid.) See Creating a Socket.

 - Macro: int ESOCKTNOSUPPORT

The socket type is not supported.

 - Macro: int EOPNOTSUPP

The operation you requested is not supported.  Some socket functions don't make sense for all types of sockets, and others may not be implemented for all communications protocols.

 - Macro: int EPFNOSUPPORT

The socket communications protocol family you requested is not supported.

 - Macro: int EAFNOSUPPORT

The address family specified for a socket is not supported; it is inconsistent with the protocol being used on the socket.  See Sockets.

 - Macro: int EADDRINUSE

The requested socket address is already in use.  See Socket Addresses.

 - Macro: int EADDRNOTAVAIL

The requested socket address is not available; for example, you tried to give a socket a name that doesn't match the local host name.  See Socket Addresses.

 - Macro: int ENETDOWN

A socket operation failed because the network was down.

 - Macro: int ENETUNREACH

A socket operation failed because the subnet containing the remost host was unreachable.

 - Macro: int ENETRESET

A network connection was reset because the remote host crashed.

 - Macro: int ECONNABORTED

A network connection was aborted locally.

 - Macro: int ECONNRESET

A network connection was closed for reasons outside the control of the local host, such as by the remote machine rebooting or an unrecoverable protocol violation.

 - Macro: int ENOBUFS

The kernel's buffers for I/O operations are all in use.

 - Macro: int EISCONN

You tried to connect a socket that is already connected.  See Connecting.

 - Macro: int ENOTCONN

The socket is not connected to anything.  You get this error when you try to transmit data over a socket, without first specifying a destination for the data.

 - Macro: int ESHUTDOWN

The socket has already been shut down.

 - Macro: int ETIMEDOUT

A socket operation with a specified timeout received no response during the timeout period.

 - Macro: int ECONNREFUSED

A remote host refused to allow the network connection (typically because it is not running the requested service).

 - Macro: int ELOOP

Too many levels of symbolic links were encountered in looking up a file name.  This often indicates a cycle of symbolic links.

 - Macro: int ENAMETOOLONG

Filename too long (longer than PATH_MAX; see Limits for Files) or host name too long (in gethostname or sethostname; see Host Identification).

 - Macro: int EHOSTDOWN

The remote host for a requested network connection is down.

 - Macro: int EHOSTUNREACH

The remote host for a requested network connection is not reachable.

 - Macro: int ENOTEMPTY

Directory not empty, where an empty directory was expected.  Typically, this error occurs when you are trying to delete a directory.

 - Macro: int EUSERS

The file quota system is confused because there are too many users.

 - Macro: int EDQUOT

The user's disk quota was exceeded.

 - Macro: int ESTALE

Stale NFS file handle.  This indicates an internal confusion in the NFS system which is due to file system rearrangements on the server host.  Repairing this condition usually requires unmounting and remounting the NFS file system on the local host.

 - Macro: int EREMOTE

An attempt was made to NFS-mount a remote file system with a file name that already specifies an NFS-mounted file.  (This is an error on some operating systems, but we expect it to work properly on the GNU system, making this error code impossible.)

 - Macro: int ENOLCK

No locks available.  This is used by the file locking facilities; see See File Locks.  This error never occurs in the GNU system.

 - Macro: int ENOSYS

Function not implemented.  Some functions have commands or options defined that might not be supported in all implementations, and this is the kind of error you get if you request them and they are not supported.

 - Macro: int EBACKGROUND

In the GNU system, servers supporting the term protocol return this error for certain operations when the caller is not in the foreground process group of the terminal.  Users do not usually see this error because functions such as read and write translate it into a SIGTTIN or SIGTTOU signal.  See Job Control, for information on process groups and these signals.

 - Macro: int ED

The experienced user will know what is wrong.

 - Macro: int EGREGIOUS

You did what?

 - Macro: int EIEIO

Go home and have a glass of warm, dairy-fresh milk.

 - Macro: int EGRATUITOUS

This error code has no purpose.


Error Messages
Node: Error Messages, Next: , Prev: Error Codes, Up: Error Reporting

Error Messages

The library has functions and variables designed to make it easy for your program to report informative error messages in the customary format about the failure of a library call.  The functions strerror and perror give you the standard error message for a given error code; the variable program_invocation_short_name gives you convenient access to the name of the program that encountered the error.

 - Function: char * strerror (int errnum)

The strerror function maps the error code (see Checking for Errors) specified by the errnum argument to a descriptive error message string.  The return value is a pointer to this string.

The value errnum normally comes from the variable errno.

You should not modify the string returned by strerror.  Also, if you make subsequent calls to strerror, the string might be overwritten.  (But it's guaranteed that no library function ever calls strerror behind your back.)

The function strerror is declared in 'string.h'.

 - Function: void perror (const char *message)

This function prints an error message to the stream stderr; see See Standard Streams.

If you call perror with a message that is either a null pointer or an empty string, perror just prints the error message corresponding to errno, adding a trailing newline.

If you supply a non-null message argument, then perror prefixes its output with this string.  It adds a colon and a space character to separate the message from the error string corresponding to errno.

The function perror is declared in 'stdio.h'.

strerror and perror produce the exact same message for any given error code; the precise text varies from system to system.  On the GNU system, the messages are fairly short; there are no multi-line messages or embedded newlines.  Each error message begins with a capital letter and does not include any terminating punctuation.

Compatibility Note:  The strerror function is a new feature of ANSI C.  Many older C systems do not support this function yet.

Many programs that don't read input from the terminal are designed to exit if any system call fails.  By convention, the error message from such a program should start with the program's name, sans directories.  You can find that name in the variable program_invocation_short_name; the full file name is stored the variable program_invocation_name:

 - Variable: char * program_invocation_name

This variable's value is the name that was used to invoke the program running in the current process.  It is the same as argv[0].  Note that this is not necessarily a useful file name; often it contains no directory names.  See Program Arguments.

 - Variable: char * program_invocation_short_name

This variable's value is the name that was used to invoke the program running in the current process, with directory names removed.  (That is to say, it is the same as program_invocation_name minus everything up to the last slash, if any.)

The library initialization code sets up both of these variables before calling main.

Portability Note: These two variables are GNU extensions.  If you want your program to work with non-GNU libraries, you must save the value of argv[0] in main, and then strip off the directory names yourself.  We added these extensions to make it possible to write self-contained error-reporting subroutines that require no explicit cooperation from main.

Here is an example showing how to handle failure to open a file correctly.  The function open_sesame tries to open the named file for reading and returns a stream if successful.  The fopen library function returns a null pointer if it couldn't open the file for some reason.  In that situation, open_sesame constructs an appropriate error message using the strerror function, and terminates the program.  If we were going to make some other library calls before passing the error code to strerror, we'd have to save it in a local variable instead, because those other library functions might overwrite errno in the meantime.

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

FILE *
open_sesame (char *name)
{
  FILE *stream;

  errno = 0;
  stream = fopen (name, "r");
  if (stream == NULL)
    {
      fprintf (stderr, "%s: Couldn't open file %s; %s\n",
               program_invocation_short_name, name, strerror (errno));
      exit (EXIT_FAILURE);
    }
  else
    return stream;
}


Memory Allocation
Node: Memory Allocation, Next: Character Handling, Prev: Error Reporting, Up: Top

Memory Allocation

The GNU system provides several methods for allocating memory space under explicit program control.  They vary in generality and in efficiency.

* Menu:

Memory Concepts	An introduction to concepts and terminology.

Dynamic Allocation and C	How to get different kinds of allocation in C.

Unconstrained Allocation	The malloc facility allows fully general dynamic allocation.

Obstacks	Obstacks are less general than malloc but more efficient and convenient.

Variable Size Automatic	Allocation of variable-sized blocks of automatic storage that are freed when the calling function returns.

Relocating Allocator	Waste less memory, if you can tolerate automatic relocation of the blocks you get.

Memory Warnings	Getting warnings when memory is nearly full.


Dynamic Memory Allocation Concepts
Node: Memory Concepts, Next: Dynamic Allocation and C, Prev: , Up: Memory Allocation

Dynamic Memory Allocation Concepts

"Dynamic memory allocation" is a technique in which programs determine as they are running where to store some information.  You need dynamic allocation when the number of memory blocks you need, or how long you continue to need them, depends on the data you are working on.

For example, you may need a block to store a line read from an input file; since there is no limit to how long a line can be, you must allocate the storage dynamically and make it dynamically larger as you read more of the line.

Or, you may need a block for each record or each definition in the input data; since you can't know in advance how many there will be, you must allocate a new block for each record or definition as you read it.

When you use dynamic allocation, the allocation of a block of memory is an action that the program requests explicitly.  You call a function or macro when you want to allocate space, and specify the size with an argument.  If you want to free the space, you do so by calling another function or macro.  You can do these things whenever you want, as often as you want.


Dynamic Allocation and C
Node: Dynamic Allocation and C, Next: Unconstrained Allocation, Prev: Memory Concepts, Up: Memory Allocation

Dynamic Allocation and C

The C language supports two kinds of memory allocation through the variables in C programs:

•	"Static allocation" is what happens when you declare a static variable.  Each static variable defines one block of space, of a fixed size.  The space is allocated once, when your program is started, and is never freed.

•	"Automatic allocation" happens when you declare an automatic variable, such as a function argument or a local variable.  The space for an automatic variable is allocated when the compound statement containing the declaration is entered, and is freed when that compound statement is exited.

In GNU C, the length of the automatic storage can be an expression that varies.  In other C implementations, it must be a constant.

Dynamic allocation is not supported by C variables; there is no storage class "dynamic", and there can never be a C variable whose value is stored in dynamically allocated space.  The only way to refer to dynamically allocated space is through a pointer.  Because it is less convenient, and because the actual process of dynamic allocation requires more computation time, programmers use dynamic allocation only when neither static nor automatic allocation will serve.

For example, if you want to allocate dynamically some space to hold a struct foobar, you cannot declare a variable of type struct foobar whose contents are the dynamically allocated space.  But you can declare a variable of pointer type struct foobar * and assign it the address of the space.  Then you can use the operators '*' and '->' on this pointer variable to refer to the contents of the space:

{
  struct foobar *ptr
     = (struct foobar *) malloc (sizeof (struct foobar));
  ptr->name = x;
  ptr->next = current_foobar;
  current_foobar = ptr;
}


Unconstrained Allocation
Node: Unconstrained Allocation, Next: Obstacks, Prev: Dynamic Allocation and C, Up: Memory Allocation

Unconstrained Allocation

The most general dynamic allocation facility is malloc.  It allows you to allocate blocks of memory of any size at any time, make them bigger or smaller at any time, and free the blocks individually at any time (or never).

* Menu:

Basic Allocation	Simple use of malloc.

Malloc Examples	Examples of malloc.  xmalloc.

Freeing after Malloc	Use free to free a block you got with malloc.

Changing Block Size	Use realloc to make a block bigger or smaller.

Allocating Cleared Space	Use calloc to allocate a block and clear it.

Efficiency and Malloc	Efficiency considerations in use of these functions.

Aligned Memory Blocks	Allocating specially aligned memory: memalign and valloc.

Heap Consistency Checking	Automatic checking for errors.

Hooks for Malloc	You can use these hooks for debugging programs that use malloc.

Statistics of Malloc	Getting information about how much memory your program is using.

Summary of Malloc	Summary of malloc and related functions.


Basic Storage Allocation
Node: Basic Allocation, Next: Malloc Examples, Prev: , Up: Unconstrained Allocation

Basic Storage Allocation

To allocate a block of memory, call malloc.  The prototype for this function is in 'stdlib.h'.  K

 - Function: void * malloc (size_t size)

This function returns a pointer to a newly allocated block size bytes long, or a null pointer if the block could not be allocated.

The contents of the block are undefined; you must initialize it yourself (or use calloc instead; see Allocating Cleared Space).  Normally you would cast the value as a pointer to the kind of object that you want to store in the block.  Here we show an example of doing so, and of initializing the space with zeros using the library function memset (see Copying and Concatenation):

struct foo *ptr;
...
ptr = (struct foo *) malloc (sizeof (struct foo));
if (ptr == 0) abort ();
memset (ptr, 0, sizeof (struct foo));

You can store the result of malloc into any pointer variable without a cast, because ANSI C automatically converts the type void * to another type of pointer when necessary.  But the cast is necessary in contexts other than assignment operators or if you might want your code to run in traditional C.

Remember that when allocating space for a string, the argument to malloc must be one plus the length of the string.  This is because a string is terminated with a null character that doesn't count in the "length" of the string but does need space.  For example:

char *ptr;
...
ptr = (char *) malloc (length + 1);

See Representation of Strings, for more information about this.


Examples of malloc
Node: Malloc Examples, Next: Freeing after Malloc, Prev: Basic Allocation, Up: Unconstrained Allocation

Examples of malloc

If no more space is available, malloc returns a null pointer.  You should check the value of every call to malloc.  It is useful to write a subroutine that calls malloc and reports an error if the value is a null pointer, returning only if the value is nonzero.  This function is conventionally called xmalloc.  Here it is:

void *
xmalloc (size_t size)
{
  register void *value = malloc (size);
  if (value == 0)
    fatal ("virtual memory exhausted");
  return value;
}

Here is a real example of using malloc (by way of xmalloc).  The function savestring will copy a sequence of characters into a newly allocated null-terminated string:

char *
savestring (const char *ptr, size_t len)
{
  register char *value = (char *) xmalloc (len + 1);
  memcpy (value, ptr, len);
  value[len] = '\0';
  return value;
}

The block that malloc gives you is guaranteed to be aligned so that it can hold any type of data.  In the GNU system, the address is always a multiple of eight; if the size of block is 16 or more, then the address is always a multiple of 16.  Only rarely is any higher boundary (such as a page boundary) necessary; for those cases, use memalign or valloc (see Aligned Memory Blocks).

Note that the memory located after the end of the block is likely to be in use for something else; perhaps a block already allocated by another call to malloc.  If you attempt to treat the block as longer than you asked for it to be, you are liable to destroy the data that malloc uses to keep track of its blocks, or you may destroy the contents of another block.  If you have already allocated a block and discover you want it to be bigger, use realloc (see Changing Block Size).


Freeing Memory Allocated with malloc
Node: Freeing after Malloc, Next: Changing Block Size, Prev: Malloc Examples, Up: Unconstrained Allocation

Freeing Memory Allocated with malloc

When you no longer need a block that you got with malloc, use the function free to make the block available to be allocated again.  The prototype for this function is in 'stdlib.h'.  K

 - Function: void free (void *ptr)

The free function deallocates the block of storage pointed at by ptr.

 - Function: void cfree (void *ptr)

This function does the same thing as free.  It's provided for backward compatibility with SunOS; you should use free instead.

Freeing a block alters the contents of the block.  Do not expect to find any data (such as a pointer to the next block in a chain of blocks) in the block after freeing it.  Copy whatever you need out of the block before freeing it!  Here is an example of the proper way to free all the blocks in a chain, and the strings that they point to:

struct chain
  {
    struct chain *next;
    char *name;
  }

void
free_chain (struct chain *chain)
{
  while (chain != 0)
    {
      struct chain *next = chain->next;
      free (chain->name);
      free (chain);
      chain = next;
    }
}

Occasionally, free can actually return memory to the operating system and make the process smaller.  Usually, all it can do is allow a later call to malloc to reuse the space.  In the meantime, the space remains in your program as part of a free-list used internally by malloc.

There is no point in freeing blocks at the end of a program, because all of the program's space is given back to the system when the process terminates.


Changing the Size of a Block
Node: Changing Block Size, Next: Allocating Cleared Space, Prev: Freeing after Malloc, Up: Unconstrained Allocation

Changing the Size of a Block

Often you do not know for certain how big a block you will ultimately need at the time you must begin to use the block.  For example, the block might be a buffer that you use to hold a line being read from a file; no matter how long you make the buffer initially, you may encounter a line that is longer.

You can make the block longer by calling realloc.  This function is declared in 'stdlib.h'.  K

 - Function: void * realloc (void *ptr, size_t newsize)

The realloc function changes the size of the block whose address is ptr to be newsize.

Since the space after the end of the block may be in use, realloc may find it necessary to copy the block to a new address where more free space is available.  The value of realloc is the new address of the block.  If the block needs to be moved, realloc copies the old contents.

If you pass a null pointer for ptr, realloc behaves just like 'malloc (newsize)'.  This can be convenient, but beware that older implementations (before ANSI C) may not support this behavior, and will probably crash when realloc is passed a null pointer.

Like malloc, realloc may return a null pointer if no memory space is available to make the block bigger.  When this happens, the original block is untouched; it has not been modified or relocated.

In most cases it makes no difference what happens to the original block when realloc fails, because the application program cannot continue when it is out of memory, and the only thing to do is to give a fatal error message.  Often it is convenient to write and use a subroutine, conventionally called xrealloc, that takes care of the error message as xmalloc does for malloc:

void *
xrealloc (void *ptr, size_t size)
{
  register void *value = realloc (ptr, size);
  if (value == 0)
    fatal ("Virtual memory exhausted");
  return value;
}

You can also use realloc to make a block smaller.  The reason you would do this is to avoid tying up a lot of memory space when only a little is needed.  Making a block smaller sometimes necessitates copying it, so it can fail if no other space is available.

If the new size you specify is the same as the old size, realloc is guaranteed to change nothing and return the same address that you gave.


Allocating Cleared Space
Node: Allocating Cleared Space, Next: Efficiency and Malloc, Prev: Changing Block Size, Up: Unconstrained Allocation

Allocating Cleared Space

The function calloc allocates memory and clears it to zero.  It is declared in 'stdlib.h'.  K

 - Function: void * calloc (size_t count, size_t eltsize)

This function allocates a block long enough to contain a vector of count elements, each of size eltsize.  Its contents are cleared to zero before calloc returns.

You could define calloc as follows:

void *
calloc (size_t count, size_t eltsize)
{
  size_t size = count * eltsize;
  void *value = malloc (size);
  if (value != 0)
    memset (value, 0, size);
  return value;
}

We rarely use calloc today, because it is equivalent to such a simple combination of other features that are more often used.  It is a historical holdover that is not quite obsolete.


Efficiency Considerations for malloc
Node: Efficiency and Malloc, Next: Aligned Memory Blocks, Prev: Allocating Cleared Space, Up: Unconstrained Allocation

Efficiency Considerations for malloc

To make the best use of malloc, it helps to know that the GNU version of malloc always dispenses small amounts of memory in blocks whose sizes are powers of two.  It keeps separate pools for each power of two.  This holds for sizes up to a page size.  Therefore, if you are free to choose the size of a small block in order to make malloc more efficient, make it a power of two.

Once a page is split up for a particular block size, it can't be reused for another size unless all the blocks in it are freed.  In many programs, this is unlikely to happen.  Thus, you can sometimes make a program use memory more efficiently by using blocks of the same size for many different purposes.

When you ask for memory blocks of a page or larger, malloc uses a different strategy; it rounds the size up to a multiple of a page, and it can coalesce and split blocks as needed.

The reason for the two strategies is that it is important to allocate and free small blocks as fast as possible, but speed is less important for a large block since the program normally spends a fair amount of time using it.  Also, large blocks are normally fewer in number.  Therefore, for large blocks, it makes sense to use a method which takes more time to minimize the wasted space.


Allocating Aligned Memory Blocks
Node: Aligned Memory Blocks, Next: Heap Consistency Checking, Prev: Efficiency and Malloc, Up: Unconstrained Allocation

Allocating Aligned Memory Blocks

The address of a block returned by malloc or realloc in the GNU system is always a multiple of eight.  If you need a block whose address is a multiple of a higher power of two than that, use memalign or valloc.  These functions are declared in 'stdlib.h'.

With the GNU library, you can use free to free the blocks that memalign and valloc return.  That does not work in BSD, however--BSD does not provide any way to free such blocks.

 - Function: void * memalign (size_t size, size_t boundary)

The memalign function allocates a block of size bytes whose address is a multiple of boundary.  The boundary must be a power of two!  The function memalign works by calling malloc to allocate a somewhat larger block, and then returning an address within the block that is on the specified boundary.

 - Function: void * valloc (size_t size)

Using valloc is like using memalign and passing the page size as the value of the second argument.  It is implemented like this:

void *
valloc (size_t size)
{
  return memalign (size, getpagesize ());
}


Heap Consistency Checking
Node: Heap Consistency Checking, Next: Hooks for Malloc, Prev: Aligned Memory Blocks, Up: Unconstrained Allocation

Heap Consistency Checking

You can ask malloc to check the consistency of dynamic storage by using the mcheck function.  This function is a GNU extension, declared in 'malloc.h'.  K

 - Function: int mcheck (void (*abortfn) (void))

Calling mcheck tells malloc to perform occasional consistency checks.  These will catch things such as writing past the end of a block that was allocated with malloc.

The abortfn argument is the function to call when an inconsistency is found.  If you supply a null pointer, the abort function is used.

It is too late to begin allocation checking once you have allocated anything with malloc.  So mcheck does nothing in that case.  The function returns -1 if you call it too late, and 0 otherwise (when it is successful).

The easiest way to arrange to call mcheck early enough is to use the option '-lmcheck' when you link your program; then you don't need to modify your program source at all.


Storage Allocation Hooks
Node: Hooks for Malloc, Next: Statistics of Malloc, Prev: Heap Consistency Checking, Up: Unconstrained Allocation

Storage Allocation Hooks

The GNU C library lets you modify the behavior of malloc, realloc, and free by specifying appropriate hook functions.  You can use these hooks to help you debug programs that use dynamic storage allocation, for example.

The hook variables are declared in 'malloc.h'.  K

 - Variable: __malloc_hook

The value of this variable is a pointer to function that malloc uses whenever it is called.  You should define this function to look like malloc; that is, like:

void *function (size_t size)

 - Variable: __realloc_hook

The value of this variable is a pointer to function that realloc uses whenever it is called.  You should define this function to look like realloc; that is, like:

void *function (void *ptr, size_t size)

 - Variable: __free_hook

The value of this variable is a pointer to function that free uses whenever it is called.  You should define this function to look like free; that is, like:

void function (void *ptr)

You must make sure that the function you install as a hook for one of these functions does not call that function recursively without restoring the old value of the hook first!  Otherwise, your program will get stuck in an infinite recursion.

Here is an example showing how to use __malloc_hook properly.  It installs a function that prints out information every time malloc is called.

static void *(*old_malloc_hook) (size_t);
static void *
my_malloc_hook (size_t size)
{
  void *result;
  __malloc_hook = old_malloc_hook;
  result = malloc (size);
  __malloc_hook = my_malloc_hook;
  printf ("malloc (%u) returns %p\n", (unsigned int) size, result);
  return result;
}

main ()
{
  ...
  old_malloc_hook = __malloc_hook;
  __malloc_hook = my_malloc_hook;
  ...
}

The mcheck function (see Heap Consistency Checking) works by installing such hooks.


Statistics for Storage Allocation with malloc
Node: Statistics of Malloc, Next: Summary of Malloc, Prev: Hooks for Malloc, Up: Unconstrained Allocation

Statistics for Storage Allocation with malloc

You can get information about dynamic storage allocation by calling the mstats function.  This function and its associated data type are declared in 'malloc.h'; they are a GNU extension.  K

 - Data Type: struct mstats

This structure type is used to return information about the dynamic storage allocator.  It contains the following members:

size_t bytes_total

This is the total size of memory managed by malloc, in bytes.

size_t chunks_used

This is the number of chunks in use.  (The storage allocator internally gets chunks of memory from the operating system, and then carves them up to satisfy individual malloc requests; see See Efficiency and Malloc.)

size_t bytes_used

This is the number of bytes in use.

size_t chunks_free

This is the number of chunks which are free - that is, that have been allocated by the operating system to your program, but which are not now being used.

size_t bytes_free

This is the number of bytes which are free.

 - Function: struct mstats mstats (void)

This function returns information about the current dynamic memory usage in a structure of type struct mstats.


Summary of malloc-Related Functions
Node: Summary of Malloc, Next: , Prev: Statistics of Malloc, Up: Unconstrained Allocation

Summary of malloc-Related Functions

Here is a summary of the functions that work with malloc:

void *malloc (size_t size)

Allocate a block of size bytes.  See Basic Allocation.

void free (void *addr)

Free a block previously allocated by malloc.  See Freeing after Malloc.

void *realloc (void *addr, size_t size)

Make a block previously allocated by malloc larger or smaller, possibly by copying it to a new location.  See Changing Block Size.

void *calloc (size_t count, size_t eltsize)

Allocate a block of count * eltsize bytes using malloc, and set its contents to zero.  See Allocating Cleared Space.

void *valloc (size_t size)

Allocate a block of size bytes, starting on a page boundary.  See Aligned Memory Blocks.

void *memalign (size_t size, size_t boundary)

Allocate a block of size bytes, starting on an address that is a multiple of boundary.  See Aligned Memory Blocks.

int mcheck (void (*abortfn) (void))

Tell malloc to perform occasional consistency checks on dynamically allocated memory, and to call abortfn when an inconsistency is found.  See Heap Consistency Checking.

void *(*__malloc_hook) (size_t size)

A pointer to a function that malloc uses whenever it is called.

void *(*__realloc_hook) (void *ptr, size_t size)

A pointer to a function that realloc uses whenever it is called.

void (*__free_hook) (void *ptr)

A pointer to a function that free uses whenever it is called.

struct mstats mstats (void)

Return information about the current dynamic memory usage.  See Statistics of Malloc.


Obstacks
Node: Obstacks, Next: Variable Size Automatic, Prev: Unconstrained Allocation, Up: Memory Allocation

Obstacks

An "obstack" is a pool of memory containing a stack of objects.  You can create any number of separate obstacks, and then allocate objects in specified obstacks.  Within each obstack, the last object allocated must always be the first one freed, but distinct obstacks are independent of each other.

Aside from this one constraint of order of freeing, obstacks are totally general: an obstack can contain any number of objects of any size.  They are implemented with macros, so allocation is usually very fast as long as the objects are usually small.  And the only space overhead per object is the padding needed to start each object on a suitable boundary.

* Menu:

Creating Obstacks	How to declare an obstack in your program.

Preparing for Obstacks	Preparations needed before you can use obstacks.

Allocation in an Obstack	Allocating objects in an obstack.

Freeing Obstack Objects	Freeing objects in an obstack.

Obstack Functions	The obstack functions are both functions and macros.

Growing Objects	Making an object bigger by stages.

Extra Fast Growing	Extra-high-efficiency (though more complicated) growing objects.

Status of an Obstack	Inquiries about the status of an obstack.

Obstacks Data Alignment	Controlling alignment of objects in obstacks.

Obstack Chunks	How obstacks obtain and release chunks; efficiency considerations.

Summary of Obstacks


Creating Obstacks
Node: Creating Obstacks, Next: Preparing for Obstacks, Prev: , Up: Obstacks

Creating Obstacks

The utilities for manipulating obstacks are declared in the header file 'obstack.h'.  K

 - Data Type: struct obstack

An obstack is represented by a data structure of type struct obstack.  This structure has a small fixed size; it records the status of the obstack and how to find the space in which objects are allocated.  It does not contain any of the objects themselves.  You should not try to access the contents of the structure directly; use only the functions described in this chapter.

You can declare variables of type struct obstack and use them as obstacks, or you can allocate obstacks dynamically like any other kind of object.  Dynamic allocation of obstacks allows your program to have a variable number of different stacks.  (You can even allocate an obstack structure in another obstack, but this is rarely useful.)

All the functions that work with obstacks require you to specify which obstack to use.  You do this with a pointer of type struct obstack *.  In the following, we often say "an obstack" when strictly speaking the object at hand is such a pointer.

The objects in the obstack are packed into large blocks called "chunks".  The struct obstack structure points to a chain of the chunks currently in use.

The obstack library obtains a new chunk whenever you allocate an object that won't fit in the previous chunk.  Since the obstack library manages chunks automatically, you don't need to pay much attention to them, but you do need to supply a function which the obstack library should use to get a chunk.  Usually you supply a function which uses malloc directly or indirectly.  You must also supply a function to free a chunk.  These matters are described in the following section.


Preparing for Using Obstacks
Node: Preparing for Obstacks, Next: Allocation in an Obstack, Prev: Creating Obstacks, Up: Obstacks

Preparing for Using Obstacks

Each source file in which you plan to use the obstack functions must include the header file 'obstack.h', like this:

#include <obstack.h>

Also, if the source file uses the macro obstack_init, it must declare or define two functions or macros that will be called by the obstack library.  One, obstack_chunk_alloc, is used to allocate the chunks of memory into which objects are packed.  The other, obstack_chunk_free, is used to return chunks when the objects in them are freed.

Usually these are defined to use malloc via the intermediary xmalloc (see Unconstrained Allocation).  This is done with the following pair of macro definitions:

#define obstack_chunk_alloc xmalloc
#define obstack_chunk_free free

Though the storage you get using obstacks really comes from malloc, using obstacks is faster because malloc is called less often, for larger blocks of memory.  See Obstack Chunks, for full details.

At run time, before the program can use a struct obstack object as an obstack, it must initialize the obstack by calling obstack_init.

 - Function: int obstack_init (struct obstack *obstack_ptr)

Initialize obstack obstack_ptr for allocation of objects.  This function calls the obstack's obstack_chunk_alloc function.  It returns 0 if obstack_chunk_alloc returns a null pointer, meaning that it is out of memory.  Otherwise, it returns 1.  If you supply an obstack_chunk_alloc function that calls exit (see Program Termination) or longjmp (see Non-Local Exits) when out of memory, you can safely ignore the value that obstack_init returns.

Here are two examples of how to allocate the space for an obstack and initialize it.  First, an obstack that is a static variable:

static struct obstack myobstack;
...
obstack_init (&myobstack);

Second, an obstack that is itself dynamically allocated:

struct obstack *myobstack_ptr
  = (struct obstack *) xmalloc (sizeof (struct obstack));

obstack_init (myobstack_ptr);


Allocation in an Obstack
Node: Allocation in an Obstack, Next: Freeing Obstack Objects, Prev: Preparing for Obstacks, Up: Obstacks

Allocation in an Obstack

The most direct way to allocate an object in an obstack is with obstack_alloc, which is invoked almost like malloc.

 - Function: void * obstack_alloc (struct obstack *obstack_ptr, size_t size)

This allocates an uninitialized block of size bytes in an obstack and returns its address.  Here obstack_ptr specifies which obstack to allocate the block in; it is the address of the struct obstack object which represents the obstack.  Each obstack function or macro requires you to specify an obstack_ptr as the first argument.

This function calls the obstack's obstack_chunk_alloc function if it needs to allocate a new chunk of memory; it returns a null pointer if obstack_chunk_alloc returns one.  In that case, it has not changed the amount of memory allocated in the obstack.  If you supply an obstack_chunk_alloc function that calls exit (see Program Termination) or longjmp (see Non-Local Exits) when out of memory, then obstack_alloc will never return a null pointer.

For example, here is a function that allocates a copy of a string str in a specific obstack, which is in the variable string_obstack:

struct obstack string_obstack;

char *
copystring (char *string)
{
  char *s = (char *) obstack_alloc (&string_obstack,
                                    strlen (string) + 1);
  memcpy (s, string, strlen (string));
  return s;
}

To allocate a block with specified contents, use the function obstack_copy, declared like this:

 - Function: void * obstack_copy (struct obstack *obstack_ptr, void *address, size_t size)

This allocates a block and initializes it by copying size bytes of data starting at address.  It can return a null pointer under the same conditions as obstack_alloc.

 - Function: void * obstack_copy0 (struct obstack *obstack_ptr, void *address, size_t size)

Like obstack_copy, but appends an extra byte containing a null character.  This extra byte is not counted in the argument size.

The obstack_copy0 function is convenient for copying a sequence of characters into an obstack as a null-terminated string.  Here is an example of its use:

char *
obstack_savestring (char *addr, size_t size)
{
  return obstack_copy0 (&myobstack, addr, size);
}

Contrast this with the previous example of savestring using malloc (see Basic Allocation).


Freeing Objects in an Obstack
Node: Freeing Obstack Objects, Next: Obstack Functions, Prev: Allocation in an Obstack, Up: Obstacks

Freeing Objects in an Obstack

To free an object allocated in an obstack, use the function obstack_free.  Since the obstack is a stack of objects, freeing one object automatically frees all other objects allocated more recently in the same obstack.

 - Function: void obstack_free (struct obstack *obstack_ptr, void *object)

If object is a null pointer, everything allocated in the obstack is freed.  Otherwise, object must be the address of an object allocated in the obstack.  Then object is freed, along with everything allocated in obstack since object.

Note that if object is a null pointer, the result is an uninitialized obstack.  To free all storage in an obstack but leave it valid for further allocation, call obstack_free with the address of the first object allocated on the obstack:

obstack_free (obstack_ptr, first_object_allocated_ptr);

Recall that the objects in an obstack are grouped into chunks.  When all the objects in a chunk become free, the obstack library automatically frees the chunk (see Preparing for Obstacks).  Then other obstacks, or non-obstack allocation, can reuse the space of the chunk.


Obstack Functions and Macros
Node: Obstack Functions, Next: Growing Objects, Prev: Freeing Obstack Objects, Up: Obstacks

Obstack Functions and Macros

The interfaces for using obstacks may be defined either as functions or as macros, depending on the compiler.  The obstack facility works with all C compilers, including both ANSI C and traditional C, but there are precautions you must take if you plan to use compilers other than GNU C.

If you are using an old-fashioned non-ANSI C compiler, all the obstack "functions" are actually defined only as macros.  You can call these macros like functions, but you cannot use them in any other way (for example, you cannot take their address).

Calling the macros requires a special precaution: namely, the first operand (the obstack pointer) may not contain any side effects, because it may be computed more than once.  For example, if you write this:

obstack_alloc (get_obstack (), 4);

you will find that get_obstack may be called several times.  If you use *obstack_list_ptr++ as the obstack pointer argument, you will get very strange results since the incrementation may occur several times.

In ANSI C, each function has both a macro definition and a function definition.  The function definition is used if you take the address of the function without calling it.  An ordinary call uses the macro definition by default, but you can request the function definition instead by writing the function name in parentheses, as shown here:

char *x;
void *(*funcp) ();
/* Use the macro.  */
x = (char *) obstack_alloc (obptr, size);
/* Call the function.  */
x = (char *) (obstack_alloc) (obptr, size);
/* Take the address of the function.  */
funcp = obstack_alloc;

This is the same situation that exists in ANSI C for the standard library functions.  See Macro Definitions.

Warning: When you do use the macros, you must observe the precaution of avoiding side effects in the first operand, even in ANSI C.

If you use the GNU C compiler, this precaution is not necessary, because various language extensions in GNU C permit defining the macros so as to compute each argument only once.


Growing Objects
Node: Growing Objects, Next: Extra Fast Growing, Prev: Obstack Functions, Up: Obstacks

Growing Objects

Because storage in obstack chunks is used sequentially, it is possible to build up an object step by step, adding one or more bytes at a time to the end of the object.  With this technique, you do not need to know how much data you will put in the object until you come to the end of it.  We call this the technique of "growing objects".  The special functions for adding data to the growing object are described in this section.

You don't need to do anything special when you start to grow an object.  Using one of the functions to add data to the object automatically starts it.  However, it is necessary to say explicitly when the object is finished.  This is done with the function obstack_finish.

The actual address of the object thus built up is not known until the object is finished.  Until then, it always remains possible that you will add so much data that the object must be copied into a new chunk.

While the obstack is in use for a growing object, you cannot use it for ordinary allocation of another object.  If you try to do so, the space already added to the growing object will become part of the other object.

 - Function: void obstack_blank (struct obstack *obstack_ptr, size_t size)

The most basic function for adding to a growing object is obstack_blank, which adds space without initializing it.

 - Function: void obstack_grow (struct obstack *obstack_ptr, void *data, size_t size)

To add a block of initialized space, use obstack_grow, which is the growing-object analogue of obstack_copy.  It adds size bytes of data to the growing object, copying the contents from data.

 - Function: void obstack_grow0 (struct obstack *obstack_ptr, void *data, size_t size)

This is the growing-object analogue of obstack_copy0.  It adds size bytes copied from data, followed by an additional null character.

 - Function: void obstack_1grow (struct obstack *obstack_ptr, char c)

To add one character at a time, use the function obstack_1grow.  It adds a single byte containing c to the growing object.

 - Function: void * obstack_finish (struct obstack *obstack_ptr)

When you are finished growing the object, use the function obstack_finish to close it off and return its final address.

Once you have finished the object, the obstack is available for ordinary allocation or for growing another object.

This function can return a null pointer under the same conditions as obstack_alloc (see Allocation in an Obstack).

When you build an object by growing it, you will probably need to know afterward how long it became.  You need not keep track of this as you grow the object, because you can find out the length from the obstack just before finishing the object with the function obstack_object_size, declared as follows:

 - Function: size_t obstack_object_size (struct obstack *obstack_ptr)

This function returns the current size of the growing object, in bytes.  Remember to call this function before finishing the object.  After it is finished, obstack_object_size will return zero.

If you have started growing an object and wish to cancel it, you should finish it and then free it, like this:

obstack_free (obstack_ptr, obstack_finish (obstack_ptr));

This has no effect if no object was growing.

You can use obstack_blank with a negative size argument to make the current object smaller.  Just don't try to shrink it beyond zero length--there's no telling what will happen if you do that.


Extra Fast Growing Objects
Node: Extra Fast Growing, Next: Status of an Obstack, Prev: Growing Objects, Up: Obstacks

Extra Fast Growing Objects

The usual functions for growing objects incur overhead for checking whether there is room for the new growth in the current chunk.  If you are frequently constructing objects in small steps of growth, this overhead can be significant.

You can reduce the overhead by using special "fast growth" functions that grow the object without checking.  In order to have a robust program, you must do the checking yourself.  If you do this checking in the simplest way each time you are about to add data to the object, you have not saved anything, because that is what the ordinary growth functions do.  But if you can arrange to check less often, or check more efficiently, then you make the program faster.

The function obstack_room returns the amount of room available in the current chunk.  It is declared as follows:

 - Function: size_t obstack_room (struct obstack *obstack_ptr)

This returns the number of bytes that can be added safely to the current growing object (or to an object about to be started) in obstack obstack using the fast growth functions.

While you know there is room, you can use these fast growth functions for adding data to a growing object:

 - Function: void obstack_1grow_fast (struct obstack *obstack_ptr, char c)

The function obstack_1grow_fast adds one byte containing the character c to the growing object in obstack obstack_ptr.

 - Function: void obstack_blank_fast (struct obstack *obstack_ptr, size_t size)

The function obstack_blank_fast adds size bytes to the growing object in obstack obstack_ptr without initializing them.

When you check for space using obstack_room and there is not enough room for what you want to add, the fast growth functions are not safe.  In this case, simply use the corresponding ordinary growth function instead.  Very soon this will copy the object to a new chunk; then there will be lots of room available again.

So, each time you use an ordinary growth function, check afterward for sufficient space using obstack_room.  Once the object is copied to a new chunk, there will be plenty of space again, so the program will start using the fast growth functions again.

Here is an example:

void
add_string (struct obstack *obstack, char *ptr, size_t len)
{
  while (len > 0)
    {
      if (obstack_room (obstack) > len)
        {
          /* We have enough room: add everything fast.  */
          while (len-- > 0)
            obstack_1grow_fast (obstack, *ptr++);
        }
      else
        {
          /* Not enough room. Add one character slowly,
             which may copy to a new chunk and make room.  */
          obstack_1grow (obstack, *ptr++);
          len--;
        }
    }
}


Status of an Obstack
Node: Status of an Obstack, Next: Obstacks Data Alignment, Prev: Extra Fast Growing, Up: Obstacks

Status of an Obstack

Here are functions that provide information on the current status of allocation in an obstack.  You can use them to learn about an object while still growing it.

 - Function: void * obstack_base (struct obstack *obstack_ptr)

This function returns the tentative address of the beginning of the currently growing object in obstack_ptr.  If you finish the object immediately, it will have that address.  If you make it larger first, it may outgrow the current chunk--then its address will change!

If no object is growing, this value says where the next object you allocate will start (once again assuming it fits in the current chunk).

 - Function: void * obstack_next_free (struct obstack *obstack_ptr)

This function returns the address of the first free byte in the current chunk of obstack obstack_ptr.  This is the end of the currently growing object.  If no object is growing, obstack_next_free returns the same value as obstack_base.

 - Function: size_t obstack_object_size (struct obstack *obstack_ptr)

This function returns the size in bytes of the currently growing object.  This is equivalent to

obstack_next_free (obstack_ptr) - obstack_base (obstack_ptr)


Alignment of Data in Obstacks
Node: Obstacks Data Alignment, Next: Obstack Chunks, Prev: Status of an Obstack, Up: Obstacks

Alignment of Data in Obstacks

Each obstack has an "alignment boundary"; each object allocated in the obstack automatically starts on an address that is a multiple of the specified boundary.  By default, this boundary is 4 bytes.

To access an obstack's alignment boundary, use the macro obstack_alignment_mask, whose function prototype looks like this:

 - Macro: int obstack_alignment_mask (struct obstack *obstack_ptr)

The value is a bit mask; a bit that is 1 indicates that the corresponding bit in the address of an object should be 0.  The mask value should be one less than a power of 2; the effect is that all object addresses are multiples of that power of 2.  The default value of the mask is 3, so that addresses are multiples of 4.  A mask value of 0 means an object can start on any multiple of 1 (that is, no alignment is required).

The expansion of the macro obstack_alignment_mask is an lvalue, so you can alter the mask by assignment.  For example, this statement:

obstack_alignment_mask (obstack_ptr) = 0;

has the effect of turning off alignment processing in the specified obstack.

Note that a change in alignment mask does not take effect until after the next time an object is allocated or finished in the obstack.  If you are not growing an object, you can make the new alignment mask take effect immediately by calling obstack_finish.  This will finish a zero-length object and then do proper alignment for the next object.


Obstack Chunks
Node: Obstack Chunks, Next: Summary of Obstacks, Prev: Obstacks Data Alignment, Up: Obstacks

Obstack Chunks

Obstacks work by allocating space for themselves in large chunks, and then parceling out space in the chunks to satisfy your requests.  Chunks are normally 4096 bytes long unless you specify a different chunk size.  The chunk size includes 8 bytes of overhead that are not actually used for storing objects.  Regardless of the specified size, longer chunks will be allocated when necessary for long objects.

The obstack library allocates chunks by calling the function obstack_chunk_alloc, which you must define.  When a chunk is no longer needed because you have freed all the objects in it, the obstack library frees the chunk by calling obstack_chunk_free, which you must also define.

These two must be defined (as macros) or declared (as functions) in each source file that uses obstack_init (see Creating Obstacks).  Most often they are defined as macros like this:

#define obstack_chunk_alloc xmalloc
#define obstack_chunk_free free

Note that these are simple macros (no arguments).  Macro definitions with arguments will not work!  It is necessary that obstack_chunk_alloc or obstack_chunk_free, alone, expand into a function name if it is not itself a function name.

If you allocate chunks with malloc, the chunk size should be a power of 2.  The default chunk size, 4096, was chosen because it is long enough to satisfy many typical requests on the obstack yet short enough not to waste too much memory in the portion of the last chunk not yet used.

 - Macro: size_t obstack_chunk_size (struct obstack *obstack_ptr)

This returns the chunk size of the given obstack.

Since this macro expands to an lvalue, you can specify a new chunk size by assigning it a new value.  Doing so does not affect the chunks already allocated, but will change the size of chunks allocated for that particular obstack in the future.  It is unlikely to be useful to make the chunk size smaller, but making it larger might improve efficiency if you are allocating many objects whose size is comparable to the chunk size.  Here is how to do so cleanly:

if (obstack_chunk_size (obstack_ptr) < new_chunk_size)
  obstack_chunk_size (obstack_ptr) = new_chunk_size;


Summary of Obstack Functions
Node: Summary of Obstacks, Next: , Prev: Obstack Chunks, Up: Obstacks

Summary of Obstack Functions

Here is a summary of all the functions associated with obstacks.  Each takes the address of an obstack (struct obstack *) as its first argument.

void obstack_init (struct obstack *obstack_ptr)

Initialize use of an obstack.  See Creating Obstacks.

void *obstack_alloc (struct obstack *obstack_ptr, size_t size)

Allocate an object of size uninitialized bytes.  See Allocation in an Obstack.

void *obstack_copy (struct obstack *obstack_ptr, void *address, size_t size)

Allocate an object of size bytes, with contents copied from address.  See Allocation in an Obstack.

void *obstack_copy0 (struct obstack *obstack_ptr, void *address, size_t size)

Allocate an object of size+1 bytes, with size of them copied from address, followed by a null character at the end.  See Allocation in an Obstack.

void obstack_free (struct obstack *obstack_ptr, void *object)

Free object (and everything allocated in the specified obstack more recently than object).  See Freeing Obstack Objects.

void obstack_blank (struct obstack *obstack_ptr, size_t size)

Add size uninitialized bytes to a growing object.  See Growing Objects.

void obstack_grow (struct obstack *obstack_ptr, void *address, size_t size)

Add size bytes, copied from address, to a growing object.  See Growing Objects.

void obstack_grow0 (struct obstack *obstack_ptr, void *address, size_t size)

Add size bytes, copied from address, to a growing object, and then add another byte containing a null character.  See Growing Objects.

void obstack_1grow (struct obstack *obstack_ptr, char data_char)

Add one byte containing data_char to a growing object.  See Growing Objects.

void *obstack_finish (struct obstack *obstack_ptr)

Finalize the object that is growing and return its permanent address.  See Growing Objects.

size_t obstack_object_size (struct obstack *obstack_ptr)

Get the current size of the currently growing object.  See Growing Objects.

void obstack_blank_fast (struct obstack *obstack_ptr, size_t size)

Add size uninitialized bytes to a growing object without checking that there is enough room.  See Extra Fast Growing.

void obstack_1grow_fast (struct obstack *obstack_ptr, char data_char)

Add one byte containing data_char to a growing object without checking that there is enough room.  See Extra Fast Growing.

size_t obstack_room (struct obstack *obstack_ptr)

Get the amount of room now available for growing the current object.  See Extra Fast Growing.

int obstack_alignment_mask (struct obstack *obstack_ptr)

The mask used for aligning the beginning of an object.  This is an lvalue.  See Obstacks Data Alignment.

size_t obstack_chunk_size (struct obstack *obstack_ptr)

The size for allocating chunks.  This is an lvalue.  See Obstack Chunks.

void *obstack_base (struct obstack *obstack_ptr)

Tentative starting address of the currently growing object.  See Status of an Obstack.

void *obstack_next_free (struct obstack *obstack_ptr)

Address just after the end of the currently growing object.  See Status of an Obstack.


Automatic Storage with Variable Size
Node: Variable Size Automatic, Next: Relocating Allocator, Prev: Obstacks, Up: Memory Allocation

Automatic Storage with Variable Size

The function alloca supports a kind of half-dynamic allocation in which blocks are allocated dynamically but freed automatically.

Allocating a block with alloca is an explicit action; you can allocate as many blocks as you wish, and compute the size at run time.  But all the blocks are freed when you exit the function that alloca was called from, just as if they were automatic variables declared in that function.  There is no way to free the space explicitly.

The prototype for alloca is in 'stdlib.h'.  This function is a BSD extension.  K

 - Function: void * alloca (size_t size);

The return value of alloca is the address of a block of size bytes of storage, allocated in the stack frame of the calling function.

Do not use alloca inside the arguments of a function call--you will get unpredictable results, because the stack space for the alloca would appear on the stack in the middle of the space for the function arguments.  An example of what to avoid is foo (x, alloca (4), y).

* Menu:

Alloca Example	Example of using alloca.

Advantages of Alloca	Reasons to use alloca.

Disadvantages of Alloca	Reasons to avoid alloca.

GNU C Variable-Size Arrays	Only in GNU C, here is an alternative method of allocating dynamically and freeing automatically.


alloca Example
Node: Alloca Example, Next: Advantages of Alloca, Prev: , Up: Variable Size Automatic

alloca Example

As an example of use of alloca, here is a function that opens a file name made from concatenating two argument strings, and returns a file descriptor or minus one signifying failure:

int
open2 (char *str1, char *str2, int flags, int mode)
{
  char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
  strcpy (name, str1);
  strcat (name, str2);
  return open (name, flags, mode);
}

Here is how you would get the same results with malloc and free:

int
open2 (char *str1, char *str2, int flags, int mode)
{
  char *name = (char *) malloc (strlen (str1) + strlen (str2) + 1);
  int desc;
  if (name == 0)
    fatal ("virtual memory exceeded");
  strcpy (name, str1);
  strcat (name, str2);
  desc = open (name, flags, mode);
  free (name);
  return desc;
}

As you can see, it is simpler with alloca.  But alloca has other, more important advantages, and some disadvantages.


Advantages of alloca
Node: Advantages of Alloca, Next: Disadvantages of Alloca, Prev: Alloca Example, Up: Variable Size Automatic

Advantages of alloca

Here are the reasons why alloca may be preferable to malloc:

•	Using alloca wastes very little space and is very fast.  (It is open-coded by the GNU C compiler.)

•	Since alloca does not have separate pools for different sizes of block, space used for any size block can be reused for any other size.  alloca does not cause storage fragmentation.

•	Nonlocal exits done with longjmp (see Non-Local Exits) automatically free the space allocated with alloca when they exit through the function that called alloca.  This is the most important reason to use alloca.

To illustrate this, suppose you have a function open_or_report_error which returns a descriptor, like open, if it succeeds, but does not return to its caller if it fails.  If the file cannot be opened, it prints an error message and jumps out to the command level of your program using longjmp.  Let's change open2 (see Alloca Example) to use this subroutine:

int
open2 (char *str1, char *str2, int flags, int mode)
{
  char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
  strcpy (name, str1);
  strcat (name, str2);
  return open_or_report_error (name, flags, mode);
}

Because of the way alloca works, the storage it allocates is freed even when an error occurs, with no special effort required.

By contrast, the previous definition of open2 (which uses malloc and free) would develop a storage leak if it were changed in this way.  Even if you are willing to make more changes to fix it, there is no easy way to do so.


Disadvantages of alloca
Node: Disadvantages of Alloca, Next: GNU C Variable-Size Arrays, Prev: Advantages of Alloca, Up: Variable Size Automatic

Disadvantages of alloca

These are the disadvantages of alloca in comparison with malloc:

•	If you try to allocate more storage than the machine can provide, you don't get a clean error message.  Instead you get a fatal signal like the one you would get from an infinite recursion; probably a segmentation violation (see Program Error Signals).

•	Some non-GNU systems fail to support alloca, so it is less portable.  However, a slower emulation of alloca written in C is available for use on systems with this deficiency.


GNU C Variable-Size Arrays
Node: GNU C Variable-Size Arrays, Next: , Prev: Disadvantages of Alloca, Up: Variable Size Automatic

GNU C Variable-Size Arrays

In GNU C, you can replace most uses of alloca with an array of variable size.  Here is how open2 would look then:

int open2 (char *str1, char *str2, int flags, int mode)
{
  char name[strlen (str1) + strlen (str2) + 1];
  strcpy (name, str1);
  strcat (name, str2);
  return open (name, flags, mode);
}

But alloca is not always equivalent to a variable-sized array, for several reasons:

•	A variable size array's space is freed at the end of the scope of the name of the array.  The space allocated with alloca remains until the end of the function.

•	It is possible to use alloca within a loop, allocating an additional block on each iteration.  This is impossible with variable-sized arrays.

Note: If you mix use of alloca and variable-sized arrays within one function, exiting a scope in which a variable-sized array was declared frees all blocks allocated with alloca during the execution of that scope.


Relocating Allocator
Node: Relocating Allocator, Next: Memory Warnings, Prev: Variable Size Automatic, Up: Memory Allocation

Relocating Allocator

Any system of dynamic memory allocation has overhead: the amount of space it uses is more than the amount the program asks for.  The "relocating memory allocator" achieves very low overhead by moving blocks in memory as necessary, on its own initiative.

* Menu:

Relocator Concepts	How to understand relocating allocation.

Using Relocator	Functions for relocating allocation.


Concepts of Relocating Allocation
Node: Relocator Concepts, Next: Using Relocator, Prev: , Up: Relocating Allocator

Concepts of Relocating Allocation

The "relocating memory allocator" achieves very low overhead by moving blocks in memory as necessary, on its own initiative.

When you allocate a block with malloc, the address of the block never changes unless you use realloc to change its size.  Thus, you can safely store the address in various places, temporarily or permanently, as you like.  This is not safe when you use the relocating memory allocator, because any and all relocatable blocks can move whenever you allocate memory in any fashion.  Even calling malloc or realloc can move the relocatable blocks.

For each relocatable block, you must make a "handle"--a pointer object in memory, designated to store the address of that block.  The relocating allocator knows where each block's handle is, and updates the address stored there whenever it moves the block, so that the handle always points to the block.  Each time you access the contents of the block, you should fetch its address anew from the handle.

To call any of the relocating allocator functions from a signal handler is almost certainly incorrect, because the signal could happen at any time and relocate all the blocks.  The only way to make this safe is to block the signal around any access to the contents of any relocatable block--not a convenient mode of operation.  See Nonreentrancy.


Allocating and Freeing Relocatable Blocks
Node: Using Relocator, Next: , Prev: Relocator Concepts, Up: Relocating Allocator

Allocating and Freeing Relocatable Blocks

In the descriptions below, handleptr designates the address of the handle.  All the functions are declared in 'malloc.h'; all are GNU extensions.

 - Function: void * r_alloc (void **handleptr, size_t size)

This function allocates a relocatable block of size size.  It stores the block's address in *handleptr and returns a non-null pointer to indicate success.

If r_alloc can't get the space needed, it stores a null pointer in *handleptr, and returns a null pointer.

 - Function: void r_alloc_free (void **handleptr)

This function is the way to free a relocatable block.  It frees the block that *handleptr points to, and stores a null pointer in *handleptr to show it doesn't point to an allocated block any more.

 - Function: void * r_re_alloc (void **handleptr, size_t size)

The function r_re_alloc adjusts the size of the block that *handleptr points to, making it size bytes long.  It stores the address of the resized block in *handleptr and returns a non-null pointer to indicate success.

If enough memory is not available, this function returns a null pointer and does not modify *handleptr.


Memory Usage Warnings
Node: Memory Warnings, Next: , Prev: Relocating Allocator, Up: Memory Allocation

Memory Usage Warnings

You can ask for warnings as the program approaches running out of memory space, by calling memory_warnings.  This tells malloc to check memory usage every time it asks for more memory from the operating system.  This is a GNU extension declared in 'malloc.h'.

 - Function: void memory_warnings (void *start, void (*warn_func) (const char *))

Call this function to request warnings for nearing exhaustion of virtual memory.

The argument start says where data space begins, in memory.  The allocator compares this against the last address used and against the limit of data space, to determine the fraction of available memory in use.  If you supply zero for start, then a default value is used which is right in most circumstances.

For warn_func, supply a function that malloc can call to warn you.  It is called with a string (a warning message) as argument.  Normally it ought to display the string for the user to read.

The warnings come when memory becomes 75% full, when it becomes 85% full, and when it becomes 95% full.  Above 95% you get another warning each time memory usage increases.


Character Handling
Node: Character Handling, Next: String and Array Utilities, Prev: Memory Allocation, Up: Top

Character Handling

Programs that work with characters and strings often need to classify a character--is it alphabetic, is it a digit, is it whitespace, and so on--and perform case conversion operations on characters.  The functions in the header file 'ctype.h' are provided for this purpose.  KSince the choice of locale and character set can alter the classifications of particular character codes, all of these functions are affected by the current locale.  (More precisely, they are affected by the locale currently selected for character classification--the LC_CTYPE category; see See Locale Categories.)

* Menu:

Classification of Characters	Testing whether characters are letters, digits, punctuation, etc.

Case Conversion	Case mapping, and the like.


Classification of Characters
Node: Classification of Characters, Next: Case Conversion, Prev: , Up: Character Handling

Classification of Characters

This section explains the library functions for classifying characters.  For example, isalpha is the function to test for an alphabetic character.  It takes one argument, the character to test, and returns a nonzero integer if the character is alphabetic, and zero otherwise.  You would use it like this:

if (isalpha (c))
  printf ("The character `%c' is alphabetic.\n", c);

Each of the functions in this section tests for membership in a particular class of characters; each has a name starting with 'is'.  Each of them takes one argument, which is a character to test, and returns an int which is treated as a boolean value.  The character argument is passed as an int, and it may be the constant value EOF instead of a real character.

The attributes of any given character can vary between locales.  See Locales, for more information on locales.

These functions are declared in the header file 'ctype.h'.  K

- Function: int islower (int c)

Returns true if c is a lower-case letter.

- Function: int isupper (int c)

Returns true if c is an upper-case letter.

- Function: int isalpha (int c)

Returns true if c is an alphabetic character (a letter).  If islower or isupper is true of a character, then isalpha is also true.

In some locales, there may be additional characters for which isalpha is true-letters which are neither upper case nor lower case.  But in the standard "C" locale, there are no such additional characters.

- Function: int isdigit (int c)

Returns true if c is a decimal digit ('0' through '9').

- Function: int isalnum (int c)

Returns true if c is an alphanumeric character (a letter or number); in other words, if either isalpha or isdigit is true of a character, then isalnum is also true.

- Function: int isxdigit (int c)

Returns true if c is a hexadecimal digit.  Hexadecimal digits include the normal decimal digits '0' through '9' and the letters 'A' through 'F' and 'a' through 'f'.

- Function: int ispunct (int c)

Returns true if c is a punctuation character.  This means any printing character that is not alphanumeric or a space character.

- Function: int isspace (int c)

Returns true if c is a "whitespace" character.  In the standard "C" locale, isspace returns true for only the standard whitespace characters:

' '

space

'\f'

formfeed

'\n'

newline

'\r'

carriage return

'\t'

horizontal tab

'\v'

vertical tab

- Function: int isblank (int c)

Returns true if c is a blank character; that is, a space or a tab.  This function is a GNU extension.

- Function: int isgraph (int c)

Returns true if c is a graphic character; that is, a character that has a glyph associated with it.  The whitespace characters are not considered graphic.

- Function: int isprint (int c)

Returns true if c is a printing character.  Printing characters include all the graphic characters, plus the space (' ') character.

- Function: int iscntrl (int c)

Returns true if c is a control character (that is, a character that is not a printing character).

- Function: int isascii (int c)

Returns true if c is a 7-bit unsigned char value that fits into the US/UK ASCII character set.  This function is a BSD extension and is also an SVID extension.


Case Conversion
Node: Case Conversion, Next: , Prev: Classification of Characters, Up: Character Handling

Case Conversion

This section explains the library functions for performing conversions such as case mappings on characters.  For example, toupper converts any character to upper case if possible.  If the character can't be converted, toupper returns it unchanged.

These functions take one argument of type int, which is the character to convert, and return the converted character as an int.  If the conversion is not applicable to the argument given, the argument is returned unchanged.

Compatibility Note: In pre-ANSI C dialects, instead of returning the argument unchanged, these functions may fail when the argument is not suitable for the conversion.  Thus for portability, you may need to write islower(c) ? toupper(c) : c rather than just toupper(c).

These functions are declared in the header file 'ctype.h'.  K

 - Function: int tolower (int c)

If c is an upper-case letter, tolower returns the corresponding lower-case letter.  If c is not an upper-case letter, c is returned unchanged.

 - Function: int toupper (int c)

If c is a lower-case letter, tolower returns the corresponding upper-case letter.  Otherwise c is returned unchanged.

 - Function: int toascii (int c)

This function converts c to a 7-bit unsigned char value that fits into the US/UK ASCII character set, by clearing the high-order bits.  This function is a BSD extension and is also an SVID extension.

 - Function: int _tolower (int c)

This is identical to tolower, and is provided for compatibility with the SVID.  See SVID.

 - Function: int _toupper (int c)

This is identical to toupper, and is provided for compatibility with the SVID.


String and Array Utilities
Node: String and Array Utilities, Next: Extended Characters, Prev: Character Handling, Up: Top

String and Array Utilities

Operations on strings (or arrays of characters) are an important part of many programs.  The GNU C library provides an extensive set of string utility functions, including functions for copying, concatenating, comparing, and searching strings.  Many of these functions can also operate on arbitrary regions of storage; for example, the memcpy function can be used to copy the contents of any kind of array.

It's fairly common for beginning C programmers to "reinvent the wheel" by duplicating this functionality in their own code, but it pays to become familiar with the library functions and to make use of them, since this offers benefits in maintenance, efficiency, and portability.

For instance, you could easily compare one string to another in two lines of C code, but if you use the built-in strcmp function, you're less likely to make a mistake.  And, since these library functions are typically highly optimized, your program may run faster too.

* Menu:

Representation of Strings	Introduction to basic concepts.

String/Array Conventions	Whether to use a string function or an arbitrary array function.

String Length	Determining the length of a string.

Copying and Concatenation	Functions to copy the contents of strings and arrays.

String/Array Comparison	Functions for byte-wise and character-wise comparison.

Collation Functions	Functions for collating strings.

Search Functions	Searching for a specific element or substring.

Finding Tokens in a String	Splitting a string into tokens by looking for delimiters.


Representation of Strings
Node: Representation of Strings, Next: String/Array Conventions, Prev: , Up: String and Array Utilities

Representation of Strings

This section is a quick summary of string concepts for beginning C programmers.  It describes how character strings are represented in C and some common pitfalls.  If you are already familiar with this material, you can skip this section.

A "string" is an array of char objects.  But string-valued variables are usually declared to be pointers of type char *.  Such variables do not include space for the text of a string; that has to be stored somewhere else--in an array variable, a string constant, or dynamically allocated memory (see Memory Allocation).  It's up to you to store the address of the chosen memory space into the pointer variable.  Alternatively you can store a "null pointer" in the pointer variable.  The null pointer does not point anywhere, so attempting to reference the string it points to gets an error.

By convention, a "null character", '\0', marks the end of a string.  For example, in testing to see whether the char * variable p points to a null character marking the end of a string, you can write !*p or *p == '\0'.

A null character is quite different conceptually from a null pointer, although both are represented by the integer 0.

"String literals" appear in C program source as strings of characters between double-quote characters ('"').  In ANSI C, string literals can also be formed by "string concatenation": "a" "b" is the same as "ab".  Modification of string literals is not allowed by the GNU C compiler, because literals are placed in read-only storage.

Character arrays that are declared const cannot be modified either.  It's generally good style to declare non-modifiable string pointers to be of type const char *, since this often allows the C compiler to detect accidental modifications as well as providing some amount of documentation about what your program intends to do with the string.

The amount of memory allocated for the character array may extend past the null character that normally marks the end of the string.  In this document, the term "allocation size" is always used to refer to the total amount of memory allocated for the string, while the term "length" refers to the number of characters up to (but not including) the terminating null character.  KA notorious source of program bugs is trying to put more characters in a string than fit in its allocated size.  When writing code that extends strings or moves characters into a pre-allocated array, you should be very careful to keep track of the length of the text and make explicit checks for overflowing the array.  Many of the library functions do not do this for you!  Remember also that you need to allocate an extra byte to hold the null character that marks the end of the string.


String and Array Conventions
Node: String/Array Conventions, Next: String Length, Prev: Representation of Strings, Up: String and Array Utilities

String and Array Conventions

This chapter describes both functions that work on arbitrary arrays or blocks of memory, and functions that are specific to null-terminated arrays of characters.

Functions that operate on arbitrary blocks of memory have names beginning with 'mem' (such as memcpy) and invariably take an argument which specifies the size (in bytes) of the block of memory to operate on.  The array arguments and return values for these functions have type void *, and as a matter of style, the elements of these arrays are referred to as "bytes".  You can pass any kind of pointer to these functions, and the sizeof operator is useful in computing the value for the size argument.

In contrast, functions that operate specifically on strings have names beginning with 'str' (such as strcpy) and look for a null character to terminate the string instead of requiring an explicit size argument to be passed.  (Some of these functions accept a specified maximum length, but they also check for premature termination with a null character.)  The array arguments and return values for these functions have type char *, and the array elements are referred to as "characters".

In many cases, there are both 'mem' and 'str' versions of a function.  The one that is more appropriate to use depends on the exact situation.  When your program is manipulating arbitrary arrays or blocks of storage, then you should always use the 'mem' functions.  On the other hand, when you are manipulating null-terminated strings it is usually more convenient to use the 'str' functions, unless you already know the length of the string in advance.


String Length
Node: String Length, Next: Copying and Concatenation, Prev: String/Array Conventions, Up: String and Array Utilities

String Length

You can get the length of a string using the strlen function.  This function is declared in the header file 'string.h'.  K

 - Function: size_t strlen (const char *s)

The strlen function returns the length of the null-terminated string s.  (In other words, it returns the offset of the terminating null character within the array.)

For example,

strlen ("hello, world")
    => 12

When applied to a character array, the strlen function returns the length of the string stored there, not its allocation size.  You can get the allocation size of the character array that holds a string using the sizeof operator:

char string[32] = "hello, world";
sizeof (string)
    => 32
strlen (string)
    => 12


Copying and Concatenation
Node: Copying and Concatenation, Next: String/Array Comparison, Prev: String Length, Up: String and Array Utilities

Copying and Concatenation

You can use the functions described in this section to copy the contents of strings and arrays, or to append the contents of one string to another.  These functions are declared in the header file 'string.h'.  KA helpful way to remember the ordering of the arguments to the functions in this section is that it corresponds to an assignment expression, with the destination array specified to the left of the source array.  All of these functions return the address of the destination array.

Most of these functions do not work properly if the source and destination arrays overlap.  For example, if the beginning of the destination array overlaps the end of the source array, the original contents of that part of the source array may get overwritten before it is copied.  Even worse, in the case of the string functions, the null character marking the end of the string may be lost, and the copy function might get stuck in a loop trashing all the memory allocated to your program.

All functions that have problems copying between overlapping arrays are explicitly identified in this manual.  In addition to functions in this section, there are a few others like sprintf (see Formatted Output Functions) and scanf (see Formatted Input Functions).

 - Function: void * memcpy (void *to, const void *from, size_t size)

The memcpy function copies size bytes from the object beginning at from into the object beginning at to.  The behavior of this function is undefined if the two arrays to and from overlap; use memmove instead if overlapping is possible.

The value returned by memcpy is the value of to.

Here is an example of how you might use memcpy to copy the contents of an array:

struct foo *oldarray, *newarray;
int arraysize;
...
memcpy (new, old, arraysize * sizeof (struct foo));

 - Function: void * memmove (void *to, const void *from, size_t size)

memmove copies the size bytes at from into the size bytes at to, even if those two blocks of space overlap.  In the case of overlap, memmove is careful to copy the original values of the bytes in the block at from, including those bytes which also belong to the block at to.

 - Function: void * memccpy (void *to, const void *from, int c, size_t size)

This function copies no more than size bytes from from to to, stopping if a byte matching c is found.  The return value is a pointer into to one byte past where c was copied, or a null pointer if no byte matching c appeared in the first size bytes of from.

 - Function: void * memset (void *block, int c, size_t size)

This function copies the value of c (converted to an unsigned char) into each of the first size bytes of the object beginning at block.  It returns the value of block.

 - Function: char * strcpy (char *to, const char *from)

This copies characters from the string from (up to and including the terminating null character) into the string to.  Like memcpy, this function has undefined results if the strings overlap.  The return value is the value of to.

 - Function: char * strncpy (char *to, const char *from, size_t size)

This function is similar to strcpy but always copies exactly size characters into to.

If the length of from is more than size, then strncpy copies just the first size characters.

If the length of from is less than size, then strncpy copies all of from, followed by enough null characters to add up to size characters in all.  This behavior is rarely useful, but it is specified by the ANSI C standard.

The behavior of strncpy is undefined if the strings overlap.

Using strncpy as opposed to strcpy is a way to avoid bugs relating to writing past the end of the allocated space for to.  However, it can also make your program much slower in one common case: copying a string which is probably small into a potentially large buffer.  In this case, size may be large, and when it is, strncpy will waste a considerable amount of time copying null characters.

 - Function: char * strdup (const char *s)

This function copies the null-terminated string s into a newly allocated string.  The string is allocated using malloc; see See Unconstrained Allocation.  If malloc cannot allocate space for the new string, strdup returns a null pointer.  Otherwise it returns a pointer to the new string.

 - Function: char * stpcpy (char *to, const char *from)

This function is like strcpy, except that it returns a pointer to the end of the string to (that is, the address of the terminating null character) rather than the beginning.

For example, this program uses stpcpy to concatenate 'foo' and 'bar' to produce 'foobar', which it then prints.

#include <string.h>
#include <stdio.h>

int
main (void)
{
  char buffer[10];
  char *to = buffer;
  to = stpcpy (to, "foo");
  to = stpcpy (to, "bar");
  puts (buffer);
  return 0;
}

This function is not part of the ANSI or POSIX standards, and is not customary on Unix systems, but we did not invent it either.  Perhaps it comes from MS-DOG.

Its behavior is undefined if the strings overlap.

 - Function: char * strcat (char *to, const char *from)

The strcat function is similar to strcpy, except that the characters from from are concatenated or appended to the end of to, instead of overwriting it.  That is, the first character from from overwrites the null character marking the end of to.

An equivalent definition for strcat would be:

char *
strcat (char *to, const char *from)
{
  strcpy (to + strlen (to), from);
  return to;
}

This function has undefined results if the strings overlap.

 - Function: char * strncat (char *to, const char *from, size_t size)

This function is like strcat except that not more than size characters from from are appended to the end of to.  A single null character is also always appended to to, so the total allocated size of to must be at least size + 1 bytes longer than its initial length.

The strncat function could be implemented like this:

char *
strncat (char *to, const char *from, size_t size)
{
  strncpy (to + strlen (to), from, size);
  return to;
}

The behavior of strncat is undefined if the strings overlap.

Here is an example showing the use of strncpy and strncat.  Notice how, in the call to strncat, the size parameter is computed to avoid overflowing the character array buffer.

#include <string.h>
#include <stdio.h>

#define SIZE 10

static char buffer[SIZE];

main ()
{
  strncpy (buffer, "hello", SIZE);
  puts (buffer);
  strncat (buffer, ", world", SIZE - strlen (buffer) - 1);
  puts (buffer);
}

The output produced by this program looks like:

hello
hello, wo

 - Function: void * bcopy (void *from, const void *to, size_t size)

This is a partially obsolete alternative for memmove, derived from BSD.  Note that it is not quite equivalent to memmove, because the arguments are not in the same order.

 - Function: void * bzero (void *block, size_t size)

This is a partially obsolete alternative for memset, derived from BSD.  Note that it is not as general as memset, because the only value it can store is zero.  Some machines have special instructions for zeroing memory, so bzero might be more efficient than memset.


String/Array Comparison
Node: String/Array Comparison, Next: Collation Functions, Prev: Copying and Concatenation, Up: String and Array Utilities

String/Array Comparison

You can use the functions in this section to perform comparisons on the contents of strings and arrays.  As well as checking for equality, these functions can also be used as the ordering functions for sorting operations.  See Searching and Sorting, for an example of this.

Unlike most comparison operations in C, the string comparison functions return a nonzero value if the strings are not equivalent rather than if they are.  The sign of the value indicates the relative ordering of the first characters in the strings that are not equivalent:  a negative value indicates that the first string is "less" than the second, while a positive value indicates that the first string is "greater".

If you are using these functions only to check for equality, you might find it makes for a cleaner program to hide them behind a macro definition, like this:

#define str_eq(s1,s2)  (!strcmp ((s1),(s2)))

All of these functions are declared in the header file 'string.h'.  K

 - Function: int memcmp (const void *a1, const void *a2, size_t size)

The function memcmp compares the size bytes of memory beginning at a1 against the size bytes of memory beginning at a2.  The value returned has the same sign as the difference between the first differing pair of bytes (interpreted as unsigned char objects, then promoted to int).

If the contents of the two blocks are equal, memcmp returns 0.

On arbitrary arrays, the memcmp function is mostly useful for testing equality.  It usually isn't meaningful to do byte-wise ordering comparisons on arrays of things other than bytes.  For example, a byte-wise comparison on the bytes that make up floating-point numbers isn't likely to tell you anything about the relationship between the values of the floating-point numbers.

You should also be careful about using memcmp to compare objects that can contain "holes", such as the padding inserted into structure objects to enforce alignment requirements, extra space at the end of unions, and extra characters at the ends of strings whose length is less than their allocated size.  The contents of these "holes" are indeterminate and may cause strange behavior when performing byte-wise comparisons.  For more predictable results, perform an explicit component-wise comparison.

For example, given a structure type definition like:

struct foo
  {
    unsigned char tag;
    union
      {
        double f;
        long i;
        char *p;
      } value;
  };

you are better off writing a specialized comparison function to compare struct foo objects instead of comparing them with memcmp.

 - Function: int strcmp (const char *s1, const char *s2)

The strcmp function compares the string s1 against s2, returning a value that has the same sign as the difference between the first differing pair of characters (interpreted as unsigned char objects, then promoted to int).

If the two strings are equal, strcmp returns 0.

A consequence of the ordering used by strcmp is that if s1 is an initial substring of s2, then s1 is considered to be "less than" s2.

 - Function: int strcasecmp (const char *s1, const char *s2)

This function is like strcmp, except that differences in case are ignored.

strcasecmp is derived from BSD.

 - Function: int strncasecmp (const char *s1, const char *s2, size_t n)

This function is like strncmp, except that differences in case are ignored.

strncasecmp is a GNU extension.

 - Function: int strncmp (const char *s1, const char *s2, size_t size)

This function is the similar to strcmp, except that no more than size characters are compared.  In other words, if the two strings are the same in their first size characters, the return value is zero.

Here are some examples showing the use of strcmp and strncmp.  These examples assume the use of the ASCII character set.  (If some other character set--say, EBCDIC--is used instead, then the glyphs are associated with different numeric codes, and the return values and ordering may differ.)

strcmp ("hello", "hello")
    => 0    /* These two strings are the same. */
strcmp ("hello", "Hello")
    => 32   /* Comparisons are case-sensitive. */
strcmp ("hello", "world")
    => -15  /* The character 'h' comes before 'w'. */
strcmp ("hello", "hello, world")
    => -44  /* Comparing a null character against a comma. */
strncmp ("hello", "hello, world"", 5)
    => 0    /* The initial 5 characters are the same. */
strncmp ("hello, world", "hello, stupid world!!!", 5)
    => 0    /* The initial 5 characters are the same. */

 - Function: int bcmp (const void *a1, const void *a2, size_t size)

This is an obsolete alias for memcmp, derived from BSD.


Collation Functions
Node: Collation Functions, Next: Search Functions, Prev: String/Array Comparison, Up: String and Array Utilities

Collation Functions

In some locales, the conventions for lexicographic ordering differ from the strict numeric ordering of character codes.  For example, in Spanish most glyphs with diacritical marks such as accents are not considered distinct letters for the purposes of collation.  On the other hand, the two-character sequence 'll' is treated as a single letter that is collated immediately after 'l'.

You can use the functions strcoll and strxfrm (declared in the header file 'string.h') to compare strings using a collation ordering appropriate for the current locale.  The locale used by these functions in particular can be specified by setting the locale for the LC_COLLATE category; see See Locales.  KIn the standard C locale, the collation sequence for strcoll is the same as that for strcmp.

Effectively, the way these functions work is by applying a mapping to transform the characters in a string to a byte sequence that represents the string's position in the collating sequence of the current locale.  Comparing two such byte sequences in a simple fashion is equivalent to comparing the strings with the locale's collating sequence.

The function strcoll performs this translation implicitly, in order to do one comparison.  By contrast, strxfrm performs the mapping explicitly.  If you are making multiple comparisons using the same string or set of strings, it is likely to be more efficient to use strxfrm to transform all the strings just once, and subsequently compare the transformed strings with strcmp.

 - Function: int strcoll (const char *s1, const char *s2)

The strcoll function is similar to strcmp but uses the collating sequence of the current locale for collation (the LC_COLLATE locale).

Here is an example of sorting an array of strings, using strcoll to compare them.  The actual sort algorithm is not written here; it comes from qsort (see Array Sort Function).  The job of the code shown here is to say how to compare the strings while sorting them.  (Later on in this section, we will show a way to do this more efficiently using strxfrm.)

/* This is the comparison function used with qsort. */

int
compare_elements (char **p1, char **p2)
{
  return strcoll (*p1, *p2);
}

/* This is the entry point---the function to sort
   strings using the locale's collating sequence. */

void
sort_strings (char **array, int nstrings)
{
  /* Sort temp_array by comparing the strings. */
  qsort (array, sizeof (char *),
         nstrings, compare_elements);
}

- Function: size_t strxfrm (char *to, const char *from, size_t size)

The function strxfrm transforms string using the collation transformation determined by the locale currently selected for collation, and stores the transformed string in the array to.  Up to size characters (including a terminating null character) are stored.

The behavior is undefined if the strings to and from overlap; see See Copying and Concatenation.

The return value is the length of the entire transformed string.  This value is not affected by the value of size, but if it is greater than size, it means that the transformed string did not entirely fit in the array to.  In this case, only as much of the string as actually fits was stored.  To get the whole transformed string, call strxfrm again with a bigger output array.

The transformed string may be longer than the original string, and it may also be shorter.

If size is zero, no characters are stored in to.  In this case, strxfrm simply returns the number of characters that would be the length of the transformed string.  This is useful for determining what size string to allocate.  It does not matter what to is if size is zero; to may even be a null pointer.

Here is an example of how you can use strxfrm when you plan to do many comparisons.  It does the same thing as the previous example, but much faster, because it has to transform each string only once, no matter how many times it is compared with other strings.  Even the time needed to allocate and free storage is much less than the time we save, when there are many strings.

struct sorter { char *input; char *transformed; };

/* This is the comparison function used with qsort
   to sort an array of struct sorter. */

int
compare_elements (struct sorter *p1, struct sorter *p2)
{
  return strcmp (p1->transformed, p2->transformed);
}

/* This is the entry point---the function to sort
   strings using the locale's collating sequence. */

void
sort_strings_fast (char **array, int nstrings)
{
  struct sorter temp_array[nstrings];
  int i;

  /* Set up temp_array.  Each element contains
     one input string and its transformed string. */
  for (i = 0; i < nstrings; i++)
    {
      size_t length = strlen (array[i]) * 2;

      temp_array[i].input = array[i];

      /* Transform array[i].
         First try a buffer probably big enough. */
      while (1)
        {
          char *transformed = (char *) xmalloc (length);
          if (strxfrm (transformed, array[i], length) < length)
            {
              temp_array[i].transformed = transformed;
              break;
            }
          /* Try again with a bigger buffer. */
          free (transformed);
          length *= 2;
        }
    }

  /* Sort temp_array by comparing transformed strings. */
  qsort (temp_array, sizeof (struct sorter),
         nstrings, compare_elements);

  /* Put the elements back in the permanent array
     in their sorted order. */
  for (i = 0; i < nstrings; i++)
    array[i] = temp_array[i].input;

  /* Free the strings we allocated. */
  for (i = 0; i < nstrings; i++)
    free (temp_array[i].transformed);
}

Compatibility Note:  The string collation functions are a new feature of ANSI C.  Older C dialects have no equivalent feature.


Search Functions
Node: Search Functions, Next: Finding Tokens in a String, Prev: Collation Functions, Up: String and Array Utilities

Search Functions

This section describes library functions which perform various kinds of searching operations on strings and arrays.  These functions are declared in the header file 'string.h'.  K

 - Function: void * memchr (const void *block, int c, size_t size)

This function finds the first occurrence of the byte c (converted to an unsigned char) in the initial size bytes of the object beginning at block.  The return value is a pointer to the located byte, or a null pointer if no match was found.

 - Function: char * strchr (const char *string, int c)

The strchr function finds the first occurrence of the character c (converted to a char) in the null-terminated string beginning at string.  The return value is a pointer to the located character, or a null pointer if no match was found.

For example,

strchr ("hello, world", 'l')
    => "llo, world"
strchr ("hello, world", '?')
    => NULL

The terminating null character is considered to be part of the string, so you can use this function get a pointer to the end of a string by specifying a null character as the value of the c argument.

 - Function: char * index (const char *string, int c)

index is another name for strchr; they are exactly the same.

 - Function: char * strrchr (const char *string, int c)

The function strrchr is like strchr, except that it searches backwards from the end of the string string (instead of forwards from the front).

For example,

strrchr ("hello, world", 'l')
    => "ld"

 - Function: char * rindex (const char *string, int c)

rindex is another name for strrchr; they are exactly the same.

 - Function: char * strstr (const char *haystack, const char *needle)

This is like strchr, except that it searches haystack for a substring needle rather than just a single character.  It returns a pointer into the string haystack that is the first character of the substring, or a null pointer if no match was found.  If needle is an empty string, the function returns haystack.

For example,

strstr ("hello, world", "l")
    => "llo, world"
strstr ("hello, world", "wo")
    => "world"

 - Function: void * memmem (const void *needle, size_t needle_len,

const void *haystack, size_t haystack_len)

This is like strstr, but needle and haystack are byte arrays rather than null-terminated strings.  needle_len is the length of needle and haystack_len is the length of haystack.

This function is a GNU extension.

 - Function: size_t strspn (const char *string, const char *skipset)

The strspn ("string span") function returns the length of the initial substring of string that consists entirely of characters that are members of the set specified by the string skipset.  The order of the characters in skipset is not important.

For example,

strspn ("hello, world", "abcdefghijklmnopqrstuvwxyz")
    => 5

 - Function: size_t strcspn (const char *string, const char *stopset)

The strcspn ("string complement span") function returns the length of the initial substring of string that consists entirely of characters that are not members of the set specified by the string stopset.  (In other words, it returns the offset of the first character in string that is a member of the set stopset.)

For example,

strcspn ("hello, world", " \t\n,.;!?")
    => 5

 - Function: char * strpbrk (const char *string, const char *stopset)

The strpbrk ("string pointer break") function is related to strcspn, except that it returns a pointer to the first character in string that is a member of the set stopset instead of the length of the initial substring.  It returns a null pointer if no such character from stopset is found.

For example,

strpbrk ("hello, world", " \t\n,.;!?")
    => ", world"


Finding Tokens in a String
Node: Finding Tokens in a String, Next: , Prev: Search Functions, Up: String and Array Utilities

Finding Tokens in a String

It's fairly common for programs to have a need to do some simple kinds of lexical analysis and parsing, such as splitting a command string up into tokens.  You can do this with the strtok function, declared in the header file 'string.h'.  K

 - Function: char * strtok (char *newstring, const char *delimiters)

A string can be split into tokens by making a series of calls to the function strtok.

The string to be split up is passed as the newstring argument on the first call only.  The strtok function uses this to set up some internal state information.  Subsequent calls to get additional tokens from the same string are indicated by passing a null pointer as the newstring argument.  Calling strtok with another non-null newstring argument reinitializes the state information.  It is guaranteed that no other library function ever calls strtok behind your back (which would mess up this internal state information).

The delimiters argument is a string that specifies a set of delimiters that may surround the token being extracted.  All the initial characters that are members of this set are discarded.  The first character that is not a member of this set of delimiters marks the beginning of the next token.  The end of the token is found by looking for the next character that is a member of the delimiter set.  This character in the original string newstring is overwritten by a null character, and the pointer to the beginning of the token in newstring is returned.

On the next call to strtok, the searching begins at the next character beyond the one that marked the end of the previous token.  Note that the set of delimiters delimiters do not have to be the same on every call in a series of calls to strtok.

If the end of the string newstring is reached, or if the remainder of string consists only of delimiter characters, strtok returns a null pointer.

Warning: Since strtok alters the string it is parsing, you always copy the string to a temporary buffer before parsing it with strtok.  If you allow strtok to modify a string that came from another part of your program, you are asking for trouble; that string may be part of a data structure that could be used for other purposes during the parsing, when alteration by strtok makes the data structure temporarily inaccurate.

The string that you are operating on might even be a constant.  Then when strtok tries to modify it, your program will get a fatal signal for writing in read-only memory.  See Program Error Signals.

This is a special case of a general principle: if a part of a program does not have as its purpose the modification of a certain data structure, then it is error-prone to modify the data structure temporarily.

The function strtok is not reentrant.  See Nonreentrancy, for a discussion of where and why reentrancy is important.

Here is a simple example showing the use of strtok.

#include <string.h>
#include <stddef.h>

...

char string[] = "words separated by spaces -- and, punctuation!";
const char delimiters[] = " .,;:!-";
char *token;

...

token = strtok (string, delimiters);  /* token => "words" */
token = strtok (NULL, delimiters);    /* token => "separated" */
token = strtok (NULL, delimiters);    /* token => "by" */
token = strtok (NULL, delimiters);    /* token => "spaces" */
token = strtok (NULL, delimiters);    /* token => "and" */
token = strtok (NULL, delimiters);    /* token => "punctuation" */
token = strtok (NULL, delimiters);    /* token => NULL */


Input/Output Overview
Node: I/O Overview, Next: I/O on Streams, Prev: Pattern Matching, Up: Top

Input/Output Overview

Most programs need to do either input (reading data) or output (writing data), or most frequently both, in order to do anything useful.  The GNU C library provides such a large selection of input and output functions that the hardest part is often deciding which function is most appropriate!

This chapter introduces concepts and terminology relating to input and output.  Other chapters relating to the GNU I/O facilities are:

•	See I/O on Streams, which covers the high-level functions that operate on streams, including formatted input and output.

•	See Low-Level I/O, which covers the basic I/O and control functions on file descriptors.

•	See File System Interface, which covers functions for operating on directories and for manipulating file attributes such as access modes and ownership.

•	See Pipes and FIFOs, which includes information on the basic interprocess communication facilities.

•	See Sockets, which covers a more complicated interprocess communication facility with support for networking.

•	See Low-Level Terminal Interface, which covers functions for changing how input and output to terminal or other serial devices are processed.

* Menu:

I/O Concepts	Some basic information and terminology.

File Names	How to refer to a file.


Input/Output Concepts
Node: I/O Concepts, Next: File Names, Prev: , Up: I/O Overview

Input/Output Concepts

Before you can read or write the contents of a file, you must establish a connection or communications channel to the file.  This process is called "opening" the file.  You can open a file for reading, writing, or both.  KThe connection to an open file is represented either as a stream or as a file descriptor.  You pass this as an argument to the functions that do the actual read or write operations, to tell them which file to operate on.  Certain functions expect streams, and others are designed to operate on file descriptors.

When you have finished reading to or writing from the file, you can terminate the connection by "closing" the file.  Once you have closed a stream or file descriptor, you cannot do any more input or output operations on it.

* Menu:

Streams and File Descriptors	The GNU Library provides two ways to access the contents of files.

File Position	The number of bytes from the beginning of the file.


Streams and File Descriptors
Node: Streams and File Descriptors, Next: File Position, Prev: , Up: I/O Concepts

Streams and File Descriptors

When you want to do input or output to a file, you have a choice of two basic mechanisms for representing the connection between your program and the file: file descriptors and streams.  File descriptors are represented as objects of type int, while streams are represented as FILE * objects.

File descriptors provide a primitive, low-level interface to input and output operations.  Both file descriptors and streams can represent a connection to a device (such as a terminal), or a pipe or socket for communicating with another process, as well as a normal file.  But, if you want to do control operations that are specific to a particular kind of device, you must use a file descriptor; there are no facilities to use streams in this way.  You must also use file descriptors if your program needs to do input or output in special modes, such as nonblocking (or polled) input (see File Status Flags).

Streams provide a higher-level interface, layered on top of the primitive file descriptor facilities.  The stream interface treats all kinds of files pretty much alike--the sole exception being the three styles of buffering that you can choose (see Stream Buffering).

The main advantage of using the stream interface is that the set of functions for performing actual input and output operations (as opposed to control operations) on streams is much richer and more powerful than the corresponding facilities for file descriptors.  The file descriptor interface provides only simple functions for transferring blocks of characters, but the stream interface also provides powerful formatted input and output functions (printf and scanf) as well as functions for character- and line-oriented input and output.

Since streams are implemented in terms of file descriptors, you can extract the file descriptor from a stream and perform low-level operations directly on the file descriptor.  You can also initially open a connection as a file descriptor and then make a stream associated with that file descriptor.

In general, you should stick with using streams rather than file descriptors, unless there is some specific operation you want to do that can only be done on a file descriptor.  If you are a beginning programmer and aren't sure what functions to use, we suggest that you concentrate on the formatted input functions (see Formatted Input) and formatted output functions (see Formatted Output).

If you are concerned about portability of your programs to systems other than GNU, you should also be aware that file descriptors are not as portable as streams.  You can expect any system running ANSI C to support streams, but non-GNU systems may not support file descriptors at all, or may only implement a subset of the GNU functions that operate on file descriptors.  Most of the file descriptor functions in the GNU library are included in the POSIX.1 standard, however.


File Position
Node: File Position, Next: , Prev: Streams and File Descriptors, Up: I/O Concepts

File Position

One of the attributes of an open file is its "file position" that keeps track of where in the file the next character is to be read or written.  In the GNU system, and all POSIX.1 systems, the file position is simply an integer representing the number of bytes from the beginning of the file.

The file position is normally set to the beginning of the file when it is opened, and each time a character is read or written, the file position is incremented.  In other words, access to the file is normally "sequential".  KOrdinary files permit read or write operations at any position within the file.  Some other kinds of files may also permit this.  Files which do permit this are sometimes referred to as "random-access" files.  You can change the file position using the fseek function on a stream (see File Positioning) or the lseek function on a file descriptor (see I/O Primitives).  If you try to change the file position on a file that doesn't support random access, you get the ESPIPE error.  KStreams and descriptors that are opened for "append access" are treated specially for output: output to such files is always appended sequentially to the end of the file, regardless of the file position.  But, the file position is still used to control where in the file reading is done.  KIf you think about it, you'll realize that several programs can read a given file at the same time.  In order for each program to be able to read the file at its own pace, each program must have its own file pointer, which is not affected by anything the other programs do.

In fact, each opening of a file creates a separate file position.  Thus, if you open a file twice even in the same program, you get two streams or descriptors with independent file positions.

By contrast, if you open a descriptor and then duplicate it to get another descriptor, these two descriptors share the same file position: changing the file position of one descriptor will affect the other.


File Names
Node: File Names, Next: , Prev: I/O Concepts, Up: I/O Overview

File Names

In order to open a connection to a file, or to perform other operations such as deleting a file, you need some way to refer to the file.  Nearly all files have names that are strings--even files which are actually devices such as tape drives or terminals.  These strings are called "file names".  You specify the file name to say which file you want to open or operate on.

This section describes the conventions for file names and how the operating system works with them.  K

* Menu:

Directories	Directories contain entries for files.

File Name Resolution	A file name specifies how to look up a file.

File Name Errors	Error conditions relating to file names.

File Name Portability	File name portability and syntax issues.


Directories
Node: Directories, Next: File Name Resolution, Prev: , Up: File Names

Directories

In order to understand the syntax of file names, you need to understand how the file system is organized into a hierarchy of directories.

A "directory" is a file that contains information to associate other files with names; these associations are called "links" or "directory entries".  Sometimes, people speak of "files in a directory", but in reality, a directory only contains pointers to files, not the files themselves.

The name of a file contained in a directory entry is called a "file name component".  In general, a file name consists of a sequence of one or more such components, separated by the slash character ('/').  A file name which is just one component names a file with respect to its directory.  A file name with multiple components names a directory, and then a file in that directory, and so on.

Some other documents, such as the POSIX standard, use the term "pathname" for what we call a file name, and either "filename" or "pathname component" for what this manual calls a file name component.  We don't use this terminology because a "path" is something completely different (a list of directories to search), and we think that "pathname" used for something else will confuse users.  We always use "file name" and "file name component" (or sometimes just "component", where the context is obvious) in GNU documentation.

You can find more detailed information about operations on directories in See File System Interface.


File Name Resolution
Node: File Name Resolution, Next: File Name Errors, Prev: Directories, Up: File Names

File Name Resolution

A file name consists of file name components separated by slash ('/') characters.  On the systems that the GNU C library supports, multiple successive '/' characters are equivalent to a single '/' character.

The process of determining what file a file name refers to is called "file name resolution".  This is performed by examining the components that make up a file name in left-to-right order, and locating each successive component in the directory named by the previous component.  Of course, each of the files that are referenced as directories must actually exist, be directories instead of regular files, and have the appropriate permissions to be accessible by the process; otherwise the file name resolution fails.

If a file name begins with a '/', the first component in the file name is located in the "root directory" of the process (usually all processes on the system have the same root directory).  Such a file name is called an "absolute file name".

Otherwise, the first component in the file name is located in the current working directory (see Working Directory).  This kind of file name is called a "relative file name".

The file name components '.' ("dot") and '..' ("dot-dot") have special meanings.  Every directory has entries for these file name components.  The file name component '.' refers to the directory itself, while the file name component '..' refers to its "parent directory" (the directory that contains the link for the directory in question).  As a special case, '..' in the root directory refers to the root directory itself, since it has no parent; thus '/..' is the same as '/'.

Here are some examples of file names:

'/a'

The file named 'a', in the root directory.

'/a/b'

The file named 'b', in the directory named 'a' in the root directory.

'a'

The file named 'a', in the current working directory.

'/a/./b'

This is the same as '/a/b'.

'./a'

The file named 'a', in the current working directory.

'../a'

The file named 'a', in the parent directory of the current working directory.

A file name that names a directory may optionally end in a '/'.  You can specify a file name of '/' to refer to the root directory, but the empty string is not a meaningful file name.  If you want to refer to the current working directory, use a file name of '.' or './'.

Unlike some other operating systems, the GNU system doesn't have any built-in support for file types (or extensions) or file versions as part of its file name syntax.  Many programs and utilities use conventions for file names--for example, files containing C source code usually have names suffixed with '.c'--but there is nothing in the file system itself that enforces this kind of convention.


File Name Errors
Node: File Name Errors, Next: File Name Portability, Prev: File Name Resolution, Up: File Names

File Name Errors

Functions that accept file name arguments usually detect these errno error conditions relating to file name syntax.  These errors are referred to throughout this manual as the "usual file name syntax errors".

EACCES

The process does not have search permission for a directory component of the file name.

ENAMETOOLONG

This error is used when either the the total length of a file name is greater than PATH_MAX, or when an individual file name component has a length greater than NAME_MAX.  See Limits for Files.

In the GNU system, there is no imposed limit on overall file name length, but some file systems may place limits on the length of a component.

ENOENT

This error is reported when a file referenced as a directory component in the file name doesn't exist, or when a component is a symbolic link whose target file does not exist.  See Symbolic Links.

ENOTDIR

A file that is referenced as a directory component in the file name exists, but it isn't a directory.

ELOOP

Too many symbolic links were resolved while trying to look up the file name.  The system has an arbitrary limit on the number of symbolic links that may be resolved in looking up a single file name, as a primitive way to detect loops.  See Symbolic Links.


Portability of File Names
Node: File Name Portability, Next: , Prev: File Name Errors, Up: File Names

Portability of File Names

The rules for the syntax of file names discussed in See File Names, are the rules normally used by the GNU system and by other POSIX systems.  However, other operating systems may use other conventions.

There are two reasons why it can be important for you to be aware of file name portability issues:

•	If your program makes assumptions about file name syntax, or contains embedded literal file name strings, it is more difficult to get it to run under other operating systems that use different syntax conventions.

•	Even if you are not concerned about running your program on machines that run other operating systems, it may still be possible to access files that use different naming conventions.  For example, you may be able to access file systems on another computer running a different operating system over a network, or read and write disks in formats used by other operating systems.

The ANSI C standard says very little about file name syntax, only that file names are strings.  In addition to varying restrictions on the length of file names and what characters can validly appear in a file name, different operating systems use different conventions and syntax for concepts such as structured directories and file types or extensions.  Some concepts such as file versions might be supported in some operating systems and not by others.

The POSIX.1 standard allows implementations to put additional restrictions on file name syntax, concerning what characters are permitted in file names and on the length of file name and file name component strings.  However, in the GNU system, you do not need to worry about these restrictions; any character except the null character is permitted in a file name string, and there are no limits on the length of file name strings.


Input/Output on Streams
Node: I/O on Streams, Next: Low-Level I/O, Prev: I/O Overview, Up: Top

Input/Output on Streams

This chapter describes the functions for creating streams and performing input and output operations on them.  As discussed in See I/O Overview, a stream is a fairly abstract, high-level concept representing a communications channel to a file, device, or process.

* Menu:

Streams	About the data type representing a stream.

Standard Streams	Streams to the standard input and output devices are created for you.

Opening Streams	How to create a stream to talk to a file.

Closing Streams	Close a stream when you are finished with it.

Simple Output	Unformatted output by characters and lines.

Character Input	Unformatted input by characters and words.

Line Input	Reading a line or a record from a stream.

Unreading	Peeking ahead/pushing back input just read.

Formatted Output	printf and related functions.

Customizing Printf	You can define new conversion specifiers for printf and friends.

Formatted Input	scanf and related functions.

Block Input/Output	Input and output operations on blocks of data.

EOF and Errors	How you can tell if an I/O error happens.

Binary Streams	Some systems distinguish between text files and binary files.

File Positioning	About random-access streams.

Portable Positioning	Random access on peculiar ANSI C systems.

Stream Buffering	How to control buffering of streams.

Other Kinds of Streams	Streams that do not necessarily correspond to an open file.


Streams
Node: Streams, Next: Standard Streams, Prev: , Up: I/O on Streams

Streams

For historical reasons, the type of the C data structure that represents a stream is called FILE rather than "stream".  Since most of the library functions deal with objects of type FILE *, sometimes the term "file pointer" is also used to mean "stream".  This leads to unfortunate confusion over terminology in many books on C.  This manual, however, is careful to use the terms "file" and "stream" only in the technical sense.  KThe FILE type is declared in the header file 'stdio.h'.

 - Data Type: FILE

This is the data type used to represent stream objects.  A FILE object holds all of the internal state information about the connection to the associated file, including such things as the file position indicator and buffering information.  Each stream also has error and end-of-file status indicators that can be tested with the ferror and feof functions; see See EOF and Errors.

FILE objects are allocated and managed internally by the input/output library functions.  Don't try to create your own objects of type FILE; let the library do it.  Your programs should deal only with pointers to these objects (that is, FILE * values) rather than the objects themselves.


Standard Streams
Node: Standard Streams, Next: Opening Streams, Prev: Streams, Up: I/O on Streams

Standard Streams

When the main function of your program is invoked, it already has three predefined streams open and available for use.  These represent the "standard" input and output channels that have been established for the process.

These streams are declared in the header file 'stdio.h'.  K

 - Variable: FILE * stdin

The "standard input" stream, which is the normal source of input for the program.

 - Variable: FILE * stdout

The "standard output" stream, which is used for normal output from the program.

 - Variable: FILE * stderr

The "standard error" stream, which is used for error messages and diagnostics issued by the program.

In the GNU system, you can specify what files or processes correspond to these streams using the pipe and redirection facilities provided by the shell.  (The primitives shells use to implement these facilities are described in See File System Interface.)  Most other operating systems provide similar mechanisms, but the details of how to use them can vary.

In the GNU C library, stdin, stdout, and stderr are normal variables which you can set just like any others.  For example, to redirect the standard output to a file, you could do:

fclose (stdout);
stdout = fopen ("standard-output-file", "w");

Note however, that in other systems stdin, stdout, and stderr are macros that you cannot assign to in the normal way.  But you can use freopen to get the effect of closing one and reopening it.  See Opening Streams.


Opening Streams
Node: Opening Streams, Next: Closing Streams, Prev: Standard Streams, Up: I/O on Streams

Opening Streams

Opening a file with the fopen function creates a new stream and establishes a connection between the stream and a file.  This may involve creating a new file.

Everything described in this section is declared in the header file 'stdio.h'.

 - Function: FILE * fopen (const char *filename, const char *opentype)

The fopen function opens a stream for I/O to the file filename, and returns a pointer to the stream.

The opentype argument is a string that controls how the file is opened and specifies attributes of the resulting stream.  It must begin with one of the following sequences of characters:

'r'

Open an existing file for reading only.

'w'

Open the file for writing only.  If the file already exists, it is truncated to zero length.  Otherwise a new file is created.

'a'

Open a file for append access; that is, writing at the end of file only.  If the file already exists, its initial contents are unchanged and output to the stream is appended to the end of the file.  Otherwise, a new, empty file is created.

'r+'

Open an existing file for both reading and writing.  The initial contents of the file are unchanged and the initial file position is at the beginning of the file.

'w+'

Open a file for both reading and writing.  If the file already exists, it is truncated to zero length.  Otherwise, a new file is created.

'a+'

Open or create file for both reading and appending.  If the file exists, its initial contents are unchanged.  Otherwise, a new file is created.  The initial file position for reading is at the beginning of the file, but output is always appended to the end of the file.

As you can see, '+' requests a stream that can do both input and output.  The ANSI standard says that when using such a stream, you must call fflush (see Stream Buffering) or a file positioning function such as fseek (see File Positioning) when switching from reading to writing or vice versa.  Otherwise, internal buffers might not be emptied properly.  The GNU C library does not have this limitation; you can do arbitrary reading and writing operations on a stream in whatever order.

The GNU C library defines one additional character for use in opentype: the character 'x' insists on creating a new file--if a file filename already exists, fopen fails rather than opening it.  This is equivalent to the O_EXCL option to the open function (see File Status Flags).

The character 'b' in opentype has a standard meaning; it requests a binary stream rather than a text stream.  But this makes no difference in POSIX systems (including the GNU system).  If both '+' and 'b' are specified, they can appear in either order.  See Binary Streams.

Any other characters in opentype are simply ignored.  They may be meaningful in other systems.

If the open fails, fopen returns a null pointer.

You can have multiple streams (or file descriptors) pointing to the same file open at the same time.  If you do only input, this works straightforwardly, but you must be careful if any output streams are included.  See Stream/Descriptor Precautions.  This is equally true whether the streams are in one program (not usual) or in several programs (which can easily happen).  It may be advantageous to use the file locking facilities to avoid simultaneous access.  See File Locks.

 - Macro: int FOPEN_MAX

The value of this macro is an integer constant expression that represents the minimum number of streams that the implementation guarantees can be open simultaneously.  You might be able to open more than this many streams, but that is not guaranteed.  The value of this constant is at least eight, which includes the three standard streams stdin, stdout, and stderr.

 - Function: FILE * freopen (const char *filename, const char *opentype, FILE *stream)

This function is like a combination of fclose and fopen.  It first closes the stream referred to by stream, ignoring any errors that are detected in the process.  (Because errors are ignored, you should not use freopen on an output stream if you have actually done any output using the stream.)  Then the file named by filename is opened with mode opentype as for fopen, and associated with the same stream object stream.

If the operation fails, a null pointer is returned; otherwise, freopen returns stream.

freopen has traditionally been used to connect a standard stream such as stdin with a file of your own choice.  This is useful in programs in which use of a standard stream for certain purposes is hard-coded.  In the GNU C library, you can simply close the standard streams and open new ones with fopen.  But other systems lack this ability, so using freopen is more portable.


Closing Streams
Node: Closing Streams, Next: Simple Output, Prev: Opening Streams, Up: I/O on Streams

Closing Streams

When a stream is closed with fclose, the connection between the stream and the file is cancelled.  After you have closed a stream, you cannot perform any additional operations on it.

 - Function: int fclose (FILE *stream)

This function causes stream to be closed and the connection to the corresponding file to be broken.  Any buffered output is written and any buffered input is discarded.  The fclose function returns a value of 0 if the file was closed successfully, and EOF if an error was detected.

It is important to check for errors when you call fclose to close an output stream, because real, everyday errors can be detected at this time.  For example, when fclose writes the remaining buffered output, it might get an error because the disk is full.  Even if you know the buffer is empty, errors can still occur when closing a file if you are using NFS.

The function fclose is declared in 'stdio.h'.

If the main function to your program returns, or if you call the exit function (see Normal Termination), all open streams are automatically closed properly.  If your program terminates in any other manner, such as by calling the abort function (see Aborting a Program) or from a fatal signal (see Signal Handling), open streams might not be closed properly.  Buffered output may not be flushed and files may not be complete.  For more information on buffering of streams, see See Stream Buffering.


Simple Output by Characters or Lines
Node: Simple Output, Next: Character Input, Prev: Closing Streams, Up: I/O on Streams

Simple Output by Characters or Lines

This section describes functions for performing character- and line-oriented output.

These functions are declared in the header file 'stdio.h'.  K

 - Function: int fputc (int c, FILE *stream)

The fputc function converts the character c to type unsigned char, and writes it to the stream stream.  EOF is returned if a write error occurs; otherwise the character c is returned.

 - Function: int putc (int c, FILE *stream)

This is just like fputc, except that most systems implement it as a macro, making it faster.  One consequence is that it may evaluate the stream argument more than once.  putc is usually the best function to use for writing a single character.

 - Function: int putchar (int c)

The putchar function is equivalent to putc with stdout as the value of the stream argument.

 - Function: int fputs (const char *s, FILE *stream)

The function fputs writes the string s to the stream stream.  The terminating null character is not written.  This function does not add a newline character, either.  It outputs only the chars in the string.

This function returns EOF if a write error occurs, and otherwise a non-negative value.

For example:

fputs ("Are ", stdout);
fputs ("you ", stdout);
fputs ("hungry?\n", stdout);

outputs the text 'Are you hungry?' followed by a newline.

 - Function: int puts (const char *s)

The puts function writes the string s to the stream stdout followed by a newline.  The terminating null character of the string is not written.

puts is the most convenient function for printing simple messages.  For example:

puts ("This is a message.");

 - Function: int putw (int w, FILE *stream)

This function writes the word w (that is, an int) to stream.  It is provided for compatibility with SVID, but we recommend you use fwrite instead (see Block Input/Output).


Character Input
Node: Character Input, Next: Line Input, Prev: Simple Output, Up: I/O on Streams

Character Input

This section describes functions for performing character- and line-oriented input.  These functions are declared in the header file 'stdio.h'.  K

 - Function: int fgetc (FILE *stream)

This function reads the next character as an unsigned char from the stream stream and returns its value, converted to an int.  If an end-of-file condition or read error occurs, EOF is returned instead.

 - Function: int getc (FILE *stream)

This is just like fgetc, except that it is permissible (and typical) for it to be implemented as a macro that evaluates the stream argument more than once.  getc is often highly optimized, so it is usually the best function to use to read a single character.

 - Function: int getchar (void)

The getchar function is equivalent to getc with stdin as the value of the stream argument.

Here is an example of a function that does input using fgetc.  It would work just as well using getc instead, or using getchar () instead of fgetc (stdin).

int
y_or_n_p (const char *question)
{
  fputs (question, stdout);
  while (1)
    {
      int c, answer;
      /* Write a space to separate answer from question. */
      fputc (' ', stdout);
      /* Read the first character of the line.
         This should be the answer character, but might not be. */
      c = tolower (fgetc (stdin));
      answer = c;
      /* Discard rest of input line. */
      while (c != '\n')
        c = fgetc (stdin);
      /* Obey the answer if it was valid. */
      if (answer == 'y')
        return 1;
      if (answer == 'n')
        return 0;
      /* Answer was invalid: ask for valid answer. */
      fputs ("Please answer y or n:", stdout);
    }
}

 - Function: int getw (FILE *stream)

This function reads a word (that is, an int) from stream.  It's provided for compatibility with SVID.  We recommend you use fread instead (see Block Input/Output).


Line-Oriented Input
Node: Line Input, Next: Unreading, Prev: Character Input, Up: I/O on Streams

Line-Oriented Input

Since many programs interpret input on the basis of lines, it's convenient to have functions to read a line of text from a stream.

Standard C has functions to do this, but they aren't very safe: null characters and even (for gets) long lines can confuse them.  So the GNU library provides the nonstandard getline function that makes it easy to read lines reliably.

Another GNU extension, getdelim, generalizes getline.  It reads a delimited record, defined as everything through the next occurrence of a specified delimiter character.

All these functions are declared in 'stdio.h'.

 - Function: ssize_t getline (char **lineptr, size_t *n, FILE *stream)

This function reads an entire line from stream, storing the text (including the newline and a terminating null character) in a buffer and storing the buffer address in *lineptr.

Before calling getline, you should place in *lineptr the address of a buffer *n bytes long, allocated with malloc.  If this buffer is long enough to hold the line, getline stores the line in this buffer.  Otherwise, getline makes the buffer bigger using realloc, storing the new buffer address back in *lineptr and the increased size back in *n.  See Unconstrained Allocation.

If you set *lineptr to a null pointer, and *n to zero, before the call, then getline allocates the initial buffer for you by calling malloc.

In either case, when getline returns,  *lineptr is a char * which points to the text of the line.

When getline is successful, it returns the number of characters read (including the newline, but not including the terminating null).  This value enables you to distinguish null characters that are part of the line from the null character inserted as a terminator.

This function is a GNU extension, but it is the recommended way to read lines from a stream.  The alternative standard functions are unreliable.

If an error occurs or end of file is reached, getline returns -1.

 - Function: ssize_t getdelim (char **lineptr, size_t *n, int delimiter, FILE *stream)

This function is like getline except that the character which tells it to stop reading is not necessarily newline.  The argument delimiter specifies the delimiter character; getdelim keeps reading until it sees that character (or end of file).

The text is stored in lineptr, including the delimiter character and a terminating null.  Like getline, getdelim makes lineptr bigger if it isn't big enough.

getline is in fact implemented in terms of getdelim, just like this:

ssize_t
getline (char **lineptr, size_t *n, FILE *stream)
{
  return getdelim (lineptr, n, '\n', stream);
}

 - Function: char * fgets (char *s, int count, FILE *stream)

The fgets function reads characters from the stream stream up to and including a newline character and stores them in the string s, adding a null character to mark the end of the string.  You must supply count characters worth of space in s, but the number of characters read is at most count - 1.  The extra character space is used to hold the null character at the end of the string.

If the system is already at end of file when you call fgets, then the contents of the array s are unchanged and a null pointer is returned.  A null pointer is also returned if a read error occurs.  Otherwise, the return value is the pointer s.

Warning:  If the input data has a null character, you can't tell.  So don't use fgets unless you know the data cannot contain a null.  Don't use it to read files edited by the user because, if the user inserts a null character, you should either handle it properly or print a clear error message.  We recommend using getline instead of fgets.

 - Deprecated function: char * gets (char *s)

The function gets reads characters from the stream stdin up to the next newline character, and stores them in the string s.  The newline character is discarded (note that this differs from the behavior of fgets, which copies the newline character into the string).  If gets encounters a read error or end-of-file, it returns a null pointer; otherwise it returns s.

Warning: The gets function is very dangerous because it provides no protection against overflowing the string s.  The GNU library includes it for compatibility only.  You should always use fgets or getline instead.  To remind you of this, the linker (if using GNU ld) will issue a warning whenever you use gets.


Unreading
Node: Unreading, Next: Formatted Output, Prev: Line Input, Up: I/O on Streams

Unreading

In parser programs it is often useful to examine the next character in the input stream without removing it from the stream.  This is called "peeking ahead" at the input because your program gets a glimpse of the input it will read next.

Using stream I/O, you can peek ahead at input by first reading it and then "unreading" it (also called  "pushing it back" on the stream).  Unreading a character makes it available to be input again from the stream, by  the next call to fgetc or other input function on that stream.

* Menu:

Unreading Idea	An explanation of unreading with pictures.

How Unread	How to call ungetc to do unreading.


What Unreading Means
Node: Unreading Idea, Next: How Unread, Prev: , Up: Unreading

What Unreading Means

Here is a pictorial explanation of unreading.  Suppose you have a stream reading a file that contains just six characters, the letters 'foobar'.  Suppose you have read three characters so far.  The situation looks like this:

f  o  o  b  a  r
         ^

so the next input character will be 'b'.

If instead of reading 'b' you unread the letter 'o', you get a situation like this:

f  o  o  b  a  r
         |
      o--
      ^

so that the next input characters will be 'o' and 'b'.

If you unread '9' instead of 'o', you get this situation:

f  o  o  b  a  r
         |
      9--
      ^

so that the next input characters will be '9' and 'b'.


Using ungetc To Do Unreading
Node: How Unread, Next: , Prev: Unreading Idea, Up: Unreading

Using ungetc To Do Unreading

The function to unread a character is called ungetc, because it reverses the action of getc.

 - Function: int ungetc (int c, FILE *stream)

The ungetc function pushes back the character c onto the input stream stream.  So the next input from stream will read c before anything else.

If c is EOF, ungetc does nothing and just returns EOF.  This lets you call ungetc with the return value of getc without needing to check for an error from getc.

The character that you push back doesn't have to be the same as the last character that was actually read from the stream.  In fact, it isn't necessary to actually read any characters from the stream before unreading them with ungetc!  But that is a strange way to write a program; usually ungetc is used only to unread a character that was just read from the same stream.

The GNU C library only supports one character of pushback--in other words, it does not work to call ungetc twice without doing input in between.  Other systems might let you push back multiple characters; then reading from the stream retrieves the characters in the reverse order that they were pushed.

Pushing back characters doesn't alter the file; only the internal buffering for the stream is affected.  If a file positioning function (such as fseek or rewind; see File Positioning) is called, any pending pushed-back characters are discarded.

Unreading a character on a stream that is at end of file clears the end-of-file indicator for the stream, because it makes the character of input available.  After you read that character, trying to read again will encounter end of file.

Here is an example showing the use of getc and ungetc to skip over whitespace characters.  When this function reaches a non-whitespace character, it unreads that character to be seen again on the next read operation on the stream.

#include <stdio.h>
#include <ctype.h>

void
skip_whitespace (FILE *stream)
{
  int c;
  do
    /* No need to check for EOF because it is not
       isspace, and ungetc ignores EOF.  */
    c = getc (stream);
  while (isspace (c));
  ungetc (c, stream);
}


Formatted Output
Node: Formatted Output, Next: Customizing Printf, Prev: Unreading, Up: I/O on Streams

Formatted Output

The functions described in this section (printf and related functions) provide a convenient way to perform formatted output.  You call printf with a "format string" or "template string" that specifies how to format the values of the remaining arguments.

Unless your program is a filter that specifically performs line- or character-oriented processing, using printf or one of the other related functions described in this section is usually the easiest and most concise way to perform output.  These functions are especially useful for printing error messages, tables of data, and the like.

* Menu:

Formatted Output Basics	Some examples to get you started.

Output Conversion Syntax	General syntax of conversion specifications.

Table of Output Conversions	Summary of output conversions and what they do.

Integer Conversions	Details about formatting of integers.

Floating-Point Conversions	Details about formatting of floating-point numbers.

Other Output Conversions	Details about formatting of strings, characters, pointers, and the like.

Formatted Output Functions	Descriptions of the actual functions.

Dynamic Output	Functions that allocate memory for the output.

Variable Arguments Output	vprintf and friends.

Parsing a Template String	What kinds of args does a given template call for?

Example of Parsing	Sample program using parse_printf_format.


Formatted Output Basics
Node: Formatted Output Basics, Next: Output Conversion Syntax, Prev: , Up: Formatted Output

Formatted Output Basics

The printf function can be used to print any number of arguments.  The template string argument you supply in a call provides information not only about the number of additional arguments, but also about their types and what style should be used for printing them.

Ordinary characters in the template string are simply written to the output stream as-is, while "conversion specifications" introduced by a '%' character in the template cause subsequent arguments to be formatted and written to the output stream.  For example, K

int pct = 37;
char filename[] = "foo.txt";
printf ("Processing of `%s' is %d%% finished.\nPlease be patient.\n",
        filename, pct);

produces output like

Processing of `foo.txt' is 37% finished.
Please be patient.

This example shows the use of the '%d' conversion to specify that an int argument should be printed in decimal notation, the '%s' conversion to specify printing of a string argument, and the '%%' conversion to print a literal '%' character.

There are also conversions for printing an integer argument as an unsigned value in octal, decimal, or hexadecimal radix ('%o', '%u', or '%x', respectively); or as a character value ('%c').

Floating-point numbers can be printed in normal, fixed-point notation using the '%f' conversion or in exponential notation using the '%e' conversion.  The '%g' conversion uses either '%e' or '%f' format, depending on what is more appropriate for the magnitude of the particular number.

You can control formatting more precisely by writing "modifiers" between the '%' and the character that indicates which conversion to apply.  These slightly alter the ordinary behavior of the conversion.  For example, most conversion specifications permit you to specify a minimum field width and a flag indicating whether you want the result left- or right-justified within the field.

The specific flags and modifiers that are permitted and their interpretation vary depending on the particular conversion.  They're all described in more detail in the following sections.  Don't worry if this all seems excessively complicated at first; you can almost always get reasonable free-format output without using any of the modifiers at all.  The modifiers are mostly used to make the output look "prettier" in tables.


Output Conversion Syntax
Node: Output Conversion Syntax, Next: Table of Output Conversions, Prev: Formatted Output Basics, Up: Formatted Output

Output Conversion Syntax

This section provides details about the precise syntax of conversion specifications that can appear in a printf template string.

Characters in the template string that are not part of a conversion specification are printed as-is to the output stream.  Multibyte character sequences (see Extended Characters) are permitted in a template string.

The conversion specifications in a printf template string have the general form:

% flags width [ . precision ] type conversion

For example, in the conversion specifier '%-10.8ld', the '-' is a flag, '10' specifies the field width, the precision is '8', the letter 'l' is a type modifier, and 'd' specifies the conversion style.  (This particular type specifier says to print a long int argument in decimal notation, with a minimum of 8 digits left-justified in a field at least 10 characters wide.)

In more detail, output conversion specifications consist of an initial '%' character followed in sequence by:

•	Zero or more "flag characters" that modify the normal behavior of the conversion specification.  K

•	An optional decimal integer specifying the "minimum field width".  If the normal conversion produces fewer characters than this, the field is padded with spaces to the specified width.  This is a minimum value; if the normal conversion produces more characters than this, the field is not truncated.  Normally, the output is right-justified within the field.  KYou can also specify a field width of '*'.  This means that the next argument in the argument list (before the actual value to be printed) is used as the field width.  The value must be an int.  If the value is negative, this means to set the '-' flag (see below) and to use the absolute value as the field width.

•	An optional "precision" to specify the number of digits to be written for the numeric conversions.  If the precision is specified, it consists of a period ('.') followed optionally by a decimal integer (which defaults to zero if omitted).  KYou can also specify a precision of '*'.  This means that the next argument in the argument list (before the actual value to be printed) is used as the precision.  The value must be an int, and is ignored if it is negative.  If you specify '*' for both the field width and precision, the field width argument precedes the precision argument.  Other C library versions may not recognize this syntax.

•	An optional "type modifier character", which is used to specify the data type of the corresponding argument if it differs from the default type.  (For example, the integer conversions assume a type of int, but you can specify 'h', 'l', or 'L' for other integer types.) K

•	A character that specifies the conversion to be applied.

The exact options that are permitted and how they are interpreted vary between the different conversion specifiers.  See the descriptions of the individual conversions for information about the particular options that they use.


Table of Output Conversions
Node: Table of Output Conversions, Next: Integer Conversions, Prev: Output Conversion Syntax, Up: Formatted Output

Table of Output Conversions

Here is a table summarizing what all the different conversions do:

'%d', '%i'

Print an integer as a signed decimal number.  See Integer Conversions, for details.  '%d' and '%i' are synonymous for output, but are different when used with scanf for input (see Table of Input Conversions).

'%o'

Print an integer as an unsigned octal number.  See Integer Conversions, for details.

'%u'

Print an integer as an unsigned decimal number.  See Integer Conversions, for details.

'%x', '%X'

Print an integer as an unsigned hexadecimal number.  '%x' uses lower-case letters and '%X' uses upper-case.  See Integer Conversions, for details.

'%f'

Print a floating-point number in normal (fixed-point) notation.  See Floating-Point Conversions, for details.

'%e', '%E'

Print a floating-point number in exponential notation.  '%e' uses lower-case letters and '%E' uses upper-case.  See Floating-Point Conversions, for details.

'%g', '%G'

Print a floating-point number in either normal or exponential notation, whichever is more appropriate for its magnitude.  '%g' uses lower-case letters and '%G' uses upper-case.  See Floating-Point Conversions, for details.

'%c'

Print a single character.  See Other Output Conversions.

'%s'

Print a string.  See Other Output Conversions.

'%p'

Print the value of a pointer.  See Other Output Conversions.

'%n'

Get the number of characters printed so far.  See Other Output Conversions.  Note that this conversion specification never produces any output.

'%m'

Print the string corresponding to the value of errno.  (This is a GNU extension.) See Other Output Conversions.

'%%'

Print a literal '%' character.  See Other Output Conversions.

If the syntax of a conversion specification is invalid, unpredictable things will happen, so don't do this.  If there aren't enough function arguments provided to supply values for all the conversion specifications in the template string, or if the arguments are not of the correct types, the results are unpredictable.  If you supply more arguments than conversion specifications, the extra argument values are simply ignored; this is sometimes useful.


Integer Conversions
Node: Integer Conversions, Next: Floating-Point Conversions, Prev: Table of Output Conversions, Up: Formatted Output

Integer Conversions

This section describes the options for the '%d', '%i', '%o', '%u', '%x', and '%X' conversion specifications.  These conversions print integers in various formats.

The '%d' and '%i' conversion specifications both print an int argument as a signed decimal number; while '%o', '%u', and '%x' print the argument as an unsigned octal, decimal, or hexadecimal number (respectively).  The '%X' conversion specification is just like '%x' except that it uses the characters 'ABCDEF' as digits instead of 'abcdef'.

The following flags are meaningful:

'-'

Left-justify the result in the field (instead of the normal right-justification).

'+'

For the signed '%d' and '%i' conversions, print a plus sign if the value is positive.

' '

For the signed '%d' and '%i' conversions, if the result doesn't start with a plus or minus sign, prefix it with a space character instead.  Since the '+' flag ensures that the result includes a sign, this flag is ignored if you supply both of them.

'#'

For the '%o' conversion, this forces the leading digit to be '0', as if by increasing the precision.  For '%x' or '%X', this prefixes a leading '0x' or '0X' (respectively) to the result.  This doesn't do anything useful for the '%d', '%i', or '%u' conversions.  Using this flag produces output which can be parsed by the strtoul function (see Parsing of Integers) and scanf with the '%i' conversion (see Numeric Input Conversions).

'0'

Pad the field with zeros instead of spaces.  The zeros are placed after any indication of sign or base.  This flag is ignored if the '-' flag is also specified, or if a precision is specified.

If a precision is supplied, it specifies the minimum number of digits to appear; leading zeros are produced if necessary.  If you don't specify a precision, the number is printed with as many digits as it needs.  If you convert a value of zero with an explicit precision of zero, then no characters at all are produced.

Without a type modifier, the corresponding argument is treated as an int (for the signed conversions '%i' and '%d') or unsigned int (for the unsigned conversions '%o', '%u', '%x', and '%X').  Recall that since printf and friends are variadic, any char and short arguments are automatically converted to int by the default argument promotions.  For arguments of other integer types, you can use these modifiers:

'h'

Specifies that the argument is a short int or unsigned short int, as appropriate.  A short argument is converted to an int or unsigned int by the default argument promotions anyway, but the 'h' modifier says to convert it back to a short again.

'l'

Specifies that the argument is a long int or unsigned long int, as appropriate.

'L'

Specifies that the argument is a long long int.  (This type is an extension supported by the GNU C compiler.  On systems that don't support extra-long integers, this is the same as long int.)

'Z'

Specifies that the argument is a size_t.  This is a GNU extension.

Here is an example.  Using the template string:

"|%5d|%-5d|%+5d|%+-5d|% 5d|%05d|%5.0d|%5.2d|%d|\n"

to print numbers using the different options for the '%d' conversion gives results like:

|    0|0    |   +0|+0   |    0|00000|     |   00|0|
|    1|1    |   +1|+1   |    1|00001|    1|   01|1|
|   -1|-1   |   -1|-1   |   -1|-0001|   -1|  -01|-1|
|100000|100000|+100000| 100000|100000|100000|100000|100000|

In particular, notice what happens in the last case where the number is too large to fit in the minimum field width specified.

Here are some more examples showing how unsigned integers print under various format options, using the template string:

"|%5u|%5o|%5x|%5X|%#5o|%#5x|%#5X|%#10.8x|\n"

|    0|    0|    0|    0|    0|  0x0|  0X0|0x00000000|
|    1|    1|    1|    1|   01|  0x1|  0X1|0x00000001|
|100000|303240|186a0|186A0|0303240|0x186a0|0X186A0|0x000186a0|


Floating-Point Conversions
Node: Floating-Point Conversions, Next: Other Output Conversions, Prev: Integer Conversions, Up: Formatted Output

Floating-Point Conversions

This section discusses the conversion specifications for floating-point numbers: the '%f', '%e', '%E', '%g', and '%G' conversions.

The '%f' conversion prints its argument in fixed-point notation, producing output of the form [-]ddd.ddd, where the number of digits following the decimal point is controlled by the precision you specify.

The '%e' conversion prints its argument in exponential notation, producing output of the form [-]d.ddde[+|-]dd.  Again, the number of digits following the decimal point is controlled by the precision.  The exponent always contains at least two digits.  The '%E' conversion is similar but the exponent is marked with the letter 'E' instead of 'e'.

The '%g' and '%G' conversions print the argument in the style of '%e' or '%E' (respectively) if the exponent would be less than -4 or greater than or equal to the precision; otherwise they use the '%f' style.  Trailing zeros are removed from the fractional portion of the result and a decimal-point character appears only if it is followed by a digit.

The following flags can be used to modify the behavior:

'-'

Left-justify the result in the field.  Normally the result is right-justified.

'+'

Always include a plus or minus sign in the result.

' '

If the result doesn't start with a plus or minus sign, prefix it with a space instead.  Since the '+' flag ensures that the result includes a sign, this flag is ignored if you supply both of them.

'#'

Specifies that the result should always include a decimal point, even if no digits follow it.  For the '%g' and '%G' conversions, this also forces trailing zeros after the decimal point to be left in place where they would otherwise be removed.

'0'

Pad the field with zeros instead of spaces; the zeros are placed after any sign.  This flag is ignored if the '-' flag is also specified.

The precision specifies how many digits follow the decimal-point character for the '%f', '%e', and '%E' conversions.  For these conversions, the default precision is 6.  If the precision is explicitly 0, this suppresses the decimal point character entirely.  For the '%g' and '%G' conversions, the precision specifies how many significant digits to print.  Significant digits are the first digit before the decimal point, and all the digits after it.  If the precision 0 or not specified for '%g' or '%G', it is treated like a value of 1.  If the value being printed cannot be expressed precisely in the specified number of digits, the value is rounded to the nearest number that fits.

Without a type modifier, the floating-point conversions use an argument of type double.  (By the default argument promotions, any float arguments are automatically converted to double.) The following type modifier is supported:

'L'

An uppercase 'L' specifies that the argument is a long double.

Here are some examples showing how numbers print using the various floating-point conversions.  All of the numbers were printed using this template string:

"|%12.4f|%12.4e|%12.4g|\n"

Here is the output:

|      0.0000|  0.0000e+00|           0|
|      1.0000|  1.0000e+00|           1|
|     -1.0000| -1.0000e+00|          -1|
|    100.0000|  1.0000e+02|         100|
|   1000.0000|  1.0000e+03|        1000|
|  10000.0000|  1.0000e+04|       1e+04|
|  12345.0000|  1.2345e+04|   1.234e+04|
| 100000.0000|  1.0000e+05|       1e+05|
| 123456.0000|  1.2346e+05|   1.234e+05|

Notice how the '%g' conversion drops trailing zeros.


Other Output Conversions
Node: Other Output Conversions, Next: Formatted Output Functions, Prev: Floating-Point Conversions, Up: Formatted Output

Other Output Conversions

This section describes miscellaneous conversions for printf.

The '%c' conversion prints a single character.  The int argument is first converted to an unsigned char.  The '-' flag can be used to specify left-justification in the field, but no other flags are defined, and no precision or type modifier can be given.  For example:

printf ("%c%c%c%c%c", 'h', 'e', 'l', 'l', 'o');

prints 'hello'.

The '%s' conversion prints a string.  The corresponding argument must be of type char * (or const char *).  A precision can be specified to indicate the maximum number of characters to write; otherwise characters in the string up to but not including the terminating null character are written to the output stream.  The '-' flag can be used to specify left-justification in the field, but no other flags or type modifiers are defined for this conversion.  For example:

printf ("%3s%-6s", "no", "where");

prints ' nowhere '.

If you accidentally pass a null pointer as the argument for a '%s' conversion, the GNU library prints it as '(null)'.  We think this is more useful than crashing.  But it's not good practice to pass a null argument intentionally.

The '%m' conversion prints the string corresponding to the error code in errno.  See Error Messages.  Thus:

fprintf (stderr, "can't open `%s': %m\n", filename);

is equivalent to:

fprintf (stderr, "can't open `%s': %s\n", filename, strerror (errno));

The '%m' conversion is a GNU C library extension.

The '%p' conversion prints a pointer value.  The corresponding argument must be of type void *.  In practice, you can use any type of pointer.

In the GNU system, non-null pointers are printed as unsigned integers, as if a '%#x' conversion were used.  Null pointers print as '(nil)'.  (Pointers might print differently in other systems.)

For example:

printf ("%p", "testing");

prints '0x' followed by a hexadecimal number--the address of the string constant "testing".  It does not print the word 'testing'.

You can supply the '-' flag with the '%p' conversion to specify left-justification, but no other flags, precision, or type modifiers are defined.

The '%n' conversion is unlike any of the other output conversions.  It uses an argument which must be a pointer to an int, but instead of printing anything it stores the number of characters printed so far by this call at that location.  The 'h' and 'l' type modifiers are permitted to specify that the argument is of type short int * or long int * instead of int *, but no flags, field width, or precision are permitted.

For example,

int nchar;
printf ("%d %s%n\n", 3, "bears", &nchar);

prints:

3 bears

and sets nchar to 7, because '3 bears' is seven characters.

The '%%' conversion prints a literal '%' character.  This conversion doesn't use an argument, and no flags, field width, precision, or type modifiers are permitted.


Formatted Output Functions
Node: Formatted Output Functions, Next: Dynamic Output, Prev: Other Output Conversions, Up: Formatted Output

Formatted Output Functions

This section describes how to call printf and related functions.  Prototypes for these functions are in the header file 'stdio.h'.  Because these functions take a variable number of arguments, you must declare prototypes for them before using them.  Of course, the easiest way to make sure you have all the right prototypes is to just include 'stdio.h'.  K

 - Function: int printf (const char *template, ...)

The printf function prints the optional arguments under the control of the template string template to the stream stdout.  It returns the number of characters printed, or a negative value if there was an output error.

 - Function: int fprintf (FILE *stream, const char *template, ...)

This function is just like printf, except that the output is written to the stream stream instead of stdout.

 - Function: int sprintf (char *s, const char *template, ...)

This is like printf, except that the output is stored in the character array s instead of written to a stream.  A null character is written to mark the end of the string.

The sprintf function returns the number of characters stored in the array s, not including the terminating null character.

The behavior of this function is undefined if copying takes place between objects that overlap--for example, if s is also given as an argument to be printed under control of the '%s' conversion.  See Copying and Concatenation.

Warning: The sprintf function can be dangerous because it can potentially output more characters than can fit in the allocation size of the string s.  Remember that the field width given in a conversion specification is only a minimum value.

To avoid this problem, you can use snprintf or asprintf, described below.

 - Function: int snprintf (char *s, size_t size, const char *template, ...)

The snprintf function is similar to sprintf, except that the size argument specifies the maximum number of characters to produce.  The trailing null character is counted towards this limit, so you should allocate at least size characters for the string s.

The return value is the number of characters stored, not including the terminating null.  If this value equals size - 1, then there was not enough space in s for all the output.  You should try again with a bigger output string.  Here is an example of doing this:

/* Construct a message describing the value of a variable
   whose name is name and whose value is value. */
char *
make_message (char *name, char *value)
{
  /* Guess we need no more than 100 chars of space. */
  int size = 100;
  char *buffer = (char *) xmalloc (size);

while (1)
    {
      /* Try to print in the allocated space. */
      int nchars = snprintf (buffer, size,
                             "value of %s is %s",
                             name, value);
      /* If that worked, return the string. */
      if (nchars < size)
        return buffer;
      /* Else try again with twice as much space. */
      size *= 2;
      buffer = (char *) xrealloc (size, buffer);
    }
}

In practice, it is often easier just to use asprintf, below.


Dynamically Allocating Formatted Output
Node: Dynamic Output, Next: Variable Arguments Output, Prev: Formatted Output Functions, Up: Formatted Output

Dynamically Allocating Formatted Output

The functions in this section do formatted output and place the results in dynamically allocated memory.

 - Function: int asprintf (char **ptr, const char *template, ...)

This function is similar to sprintf, except that it dynamically allocates a string (as with malloc; see Unconstrained Allocation) to hold the output, instead of putting the output in a buffer you allocate in advance.  The ptr argument should be the address of a char * object, and asprintf stores a pointer to the newly allocated string at that location.

Here is how to use asprintf to get the same result as the snprintf example, but more easily:

/* Construct a message describing the value of a variable
   whose name is name and whose value is value. */
char *
make_message (char *name, char *value)
{
  char *result;
  asprintf (&result, "value of %s is %s", name, value);
  return result;
}

 - Function: int obstack_printf (struct obstack *obstack, const char *template, ...)

This function is similar to asprintf, except that it uses the obstack obstack to allocate the space.  See Obstacks.

The characters are written onto the end of the current object.  To get at them, you must finish the object with obstack_finish (see Growing Objects).


Variable Arguments Output Functions
Node: Variable Arguments Output, Next: Parsing a Template String, Prev: Dynamic Output, Up: Formatted Output

Variable Arguments Output Functions

The functions vprintf and friends are provided so that you can define your own variadic printf-like functions that make use of the same internals as the built-in formatted output functions.

The most natural way to define such functions would be to use a language construct to say, "Call printf and pass this template plus all of my arguments after the first five."  But there is no way to do this in C, and it would be hard to provide a way, since at the C language level there is no way to tell how many arguments your function received.

Since that method is impossible, we provide alternative functions, the vprintf series, which lets you pass a va_list to describe "all of my arguments after the first five."

Before calling vprintf or the other functions listed in this section, you must call va_start (see Variadic Functions) to initialize a pointer to the variable arguments.  Then you can call va_arg to fetch the arguments that you want to handle yourself.  This advances the pointer past those arguments.

Once your va_list pointer is pointing at the argument of your choice, you are ready to call vprintf.  That argument and all subsequent arguments that were passed to your function are used by vprintf along with the template that you specified separately.

In some other systems, the va_list pointer may become invalid after the call to vprintf, so you must not use va_arg after you call vprintf.  Instead, you should call va_end to retire the pointer from service.  However, you can safely call va_start on another pointer variable and begin fetching the arguments again through that pointer.  Calling vprintf does not destroy the argument list of your function, merely the particular pointer that you passed to it.

GNU C does not have such restrictions.  You can safely continue to fetch arguments from a va_list pointer after passing it to vprintf, and va_end is a no-op.  (Note, however, that subsequent va_arg calls will fetch the same arguments which vprintf previously used.)

Prototypes for these functions are declared in 'stdio.h'.  K

 - Function: int vprintf (const char *template, va_list ap)

This function is similar to printf except that, instead of taking a variable number of arguments directly, it takes an argument list pointer ap.

 - Function: int vfprintf (FILE *stream, const char *template, va_list ap)

This is the equivalent of fprintf with the variable argument list specified directly as for vprintf.

 - Function: int vsprintf (char *s, const char *template, va_list ap)

This is the equivalent of sprintf with the variable argument list specified directly as for vprintf.

 - Function: int vsnprintf (char *s, size_t size, const char *template, va_list ap)

This is the equivalent of snprintf with the variable argument list specified directly as for vprintf.

 - Function: int vasprintf (char **ptr, const char *template, va_list ap)

The vasprintf function is the equivalent of asprintf with the variable argument list specified directly as for vprintf.

 - Function: int obstack_vprintf (struct obstack *obstack, const char *template, va_list ap)

The obstack_vprintf function is the equivalent of obstack_printf with the variable argument list specified directly as for vprintf.

Here's an example showing how you might use vfprintf.  This is a function that prints error messages to the stream stderr, along with a prefix indicating the name of the program (see Error Messages, for a description of program_invocation_short_name).

#include <stdio.h>
#include <stdarg.h>

void
eprintf (char *template, ...)
{
  va_list ap;
  extern char *program_invocation_short_name;

  fprintf (stderr, "%s: ", program_invocation_short_name);
  va_start (ap, count);
  vfprintf (stderr, template, ap);
  va_end (ap);
}

You could call eprintf like this:

eprintf ("file `%s' does not exist\n", filename);


Parsing a Template String
Node: Parsing a Template String, Next: Example of Parsing, Prev: Variable Arguments Output, Up: Formatted Output

Parsing a Template String

You can use the function parse_printf_format to obtain information about the number and types of arguments that are expected by a given template string.  This function permits interpreters that provide interfaces to printf to avoid passing along invalid arguments from the user's program, which could cause a crash.

All the symbols described in this section are declared in the header file 'printf.h'.

 - Function: size_t parse_printf_format (const char *template, size_t n, int *argtypes)

This function returns information about the number and types of arguments expected by the printf template string template.  The information is stored in the array argtypes; each element of this array describes one argument.  This information is encoded using the various 'PA_' macros, listed below.

The n argument specifies the number of elements in the array argtypes.  This is the most elements that parse_printf_format will try to write.

parse_printf_format returns the total number of arguments required by template.  If this number is greater than n, then the information returned describes only the first n arguments.  If you want information about more than that many arguments, allocate a bigger array and call parse_printf_format again.

The argument types are encoded as a combination of a basic type and modifier flag bits.

 - Macro: int PA_FLAG_MASK

This macro is a bitmask for the type modifier flag bits.  You can write the expression (argtypes[i] & PA_FLAG_MASK) to extract just the flag bits for an argument, or (argtypes[i] & ~PA_FLAG_MASK) to extract just the basic type code.

Here are symbolic constants that represent the basic types; they stand for integer values.

PA_INT

This specifies that the base type is int.

PA_CHAR

This specifies that the base type is int, cast to char.

PA_STRING

This specifies that the base type is char *, a null-terminated string.

PA_POINTER

This specifies that the base type is void *, an arbitrary pointer.

PA_FLOAT

This specifies that the base type is float.

PA_DOUBLE

This specifies that the base type is double.

PA_LAST

You can define additional base types for your own programs as offsets from PA_LAST.  For example, if you have data types 'foo' and 'bar' with their own specialized printf conversions, you could define encodings for these types as:

#define PA_FOO  PA_LAST
#define PA_BAR  (PA_LAST + 1)

Here are the flag bits that modify a basic type.  They are combined with the code for the basic type using inclusive-or.

PA_FLAG_PTR

If this bit is set, it indicates that the encoded type is a pointer to the base type, rather than an immediate value.  For example, 'PA_INT|PA_FLAG_PTR' represents the type 'int *'.

PA_FLAG_SHORT

If this bit is set, it indicates that the base type is modified with short.  (This corresponds to the 'h' type modifier.)

PA_FLAG_LONG

If this bit is set, it indicates that the base type is modified with long.  (This corresponds to the 'l' type modifier.)

PA_FLAG_LONG_LONG

If this bit is set, it indicates that the base type is modified with long long.  (This corresponds to the 'L' type modifier.)

PA_FLAG_LONG_DOUBLE

This is a synonym for PA_FLAG_LONG_LONG, used by convention with a base type of PA_DOUBLE to indicate a type of long double.

For an example of using these facilitles, see See Example of Parsing.


Example of Parsing a Template String
Node: Example of Parsing, Next: , Prev: Parsing a Template String, Up: Formatted Output

Example of Parsing a Template String

Here is an example of decoding argument types for a format string.  We assume this is part of an interpreter which contains arguments of type NUMBER, CHAR, STRING and STRUCTURE (and perhaps others which are not valid here).

/* Test whether the nargs specified objects
   in the vector args are valid
   for the format string format:
   if so, return 1.
   If not, return 0 after printing an error message.  */

int
validate_args (char *format, int nargs, OBJECT *args)
{
  int *argtypes;
  int nwanted;

  /* Get the information about the arguments.
     Each conversion specification must be at least two characters
     long, so there cannot be more specifications than half the
     length of the string.  */

  argtypes = (int *) alloca (strlen (format) / 2 * sizeof (int));
  nwanted = parse_printf_format (string, nelts, argtypes);

  /* Check the number of arguments.  */
  if (nwanted > nargs)
    {
      error ("too few arguments (at least %d required)", nwanted);
      return 0;
    }

  /* Check the C type wanted for each argument
     and see if the object given is suitable.  */
  for (i = 0; i < nwanted; i++)
    {
      int wanted;

      if (argtypes[i] & PA_FLAG_PTR)
        wanted = STRUCTURE;
      else
        switch (argtypes[i] & ~PA_FLAG_MASK)
          {
          case PA_INT:
          case PA_FLOAT:
          case PA_DOUBLE:
            wanted = NUMBER;
            break;
          case PA_CHAR:
            wanted = CHAR;
            break;
          case PA_STRING:
            wanted = STRING;
            break;
          case PA_POINTER:
            wanted = STRUCTURE;
            break;
          }
      if (TYPE (args[i]) != wanted)
        {
          error ("type mismatch for arg number %d", i);
          return 0;
        }
    }
  return 1;
}


Customizing printf
Node: Customizing Printf, Next: Formatted Input, Prev: Formatted Output, Up: I/O on Streams

Customizing printf

The GNU C library lets you define your own custom conversion specifiers for printf template strings, to teach printf clever ways to print the important data structures of your program.

The way you do this is by registering the conversion with the function register_printf_function; see See Registering New Conversions.  One of the arguments you pass to this function is a pointer to a handler function that produces the actual output; see See Defining the Output Handler, for information on how to write this function.

You can also install a function that just returns information about the number and type of arguments expected by the conversion specifier.  See Parsing a Template String, for information about this.

The facilities of this section are declared in the header file 'printf.h'.

* Menu:

Registering New Conversions	Using register_printf_function to register a new output conversion.

Conversion Specifier Options	The handler must be able to get the options specified in the template when it is called.

Defining the Output Handler	Defining the handler and arginfo functions that are passed as arguments to register_printf_function.

Printf Extension Example	How to define a printf handler function.

Portability Note: The ability to extend the syntax of printf template strings is a GNU extension.  ANSI standard C has nothing similar.


Registering New Conversions
Node: Registering New Conversions, Next: Conversion Specifier Options, Prev: , Up: Customizing Printf

Registering New Conversions

The function to register a new output conversion is register_printf_function, declared in 'printf.h'.  K

 - Function: int register_printf_function (int spec, printf_function handler_function, printf_arginfo_function arginfo_function)

This function defines the conversion specifier character spec.  Thus, if spec is 'q', it defines the conversion '%q'.

The handler_function is the function called by printf and friends when this conversion appears in a template string.  See Defining the Output Handler, for information about how to define a function to pass as this argument.  If you specify a null pointer, any existing handler function for spec is removed.

The arginfo_function is the function called by parse_printf_format when this conversion appears in a template string.  See Parsing a Template String, for information about this.

Normally, you install both functions for a conversion at the same time, but if you are never going to call parse_printf_format, you do not need to define an arginfo function.

The return value is 0 on success, and -1 on failure (which occurs if spec is out of range).

You can redefine the standard output conversions, but this is probably not a good idea because of the potential for confusion.  Library routines written by other people could break if you do this.


Conversion Specifier Options
Node: Conversion Specifier Options, Next: Defining the Output Handler, Prev: Registering New Conversions, Up: Customizing Printf

Conversion Specifier Options

If you define a meaning for '%q', what if the template contains '%+23q' or '%-#q'?  To implement a sensible meaning for these, the handler when called needs to be able to get the options specified in the template.

Both the handler_function and arginfo_function arguments to register_printf_function accept an argument of type struct printf_info, which contains information about the options appearing in an instance of the conversion specifier.  This data type is declared in the header file 'printf.h'.  K

 - Type: struct printf_info

This structure is used to pass information about the options appearing in an instance of a conversion specifier in a printf template string to the handler and arginfo functions for that specifier.  It contains the following members:

int prec

This is the precision specified.  The value is -1 if no precision was specified.  If the precision was given as '*', the printf_info structure passed to the handler function contains the actual value retrieved from the argument list.  But the structure passed to the arginfo function contains a value of INT_MIN, since the actual value is not known.

int width

This is the minimum field width specified.  The value is 0 if no width was specified.  If the field width was given as '*', the printf_info structure passed to the handler function contains the actual value retrieved from the argument list.  But the structure passed to the arginfo function contains a value of INT_MIN, since the actual value is not known.

char spec

This is the conversion specifier character specified.  It's stored in the structure so that you can register the same handler function for multiple characters, but still have a way to tell them apart when the handler function is called.

unsigned int is_long_double

This is a boolean that is true if the 'L' type modifier was specified.

unsigned int is_short

This is a boolean that is true if the 'h' type modifier was specified.

unsigned int is_long

This is a boolean that is true if the 'l' type modifier was specified.

unsigned int alt

This is a boolean that is true if the '#' flag was specified.

unsigned int space

This is a boolean that is true if the ' ' flag was specified.

unsigned int left

This is a boolean that is true if the '-' flag was specified.

unsigned int showsign

This is a boolean that is true if the '+' flag was specified.

char pad

This is the character to use for padding the output to the minimum field width.  The value is '0' if the '0' flag was specified, and ' ' otherwise.


Defining the Output Handler
Node: Defining the Output Handler, Next: Printf Extension Example, Prev: Conversion Specifier Options, Up: Customizing Printf

Defining the Output Handler

Now let's look at how to define the handler and arginfo functions which are passed as arguments to register_printf_function.

You should define your handler functions with a prototype like:

int function (FILE *stream, const struct printf_info *info,
                    va_list *ap_pointer)

The stream argument passed to the handler function is the stream to which it should write output.

The info argument is a pointer to a structure that contains information about the various options that were included with the conversion in the template string.  You should not modify this structure inside your handler function.  See Conversion Specifier Options, for a description of this data structure.

The ap_pointer argument is used to pass the tail of the variable argument list containing the values to be printed to your handler.  Unlike most other functions that can be passed an explicit variable argument list, this is a pointer to a va_list, rather than the va_list itself.  Thus, you should fetch arguments by means of va_arg (type, *ap_pointer).

(Passing a pointer here allows the function that calls your handler function to update its own va_list variable to account for the arguments that your handler processes.  See Variadic Functions.)

Your handler function should return a value just like printf does: it should return the number of characters it has written, or a negative value to indicate an error.

 - Data Type: printf_function

This is the data type that a handler function should have.

If you are going to use parse_printf_format in your application, you should also define a function to pass as the arginfo_function argument for each new conversion you install with register_printf_function.

You should define these functions with a prototype like:

int function (const struct printf_info *info,
                    size_t n, int *argtypes)

The return value from the function should be the number of arguments the conversion expects.  The function should also fill in no more than n elements of the argtypes array with information about the types of each of these arguments.  This information is encoded using the various 'PA_' macros.  (You will notice that this is the same calling convention parse_printf_format itself uses.)

 - Data Type: printf_arginfo_function

This type is used to describe functions that return information about the number and type of arguments used by a conversion specifier.


printf Extension Example
Node: Printf Extension Example, Next: , Prev: Defining the Output Handler, Up: Customizing Printf

printf Extension Example

Here is an example showing how to define a printf handler function.  This program defines a data structure called a Widget and defines the '%W' conversion to print information about Widget * arguments, including the pointer value and the name stored in the data structure.  The '%W' conversion supports the minimum field width and left-justification options, but ignores everything else.

#include <stdio.h>
#include <printf.h>
#include <stdarg.h>

typedef struct
  {
    char *name;
  } Widget;

int
print_widget (FILE *stream, const struct printf_info *info, va_list *app)
{
  Widget *w;
  char *buffer;
  int len;

  /* Format the output into a string. */
  w = va_arg (*app, Widget *);
  len = asprintf (&buffer, "<Widget %p: %s>", w, w->name);
  if (len == -1)
    return -1;

  /* Pad to the minimum field width and print to the stream. */
  len = fprintf (stream, "%*s",
                 (info->left ? - info->width : info->width),
                 buffer);

  /* Clean up and return. */
  free (buffer);
  return len;
}

int
main (void)
{
  /* Make a widget to print. */
  Widget mywidget;
  mywidget.name = "mywidget";

  /* Register the print function for widgets. */
  register_printf_function ('W', print_widget, NULL); /* No arginfo. */

  /* Now print the widget. */
  printf ("|%W|\n", &mywidget);
  printf ("|%35W|\n", &mywidget);
  printf ("|%-35W|\n", &mywidget);

  return 0;
}

The output produced by this program looks like:

|<Widget 0xffeffb7c: mywidget>|
|      <Widget 0xffeffb7c: mywidget>|
|<Widget 0xffeffb7c: mywidget>      |


Formatted Input
Node: Formatted Input, Next: Block Input/Output, Prev: Customizing Printf, Up: I/O on Streams

Formatted Input

The functions described in this section (scanf and related functions) provide facilities for formatted input analogous to the formatted output facilities.  These functions provide a mechanism for reading arbitrary values under the control of a "format string" or "template string".

* Menu:

Formatted Input Basics	Some basics to get you started.

Input Conversion Syntax	Syntax of conversion specifications.

Table of Input Conversions	Summary of input conversions and what they do.

Numeric Input Conversions	Details of conversions for reading numbers.

String Input Conversions	Details of conversions for reading strings.

Dynamic String Input	String conversions that malloc the buffer.

Other Input Conversions	Details of miscellaneous other conversions.

Formatted Input Functions	Descriptions of the actual functions.

Variable Arguments Input	vscanf and friends.


Formatted Input Basics
Node: Formatted Input Basics, Next: Input Conversion Syntax, Prev: , Up: Formatted Input

Formatted Input Basics

Calls to scanf are superficially similar to calls to printf in that arbitrary arguments are read under the control of a template string.  While the syntax of the conversion specifications in the template is very similar to that for printf, the interpretation of the template is oriented more towards free-format input and simple pattern matching, rather than fixed-field formatting.  For example, most scanf conversions skip over any amount of "white space" (including spaces, tabs, and newlines) in the input file, and there is no concept of precision for the numeric input conversions as there is for the corresponding output conversions.  Ordinarily, non-whitespace characters in the template are expected to match characters in the input stream exactly, but a matching failure is distinct from an input error on the stream.  KAnother area of difference between scanf and printf is that you must remember to supply pointers rather than immediate values as the optional arguments to scanf; the values that are read are stored in the objects that the pointers point to.  Even experienced programmers tend to forget this occasionally, so if your program is getting strange errors that seem to be related to scanf, you might want to double-check this.

When a "matching failure" occurs, scanf returns immediately, leaving the first non-matching character as the next character to be read from the stream.  The normal return value from scanf is the number of values that were assigned, so you can use this to determine if a matching error happened before all the expected values were read.  KThe scanf function is typically used for things like reading in the contents of tables.  For example, here is a function that uses scanf to initialize an array of double:

void
readarray (double *array, int n)
{
  int i;
  for (i=0; i<n; i++)
    if (scanf (" %lf", &(array[i])) != 1)
      invalid_input_error ();
}

The formatted input functions are not used as frequently as the formatted output functions.  Partly, this is because it takes some care to use them properly.  Another reason is that it is difficult to recover from a matching error.

If you are trying to read input that doesn't match a single, fixed pattern, you may be better off using a tool such as Flex to generate a lexical scanner, or Bison to generate a parser, rather than using scanf.  For more information about this, see See Flex, and See Bison.


Input Conversion Syntax
Node: Input Conversion Syntax, Next: Table of Input Conversions, Prev: Formatted Input Basics, Up: Formatted Input

Input Conversion Syntax

A scanf template string is a string that contains ordinary multibyte characters interspersed with conversion specifications that start with '%'.

Any whitespace character (as defined by the isspace function; see Classification of Characters) in the template causes any number of whitespace characters in the input stream to be read and discarded.  The whitespace characters that are matched need not be exactly the same whitespace characters that appear in the template string.  For example, write ' , ' in the template to recognize a comma with optional whitespace before and after.

Other characters in the template string that are not part of conversion specifications must match characters in the input stream exactly; if this is not the case, a matching failure occurs.

The conversion specifications in a scanf template string have the general form:

% flags width type conversion

In more detail, an input conversion specification consists of an initial '%' character followed in sequence by:

•	An optional "flag character" '*', which says to ignore the text read for this specification.  When scanf finds a conversion specification that uses this flag, it reads input as directed by the rest of the conversion specification, but it discards this input, does not use a pointer argument, and does not increment the count of successful assignments.  K

•	An optional flag character 'a' (valid with string conversions only) which requests allocation of a buffer long enough to store the string in.  (This is a GNU extension.) See Dynamic String Input.

•	An optional decimal integer that specifies the "maximum field width".  Reading of characters from the input stream stops either when this maximum is reached or when a non-matching character is found, whichever happens first.  Most conversions discard initial whitespace characters (those that don't are explicitly documented), and these discarded characters don't count towards the maximum field width.  String input conversions store a null character to mark the end of the input; the maximum field width does not include this terminator.  K

•	An optional "type modifier character".  For example, you can specify a type modifier of 'l' with integer conversions such as '%d' to specify that the argument is a pointer to a long int rather than a pointer to an int.  K

•	A character that specifies the conversion to be applied.

The exact options that are permitted and how they are interpreted vary between the different conversion specifiers.  See the descriptions of the individual conversions for information about the particular options that they allow.


Table of Input Conversions
Node: Table of Input Conversions, Next: Numeric Input Conversions, Prev: Input Conversion Syntax, Up: Formatted Input

Table of Input Conversions

Here is a table that summarizes the various conversion specifications:

'%d'

Matches an optionally signed integer written in decimal.  See Numeric Input Conversions.

'%i'

Matches an optionally signed integer in any of the formats that the C language defines for specifying an integer constant.  See Numeric Input Conversions.

'%o'

Matches an unsigned integer written in octal radix.  See Numeric Input Conversions.

'%u'

Matches an unsigned integer written in decimal radix.  See Numeric Input Conversions.

'%x', '%X'

Matches an unsigned integer written in hexadecimal radix.  See Numeric Input Conversions.

'%e', '%f', '%g', '%E', '%G'

Matches an optionally signed floating-point number.  See Numeric Input Conversions.

'%s'

Matches a string containing only non-whitespace characters.  See String Input Conversions.

'%['

Matches a string of characters that belong to a specified set.  See String Input Conversions.

'%c'

Matches a string of one or more characters; the number of characters read is controlled by the maximum field width given for the conversion.  See String Input Conversions.

'%p'

Matches a pointer value in the same implementation-defined format used by the '%p' output conversion for printf.  See Other Input Conversions.

'%n'

This conversion doesn't read any characters; it records the number of characters read so far by this call.  See Other Input Conversions.

'%%'

This matches a literal '%' character in the input stream.  No corresponding argument is used.  See Other Input Conversions.

If the syntax of a conversion specification is invalid, the behavior is undefined.  If there aren't enough function arguments provided to supply addresses for all the conversion specifications in the template strings that perform assignments, or if the arguments are not of the correct types, the behavior is also undefined.  On the other hand, extra arguments are simply ignored.


Numeric Input Conversions
Node: Numeric Input Conversions, Next: String Input Conversions, Prev: Table of Input Conversions, Up: Formatted Input

Numeric Input Conversions

This section describes the scanf conversions for reading numeric values.

The '%d' conversion matches an optionally signed integer in decimal radix.  The syntax that is recognized is the same as that for the strtol function (see Parsing of Integers) with the value 10 for the base argument.

The '%i' conversion matches an optionally signed integer in any of the formats that the C language defines for specifying an integer constant.  The syntax that is recognized is the same as that for the strtol function (see Parsing of Integers) with the value 0 for the base argument.  (You can print integers in this syntax with printf by using the '#' flag character with the '%x', '%o', or '%d' conversion.  See Integer Conversions.)

For example, any of the strings '10', '0xa', or '012' could be read in as integers under the '%i' conversion.  Each of these specifies a number with decimal value 10.

The '%o', '%u', and '%x' conversions match unsigned integers in octal, decimal, and hexadecimal radices, respectively.  The syntax that is recognized is the same as that for the strtoul function (see Parsing of Integers) with the appropriate value (8, 10, or 16) for the base argument.

The '%X' conversion is identical to the '%x' conversion.  They both permit either uppercase or lowercase letters to be used as digits.

The default type of the corresponding argument for the %d and %i conversions is int *, and unsigned int * for the other integer conversions.  You can use the following type modifiers to specify other sizes of integer:

'h'

Specifies that the argument is a short int * or unsigned short int *.

'l'

Specifies that the argument is a long int * or unsigned long int *.

'L'

Specifies that the argument is a long long int * or unsigned long long int *.  (The long long type is an extension supported by the GNU C compiler.  For systems that don't provide extra-long integers, this is the same as long int.)

All of the '%e', '%f', '%g', '%E', and '%G' input conversions are interchangeable.  They all match an optionally signed floating point number, in the same syntax as for the strtod function (see Parsing of Floats).

For the floating-point input conversions, the default argument type is float *.  (This is different from the corresponding output conversions, where the default type is double; remember that float arguments to printf are converted to double by the default argument promotions, but float * arguments are not promoted to double *.)  You can specify other sizes of float using these type modifiers:

'l'

Specifies that the argument is of type double *.

'L'

Specifies that the argument is of type long double *.


String Input Conversions
Node: String Input Conversions, Next: Dynamic String Input, Prev: Numeric Input Conversions, Up: Formatted Input

String Input Conversions

This section describes the scanf input conversions for reading string and character values: '%s', '%[', and '%c'.

You have two options for how to receive the input from these conversions:

•	Provide a buffer to store it in.  This is the default.  You should provide an argument of type char *.

Warning: To make a robust program, you must make sure that the input (plus its terminating null) cannot possibly exceed the size of the buffer you provide.  In general, the only way to do this is to specify a maximum field width one less than the buffer size.  If you provide the buffer, always specify a maximum field width to prevent overflow.

•	Ask scanf to allocate a big enough buffer, by specifying the 'a' flag character.  This is a GNU extension.  You should provide an argument of type char ** for the buffer address to be stored in.  See Dynamic String Input.

The '%c' conversion is the simplest: it matches a fixed number of characters, always.  The maximum field with says how many characters to read; if you don't specify the maximum, the default is 1.  This conversion doesn't append a null character to the end of the text it reads.  It also does not skip over initial whitespace characters.  It reads precisely the next n characters, and fails if it cannot get that many.  Since there is always a maximum field width with '%c' (whether specified, or 1 by default), you can always prevent overflow by making the buffer long enough.

The '%s' conversion matches a string of non-whitespace characters.  It skips and discards initial whitespace, but stops when it encounters more whitespace after having read something.  It stores a null character at the end of the text that it reads.

For example, reading the input:

 hello, world

with the conversion '%10c' produces " hello, wo", but reading the same input with the conversion '%10s' produces "hello,".

Warning: If you do not specify a field width for '%s', then the number of characters read is limited only by where the next whitespace character appears.  This almost certainly means that invalid input can make your program crash--which is a bug.

To read in characters that belong to an arbitrary set of your choice, use the '%[' conversion.  You specify the set between the '[' character and a following ']' character, using the same syntax used in regular expressions.  As special cases:

•	A literal ']' character can be specified as the first character of the set.

•	An embedded '-' character (that is, one that is not the first or last character of the set) is used to specify a range of characters.

•	If a caret character '^' immediately follows the initial '[', then the set of allowed input characters is the everything except the characters listed.

The '%[' conversion does not skip over initial whitespace characters.

Here are some examples of '%[' conversions and what they mean:

'%25[1234567890]'

Matches a string of up to 25 digits.

'%25[][]'

Matches a string of up to 25 square brackets.

'%25[^ \f\n\r\t\v]'

Matches a string up to 25 characters long that doesn't contain any of the standard whitespace characters.  This is slightly different from '%s', because if the input begins with a whitespace character, '%[' reports a matching failure while '%s' simply discards the initial whitespace.

'%25[a-z]'

Matches up to 25 lowercase characters.

One more reminder: the '%s' and '%[' conversions are dangerous if you don't specify a maximum width or use the 'a' flag, because input too long would overflow whatever buffer you have provided for it.  No matter how long your buffer is, a user could supply input that is longer.  A well-written program reports invalid input with a comprehensible error message, not with a crash.


Dynamically Allocating String Conversions
Node: Dynamic String Input, Next: Other Input Conversions, Prev: String Input Conversions, Up: Formatted Input

Dynamically Allocating String Conversions

A GNU extension to formatted input lets you safely read a string with no maximum size.  Using this feature, you don't supply a buffer; instead, scanf allocates a buffer big enough to hold the data and gives you its address.  To use this feature, write 'a' as a flag character, as in '%as' or '%a[0-9a-z]'.

The pointer argument you supply for where to store the input should have type char **.  The scanf function allocates a buffer and stores its address in the word that the argument points to.  You should free the buffer with free when you no longer need it.

Here is an example of using the 'a' flag with the '%[...]' conversion specification to read a "variable assignment" of the form 'variable = value'.

{
  char *variable, *value;

  if (2 > scanf ("%a[a-zA-Z0-9] = %a[^\n]\n",
                 &variable, &value))
    {
      invalid_input_error ();
      return 0;
    }

  ...
}


Other Input Conversions
Node: Other Input Conversions, Next: Formatted Input Functions, Prev: Dynamic String Input, Up: Formatted Input

Other Input Conversions

This section describes the miscellaneous input conversions.

The '%p' conversion is used to read a pointer value.  It recognizes the same syntax as is used by the '%p' output conversion for printf (see Other Output Conversions); that is, a hexadecimal number just as the '%x' conversion accepts.  The corresponding argument should be of type void **; that is, the address of a place to store a pointer.

The resulting pointer value is not guaranteed to be valid if it was not originally written during the same program execution that reads it in.

The '%n' conversion produces the number of characters read so far by this call.  The corresponding argument should be of type int *.  This conversion works in the same way as the '%n' conversion for printf; see See Other Output Conversions, for an example.

The '%n' conversion is the only mechanism for determining the success of literal matches or conversions with suppressed assignments.  If the '%n' follows the locus of a matching failure, then no value is stored for it since scanf returns before processing the '%n'.  If you store -1 in that argument slot before calling scanf, the presence of -1 after scanf indicates an error occurred before the '%n' was reached.

Finally, the '%%' conversion matches a literal '%' character in the input stream, without using an argument.  This conversion does not permit any flags, field width, or type modifier to be specified.


Formatted Input Functions
Node: Formatted Input Functions, Next: Variable Arguments Input, Prev: Other Input Conversions, Up: Formatted Input

Formatted Input Functions

Here are the descriptions of the functions for performing formatted input.  Prototypes for these functions are in the header file 'stdio.h'.  K

 - Function: int scanf (const char *template, ...)

The scanf function reads formatted input from the stream stdin under the control of the template string template.  The optional arguments are pointers to the places which receive the resulting values.

The return value is normally the number of successful assignments.  If an end-of-file condition is detected before any matches are performed (including matches against whitespace and literal characters in the template), then EOF is returned.

 - Function: int fscanf (FILE *stream, const char *template, ...)

This function is just like scanf, except that the input is read from the stream stream instead of stdin.

 - Function: int sscanf (const char *s, const char *template, ...)

This is like scanf, except that the characters are taken from the null-terminated string s instead of from a stream.  Reaching the end of the string is treated as an end-of-file condition.

The behavior of this function is undefined if copying takes place between objects that overlap--for example, if s is also given as an argument to receive a string read under control of the '%s' conversion.


Variable Arguments Input Functions
Node: Variable Arguments Input, Next: , Prev: Formatted Input Functions, Up: Formatted Input

Variable Arguments Input Functions

The functions vscanf and friends are provided so that you can define your own variadic scanf-like functions that make use of the same internals as the built-in formatted output functions.  These functions are analogous to the vprintf series of output functions.  See Variable Arguments Output, for important information on how to use them.

Portability Note: The functions listed in this section are GNU extensions.

 - Function: int vscanf (const char *template, va_list ap)

This function is similar to scanf except that, instead of taking a variable number of arguments directly, it takes an argument list pointer ap of type va_list (see Variadic Functions).

 - Function: int vfscanf (FILE *stream, const char *template, va_list ap)

This is the equivalent of fscanf with the variable argument list specified directly as for vscanf.

 - Function: int vsscanf (const char *s, const char *template, va_list ap)

This is the equivalent of sscanf with the variable argument list specified directly as for vscanf.


Block Input/Output
Node: Block Input/Output, Next: EOF and Errors, Prev: Formatted Input, Up: I/O on Streams

Block Input/Output

This section describes how to do input and output operations on blocks of data.  You can use these functions to read and write binary data, as well as to read and write text in fixed-size blocks instead of by characters or lines.  KBinary files are typically used to read and write blocks of data in the same format as is used to represent the data in a running program.  In other words, arbitrary blocks of memory--not just character or string objects--can be written to a binary file, and meaningfully read in again by the same program.

Storing data in binary form is often considerably more efficient than using the formatted I/O functions.  Also, for floating-point numbers, the binary form avoids possible loss of precision in the conversion process.  On the other hand, binary files can't be examined or modified easily using many standard file utilities (such as text editors), and are not portable between different implementations of the language, or different kinds of computers.

These functions are declared in 'stdio.h'.  K

 - Function: size_t fread (void *data, size_t size, size_t count, FILE *stream)

This function reads up to count objects of size size into the array data, from the stream stream.  It returns the number of objects actually read, which might be less than count if a read error occurs or the end of the file is reached.  This function returns a value of zero (and doesn't read anything) if either size or count is zero.

If fread encounters end of file in the middle of an object, it returns the number of complete objects read, and discards the partial object.  Therefore, the stream remains at the actual end of the file.

 - Function: size_t fwrite (const void *data, size_t size, size_t count, FILE *stream)

This function writes up to count objects of size size from the array data, to the stream stream.  The return value is normally count, if the call succeeds.  Any other value indicates some sort of error, such as running out of space.


End-Of-File and Errors
Node: EOF and Errors, Next: Binary Streams, Prev: Block Input/Output, Up: I/O on Streams

End-Of-File and Errors

Many of the functions described in this chapter return the value of the macro EOF to indicate unsuccessful completion of the operation.  Since EOF is used to report both end of file and random errors, it's often better to use the feof function to check explicitly for end of file and ferror to check for errors.  These functions check indicators that are part of the internal state of the stream object, indicators set if the appropriate condition was detected by a previous I/O operation on that stream.

These symbols are declared in the header file 'stdio.h'.  K

 - Macro: int EOF

This macro is an integer value that is returned by a number of functions to indicate an end-of-file condition, or some other error situation.  With the GNU library, EOF is -1.  In other libraries, its value may be some other negative number.

 - Function: void clearerr (FILE *stream)

This function clears the end-of-file and error indicators for the stream stream.

The file positioning functions (see File Positioning) also clear the end-of-file indicator for the stream.

 - Function: int feof (FILE *stream)

The feof function returns nonzero if and only if the end-of-file indicator for the stream stream is set.

 - Function: int ferror (FILE *stream)

The ferror function returns nonzero if and only if the error indicator for the stream stream is set, indicating that an error has occurred on a previous operation on the stream.

In addition to setting the error indicator associated with the stream, the functions that operate on streams also set errno in the same way as the corresponding low-level functions that operate on file descriptors.  For example, all of the functions that perform output to a stream--such as fputc, printf, and fflush--are implemented in terms of write, and all of the errno error conditions defined for write are meaningful for these functions.  For more information about the descriptor-level I/O functions, see See Low-Level I/O.


Text and Binary Streams
Node: Binary Streams, Next: File Positioning, Prev: EOF and Errors, Up: I/O on Streams

Text and Binary Streams

The GNU system and other POSIX-compatible operating systems organize all files as uniform sequences of characters.  However, some other systems make a distinction between files containing text and files containing binary data, and the input and output facilities of ANSI C provide for this distinction.  This section tells you how to write programs portable to such systems.

When you open a stream, you can specify either a "text stream" or a "binary stream".  You indicate that you want a binary stream by specifying the 'b' modifier in the opentype argument to fopen; see See Opening Streams.  Without this option, fopen opens the file as a text stream.

Text and binary streams differ in several ways:

•	The data read from a text stream is divided into "lines" which are terminated by newline ('\n') characters, while a binary stream is simply a long series of characters.  A text stream might on some systems fail to handle lines more than 254 characters long (including the terminating newline character).  K

•	On some systems, text files can contain only printing characters, horizontal tab characters, and newlines, and so text streams may not support other characters.  However, binary streams can handle any character value.

•	Space characters that are written immediately preceding a newline character in a text stream may disappear when the file is read in again.

•	More generally, there need not be a one-to-one mapping between characters that are read from or written to a text stream, and the characters in the actual file.

Since a binary stream is always more capable and more predictable than a text stream, you might wonder what purpose text streams serve.  Why not simply always use binary streams?  The answer is that on these operating systems, text and binary streams use different file formats, and the only way to read or write "an ordinary file of text" that can work with other text-oriented programs is through a text stream.

In the GNU library, and on all POSIX systems, there is no difference between text streams and binary streams.  When you open a stream, you get the same kind of stream regardless of whether you ask for binary.  This stream can handle any file content, and has none of the restrictions that text streams sometimes have.


File Positioning
Node: File Positioning, Next: Portable Positioning, Prev: Binary Streams, Up: I/O on Streams

File Positioning

The "file position" of a stream describes where in the file the stream is currently reading or writing.  I/O on the stream advances the file position through the file.  In the GNU system, the file position is represented as an integer, which counts the number of bytes from the beginning of the file.  See File Position.

During I/O to an ordinary disk file, you can change the file position whenever you wish, so as to read or write any portion of the file.  Some other kinds of files may also permit this.  Files which support changing the file position are sometimes referred to as "random-access" files.

You can use the functions in this section to examine or modify the file position indicator associated with a stream.  The symbols listed below are declared in the header file 'stdio.h'.  K

 - Function: long int ftell (FILE *stream)

This function returns the current file position of the stream stream.

This function can fail if the stream doesn't support file positioning, or if the file position can't be represented in a long int, and possibly for other reasons as well.  If a failure occurs, a value of -1 is returned.

 - Function: int fseek (FILE *stream, long int offset, int whence)

The fseek function is used to change the file position of the stream stream.  The value of whence must be one of the constants SEEK_SET, SEEK_CUR, or SEEK_END, to indicate whether the offset is relative to the beginning of the file, the current file position, or the end of the file, respectively.

This function returns a value of zero if the operation was successful, and a nonzero value to indicate failure.  A successful call also clears the end-of-file indicator of stream and discards any characters that were "pushed back" by the use of ungetc.

fseek either flushes any buffered output before setting the file position or else remembers it so it will be written later in its proper place in the file.

Portability Note: In non-POSIX systems, ftell and fseek might work reliably only on binary streams.  See Binary Streams.

The following symbolic constants are defined for use as the whence argument to fseek.  They are also used with the lseek function (see I/O Primitives) and to specify offsets for file locks (see Control Operations).

 - Macro: int SEEK_SET

This is an integer constant which, when used as the whence argument to the fseek function, specifies that the offset provided is relative to the beginning of the file.

 - Macro: int SEEK_CUR

This is an integer constant which, when used as the whence argument to the fseek function, specifies that the offset provided is relative to the current file position.

 - Macro: int SEEK_END

This is an integer constant which, when used as the whence argument to the fseek function, specifies that the offset provided is relative to the end of the file.

 - Function: void rewind (FILE *stream)

The rewind function positions the stream stream at the begining of the file.  It is equivalent to calling fseek on the stream with an offset argument of 0L and a whence argument of SEEK_SET, except that the return value is discarded and the error indicator for the stream is reset.

These three aliases for the 'SEEK_...' constants exist for the sake of compatibility with older BSD systems.  They are defined in two different header files: 'fcntl.h' and 'sys/file.h'.

L_SET

An alias for SEEK_SET.

L_INCR

An alias for SEEK_CUR.

L_XTND

An alias for SEEK_END.


Portable File-Position Functions
Node: Portable Positioning, Next: Stream Buffering, Prev: File Positioning, Up: I/O on Streams

Portable File-Position Functions

On the GNU system, the file position is truly a character count.  You can specify any character count value as an argument to fseek and get reliable results for any random access file.  However, some ANSI C systems do not represent file positions in this way.

On some systems where text streams truly differ from binary streams, it is impossible to represent the file position of a text stream as a count of characters from the beginning of the file.  For example, the file position on some systems must encode both a record offset within the file, and a character offset within the record.

As a consequence, if you want your programs to be portable to these systems, you must observe certain rules:

•	The value returned from ftell on a text stream has no predictable relationship to the number of characters you have read so far.  The only thing you can rely on is that you can use it subsequently as the offset argument to fseek to move back to the same file position.

•	In a call to fseek on a text stream, either the offset must either be zero; or whence must be SEEK_SET and the offset must be the result of an earlier call to ftell on the same stream.

•	The value of the file position indicator of a text stream is undefined while there are characters that have been pushed back with ungetc that haven't been read or discarded.  See Unreading.

But even if you observe these rules, you may still have trouble for long files, because ftell and fseek use a long int value to represent the file position.  This type may not have room to encode all the file positions in a large file.

So if you do want to support systems with peculiar encodings for the file positions, it is better to use the functions fgetpos and fsetpos instead.  These functions represent the file position using the data type fpos_t, whose internal representation varies from system to system.

These symbols are declared in the header file 'stdio.h'.  K

 - Data Type: fpos_t

This is the type of an object that can encode information about the file position of a stream, for use by the functions fgetpos and fsetpos.

In the GNU system, fpos_t is equivalent to off_t or long int.  In other systems, it might have a different internal representation.

 - Function: int fgetpos (FILE *stream, fpos_t *position)

This function stores the value of the file position indicator for the stream stream in the fpos_t object pointed to by position.  If successful, fgetpos returns zero; otherwise it returns a nonzero value and stores an implementation-defined positive value in errno.

 - Function: int fsetpos (FILE *stream, const fpos_t position)

This function sets the file position indicator for the stream stream to the position position, which must have been set by a previous call to fgetpos on the same stream.  If successful, fsetpos clears the end-of-file indicator on the stream, discards any characters that were "pushed back" by the use of ungetc, and returns a value of zero.  Otherwise, fsetpos returns a nonzero value and stores an implementation-defined positive value in errno.


Stream Buffering
Node: Stream Buffering, Next: Other Kinds of Streams, Prev: Portable Positioning, Up: I/O on Streams

Stream Buffering

Characters that are written to a stream are normally accumulated and transmitted asynchronously to the file in a block, instead of appearing as soon as they are output by the application program.  Similarly, streams often retrieve input from the host environment in blocks rather than on a character-by-character basis.  This is called "buffering".

If you are writing programs that do interactive input and output using streams, you need to understand how buffering works when you design the user interface to your program.  Otherwise, you might find that output (such as progress or prompt messages) doesn't appear when you intended it to, or that input typed by the user is made available by lines instead of by single characters, or other unexpected behavior.

This section deals only with controlling when characters are transmitted between the stream and the file or device, and not with how things like echoing, flow control, and the like are handled on specific classes of devices.  For information on common control operations on terminal devices, see See Low-Level Terminal Interface.

You can bypass the stream buffering facilities altogether by using the low-level input and output functions that operate on file descriptors instead.  See Low-Level I/O.

* Menu:

Buffering Concepts	Terminology is defined here.

Flushing Buffers	How to ensure that output buffers are flushed.

Controlling Buffering	How to specify what kind of buffering to use.


Buffering Concepts
Node: Buffering Concepts, Next: Flushing Buffers, Prev: , Up: Stream Buffering

Buffering Concepts

There are three different kinds of buffering strategies:

•	Characters written to or read from an "unbuffered" stream are transmitted individually to or from the file as soon as possible.  K

•	Characters written to or read from a "line buffered" stream are transmitted to or from the file in blocks when a newline character is encountered.  K

•	Characters written to or read from a "fully buffered" stream are transmitted to or from the file in blocks of arbitrary size.  KNewly opened streams are normally fully buffered, with one exception: a stream connected to an interactive device such as a terminal is initially line buffered.  See Controlling Buffering, for information on how to select a different kind of buffering.

The use of line buffering for interactive devices implies that output messages ending in a newline will appear immediately--which is usually what you want.  Output that doesn't end in a newline might or might not show up immediately, so if you want them to appear immediately, you should flush buffered output explicitly with fflush, as described in See Flushing Buffers.

Line buffering is a good default for terminal input as well, because most interactive programs read commands that are normally single lines.  The program should be able to execute each line right away.  A line buffered stream permits this, whereas a fully buffered stream would always read enough text to fill the buffer before allowing the program to read any of it.  Line buffering also fits in with the usual input-editing facilities of most operating systems, which work within a line of input.

Some programs need an unbuffered terminal input stream.  These include programs that read single-character commands (like Emacs) and programs that do their own input editing (such as those that use readline).  In order to read a character at a time, it is not enough to turn off buffering in the input stream; you must also turn off input editing in the operating system.  This requires changing the terminal mode (see Terminal Modes).  If you want to change the terminal modes, you have to do this separately--merely using an unbuffered stream does not change the modes.


Flushing Buffers
Node: Flushing Buffers, Next: Controlling Buffering, Prev: Buffering Concepts, Up: Stream Buffering

Flushing Buffers

"Flushing" output on a buffered stream means transmitting all accumulated characters to the file.  There are many circumstances when buffered output on a stream is flushed automatically:

•	When you try to do output and the output buffer is full.

•	When the stream is closed.  See Closing Streams.

•	When the program terminates by calling exit.  See Normal Termination.

•	When a newline is written, if the stream is line buffered.

•	Whenever an input operation on any stream actually reads data from its file.

If you want to flush the buffered output at another time, call fflush, which is declared in the header file 'stdio.h'.  K

 - Function: int fflush (FILE *stream)

This function causes any buffered output on stream to be delivered to the file.  If stream is a null pointer, then fflush causes buffered output on all open output streams to be flushed.

This function returns EOF if a write error occurs, or zero otherwise.

Compatibility Note: Some brain-damaged operating systems have been known to be so thoroughly fixated on line-oriented input and output that flushing a line buffered stream causes a newline to be written!  Fortunately, this "feature" seems to be becoming less common.  You do not need to worry about this in the GNU system.


Controlling Which Kind of Buffering
Node: Controlling Buffering, Next: , Prev: Flushing Buffers, Up: Stream Buffering

Controlling Which Kind of Buffering

After opening a stream (but before any other operations have been performed on it), you can explicitly specify what kind of buffering you want it to have using the setvbuf function.  KThe facilities listed in this section are declared in the header file 'stdio.h'.  K

 - Function: int setvbuf (FILE *stream, char *buf, int mode, size_t size)

This function is used to specify that the stream stream should have the buffering mode mode, which can be either _IOFBF (for full buffering), _IOLBF (for line buffering), or _IONBF (for unbuffered input/output).

If you specify a null pointer as the buf argument, then setvbuf allocates a buffer itself using malloc.  This buffer will be freed when you close the stream.

Otherwise, buf should be a character array that can hold at least size characters.  You should not free the space for this array as long as the stream remains open and this array remains its buffer.  You should usually either allocate it statically, or malloc (see Unconstrained Allocation) the buffer.  Using an automatic array is not a good idea unless you close the file before exiting the block that declares the array.

While the array remains a stream buffer, the stream I/O functions will use the buffer for their internal purposes.  You shouldn't try to access the values in the array directly while the stream is using it for buffering.

The setvbuf function returns zero on success, or a nonzero value if the value of mode is not valid or if the request could not be honored.

 - Macro: int _IOFBF

The value of this macro is an integer constant expression that can be used as the mode argument to the setvbuf function to specify that the stream should be fully buffered.

 - Macro: int _IOLBF

The value of this macro is an integer constant expression that can be used as the mode argument to the setvbuf function to specify that the stream should be line buffered.

 - Macro: int _IONBF

The value of this macro is an integer constant expression that can be used as the mode argument to the setvbuf function to specify that the stream should be unbuffered.

 - Macro: int BUFSIZ

The value of this macro is an integer constant expression that is good to use for the size argument to setvbuf.  This value is guaranteed to be at least 256.

The value of BUFSIZ is chosen on each system so as to make stream I/O efficient.  So it is a good idea to use BUFSIZ as the size for the buffer when you call setvbuf.

Actually, you can get an even better value to use for the buffer size by means of the fstat system call: it is found in the st_blksize field of the file attributes.  See Attribute Meanings.

Sometimes people also use BUFSIZ as the allocation size of buffers used for related purposes, such as strings used to receive a line of input with fgets (see Character Input).  There is no particular reason to use BUFSIZ for this instead of any other integer, except that it might lead to doing I/O in chunks of an efficient size.

 - Function: void setbuf (FILE *stream, char *buf)

If buf is a null pointer, the effect of this function is equivalent to calling setvbuf with a mode argument of _IONBF.  Otherwise, it is equivalent to calling setvbuf with buf, and a mode of _IOFBF and a size argument of BUFSIZ.

The setbuf function is provided for compatibility with old code; use setvbuf in all new programs.

 - Function: void setbuffer (FILE *stream, char *buf, size_t size)

If buf is a null pointer, this function makes stream unbuffered.  Otherwise, it makes stream fully buffered using buf as the buffer.  The size argument specifies the length of buf.

This function is provided for compatibility with old BSD code.  Use setvbuf instead.

 - Function: void setlinebuf (FILE *stream)

This function makes stream be line buffered, and allocates the buffer for you.

This function is provided for compatibility with old BSD code.  Use setvbuf instead.


Other Kinds of Streams
Node: Other Kinds of Streams, Next: , Prev: Stream Buffering, Up: I/O on Streams

Other Kinds of Streams

The GNU library provides ways for you to define additional kinds of streams that do not necessarily correspond to an open file.

One such type of stream takes input from or writes output to a string.  These kinds of streams are used internally to implement the sprintf and sscanf functions.  You can also create such a stream explicitly, using the functions described in See String Streams.

More generally, you can define streams that do input/output to arbitrary objects using functions supplied by your program.  This protocol is discussed in See Custom Streams.

Portability Note: The facilities described in this section are specific to GNU.  Other systems or C implementations might or might not provide equivalent functionality.

* Menu:

String Streams	Streams that get data from or put data in a string or memory buffer.

Obstack Streams	Streams that store data in an obstack.

Custom Streams	Defining your own streams with an arbitrary input data source and/or output data sink.


String Streams
Node: String Streams, Next: Obstack Streams, Prev: , Up: Other Kinds of Streams

String Streams

The fmemopen and open_memstream functions allow you to do I/O to a string or memory buffer.  These facilities are declared in 'stdio.h'.  K

 - Function: FILE * fmemopen (void *buf, size_t size, const char *opentype)

This function opens a stream that allows the access specified by the opentype argument, that reads from or writes to the buffer specified by the argument buf.  This array must be at least size bytes long.

If you specify a null pointer as the buf argument, fmemopen dynamically allocates (as with malloc; see Unconstrained Allocation) an array size bytes long.  This is really only useful if you are going to write things to the buffer and then read them back in again, because you have no way of actually getting a pointer to the buffer (for this, try open_memstream, below).  The buffer is freed when the stream is open.

The argument opentype is the same as in fopen (See Opening Streams).  If the opentype specifies append mode, then the initial file position is set to the first null character in the buffer.  Otherwise the initial file position is at the beginning of the buffer.

When a stream open for writing is flushed or closed, a null character (zero byte) is written at the end of the buffer if it fits.  You should add an extra byte to the size argument to account for this.  Attempts to write more than size bytes to the buffer result in an error.

For a stream open for reading, null characters (zero bytes) in the buffer do not count as "end of file".  Read operations indicate end of file only when the file position advances past size bytes.  So, if you want to read characters from a null-terminated string, you should supply the length of the string as the size argument.

Here is an example of using fmemopen to create a stream for reading from a string:

#include <stdio.h>

static char buffer[] = "foobar";

int
main (void)
{
  int ch;
  FILE *stream;

  stream = fmemopen (buffer, strlen (buffer), "r");
  while ((ch = fgetc (stream)) != EOF)
    printf ("Got %c\n", ch);
  fclose (stream);

  return 0;
}

This program produces the following output:

Got f
Got o
Got o
Got b
Got a
Got r

 - Function: FILE * open_memstream (char **ptr, size_t *sizeloc)

This function opens a stream for writing to a buffer.  The buffer is allocated dynamically (as with malloc; see Unconstrained Allocation) and grown as necessary.

When the stream is closed with fclose or flushed with fflush, the locations ptr and sizeloc are updated to contain the pointer to the buffer and its size.  The values thus stored remain valid only as long as no further output on the stream takes place.  If you do more output, you must flush the stream again to store new values before you use them again.

A null character is written at the end of the buffer.  This null character is not included in the size value stored at sizeloc.

You can move the stream's file position with fseek (see File Positioning).  Moving the file position past the end of the data already written fills the intervening space with zeroes.

Here is an example of using open_memstream:

#include <stdio.h>

int
main (void)
{
  char *bp;
  size_t size;
  FILE *stream;

  stream = open_memstream (&bp, &size);
  fprintf (stream, "hello");
  fflush (stream);
  printf ("buf = %s, size = %d\n", bp, size);
  fprintf (stream, ", world");
  fclose (stream);
  printf ("buf = %s, size = %d\n", bp, size);

  return 0;
}

This program produces the following output:

buf = `hello', size = 5
buf = `hello, world', size = 12


Obstack Streams
Node: Obstack Streams, Next: Custom Streams, Prev: String Streams, Up: Other Kinds of Streams

Obstack Streams

You can open an output stream that puts it data in an obstack.  See Obstacks.

 - Function: FILE * open_obstack_stream (struct obstack *obstack)

This function opens a stream for writing data into the obstack obstack.  This starts an object in the obstack and makes it grow as data is written (see Growing Objects).

Calling fflush on this stream updates the current size of the object to match the amount of data that has been written.  After a call to fflush, you can examine the object temporarily.

You can move the file position of an obstack stream with fseek (see File Positioning).  Moving the file position past the end of the data written fills the intervening space with zeros.

To make the object permanent, update the obstack with fflush, and then use obstack_finish to finalize the object and get its address.  The following write to the stream starts a new object in the obstack, and later writes add to that object until you do another fflush and obstack_finish.

But how do you find out how long the object is?  You can get the length in bytes by calling obstack_object_size (see Status of an Obstack), or you can null-terminate the object like this:

obstack_1grow (obstack, 0);

Whichever one you do, you must do it before calling obstack_finish.  (You can do both if you wish.)

Here is a sample function that uses open_obstack_stream:

char *
make_message_string (const char *a, int b)
{
  FILE *stream = open_obstack_stream (&message_obstack);
  output_task (stream);
  fprintf (stream, ": ");
  fprintf (stream, a, b);
  fprintf (stream, "\n");
  fclose (stream);
  obstack_1grow (&message_obstack, 0);
  return obstack_finish (&message_obstack);
}


Programming Your Own Custom Streams
Node: Custom Streams, Next: , Prev: Obstack Streams, Up: Other Kinds of Streams

Programming Your Own Custom Streams

This section describes how you can make a stream that gets input from an arbitrary data source or writes output to an arbitrary data sink programmed by you.  We call these "custom streams".

* Menu:

Streams and Cookies	The "cookie" records where to fetch or store data that is read or written.

Hook Functions	How you should define the four "hook functions" that a custom stream needs.


Custom Streams and Cookies
Node: Streams and Cookies, Next: Hook Functions, Prev: , Up: Custom Streams

Custom Streams and Cookies

Inside every custom stream is a special object called the "cookie".  This is an object supplied by you which records where to fetch or store the data read or written.  It is up to you to define a data type to use for the cookie.  The stream functions in the library never refer directly to its contents, and they don't even know what the type is; they record its address with type void *.

To implement a custom stream, you must specify how to fetch or store the data in the specified place.  You do this by defining "hook functions" to read, write, change "file position", and close the stream.  All four of these functions will be passed the stream's cookie so they can tell where to fetch or store the data.  The library functions don't know what's inside the cookie, but your functions will know.

When you create a custom stream, you must specify the cookie pointer, and also the four hook functions stored in a structure of type cookie_io_functions_t.

These facilities are declared in 'stdio.h'.  K

 - Data Type: cookie_io_functions_t

This is a structure type that holds the functions that define the communications protocol between the stream and its cookie.  It has the following members:

cookie_read_function_t *read

This is the function that reads data from the cookie.  If the value is a null pointer instead of a function, then read operations on ths stream always return EOF.

cookie_write_function_t *write

This is the function that writes data to the cookie.  If the value is a null pointer instead of a function, then data written to the stream is discarded.

cookie_seek_function_t *seek

This is the function that performs the equivalent of file positioning on the cookie.  If the value is a null pointer instead of a function, calls to fseek on this stream can only seek to locations within the buffer; any attempt to seek outside the buffer will return an ESPIPE error.

cookie_close_function_t *close

This function performs any appropriate cleanup on the cookie when closing the stream.  If the value is a null pointer instead of a function, nothing special is done to close the cookie when the stream is closed.

 - Function: FILE * fopencookie (void *cookie, const char *opentype, cookie_io_functions_t io_functions)

This function actually creates the stream for communicating with the cookie using the functions in the io_functions argument.  The opentype argument is interpreted as for fopen; see See Opening Streams.  (But note that the "truncate on open" option is ignored.)  The new stream is fully buffered.

The fopencookie function returns the newly created stream, or a null pointer in case of an error.


Custom Stream Hook Functions
Node: Hook Functions, Next: , Prev: Streams and Cookies, Up: Custom Streams

Custom Stream Hook Functions

Here are more details on how you should define the four hook functions that a custom stream needs.

You should define the function to read data from the cookie as:

ssize_t reader (void *cookie, void *buffer, size_t size)

This is very similar to the read function; see See I/O Primitives.  Your function should transfer up to size bytes into the buffer, and return the number of bytes read, or zero to indicate end-of-file.  You can return a value of -1 to indicate an error.

You should define the function to write data to the cookie as:

ssize_t writer (void *cookie, const void *buffer, size_t size)

This is very similar to the write function; see See I/O Primitives.  Your function should transfer up to size bytes from the buffer, and return the number of bytes written.  You can return a value of -1 to indicate an error.

You should define the function to perform seek operations on the cookie as:

int seeker (void *cookie, fpos_t *position, int whence)

For this function, the position and whence arguments are interpreted as for fgetpos; see See Portable Positioning.  In the GNU library, fpos_t is equivalent to off_t or long int, and simply represents the number of bytes from the beginning of the file.

After doing the seek operation, your function should store the resulting file position relative to the beginning of the file in position.  Your function should return a value of 0 on success and -1 to indicate an error.

You should define the function to do cleanup operations on the cookie appropriate for closing the stream as:

int cleaner (void *cookie)

Your function should return -1 to indicate an error, and 0 otherwise.

 - Data Type: cookie_read_function

This is the data type that the read function for a custom stream should have.  If you declare the function as shown above, this is the type it will have.

 - Data Type: cookie_write_function

The data type of the write function for a custom stream.

 - Data Type: cookie_seek_function

The data type of the seek function for a custom stream.

 - Data Type: cookie_close_function

The data type of the close function for a custom stream.


Low-Level Input/Output
Node: Low-Level I/O, Next: File System Interface, Prev: I/O on Streams, Up: Top

Low-Level Input/Output

This chapter describes functions for performing low-level input/output operations on file descriptors.  These functions include the primitives for the higher-level I/O functions described in See I/O on Streams, as well as functions for performing low-level control operations for which there are no equivalents on streams.

Stream-level I/O is more flexible and usually more convenient; therefore, programmers generally use the descriptor-level functions only when necessary.  These are some of the usual reasons:

•	For reading binary files in large chunks.

•	For reading an entire file into core before parsing it.

•	To perform operations other than data transfer, which can only be done with a descriptor.  (You can use fileno to get the descriptor corresponding to a stream.)

•	To pass descriptors to a child process.  (The child can create its own stream to use a descriptor that it inherits, but cannot inherit a stream directly.)

* Menu:

Opening and Closing Files	How to open and close file descriptors.

I/O Primitives	Reading and writing data.

File Position Primitive	Setting a descriptor's file position.

Descriptors and Streams	Converting descriptor to stream or vice-versa.

Stream/Descriptor Precautions	Precautions needed if you use both descriptors and streams.

Waiting for I/O	How to check for input or output on multiple file descriptors.

Control Operations	Various other operations on file descriptors.

Duplicating Descriptors	Fcntl commands for duplicating file descriptors.

Descriptor Flags	Fcntl commands for manipulating flags associated with file descriptors.

File Status Flags	Fcntl commands for manipulating flags associated with open files.

File Locks	Fcntl commands for implementing file locking.

Interrupt Input	Getting an asynchronous signal when input arrives.


Opening and Closing Files
Node: Opening and Closing Files, Next: I/O Primitives, Prev: , Up: Low-Level I/O

Opening and Closing Files

This section describes the primitives for opening and closing files using file descriptors.  The open and creat functions are declared in the header file 'fcntl.h', while close is declared in 'unistd.h'.  K

 - Function: int open (const char *filename, int flags[, mode_t mode])

The open function creates and returns a new file descriptor for the file named by filename.  Initially, the file position indicator for the file is at the beginning of the file.  The argument mode is used only when a file is created, but it doesn't hurt to supply the argument in any case.

The flags argument controls how the file is to be opened.  This is a bit mask; you create the value by the bitwise OR of the appropriate parameters (using the '|' operator in C).

The flags argument must include exactly one of these values to specify the file access mode:

O_RDONLY

Open the file for read access.

O_WRONLY

Open the file for write access.

O_RDWR

Open the file for both reading and writing.

The flags argument can also include any combination of these flags:

O_APPEND

If set, then all write operations write the data at the end of the file, extending it, regardless of the current file position.

O_CREAT

If set, the file will be created if it doesn't already exist.  K

O_EXCL

If both O_CREAT and O_EXCL are set, then open fails if the specified file already exists.

O_NOCTTY

If filename names a terminal device, don't make it the controlling terminal for the process.  See Job Control, for information about what it means to be the controlling terminal.

O_NONBLOCK

This sets nonblocking mode.  This option is usually only useful for special files such as FIFOs (see Pipes and FIFOs) and devices such as terminals.  Normally, for these files, open blocks until the file is "ready".  If O_NONBLOCK is set, open returns immediately.

The O_NONBLOCK bit also affects read and write: It permits them to return immediately with a failure status if there is no input immediately available (read), or if the output can't be written immediately (write).

O_TRUNC

If the file exists and is opened for write access, truncate it to zero length.  This option is only useful for regular files, not special files such as directories or FIFOs.

For more information about these symbolic constants, see See File Status Flags.

The normal return value from open is a non-negative integer file descriptor.  In the case of an error, a value of -1 is returned instead.  In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for this function:

EACCES

The file exists but is not readable/writable as requested by the flags argument.

EEXIST

Both O_CREAT and O_EXCL are set, and the named file already exists.

EINTR

The open operation was interrupted by a signal.  See Interrupted Primitives.

EISDIR

The flags argument specified write access, and the file is a directory.

EMFILE

The process has too many files open.

ENFILE

The entire system, or perhaps the file system which contains the directory, cannot support any additional open files at the moment.  (This problem cannot happen on the GNU system.)

ENOENT

The named file does not exist, but O_CREAT is not specified.

ENOSPC

The directory or file system that would contain the new file cannot be extended, because there is no disk space left.

ENXIO

O_NONBLOCK and O_WRONLY are both set in the flags argument, the file named by filename is a FIFO (see Pipes and FIFOs), and no process has the file open for reading.

EROFS

The file resides on a read-only file system and any of O_WRONLY, O_RDWR, O_CREAT, and O_TRUNC are set in the flags argument.

The open function is the underlying primitive for the fopen and freopen functions, that create streams.

 - Obsolete function: int creat (const char *filename, mode_t mode)

This function is obsolete.  The call:

creat (filename, mode)

is equivalent to:

open (filename, O_WRONLY | O_CREAT | O_TRUNC, mode)

 - Function: int close (int filedes)

The function close closes the file descriptor filedes.  Closing a file has the following consequences:

•	The file descriptor is deallocated.

•	Any record locks owned by the process on the file are unlocked.

•	When all file descriptors associated with a pipe or FIFO have been closed, any unread data is discarded.

The normal return value from close is 0; a value of -1 is returned in case of failure.  The following errno error conditions are defined for this function:

EBADF

The filedes argument is not a valid file descriptor.

EINTR

The close call was interrupted by a signal.  See Interrupted Primitives.  Here is an example of how to handle EINTR properly:

TEMP_FAILURE_RETRY (close (desc));

To close a stream, call fclose (see Closing Streams) instead of trying to close its underlying file descriptor with close.  This flushes any buffered output and updates the stream object to indicate that it is closed.


Input and Output Primitives
Node: I/O Primitives, Next: File Position Primitive, Prev: Opening and Closing Files, Up: Low-Level I/O

Input and Output Primitives

This section describes the functions for performing primitive input and output operations on file descriptors: read, write, and lseek.  These functions are declared in the header file 'unistd.h'.  K

 - Data Type: ssize_t

This data type is used to represent the sizes of blocks that can be read or written in a single operation.  It is similar to size_t, but must be a signed type.

- Function: ssize_t read (int filedes, void *buffer, size_t size)

The read function reads up to size bytes from the file with descriptor filedes, storing the results in the buffer.  (This is not necessarily a character string and there is no terminating null character added.)

The return value is the number of bytes actually read.  This might be less than size; for example, if there aren't that many bytes left in the file or if there aren't that many bytes immediately available.  The exact behavior depends on what kind of file it is.  Note that reading less than size bytes is not an error.

A value of zero indicates end-of-file (except if the value of the size argument is also zero).  This is not considered an error.  If you keep calling read while at end-of-file, it will keep returning zero and doing nothing else.

If read returns at least one character, there is no way you can tell whether end-of-file was reached.  But if you did reach the end, the next read will return zero.

In case of an error, read returns -1.  The following errno error conditions are defined for this function:

EAGAIN

Normally, when no input is immediately available, read waits for some input.  But if the O_NONBLOCK flag is set for the file (see File Status Flags), read returns immediately without reading any data, and reports this error.

Compatibility Note: Most versions of BSD Unix use a different error code for this: EWOULDBLOCK.  In the GNU library, EWOULDBLOCK is an alias for EAGAIN, so it doesn't matter which name you use.

On some systems, reading a large amount of data from a character special file can also fail with EAGAIN if the kernel cannot find enough physical memory to lock down the user's pages.  This is limited to devices that transfer with direct memory access into the user's memory, which means it does not include terminals, since they always use separate buffers inside the kernel.

EBADF

The filedes argument is not a valid file descriptor.

EINTR

read was interrupted by a signal while it was waiting for input.  See Interrupted Primitives.

EIO

For many devices, and for disk files, this error code indicates a hardware error.

EIO also occurs when a background process tries to read from the controlling terminal, and the normal action of stopping the process by sending it a SIGTTIN signal isn't working.  This might happen if signal is being blocked or ignored, or because the process group is orphaned.  See Job Control, for more information about job control, and See Signal Handling, for information about signals.

The read function is the underlying primitive for all of the functions that read from streams, such as fgetc.

- Function: ssize_t write (int filedes, const void *buffer, size_t size)

The write function writes up to size bytes from buffer to the file with descriptor filedes.  The data in buffer is not necessarily a character string and a null character output like any other character.

The return value is the number of bytes actually written.  This is normally the same as size, but might be less (for example, if the physical media being written to fills up).

In the case of an error, write returns -1.  The following errno error conditions are defined for this function:

EAGAIN

Normally, write blocks until the write operation is complete.  But if the O_NONBLOCK flag is set for the file (see Control Operations), it returns immediately without writing any data, and reports this error.  An example of a situation that might cause the process to block on output is writing to a terminal device that supports flow control, where output has been suspended by receipt of a STOP character.

Compatibility Note: Most versions of BSD Unix use a different error code for this: EWOULDBLOCK.  In the GNU library, EWOULDBLOCK is an alias for EAGAIN, so it doesn't matter which name you use.

On some systems, writing a large amount of data from a character special file can also fail with EAGAIN if the kernel cannot find enough physical memory to lock down the user's pages.  This is limited to devices that transfer with direct memory access into the user's memory, which means it does not include terminals, since they always use separate buffers inside the kernel.

EBADF

The filedes argument is not a valid file descriptor.

EFBIG

The size of the file is larger than the implementation can support.

EINTR

The write operation was interrupted by a signal while it was blocked waiting for completion.  See Interrupted Primitives.

EIO

For many devices, and for disk files, this error code indicates a hardware error.

EIO also occurs when a background process tries to write to the controlling terminal, and the normal action of stopping the process by sending it a SIGTTOU signal isn't working.  This might happen if the signal is being blocked or ignored.  See Job Control, for more information about job control, and See Signal Handling, for information about signals.

ENOSPC

The device is full.

EPIPE

This error is returned when you try to write to a pipe or FIFO that isn't open for reading by any process.  When this happens, a SIGPIPE signal is also sent to the process; see See Signal Handling.

Unless you have arranged to prevent EINTR failures, you should check errno after each failing call to write, and if the error was EINTR, you should simply repeat the call.  See Interrupted Primitives.  The easy way to do this is with the macro TEMP_FAILURE_RETRY, as follows:

nbytes = TEMP_FAILURE_RETRY (write (desc, buffer, count));

The write function is the underlying primitive for all of the functions that write to streams, such as fputc.


Setting the File Position of a Descriptor
Node: File Position Primitive, Next: Descriptors and Streams, Prev: I/O Primitives, Up: Low-Level I/O

Setting the File Position of a Descriptor

Just as you can set the file position of a stream with fseek, you can set the file position of a descriptor with lseek.  This specifies the position in the file for the next read or write operation.  See File Positioning, for more information on the file position and what it means.

To read the current file position value from a descriptor, use lseek (desc, 0, SEEK_CUR).

- Function: off_t lseek (int filedes, off_t offset, int whence)

The lseek function is used to change the file position of the file with descriptor filedes.

The whence argument specifies how the offset should be interpreted in the same way as for the fseek function, and can be one of the symbolic constants SEEK_SET, SEEK_CUR, or SEEK_END.

SEEK_SET

Specifies that whence is a count of characters from the beginning of the file.

SEEK_CUR

Specifies that whence is a count of characters from the current file position.  This count may be positive or negative.

SEEK_END

Specifies that whence is a count of characters from the end of the file.  A negative count specifies a position within the current extent of the file; a positive count specifies a position past the current end.  If you set the position past the current end, and actually write data, you will extend the file with zeros up to that position.

The return value from lseek is normally the resulting file position, measured in bytes from the beginning of the file.  You can use this feature together with SEEK_CUR to read the current file position.

You can set the file position past the current end of the file.  This does not by itself make the file longer; lseek never changes the file.  But subsequent output at that position will extend the file's size.

If the file position cannot be changed, or the operation is in some way invalid, lseek returns a value of -1.  The following errno error conditions are defined for this function:

EBADF

The filedes is not a valid file descriptor.

EINVAL

The whence argument value is not valid, or the resulting file offset is not valid.

ESPIPE

The filedes corresponds to a pipe or FIFO, which cannot be positioned.  (There may be other kinds of files that cannot be positioned either, but the behavior is not specified in those cases.)

The lseek function is the underlying primitive for the fseek, ftell and rewind functions, which operate on streams instead of file descriptors.

You can have multiple descriptors for the same file if you open the file more than once, or if you duplicate a descriptor with dup.  Descriptors that come from separate calls to open have independent file positions; using lseek on one descriptor has no effect on the other.  For example,

{
  int d1, d2;
  char buf[4];
  d1 = open ("foo", O_RDONLY);
  d2 = open ("foo", O_RDONLY);
  lseek (d1, 1024, SEEK_SET);
  read (d2, buf, 4);
}

will read the first four characters of the file 'foo'.  (The error-checking code necessary for a real program has been omitted here for brevity.)

By contrast, descriptors made by duplication share a common file position with the original descriptor that was duplicated.  Anything which alters the file position of one of the duplicates, including reading or writing data, affects all of them alike.  Thus, for example,

{
  int d1, d2, d3;
  char buf1[4], buf2[4];
  d1 = open ("foo", O_RDONLY);
  d2 = dup (d1);
  d3 = dup (d2);
  lseek (d3, 1024, SEEK_SET);
  read (d1, buf1, 4);
  read (d2, buf2, 4);
}

will read four characters starting with the 1024'th character of 'foo', and then four more characters starting with the 1028'th character.

 - Data Type: off_t

This is an arithmetic data type used to represent file sizes.  In the GNU system, this is equivalent to fpos_t or long int.

These three aliases for the 'SEEK_...' constants exist for the sake of compatibility with older BSD systems.  They are defined in two different header files: 'fcntl.h' and 'sys/file.h'.

L_SET

An alias for SEEK_SET.

L_INCR

An alias for SEEK_CUR.

L_XTND

An alias for SEEK_END.


Descriptors and Streams
Node: Descriptors and Streams, Next: Stream/Descriptor Precautions, Prev: File Position Primitive, Up: Low-Level I/O

Descriptors and Streams

Given an open file descriptor, you can create a stream for it with the fdopen function.  You can get the underlying file descriptor for an existing stream with the fileno function.  These functions are declared in the header file 'stdio.h'.  K

 - Function: FILE * fdopen (int filedes, const char *opentype)

The fdopen function returns a new stream for the file descriptor filedes.

The opentype argument is interpreted in the same way as for the fopen function (see Opening Streams), except that the 'b' option is not permitted; this is because GNU makes no distinction between text and binary files.  Also, "w" and "w+" do not cause truncation of the file; these have affect only when opening a file, and in this case the file has already been opened.  You must make sure that the opentype argument matches the actual mode of the open file descriptor.

The return value is the new stream.  If the stream cannot be created (for example, if the modes for the file indicated by the file descriptor do not permit the access specified by the opentype argument), a null pointer is returned instead.

For an example showing the use of the fdopen function, see See Creating a Pipe.

 - Function: int fileno (FILE *stream)

This function returns the file descriptor associated with the stream stream.  If an error is detected (for example, if the stream is not valid) or if stream does not do I/O to a file, fileno returns -1.

There are also symbolic constants defined in 'unistd.h' for the file descriptors belonging to the standard streams stdin, stdout, and stderr; see See Standard Streams.  K

STDIN_FILENO

This macro has value 0, which is the file descriptor for standard input.  K

STDOUT_FILENO

This macro has value 1, which is the file descriptor for standard output.  K

STDERR_FILENO

This macro has value 2, which is the file descriptor for standard error output.


Dangers of Mixing Streams and Descriptors
Node: Stream/Descriptor Precautions, Next: Waiting for I/O, Prev: Descriptors and Streams, Up: Low-Level I/O

Dangers of Mixing Streams and Descriptors

You can have multiple file descriptors and streams (let's call both streams and descriptors "channels" for short) connected to the same file, but you must take care to avoid confusion between channels.  There are two cases to consider: "linked" channels that share a single file position value, and "independent" channels that have their own file positions.

It's best to use just one channel in your program for actual data transfer to any given file, except when all the access is for input.  For example, if you open a pipe (something you can only do at the file descriptor level), either do all I/O with the descriptor, or construct a stream from the descriptor with fdopen and then do all I/O with the stream.

* Menu:

Linked Channels	Dealing with channels sharing a file position.

Independent Channels	Dealing with separately opened, unlinked channels.

Cleaning Streams	Cleaning a stream makes it safe to use another channel.


Linked Channels
Node: Linked Channels, Next: Independent Channels, Prev: , Up: Stream/Descriptor Precautions

Linked Channels

Channels that come from a single opening share the same file position; we call them "linked" channels.  Linked channels result when you make a stream from a descriptor using fdopen, when you get a descriptor from a stream with fileno, and when you copy a descriptor with dup or dup2.  For files that don't support random access, such as terminals and pipes, all channels are effectively linked.  On random-access files, all append-type output streams are effectively linked to each other.

If you have been using a stream for I/O, and you want to do I/O using another channel (either a stream or a descriptor) that is linked to it, you must first "clean up" the stream that you have been using.  See Cleaning Streams.

Terminating a process, or executing a new program in the process, destroys all the streams in the process.  If descriptors linked to these streams persist in other processes, their file positions become undefined as a result.  To prevent this, you must clean up the streams before destroying them.


Independent Channels
Node: Independent Channels, Next: Cleaning Streams, Prev: Linked Channels, Up: Stream/Descriptor Precautions

Independent Channels

When you open channels (streams or descriptors) separately on a seekable file, each channel has its own file position.  These are called "independent channels".

The system handles each channel independently.  Most of the time, this is quite predictable and natural (especially for input): each channel can read or write sequentially at its own place in the file.  However, if some of the channels are streams, you must take these precautions:

•	You should clean an output stream after use, before doing anything else that might read or write from the same part of the file.

•	You should clean an input stream before reading data that may have been modified using an independent channel.  Otherwise, you might read obsolete data that had been in the stream's buffer.

If you do output to one channel at the end of the file, this will certainly leave the other independent channels positioned somewhere before the new end.  If you want them to output at the end, you must set their file positions to end of file, first.  (This is not necessary if you use an append-type descriptor or stream; they always output at the current end of the file.)  In order to make the end-of-file position accurate, you must clean the output channel you were using, if it is a stream.  (This is necessary even if you plan to use an append-type channel next.)

It's impossible for two channels to have separate file pointers for a file that doesn't support random access.  Thus, channels for reading or writing such files are always linked, never independent.  Append-type channels are also always linked.  For these channels, follow the rules for linked channels; see See Linked Channels.


Cleaning Streams
Node: Cleaning Streams, Next: , Prev: Independent Channels, Up: Stream/Descriptor Precautions

Cleaning Streams

On the GNU system, you can clean up any stream with fclean:

 - Function: int fclean (FILE *stream)

Clean up the stream stream so that its buffer is empty.  If stream is doing output, force it out.  If stream is doing input, give the data in the buffer back to the system, arranging to reread it.

On other systems, you can use fflush to clean a stream in most cases.

You can skip the fclean or fflush if you know the stream is already clean.  A stream is clean whenever its buffer is empty.  For example, an unbuffered stream is always clean.  An input stream that is at end-of-file is clean.  A line-buffered stream is clean when the last character output was a newline.

There is one case in which cleaning a stream is impossible on most systems.  This is when the stream is doing input from a file that is not random-access.  Such streams typically read ahead, and when the file is not random access, there is no way to give back the excess data already read.  When an input stream reads from a random-access file, fflush does clean the stream, but leaves the file pointer at an unpredictable place; you must set the file pointer before doing any further I/O.  On the GNU system, using fclean avoids both of these problems.

Closing an output-only stream also does fflush, so this is a valid way of cleaning an output stream.  On the GNU system, closing an input stream does fclean.

You need not clean a stream before using its descriptor for control operations such as setting terminal modes; these operations don't affect the file position and are not affected by it.  You can use any descriptor for these operations, and all channels are affected simultaneously.  However, text already "output" to a stream but still buffered by the stream will be subject to the new terminal modes when subsequently flushed.  To make sure "past" output is covered by the terminal settings that were in effect at the time, flush the output streams for that terminal before setting the modes.  See Terminal Modes.


Waiting for Input or Output
Node: Waiting for I/O, Next: Control Operations, Prev: Stream/Descriptor Precautions, Up: Low-Level I/O

Waiting for Input or Output

Sometimes a program needs to accept input on multiple input channels whenever input arrives.  For example, some workstations may have devices such as a digitizing tablet, function button box, or dial box that are connected via normal asynchronous serial interfaces; good user interface style requires responding immediately to input on any device.  Another example is a program that acts as a server to several other processes via pipes or sockets.

You cannot normally use read for this purpose, because this blocks the program until input is available on one particular file descriptor; input on other channels won't wake it up.  You could set nonblocking mode and poll each file descriptor in turn, but this is very inefficient.

A better solution is to use the select function.  This blocks the program until input or output is ready on a specified set of file descriptors, or until timer expires, whichever comes first.  This facility is declared in the header file 'sys/types.h'.  KThe file descriptor sets for the select function are specified as fd_set objects.  Here is the description of the data type and some macros for manipulating these objects.

 - Data Type: fd_set

The fd_set data type represents file descriptor sets for the select function.  It is actually a bit array.

 - Macro: int FD_SETSIZE

The value of this macro is the maximum number of file descriptors that a fd_set object can hold information about.  On systems with a fixed maximum number, FD_SETSIZE is at least that number.  On some systems, including GNU, there is no absolute limit on the number of descriptors open, but this macro still has a constant value which controls the number of bits in an fd_set.

 - Macro: void FD_ZERO (fd_set *set)

This macro initializes the file descriptor set set to be the empty set.

 - Macro: void FD_SET (int filedes, fd_set *set)

This macro adds filedes to the file descriptor set set.

 - Macro: void FD_CLR (int filedes, fd_set *set)

This macro removes filedes from the file descriptor set set.

 - Macro: int FD_ISSET (int filedes, fd_set *set)

This macro returns a nonzero value (true) if filedes is a member of the the file descriptor set set, and zero (false) otherwise.

Next, here is the description of the select function itself.

 - Function: int select (int nfds, fd_set *read_fds, fd_set *write_fds, fd_set *except_fds, struct timeval *timeout)

The select function blocks the calling process until there is activity on any of the specified sets of file descriptors, or until the timeout period has expired.

The file descriptors specified by the read_fds argument are checked to see if they are ready for reading; the write_fds file descriptors are checked to see if they are ready for writing; and the except_fds file descriptors are checked for exceptional conditions.  You can pass a null pointer for any of these arguments if you are not interested in checking for that kind of condition.

"Exceptional conditions" does not mean errors--errors are reported immediately when an erroneous system call is executed, and do not constitute a state of the descriptor.  Rather, they include conditions such as the presence of an urgent message on a socket.  (See Sockets, for information on urgent messages.)

The select function checks only the first nfds file descriptors.  The usual thing is to pass FD_SETSIZE as the value of this argument.

The timeout specifies the maximum time to wait.  If you pass a null pointer for this argument, it means to block indefinitely until one of the file descriptors is ready.  Otherwise, you should provide the time in struct timeval format; see See High-Resolution Calendar.  Specify zero as the time (a struct timeval containing all zeros) if you want to find out which descriptors are ready without waiting if none are ready.

The normal return value from select is the total number of ready file descriptors in all of the sets.  Each of the argument sets is overwritten with information about the descriptors that are ready for the corresponding operation.  Thus, to see if a particular descriptor desc has input, use FD_ISSET (desc, read_fds) after select returns.

If select returns because the timeout period expires, it returns a value of zero.

Any signal will cause select to return immediately.  So if your program uses signals, you can't rely on select to keep waiting for the full time specified.  If you want to be sure of waiting for a particular amount of time, you must check for EINTR and repeat the select with a newly calculated timeout based on the current time.  See the example below.  See also See Interrupted Primitives.

If an error occurs, select returns -1 and does not modify the argument file descriptor sets.  The following errno error conditions are defined for this function:

EBADF

One of the file descriptor sets specified an invalid file descriptor.

EINTR

The operation was interrupted by a signal.  See Interrupted Primitives.

EINVAL

The timeout argument is invalid; one of the components is negative or too large.

Portability Note:  The select function is a BSD Unix feature.

Here is an example showing how you can use select to establish a timeout period for reading from a file descriptor.  The input_timeout function blocks the calling process until input is available on the file descriptor, or until the timeout period expires.

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/time.h>

int
input_timeout (int filedes, unsigned int seconds)
{
  fd_set set;
  struct timeval timeout;

  /* Initialize the file descriptor set. */
  FD_ZERO (&set);
  FD_SET (filedes, &set);

  /* Initialize the timeout data structure. */
  timeout.tv_sec = seconds;
  timeout.tv_usec = 0;

  /* select returns 0 if timeout, 1 if input available, -1 if error. */
  return TEMP_FAILURE_RETRY (select (FD_SETSIZE,
                                     &set, NULL, NULL,
                                     &timeout));
}

int
main (void)
{
  fprintf (stderr, "select returned %d.\n",
           input_timeout (STDIN_FILENO, 5));
  return 0;
}

There is another example showing the use of select to multiplex input from multiple sockets in See Server Example.


Control Operations on Files
Node: Control Operations, Next: Duplicating Descriptors, Prev: Waiting for I/O, Up: Low-Level I/O

Control Operations on Files

This section describes how you can perform various other operations on file descriptors, such as inquiring about or setting flags describing the status of the file descriptor, manipulating record locks, and the like.  All of these operations are performed by the function fcntl.

The second argument to the fcntl function is a command that specifies which operation to perform.  The function and macros that name various flags that are used with it are declared in the header file 'fcntl.h'.  (Many of these flags are also used by the open function; see See Opening and Closing Files.) K

 - Function: int fcntl (int filedes, int command, ...)

The fcntl function performs the operation specified by command on the file descriptor filedes.  Some commands require additional arguments to be supplied.  These additional arguments and the return value and error conditions are given in the detailed descriptions of the individual commands.

Briefly, here is a list of what the various commands are.

F_DUPFD

Duplicate the file descriptor (return another file descriptor pointing to the same open file).  See Duplicating Descriptors.

F_GETFD

Get flags associated with the file descriptor.  See Descriptor Flags.

F_SETFD

Set flags associated with the file descriptor.  See Descriptor Flags.

F_GETFL

Get flags associated with the open file.  See File Status Flags.

F_SETFL

Set flags associated with the open file.  See File Status Flags.

F_GETLK

Get a file lock.  See File Locks.

F_SETLK

Set or clear a file lock.  See File Locks.

F_SETLKW

Like F_SETLK, but wait for completion.  See File Locks.

F_GETOWN

Get process or process group ID to receive SIGIO signals.  See Interrupt Input.

F_SETOWN

Set process or process group ID to receive SIGIO signals.  See Interrupt Input.


Duplicating Descriptors
Node: Duplicating Descriptors, Next: Descriptor Flags, Prev: Control Operations, Up: Low-Level I/O

Duplicating Descriptors

You can "duplicate" a file descriptor, or allocate another file descriptor that refers to the same open file as the original.  Duplicate descriptors share one file position and one set of file status flags (see File Status Flags), but each has its own set of file descriptor flags (see Descriptor Flags).

The major use of duplicating a file descriptor is to implement "redirection" of input or output:  that is, to change the file or pipe that a particular file descriptor corresponds to.

You can perform this operation using the fcntl function with the F_DUPFD command, but there are also convenient functions dup and dup2 for duplicating descriptors.

The fcntl function and flags are declared in 'fcntl.h', while prototypes for dup and dup2 are in the header file 'unistd.h'.

 - Function: int dup (int old)

This function copies descriptor old to the first available descriptor number (the first number not currently open).  It is equivalent to fcntl (old, F_DUPFD, 0).

 - Function: int dup2 (int old, int new)

This function copies the descriptor old to descriptor number new.

If old is an invalid descriptor, then dup2 does nothing; it does not close new.  Otherwise, the new duplicate of old replaces any previous meaning of descriptor new, as if new were closed first.

If old and new are different numbers, and old is a valid descriptor number, then dup2 is equivalent to:

close (new);
fcntl (old, F_DUPFD, new)

However, dup2 does this atomically; there is no instant in the middle of calling dup2 at which new is closed and not yet a duplicate of old.

 - Macro: int F_DUPFD

This macro is used as the command argument to fcntl, to copy the file descriptor given as the first argument.

The form of the call in this case is:

fcntl (old, F_DUPFD, next_filedes)

The next_filedes argument is of type int and specifies that the file descriptor returned should be the next available one greater than or equal to this value.

The return value from fcntl with this command is normally the value of the new file descriptor.  A return value of -1 indicates an error.  The following errno error conditions are defined for this command:

EBADF

The old argument is invalid.

EINVAL

The next_filedes argument is invalid.

EMFILE

There are no more file descriptors available--your program is already using the maximum.

ENFILE is not a possible error code for dup2 because dup2 does not create a new opening of a file; duplicate descriptors do not count toward the limit which ENFILE indicates.  EMFILE is possible because it refers to the limit on distinct descriptor numbers in use in one process.

Here is an example showing how to use dup2 to do redirection.  Typically, redirection of the standard streams (like stdin) is done by a shell or shell-like program before calling one of the exec functions (see Executing a File) to execute a new program in a child process.  When the new program is executed, it creates and initializes the standard streams to point to the corresponding file descriptors, before its main function is invoked.

So, to redirect standard input to a file, the shell could do something like:

pid = fork ();
if (pid == 0)
  {
    char *filename;
    char *program;
    int file;
    ...
    file = TEMP_FAILURE_RETRY (open (filename, O_RDONLY));
    dup2 (file, STDIN_FILENO);
    TEMP_FAILURE_RETRY (close (file));
    execv (program, NULL);
  }

There is also a more detailed example showing how to implement redirection in the context of a pipeline of processes in See Launching Jobs.


File Descriptor Flags
Node: Descriptor Flags, Next: File Status Flags, Prev: Duplicating Descriptors, Up: Low-Level I/O

File Descriptor Flags

"File descriptor flags" are miscellaneous attributes of a file descriptor.  These flags are associated with particular file descriptors, so that if you have created duplicate file descriptors from a single opening of a file, each descriptor has its own set of flags.

Currently there is just one file descriptor flag: FD_CLOEXEC, which causes the descriptor to be closed if you use any of the exec... functions (see Executing a File).

The symbols in this section are defined in the header file 'fcntl.h'.  K

 - Macro: int F_GETFD

This macro is used as the command argument to fcntl, to specify that it should return the file descriptor flags associated with the filedes argument.

The normal return value from fcntl with this command is a nonnegative number which can be interpreted as the bitwise OR of the individual flags (except that currently there is only one flag to use).

In case of an error, fcntl returns -1.  The following errno error conditions are defined for this command:

EBADF

The filedes argument is invalid.

 - Macro: int F_SETFD

This macro is used as the command argument to fcntl, to specify that it should set the file descriptor flags associated with the filedes argument.  This requires a third int argument to specify the new flags, so the form of the call is:

fcntl (filedes, F_SETFD, new_flags)

The normal return value from fcntl with this command is an unspecified value other than -1, which indicates an error.  The flags and error conditions are the same as for the F_GETFD command.

The following macro is defined for use as a file descriptor flag with the fcntl function.  The value is an integer constant usable as a bit mask value.

 - Macro: int FD_CLOEXEC

This flag specifies that the file descriptor should be closed when an exec function is invoked; see See Executing a File.  When a file descriptor is allocated (as with open or dup), this bit is initially cleared on the new file descriptor, meaning that descriptor will survive into the new program after exec.

If you want to modify the file descriptor flags, you should get the current flags with F_GETFD and modify the value.  Don't assume that the flags listed here are the only ones that are implemented; your program may be run years from now and more flags may exist then.  For example, here is a function to set or clear the flag FD_CLOEXEC without altering any other flags:

/* Set the FD_CLOEXEC flag of desc if value is nonzero,
   or clear the flag if value is 0.
   Return 0 on success, or -1 on error with errno set. */

int
set_cloexec_flag (int desc, int value)
{
  int oldflags = fcntl (desc, F_GETFD, 0);
  /* If reading the flags failed, return error indication now.
  if (oldflags < 0)
    return oldflags;
  /* Set just the flag we want to set. */
  if (value != 0)
    oldflags |= FD_CLOEXEC;
  else
    oldflags &= ~FD_CLOEXEC;
  /* Store modified flag word in the descriptor. */
  return fcntl (desc, F_SETFD, oldflags);
}


File Status Flags
Node: File Status Flags, Next: File Locks, Prev: Descriptor Flags, Up: Low-Level I/O

File Status Flags

"File status flags" are used to specify attributes of the opening of a file.  Unlike the file descriptor flags discussed in See Descriptor Flags, the file status flags are shared by duplicated file descriptors resulting from a single opening of the file.

The file status flags are initialized by the open function from the flags argument of the open function.  Some of the flags are meaningful only in open and are not remembered subsequently; many of the rest cannot subsequently be changed, though you can read their values by examining the file status flags.

A few file status flags can be changed at any time using fcntl.  These include O_APPEND and O_NONBLOCK.

The symbols in this section are defined in the header file 'fcntl.h'.  K

 - Macro: int F_GETFL

This macro is used as the command argument to fcntl, to read the file status flags for the open file with descriptor filedes.

The normal return value from fcntl with this command is a nonnegative number which can be interpreted as the bitwise OR of the individual flags.  The flags are encoded like the flags argument to open (see Opening and Closing Files), but only the file access modes and the O_APPEND and O_NONBLOCK flags are meaningful here.  Since the file access modes are not single-bit values, you can mask off other bits in the returned flags with O_ACCMODE to compare them.

In case of an error, fcntl returns -1.  The following errno error conditions are defined for this command:

EBADF

The filedes argument is invalid.

 - Macro: int F_SETFL

This macro is used as the command argument to fcntl, to set the file status flags for the open file corresponding to the filedes argument.  This command requires a third int argument to specify the new flags, so the call looks like this:

fcntl (filedes, F_SETFL, new_flags)

You can't change the access mode for the file in this way; that is, whether the file descriptor was opened for reading or writing.  You can only change the O_APPEND and O_NONBLOCK flags.

The normal return value from fcntl with this command is an unspecified value other than -1, which indicates an error.  The error conditions are the same as for the F_GETFL command.

The following macros are defined for use in analyzing and constructing file status flag values:

O_APPEND

The bit that enables append mode for the file.  If set, then all write operations write the data at the end of the file, extending it, regardless of the current file position.

O_NONBLOCK

The bit that enables nonblocking mode for the file.  If this bit is set, read requests on the file can return immediately with a failure status if there is no input immediately available, instead of blocking.  Likewise, write requests can also return immediately with a failure status if the output can't be written immediately.

O_NDELAY

This is a synonym for O_NONBLOCK, provided for compatibility with BSD.

 - Macro: int O_ACCMODE

This macro stands for a mask that can be bitwise-ANDed with the file status flag value to produce a value representing the file access mode.  The mode will be O_RDONLY, O_WRONLY, or O_RDWR.

O_RDONLY

Open the file for read access.

O_WRONLY

Open the file for write access.

O_RDWR

Open the file for both reading and writing.

If you want to modify the file status flags, you should get the current flags with F_GETFL and modify the value.  Don't assume that the flags listed here are the only ones that are implemented; your program may be run years from now and more flags may exist then.  For example, here is a function to set or clear the flag O_NONBLOCK without altering any other flags:

/* Set the O_NONBLOCK flag of desc if value is nonzero,
   or clear the flag if value is 0.
   Return 0 on success, or -1 on error with errno set. */

int
set_nonblock_flag (int desc, int value)
{
  int oldflags = fcntl (desc, F_GETFL, 0);
  /* If reading the flags failed, return error indication now. */
  if (oldflags < 0)
    return oldflags;
  /* Set just the flag we want to set. */
  if (value != 0)
    oldflags |= O_NONBLOCK;
  else
    oldflags &= ~O_NONBLOCK;
  /* Store modified flag word in the descriptor. */
  return fcntl (desc, F_SETFL, oldflags);
}


File Locks
Node: File Locks, Next: Interrupt Input, Prev: File Status Flags, Up: Low-Level I/O

File Locks

The remaining fcntl commands are used to support "record locking", which permits multiple cooperating programs to prevent each other from simultaneously accessing parts of a file in error-prone ways.

An "exclusive" or "write" lock gives a process exclusive access for writing to the specified part of the file.  While a write lock is in place, no other process can lock that part of the file.

A "shared" or "read" lock prohibits any other process from requesting a write lock on the specified part of the file.  However, other processes can request read locks.

The read and write functions do not actually check to see whether there are any locks in place.  If you want to implement a locking protocol for a file shared by multiple processes, your application must do explicit fcntl calls to request and clear locks at the appropriate points.

Locks are associated with processes.  A process can only have one kind of lock set for each byte of a given file.  When any file descriptor for that file is closed by the process, all of the locks that process holds on that file are released, even if the locks were made using other descriptors that remain open.  Likewise, locks are released when a process exits, and are not inherited by child processes created using fork (see Creating a Process).

When making a lock, use a struct flock to specify what kind of lock and where.  This data type and the associated macros for the fcntl function are declared in the header file 'fcntl.h'.  K

 - Data Type: struct flock

This structure is used with the fcntl function to describe a file lock.  It has these members:

short int l_type

Specifies the type of the lock; one of F_RDLCK, F_WRLCK, or F_UNLCK.

short int l_whence

This corresponds to the whence argument to fseek or lseek, and specifies what the offset is relative to.  Its value can be one of SEEK_SET, SEEK_CUR, or SEEK_END.

off_t l_start

This specifies the offset of the start of the region to which the lock applies, and is given in bytes relative to the point specified by l_whence member.

off_t l_len

This specifies the length of the region to be locked.  A value of 0 is treated specially; it means the region extends to the end of the file.

pid_t l_pid

This field is the process ID (see Process Creation Concepts) of the process holding the lock.  It is filled in by calling fcntl with the F_GETLK command, but is ignored when making a lock.

 - Macro: int F_GETLK

This macro is used as the command argument to fcntl, to specify that it should get information about a lock.  This command requires a third argument of type struct flock * to be passed to fcntl, so that the form of the call is:

fcntl (filedes, F_GETLK, lockp)

If there is a lock already in place that would block the lock described by the lockp argument, information about that lock overwrites *lockp.  Existing locks are not reported if they are compatible with making a new lock as specified.  Thus, you should specify a lock type of F_WRLCK if you want to find out about both read and write locks, or F_RDLCK if you want to find out about write locks only.

There might be more than one lock affecting the region specified by the lockp argument, but fcntl only returns information about one of them.  The l_whence member of the lockp structure is set to SEEK_SET and the l_start and l_len fields set to identify the locked region.

If no lock applies, the only change to the lockp structure is to update the l_type to a value of F_UNLCK.

The normal return value from fcntl with this command is an unspecified value other than -1, which is reserved to indicate an error.  The following errno error conditions are defined for this command:

EBADF

The filedes argument is invalid.

EINVAL

Either the lockp argument doesn't specify valid lock information, or the file associated with filedes doesn't support locks.

 - Macro: int F_SETLK

This macro is used as the command argument to fcntl, to specify that it should set or clear a lock.  This command requires a third argument of type struct flock * to be passed to fcntl, so that the form of the call is:

fcntl (filedes, F_SETLK, lockp)

If the process already has a lock on any part of the region, the old lock on that part is replaced with the new lock.  You can remove a lock by specifying a lock type of F_UNLCK.

If the lock cannot be set, fcntl returns immediately with a value of -1.  This function does not block waiting for other processes to release locks.  If fcntl succeeds, it return a value other than -1.

The following errno error conditions are defined for this function:

EACCES

EAGAIN

The lock cannot be set because it is blocked by an existing lock on the file.  Some systems use EAGAIN in this case, and other systems use EACCES; your program should treat them alike, after F_SETLK.

EBADF

Either: the filedes argument is invalid; you requested a read lock but the filedes is not open for read access; or, you requested a write lock but the filedes is not open for write access.

EINVAL

Either the lockp argument doesn't specify valid lock information, or the file associated with filedes doesn't support locks.

ENOLCK

The system has run out of file lock resources; there are already too many file locks in place.

Well-designed file systems never report this error, because they have no limitation on the number of locks.  However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine.

 - Macro: int F_SETLKW

This macro is used as the command argument to fcntl, to specify that it should set or clear a lock.  It is just like the F_SETLK command, but causes the process to block (or wait) until the request can be specified.

This command requires a third argument of type struct flock *, as for the F_SETLK command.

The fcntl return values and errors are the same as for the F_SETLK command, but these additional errno error conditions are defined for this command:

EINTR

The function was interrupted by a signal while it was waiting.  See Interrupted Primitives.

EDEADLK

A deadlock condition was detected.  This can happen if two processes each already controlling a locked region request a lock on the same region locked by the other process.

The following macros are defined for use as values for the l_type member of the flock structure.  The values are integer constants.

F_RDLCK

This macro is used to specify a read (or shared) lock.

F_WRLCK

This macro is used to specify a write (or exclusive) lock.

F_UNLCK

This macro is used to specify that the region is unlocked.

As an example of a situation where file locking is useful, consider a program that can be run simultaneously by several different users, that logs status information to a common file.  One example of such a program might be a game that uses a file to keep track of high scores.  Another example might be a program that records usage or accounting information for billing purposes.

Having multiple copies of the program simultaneously writing to the file could cause the contents of the file to become mixed up.  But you can prevent this kind of problem by setting a write lock on the file before actually writing to the file.

If the program also needs to read the file and wants to make sure that the contents of the file are in a consistent state, then it can also use a read lock.  While the read lock is set, no other process can lock that part of the file for writing.

Remember that file locks are only a voluntary protocol for controlling access to a file.  There is still potential for access to the file by programs that don't use the lock protocol.


Interrupt-Driven Input
Node: Interrupt Input, Next: , Prev: File Locks, Up: Low-Level I/O

Interrupt-Driven Input

If you set the FASYNC status flag on a file descriptor (see File Status Flags), a SIGIO signal is sent whenever input or output becomes possible on that file descriptor.  The process or process group to receive the signal can be selected by using the F_SETOWN command to the fcntl function.  If the file descriptor is a socket, this also selects the recipient of SIGURG signals that are delivered when out-of-band data arrives on that socket; see See Out-of-Band Data.

If the file descriptor corresponds to a terminal device, then SIGIO signals are sent to the foreground process group of the terminal.  See Job Control.

The symbols in this section are defined in the header file 'fcntl.h'.

 - Macro: int F_GETOWN

This macro is used as the command argument to fcntl, to specify that it should get information about the process or process group to which SIGIO signals are sent.  (For a terminal, this is actually the foreground process group ID, which you can get using tcgetpgrp; see See Terminal Access Functions.)

The return value is interpreted as a process ID; if negative, its absolute value is the process group ID.

The following errno error condition is defined for this command:

EBADF

The filedes argument is invalid.

 - Macro: int F_SETOWN

This macro is used as the command argument to fcntl, to specify that it should set the process or process group to which SIGIO signals are sent.  This command requires a third argument of type pid_t to be passed to fcntl, so that the form of the call is:

fcntl (filedes, F_SETOWN, pid)

The pid argument should be a process ID.  You can also pass a negative number whose absolute value is a process group ID.

The return value from fcntl with this command is -1 in case of error and some other value if successful.  The following errno error conditions are defined for this command:

EBADF

The filedes argument is invalid.

ESRCH

There is no process or process group corresponding to pid.


File System Interface
Node: File System Interface, Next: Pipes and FIFOs, Prev: Low-Level I/O, Up: Top

File System Interface

This chapter describes the GNU C library's functions for manipulating files.  Unlike the input and output functions described in See I/O on Streams and See Low-Level I/O, these functions are concerned with operating on the files themselves, rather than on their contents.

Among the facilities described in this chapter are functions for examining or modifying directories, functions for renaming and deleting files, and functions for examining and setting file attributes such as access permissions and modification times.

* Menu:

Working Directory	This is used to resolve relative file names.

Accessing Directories	Finding out what files a directory contains.

Hard Links	Adding alternate names to a file.

Symbolic Links	A file that "points to" a file name.

Deleting Files	How to delete a file, and what that means.

Renaming Files	Changing a file's name.

Creating Directories	A system call just for creating a directory.

File Attributes	Attributes of individual files.

Making Special Files	How to create special files.

Temporary Files	Naming and creating temporary files.


Working Directory
Node: Working Directory, Next: Accessing Directories, Prev: , Up: File System Interface

Working Directory

Each process has associated with it a directory, called its "current working directory" or simply "working directory", that is used in the resolution of relative file names (see File Name Resolution).

When you log in and begin a new session, your working directory is initially set to the home directory associated with your login account in the system user database.  You can find any user's home directory using the getpwuid or getpwnam functions; see See User Database.

Users can change the working directory using shell commands like cd.  The functions described in this section are the primitives used by those commands and by other programs for examining and changing the working directory.  KPrototypes for these functions are declared in the header file 'unistd.h'.  K

 - Function: char * getcwd (char *buffer, size_t size)

The getcwd function returns an absolute file name representing the current working directory, storing it in the character array buffer that you provide.  The size argument is how you tell the system the allocation size of buffer.

The GNU library version of this function also permits you to specify a null pointer for the buffer argument.  Then getcwd allocates a buffer automatically, as with malloc (see Unconstrained Allocation).  If the size is greater than zero, then the buffer is that large; otherwise, the buffer is as large as necessary to hold the result.

The return value is buffer on success and a null pointer on failure.  The following errno error conditions are defined for this function:

EINVAL

The size argument is zero and buffer is not a null pointer.

ERANGE

The size argument is less than the length of the working directory name.  You need to allocate a bigger array and try again.

EACCES

Permission to read or search a component of the file name was denied.

Here is an example showing how you could implement the behavior of GNU's getcwd (NULL, 0) using only the standard behavior of getcwd:

char *
gnu_getcwd ()
{
  int size = 100;
  char *buffer = (char *) xmalloc (size);

  while (1)
    {
      char *value = getcwd (buffer, size);
      if (value != 0)
        return buffer;
      size *= 2;
      free (buffer);
      buffer = (char *) xmalloc (size);
    }
}

See Malloc Examples, for information about xmalloc, which is not a library function but is a customary name used in most GNU software.

 - Function: char * getwd (char *buffer)

This is similar to getcwd.  The GNU library provides getwd for backwards compatibility with BSD.  The buffer should be a pointer to an array at least PATH_MAX bytes long.

 - Function: int chdir (const char *filename)

This function is used to set the process's working directory to filename.

The normal, successful return value from chdir is 0.  A value of -1 is returned to indicate an error.  The errno error conditions defined for this function are the usual file name syntax errors (see File Name Errors), plus ENOTDIR if the file filename is not a directory.


Accessing Directories
Node: Accessing Directories, Next: Hard Links, Prev: Working Directory, Up: File System Interface

Accessing Directories

The facilities described in this section let you read the contents of a directory file.  This is useful if you want your program to list all the files in a directory, perhaps as part of a menu.

The opendir function opens a "directory stream" whose elements are directory entries.  You use the readdir function on the directory stream to retrieve these entries, represented as struct dirent objects.  The name of the file for each entry is stored in the d_name member of this structure.  There are obvious parallels here to the stream facilities for ordinary files, described in See I/O on Streams.

* Menu:

Directory Entries	Format of one directory entry.

Opening a Directory	How to open a directory stream.

Reading/Closing Directory	How to read directory entries from the stream.

Simple Directory Lister	A very simple directory listing program.

Random Access Directory	Rereading part of the directory already read with the same stream.


Format of a Directory Entry
Node: Directory Entries, Next: Opening a Directory, Prev: , Up: Accessing Directories

Format of a Directory Entry

This section describes what you find in a single directory entry, as you might obtain it from a directory stream.  All the symbols are declared in the header file 'dirent.h'.

 - Data Type: struct dirent

This is a structure type used to return information about directory entries.  It contains the following fields:

char *d_name

This is the null-terminated file name component.  This is the only field you can count on in all POSIX systems.

ino_t d_fileno

This is the file serial number.  For BSD compatibility, you can also refer to this member as d_ino.

size_t d_namlen

This is the length of the file name, not including the terminating null character.

This structure may contain additional members in the future.

When a file has multiple names, each name has its own directory entry.  The only way you can tell that the directory entries belong to a single file is that they have the same value for the d_fileno field.

File attributes such as size, modification times, and the like are part of the file itself, not any particular directory entry.  See File Attributes.


Opening a Directory Stream
Node: Opening a Directory, Next: Reading/Closing Directory, Prev: Directory Entries, Up: Accessing Directories

Opening a Directory Stream

This section describes how to open a directory stream.  All the symbols are declared in the header file 'dirent.h'.

 - Data Type: DIR

The DIR data type represents a directory stream.

You shouldn't ever allocate objects of the struct dirent or DIR data types, since the directory access functions do that for you.  Instead, you refer to these objects using the pointers returned by the following functions.

 - Function: DIR * opendir (const char *dirname)

The opendir function opens and returns a directory stream for reading the directory whose file name is dirname.  The stream has type DIR *.

If unsuccessful, opendir returns a null pointer.  In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for this function:

EACCES

Read permission is denied for the directory named by dirname.

EMFILE

The process has too many files open.

ENFILE

The entire system, or perhaps the file system which contains the directory, cannot support any additional open files at the moment.  (This problem cannot happen on the GNU system.)

The DIR type is typically implemented using a file descriptor, and the opendir function in terms of the open function.  See Low-Level I/O.  Directory streams and the underlying file descriptors are closed on exec (see Executing a File).


Reading and Closing a Directory Stream
Node: Reading/Closing Directory, Next: Simple Directory Lister, Prev: Opening a Directory, Up: Accessing Directories

Reading and Closing a Directory Stream

This section describes how to read directory entries from a directory stream, and how to close the stream when you are done with it.  All the symbols are declared in the header file 'dirent.h'.

 - Function: struct dirent * readdir (DIR *dirstream)

This function reads the next entry from the directory.  It normally returns a pointer to a structure containing information about the file.  This structure is statically allocated and can be rewritten by a subsequent call.

Portability Note: On some systems, readdir may not return entries for '.' and '..', even though these are always valid file names in any directory.  See File Name Resolution.

If there are no more entries in the directory or an error is detected, readdir returns a null pointer.  The following errno error conditions are defined for this function:

EBADF

The dirstream argument is not valid.

 - Function: int closedir (DIR *dirstream)

This function closes the directory stream dirstream.  It returns 0 on success and -1 on failure.

The following errno error conditions are defined for this function:

EBADF

The dirstream argument is not valid.


Simple Program to List a Directory
Node: Simple Directory Lister, Next: Random Access Directory, Prev: Reading/Closing Directory, Up: Accessing Directories

Simple Program to List a Directory

Here's a simple program that prints the names of the files in the current working directory:

#include <stddef.h>
#include <stdio.h>
#include <sys/types.h>
#include <dirent.h>

int
main (void)
{
  DIR *dp;
  struct dirent *ep;

  dp = opendir ("./");
  if (dp != NULL)
    {
      while (ep = readdir (dp))
        puts (ep->d_name);
      (void) closedir (dp);
    }
  else
    puts ("Couldn't open the directory.");

  return 0;
}

The order in which files appear in a directory tends to be fairly random.  A more useful program would sort the entries (perhaps by alphabetizing them) before printing them; see See Array Sort Function.


Random Access in a Directory Stream
Node: Random Access Directory, Next: , Prev: Simple Directory Lister, Up: Accessing Directories

Random Access in a Directory Stream

This section describes how to reread parts of a directory that you have already read from an open directory stream.  All the symbols are declared in the header file 'dirent.h'.

 - Function: void rewinddir (DIR *dirstream)

The rewinddir function is used to reinitialize the directory stream dirstream, so that if you call readdir it returns information about the first entry in the directory again.  This function also notices if files have been added or removed to the directory since it was opened with opendir.  (Entries for these files might or might not be returned by readdir if they were added or removed since you last called opendir or rewinddir.)

 - Function: off_t telldir (DIR *dirstream)

The telldir function returns the file position of the directory stream dirstream.  You can use this value with seekdir to restore the directory stream to that position.

 - Function: void seekdir (DIR *dirstream, off_t pos)

The seekdir function sets the file position of the directory stream dirstream to pos.  The value pos must be the result of a previous call to telldir on this particular stream; closing and reopening the directory can invalidate values returned by telldir.


Hard Links
Node: Hard Links, Next: Symbolic Links, Prev: Accessing Directories, Up: File System Interface

Hard Links

In POSIX systems, one file can have many names at the same time.  All of the names are equally real, and no one of them is preferred to the others.

To add a name to a file, use the link function.  (The new name is also called a "hard link" to the file.)  Creating a new link to a file does not copy the contents of the file; it simply makes a new name by which the file can be known, in addition to the file's existing name or names.

One file can have names in several directories, so the the organization of the file system is not a strict hierarchy or tree.

Since a particular file exists within a single file system, all its names must be in directories in that file system.  link reports an error if you try to make a hard link to the file from another file system.

The prototype for the link function is declared in the header file 'unistd.h'.  K

 - Function: int link (const char *oldname, const char *newname)

The link function makes a new link to the existing file named by oldname, under the new name newname.

This function returns a value of 0 if it is successful and -1 on failure.  In addition to the usual file name syntax errors (see File Name Errors) for both oldname and newname, the following errno error conditions are defined for this function:

EACCES

The directory in which the new link is to be written is not writable.

EEXIST

There is already a file named newname.  If you want to replace this link with a new link, you must remove the old link explicitly first.

EMLINK

There are already too many links to the file named by oldname.  (The maximum number of links to a file is LINK_MAX; see See Limits for Files.)

Well-designed file systems never report this error, because they permit more links than your disk could possibly hold.  However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine.

ENOENT

The file named by oldname doesn't exist.  You can't make a link to a file that doesn't exist.

ENOSPC

The directory or file system that would contain the new link is "full" and cannot be extended.

EPERM

Some implementations only allow privileged users to make links to directories, and others prohibit this operation entirely.  This error is used to report the problem.

EROFS

The directory containing the new link can't be modified because it's on a read-only file system.

EXDEV

The directory specified in newname is on a different file system than the existing file.


Symbolic Links
Node: Symbolic Links, Next: Deleting Files, Prev: Hard Links, Up: File System Interface

Symbolic Links

The GNU system supports "soft links" or "symbolic links".  This is a kind of "file" that is essentially a pointer to another file name.  Unlike hard links, symbolic links can be made to directories or across file systems with no restrictions.  You can also make a symbolic link to a name which is not the name of any file.  (Opening this link will fail until a file by that name is created.)  Likewise, if the symbolic link points to an existing file which is later deleted, the symbolic link continues to point to the same file name even though the name no longer names any file.

The reason symbolic links work the way they do is that special things happen when you try to open the link.  The open function realizes you have specified the name of a link, reads the file name contained in the link, and opens that file name instead.  The stat function likewise operates on the file that the symbolic link points to, instead of on the link itself.  So does link, the function that makes a hard link.

By contrast, other operations such as deleting or renaming the file operate on the link itself.  The functions readlink and lstat also refrain from following symbolic links, because their purpose is to obtain information about the link.

Prototypes for the functions listed in this section are in 'unistd.h'.  K

 - Function: int symlink (const char *oldname, const char *newname)

The symlink function makes a symbolic link to oldname named newname.

The normal return value from symlink is 0.  A return value of -1 indicates an error.  In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for this function:

EEXIST

There is already an existing file named newname.

EROFS

The file newname would exist on a read-only file system.

ENOSPC

The directory or file system cannot be extended to make the new link.

EIO

A hardware error occurred while reading or writing data on the disk.

 - Function: int readlink (const char *filename, char *buffer, size_t size)

The readlink function gets the value of the symbolic link filename.  The file name that the link points to is copied into buffer.  This file name string is not null-terminated; readlink normally returns the number of characters copied.  The size argument specifies the maximum number of characters to copy, usually the allocation size of buffer.

If the return value equals size, you cannot tell whether or not there was room to return the entire name.  So make a bigger buffer and call readlink again.  Here is an example:

char *
readlink_malloc (char *filename)
{
  int size = 100;

  while (1)
    {
      char *buffer = (char *) xmalloc (size);
      int nchars = readlink (filename, buffer, size);
      if (nchars < size)
        return buffer;
      free (buffer);
      size *= 2;
    }
}

A value of -1 is returned in case of error.  In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for this function:

EINVAL

The named file is not a symbolic link.

EIO

A hardware error occurred while reading or writing data on the disk.


Deleting Files
Node: Deleting Files, Next: Renaming Files, Prev: Symbolic Links, Up: File System Interface

Deleting Files

You can delete a file with the functions unlink or remove.  (These names are synonymous.)

Deletion actually deletes a file name.  If this is the file's only name, then the file is deleted as well.  If the file has other names as well (see Hard Links), it remains accessible under its other names.

 - Function: int unlink (const char *filename)

The unlink function deletes the file name filename.  If this is a file's sole name, the file itself is also deleted.  (Actually, if any process has the file open when this happens, deletion is postponed until all processes have closed the file.)

The function unlink is declared in the header file 'unistd.h'.

This function returns 0 on successful completion, and -1 on error.  In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for this function:

EACCESS

Write permission is denied for the directory from which the file is to be removed.

EBUSY

This error indicates that the file is being used by the system in such a way that it can't be unlinked.  Examples of situations where you might see this error are if the file name specifies the root directory or a mount point for a file system.

ENOENT

The file name to be deleted doesn't exist.

EPERM

On some systems, unlink cannot be used to delete the name of a directory, or can only be used this way by a privileged user.  To avoid such problems, use rmdir to delete directories.

EROFS

The directory in which the file name is to be deleted is on a read-only file system, and can't be modified.

 - Function: int remove (const char *filename)

The remove function is another name for unlink.  remove is the ANSI C name, whereas unlink is the POSIX.1 name.  The name remove is declared in 'stdio.h'.  K

- Function: int rmdir (const char *filename)

The rmdir function deletes a directory.  The directory must be empty before it can be removed; in other words, it can only contain entries for '.' and '..'.

In most other respects, rmdir behaves like unlink.  There are two additional errno error conditions defined for rmdir:

EEXIST

ENOTEMPTY

The directory to be deleted is not empty.

These two error codes are synonymous; some systems use one, and some use the other.

The prototype for this function is declared in the header file 'unistd.h'.  K


Renaming Files
Node: Renaming Files, Next: Creating Directories, Prev: Deleting Files, Up: File System Interface

Renaming Files

The rename function is used to change a file's name.

- Function: int rename (const char *oldname, const char *newname)

The rename function renames the file name oldname with newname.  The file formerly accessible under the name oldname is afterward accessible as newname instead.  (If the file had any other names aside from oldname, it continues to have those names.)

The directory containing the name newname must be on the same file system as the file (as indicated by the name oldname).

One special case for rename is when oldname and newname are two names for the same file.  The consistent way to handle this case is to delete oldname.  However, POSIX says that in this case rename does nothing and reports success--which is inconsistent.  We don't know what your operating system will do.  The GNU system, when completed, will probably do the right thing (delete oldname) unless you explicitly request strict POSIX compatibility "even when it hurts".

If the oldname is not a directory, then any existing file named newname is removed during the renaming operation.  However, if newname is the name of a directory, rename fails in this case.

If the oldname is a directory, then either newname must not exist or it must name a directory that is empty.  In the latter case, the existing directory named newname is deleted first.  The name newname must not specify a subdirectory of the directory oldname which is being renamed.

One useful feature of rename is that the meaning of the name newname changes "atomically" from any previously existing file by that name to its new meaning (the file that was called oldname).  There is no instant at which newname is nonexistent "in between" the old meaning and the new meaning.

If rename fails, it returns -1.  In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for this function:

EACCES

One of the directories containing newname or oldname refuses write permission; or newname and oldname are directories and write permission is refused for one of them.

EBUSY

A directory named by oldname or newname is being used by the system in a way that prevents the renaming from working.  This includes directories that are mount points for filesystems, and directories that are the current working directories of processes.

EEXIST

The directory newname isn't empty.

ENOTEMPTY

The directory newname isn't empty.

EINVAL

The oldname is a directory that contains newname.

EISDIR

The newname names a directory, but the oldname doesn't.

EMLINK

The parent directory of newname would have too many links.

Well-designed file systems never report this error, because they permit more links than your disk could possibly hold.  However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine.

ENOENT

The file named by oldname doesn't exist.

ENOSPC

The directory that would contain newname has no room for another entry, and there is no space left in the file system to expand it.

EROFS

The operation would involve writing to a directory on a read-only file system.

EXDEV

The two file names newname and oldnames are on different file systems.


Creating Directories
Node: Creating Directories, Next: File Attributes, Prev: Renaming Files, Up: File System Interface

Creating Directories

Directories are created with the mkdir function.  (There is also a shell command mkdir which does the same thing.)

 - Function: int mkdir (const char *filename, mode_t mode)

The mkdir function creates a new, empty directory whose name is filename.

The argument mode specifies the file permissions for the new directory file.  See Permission Bits, for more information about this.

A return value of 0 indicates successful completion, and -1 indicates failure.  In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for this function:

EACCES

Write permission is denied for the parent directory in which the new directory is to be added.

EEXIST

A file named filename already exists.

EMLINK

The parent directory has too many links.

Well-designed file systems never report this error, because they permit more links than your disk could possibly hold.  However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine.

ENOSPC

The file system doesn't have enough room to create the new directory.

EROFS

The parent directory of the directory being created is on a read-only file system, and cannot be modified.

To use this function, your program should include the header file 'sys/stat.h'.  K


File Attributes
Node: File Attributes, Next: Making Special Files, Prev: Creating Directories, Up: File System Interface

File Attributes

When you issue an 'ls -l' shell command on a file, it gives you information about the size of the file, who owns it, when it was last modified, and the like.  This kind of information is called the "file attributes"; it is associated with the file itself and not a particular one of its names.

This section contains information about how you can inquire about and modify these attributes of files.

* Menu:

Attribute Meanings	The names of the file attributes, and what their values mean.

Reading Attributes	How to read the attributes of a file.

Testing File Type	Distinguishing ordinary files, directories, links...

File Owner	How ownership for new files is determined, and how to change it.

Permission Bits	How information about a file's access mode is stored.

Access Permission	How the system decides who can access a file.

Setting Permissions	How permissions for new files are assigned, and how to change them.

Testing File Access	How to find out if your process can access a file.

File Times	About the time attributes of a file.


What the File Attribute Values Mean
Node: Attribute Meanings, Next: Reading Attributes, Prev: , Up: File Attributes

What the File Attribute Values Mean

When you read the attributes of a file, they come back in a structure called struct stat.  This section describes the names of the attributes, their data types, and what they mean.  For the functions to read the attributes of a file, see See Reading Attributes.

The header file 'sys/stat.h' declares all the symbols defined in this section.  K

 - Data Type: struct stat

The stat structure type is used to return information about the attributes of a file.  It contains at least the following members:

mode_t st_mode

Specifies the mode of the file.  This includes file type information (see Testing File Type) and the file permission bits (see Permission Bits).

ino_t st_ino

The file serial number, which distinguishes this file from all other files on the same device.

dev_t st_dev

Identifies the device containing the file.  The st_ino and st_dev, taken together, uniquely identify the file.

nlink_t st_nlink

The number of hard links to the file.  This count keeps track of how many directories have entries for this file.  If the count is ever decremented to zero, then the file itself is discarded.  Symbolic links are not counted in the total.

uid_t st_uid

The user ID of the file's owner.  See File Owner.

gid_t st_gid

The group ID of the file.  See File Owner.

off_t st_size

This specifies the size of a regular file in bytes.  For files that are really devices and the like, this field isn't usually meaningful.

time_t st_atime

This is the last access time for the file.  See File Times.

unsigned long int st_atime_usec

This is the fractional part of the last access time for the file.  See File Times.

time_t st_mtime

This is the time of the last modification to the contents of the file.  See File Times.

unsigned long int st_mtime_usec

This is the fractional part of the time of last modification to the contents of the file.  See File Times.

time_t st_ctime

This is the time of the last modification to the attributes of the file.  See File Times.

unsigned long int st_ctime_usec

This is the fractional part of the time of last modification to the attributes of the file.  See File Times.

unsigned int st_nblocks

This is the amount of disk space that the file occupies, measured in units of 512-byte blocks.

The number of disk blocks is not strictly proportional to the size of the file, for two reasons: the file system may use some blocks for internal record keeping; and the file may be sparse--it may have "holes" which contain zeros but do not actually take up space on the disk.

You can tell (approximately) whether a file is sparse by comparing this value with st_size, like this:

(st.st_blocks * 512 < st.st_size)

This test is not perfect because a file that is just slightly sparse might not be detected as sparse at all.  For practical applications, this is not a problem.

unsigned int st_blksize

The optimal block size for reading of writing this file.  You might use this size for allocating the buffer space for reading of writing the file.

Some of the file attributes have special data type names which exist specifically for those attributes.  (They are all aliases for well-known integer types that you know and love.)  These typedef names are defined in the header file 'sys/types.h' as well as in 'sys/stat.h'.  Here is a list of them.

 - Data Type: mode_t

This is an integer data type used to represent file modes.  In the GNU system, this is equivalent to unsigned int.

- Data Type: ino_t

This is an arithmetic data type used to represent file serial numbers.  (In Unix jargon, these are sometimes called "inode numbers".) In the GNU system, this type is equivalent to unsigned long int.

 - Data Type: dev_t

This is an arithmetic data type used to represent file device numbers.  In the GNU system, this is equivalent to int.

 - Data Type: nlink_t

This is an arithmetic data type used to represent file link counts.  In the GNU system, this is equivalent to unsigned short int.


Reading the Attributes of a File
Node: Reading Attributes, Next: Testing File Type, Prev: Attribute Meanings, Up: File Attributes

Reading the Attributes of a File

To examine the attributes of files, use the functions stat, fstat and lstat.  They return the attribute information in a struct stat object.  All three functions are declared in the header file 'sys/stat.h'.

 - Function: int stat (const char *filename, struct stat *buf)

The stat function returns information about the attributes of the file named by filename in the structure pointed at by buf.

If filename is the name of a symbolic link, the attributes you get describe the file that the link points to.  If the link points to a nonexistent file name, then stat fails, reporting a nonexistent file.

The return value is 0 if the operation is successful, and -1 on failure.  In addition to the usual file name syntax errors (see File Name Errors, the following errno error conditions are defined for this function:

ENOENT

The file named by filename doesn't exist.

 - Function: int fstat (int filedes, struct stat *buf)

The fstat function is like stat, except that it takes an open file descriptor as an argument instead of a file name.  See Low-Level I/O.

Like stat, fstat returns 0 on success and -1 on failure.  The following errno error conditions are defined for fstat:

EBADF

The filedes argument is not a valid file descriptor.

 - Function: int lstat (const char *filename, struct stat *buf)

The lstat function is like stat, except that it does not follow symbolic links.  If filename is the name of a symbolic link, lstat returns information about the link itself; otherwise, lstat works like stat.  See Symbolic Links.


Testing the Type of a File
Node: Testing File Type, Next: File Owner, Prev: Reading Attributes, Up: File Attributes

Testing the Type of a File

The "file mode", stored in the st_mode field of the file attributes, contains two kinds of information: the file type code, and the access permission bits.  This section discusses only the type code, which you can use to tell whether the file is a directory, whether it is a socket, and so on.  For information about the access permission, See Permission Bits.

There are two predefined ways you can access the file type portion of the file mode.  First of all, for each type of file, there is a "predicate macro" which examines a file mode value and returns true or false--is the file of that type, or not.  Secondly, you can mask out the rest of the file mode to get just a file type code.  You can compare this against various constants for the supported file types.

All of the symbols listed in this section are defined in the header file 'sys/stat.h'.  KThe following predicate macros test the type of a file, given the value m which is the st_mode field returned by stat on that file:

 - Macro: int S_ISDIR (mode_t m)

This macro returns nonzero if the file is a directory.

 - Macro: int S_ISCHR (mode_t m)

This macro returns nonzero if the file is a character special file (a device like a terminal).

 - Macro: int S_ISBLK (mode_t m)

This macro returns nonzero if the file is a block special file (a device like a disk).

 - Macro: int S_ISREG (mode_t m)

This macro returns nonzero if the file is a regular file.

 - Macro: int S_ISFIFO (mode_t m)

This macro returns nonzero if the file is a FIFO special file, or a pipe.  See Pipes and FIFOs.

 - Macro: int S_ISLNK (mode_t m)

This macro returns nonzero if the file is a symbolic link.  See Symbolic Links.

 - Macro: int S_ISSOCK (mode_t m)

This macro returns nonzero if the file is a socket.  See Sockets.

An alterate non-POSIX method of testing the file type is supported for compatibility with BSD.  The mode can be bitwise ANDed with S_IFMT to extract the file type code, and compared to the appropriate type code constant.  For example,

S_ISCHR (mode)

is equivalent to:

((mode & S_IFMT) == S_IFCHR)

 - Macro: int S_IFMT

This is a bit mask used to extract the file type code portion of a mode value.

These are the symbolic names for the different file type codes:

S_IFDIR

This macro represents the value of the file type code for a directory file.

S_IFCHR

This macro represents the value of the file type code for a character-oriented device file.

S_IFBLK

This macro represents the value of the file type code for a block-oriented device file.

S_IFREG

This macro represents the value of the file type code for a regular file.

S_IFLNK

This macro represents the value of the file type code for a symbolic link.

S_IFSOCK

This macro represents the value of the file type code for a socket.

S_IFIFO

This macro represents the value of the file type code for a FIFO or pipe.


File Owner
Node: File Owner, Next: Permission Bits, Prev: Testing File Type, Up: File Attributes

File Owner

Every file has an "owner" which is one of the registered user names defined on the system.  Each file also has a "group", which is one of the defined groups.  The file owner can often be useful for showing you who edited the file (especially when you edit with GNU Emacs), but its main purpose is for access control.

The file owner and group play a role in determining access because the file has one set of access permission bits for the user that is the owner, another set that apply to users who belong to the file's group, and a third set of bits that apply to everyone else.  See Access Permission, for the details of how access is decided based on this data.

When a file is created, its owner is set from the effective user ID of the process that creates it (see Process Persona).  The file's group ID may be set from either effective group ID of the process, or the group ID of the directory that contains the file, depending on the system where the file is stored.  When you access a remote file system, it behaves according to its own rule, not according to the system your program is running on.  Thus, your program must be prepared to encounter either kind of behavior, no matter what kind of system you run it on.

You can change the owner and/or group owner of an existing file using the chown function.  This is the primitive for the chown and chgrp shell commands.

The prototype for this function is declared in 'unistd.h'.

 - Function: int chown (const char *filename, uid_t owner, gid_t group)

The chown function changes the owner of the file filename to owner, and its group owner to group.

Changing the owner of the file on certain systems clears the set-user-ID and set-group-ID bits of the file's permissions.  (This is because those bits may not be appropriate for the new owner.)  The other file permission bits are not changed.

The return value is 0 on success and -1 on failure.  In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for this function:

EPERM

This process lacks permission to make the requested change.

Only privileged users or the file's owner can change the file's group.  On most file systems, only privileged users can change the file owner; some file systems allow you to change the owner if you are currently the owner.  When you access a remote file system, the behavior you encounter is determined by the system that actually holds the file, not by the system your program is running on.

See Options for Files, for information about the _POSIX_CHOWN_RESTRICTED macro.

EROFS

The file is on a read-only file system.

 - Function: int fchown (int filedes, int owner, int group)

This is like chown, except that it changes the owner of the file with open file descriptor filedes.

The return value from fchown is 0 on success and -1 on failure.  The following errno error codes are defined for this function:

EBADF

The filedes argument is not a valid file descriptor.

EINVAL

The filedes argument corresponds to a pipe or socket, not an ordinary file.

EPERM

This process lacks permission to make the requested change.  For details, see chmod, above.

EROFS

The file resides on a read-only file system.


The Mode Bits for Access Permission
Node: Permission Bits, Next: Access Permission, Prev: File Owner, Up: File Attributes

The Mode Bits for Access Permission

The "file mode", stored in the st_mode field of the file attributes, contains two kinds of information: the file type code, and the access permission bits.  This section discusses only the access permission bits, which control who can read or write the file.  See Testing File Type, for information about the file type code.

All of the symbols listed in this section are defined in the header file 'sys/stat.h'.  KThese symbolic constants are defined for the file mode bits that control access permission for the file:

S_IRUSR

S_IREAD

Read permission bit for the owner of the file.  On many systems, this bit is 0400.  S_IREAD is an obsolete synonym provided for BSD compatibility.

S_IWUSR

S_IWRITE

Write permission bit for the owner of the file.  Usually 0200.  S_IWRITE is an obsolete synonym provided for BSD compatibility.

S_IXUSR

S_IEXEC

Execute (for ordinary files) or search (for directories) permission bit for the owner of the file.  Usually 0100.  S_IEXEC is an obsolete synonym provided for BSD compatibility.

S_IRWXU

This is equivalent to '(S_IRUSR | S_IWUSR | S_IXUSR)'.

S_IRGRP

Read permission bit for the group owner of the file.  Usually 040.

S_IWGRP

Write permission bit for the group owner of the file.  Usually 020.

S_IXGRP

Execute or search permission bit for the group owner of the file.  Usually 010.

S_IRWXG

This is equivalent to '(S_IRGRP | S_IWGRP | S_IXGRP)'.

S_IROTH

Read permission bit for other users.  Usually 04.

S_IWOTH

Write permission bit for other users.  Usually 02.

S_IXOTH

Execute or search permission bit for other users.  Usually 01.

S_IRWXO

This is equivalent to '(S_IROTH | S_IWOTH | S_IXOTH)'.

S_ISUID

This is the set-user-ID on execute bit, usually 04000.  See How Change Persona.

S_ISGID

This is the set-group-ID on execute bit, usually 02000.  See How Change Persona.

S_ISVTX

This is the "sticky" bit, usually 01000.

On an executable file, it modifies the swapping policies of the system.  Normally, when a program terminates, its pages in core are immediately freed and reused.  If the sticky bit is set on the executable file, the system keeps the pages in core for a while as if the program were still running.  This is advantageous for a program that is likely to be run many times in succession.

On a directory, the sticky bit gives permission to delete a file in the directory if you can write the contents of that file.  Ordinarily, a user either can delete all the files in the directory or cannot delete any of them (based on whether the user has write permission for the directory).  The sticky bit makes it possible to control deletion for individual files.

The actual bit values of the symbols are listed in the table above so you can decode file mode values when debugging your programs.  These bit values are correct for most systems, but they are not guaranteed.

Warning: Writing explicit numbers for file permissions is bad practice.  It is not only nonportable, it also requires everyone who reads your program to remember what the bits mean.  To make your program clean, use the symbolic names.


How Your Access to a File is Decided
Node: Access Permission, Next: Setting Permissions, Prev: Permission Bits, Up: File Attributes

How Your Access to a File is Decided

Recall that the operating system normally decides access permission for a file based on the effective user and group IDs of the process, and its supplementary group IDs, together with the file's owner, group and permission bits.  These concepts are discussed in detail in See Process Persona.

If the effective user ID of the process matches the owner user ID of the file, then permissions for read, write, and execute/search are controlled by the corresponding "user" (or "owner") bits.  Likewise, if any of the effective group ID or supplementary group IDs of the process matches the group owner ID of the file, then permissions are controlled by the "group" bits.  Otherwise, permissions are controlled by the "other" bits.

Privileged users, like 'root', can access any file, regardless of its file permission bits.  As a special case, for a file to be executable even for a privileged user, at least one of its execute bits must be set.


Assigning File Permissions
Node: Setting Permissions, Next: Testing File Access, Prev: Access Permission, Up: File Attributes

Assigning File Permissions

The primitive functions for creating files (for example, open or mkdir) take a mode argument, which specifies the file permissions for the newly created file.  But the specified mode is modified by the process's "file creation mask", or "umask", before it is used.

The bits that are set in the file creation mask identify permissions that are always to be disabled for newly created files.  For example, if you set all the "other" access bits in the mask, then newly created files are not accessible at all to processes in the "other" category, even if the mode argument specified to the creation function would permit such access.  In other words, the file creation mask is the complement of the ordinary access permissions you want to grant.

Programs that create files typically specify a mode argument that includes all the permissions that make sense for the particular file.  For an ordinary file, this is typically read and write permission for all classes of users.  These permissions are then restricted as specified by the individual user's own file creation mask.

To change the permission of an existing file given its name, call chmod.  This function ignores the file creation mask; it uses exactly the specified permission bits.

In normal use, the file creation mask is initialized in the user's login shell (using the umask shell command), and inherited by all subprocesses.  Application programs normally don't need to worry about the file creation mask.  It will do automatically what it is supposed to do.

When your program should create a file and bypass the umask for its access permissions, the easiest way to do this is to use fchmod after opening the file, rather than changing the umask.

In fact, changing the umask is usually done only by shells.  They use the umask function.

The functions in this section are declared in 'sys/stat.h'.  K

 - Function: mode_t umask (mode_t mask)

The umask function sets the file creation mask of the current process to mask, and returns the previous value of the file creation mask.

Here is an example showing how to read the mask with umask without changing it permanently:

mode_t
read_umask (void)
{
  mask = umask (0);
  umask (mask);
}

However, it is better to use getumask if you just want to read the mask value, because that is reentrant (at least if you use the GNU operating system).

 - Function: mode_t getumask (void)

Return the current value of the file creation mask for the current process.  This function is a GNU extension.

 - Function: int chmod (const char *filename, mode_t mode)

The chmod function sets the access permission bits for the file named by filename to mode.

If the filename names a symbolic link, chmod changes the permission of the file pointed to by the link, not those of the link itself.  There is actually no way to set the mode of a link, which is always -1.

This function returns 0 if successful and -1 if not.  In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for this function:

ENOENT

The named file doesn't exist.

EPERM

This process does not have permission to change the access permission of this file.  Only the file's owner (as judged by the effective user ID of the process) or a privileged user can change them.

EROFS

The file resides on a read-only file system.

 - Function: int fchmod (int filedes, int mode)

This is like chmod, except that it changes the permissions of the file currently open via descriptor filedes.

The return value from fchmod is 0 on success and -1 on failure.  The following errno error codes are defined for this function:

EBADF

The filedes argument is not a valid file descriptor.

EINVAL

The filedes argument corresponds to a pipe or socket, or something else that doesn't really have access permissions.

EPERM

This process does not have permission to change the access permission of this file.  Only the file's owner (as judged by the effective user ID of the process) or a privileged user can change them.

EROFS

The file resides on a read-only file system.


Testing Permission to Access a File
Node: Testing File Access, Next: File Times, Prev: Setting Permissions, Up: File Attributes

Testing Permission to Access a File

When a program runs as a privileged user, this permits it to access files off-limits to ordinary users--for example, to modify '/etc/passwd'.  Programs designed to be run by ordinary users but access such files use the setuid bit feature so that they always run with root as the effective user ID.

Such a program may also access files specified by the user, files which conceptually are being accessed explicitly by the user.  Since the program runs as root, it has permission to access whatever file the user specifies--but usually the desired behavior is to permit only those files which the user could ordinarily access.

The program therefore must explicitly check whether the user would have the necessary access to a file, before it reads or writes the file.

To do this, use the function access, which checks for access permission based on the process's real user ID rather than the effective user ID.  (The setuid feature does not alter the real user ID, so it reflects the user who actually ran the program.)

There is another way you could check this access, which is easy to describe, but very hard to use.  This is to examine the file mode bits and mimic the system's own access computation.  This method is undesirable because many systems have additional access control features; your program cannot portably mimic them, and you would not want to try to keep track of the diverse features that different systems have.  Using access is simple and automatically does whatever is appropriate for the system you are using.

The symbols in this section are declared in 'unistd.h'.

 - Function: int access (const char *filename, int how)

The access function checks to see whether the file named by filename can be accessed in the way specified by the how argument.  The how argument either can be the bitwise OR of the flags R_OK, W_OK, X_OK, or the existence test F_OK.

This function uses the real user and group ID's of the calling process, rather than the effective ID's, to check for access permission.  As a result, if you use the function from a setuid or setgid program (see How Change Persona), it gives information relative to the user who actually ran the program.

The return value is 0 if the access is permitted, and -1 otherwise.  (In other words, treated as a predicate function, access returns true if the requested access is denied.)

In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for this function:

EACCES

The access specified by how is denied.

ENOENT

The file doesn't exist.

EROFS

Write permission was requested for a file on a read-only file system.

These macros are defined in the header file 'unistd.h' for use as the how argument to the access function.  The values are integer constants.  K

 - Macro: int R_OK

Argument that means, test for read permission.

 - Macro: int W_OK

Argument that means, test for write permission.

 - Macro: int X_OK

Argument that means, test for execute/search permission.

 - Macro: int F_OK

Argument that means, test for existence of the file.


File Times
Node: File Times, Next: , Prev: Testing File Access, Up: File Attributes

File Times

Each file has three timestamps associated with it:  its access time, its modification time, and its attribute modification time.  These correspond to the st_atime, st_mtime, and st_ctime members of the stat structure; see See File Attributes.

All of these times are represented in calendar time format, as time_t objects.  This data type is defined in 'time.h'.  For more information about representation and manipulation of time values, see See Calendar Time.  KWhen an existing file is opened, its attribute change time and modification time fields are updated.  Reading from a file updates its access time attribute, and writing updates its modification time.

When a file is created, all three timestamps for that file are set to the current time.  In addition, the attribute change time and modification time fields of the directory that contains the new entry are updated.

Adding a new name for a file with the link function updates the attribute change time field of the file being linked, and both the attribute change time and modification time fields of the directory containing the new name.  These same fields are affected if a file name is deleted with unlink, remove, or rmdir.  Renaming a file with rename affects only the attribute change time and modification time fields of the two parent directories involved, and not the times for the file being renamed.

Changing attributes of a file (for example, with chmod) updates its attribute change time field.

You can also change some of the timestamps of a file explicitly using the utime function--all except the attribute change time.  You need to include the header file 'utime.h' to use this facility.  K

 - Data Type: struct utimbuf

The utimbuf structure is used with the utime function to specify new access and modification times for a file.  It contains the following members:

time_t actime

This is the access time for the file.

time_t modtime

This is the modification time for the file.

 - Function: int utime (const char *filename, const struct utimbuf *times)

This function is used to modify the file times associated with the file named filename.

If times is a null pointer, then the access and modification times of the file are set to the current time.  Otherwise, they are set to the values from the actime and modtime members (respectively) of the utimbuf structure pointed at by times.

The attribute modification time for the file is set to the current time in either case (since changing the timestamps is itself a modification of the file attributes).

The utime function returns 0 if successful and -1 on failure.  In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for this function:

EACCES

There is a permission problem in the case where a null pointer was passed as the times argument.  In order to update the timestamp on the file, you must either be the owner of the file, have write permission on the file, or be a privileged user.

ENOENT

The file doesn't exist.

EPERM

If the times argument is not a null pointer, you must either be the owner of the file or be a privileged user.  This error is used to report the problem.

EROFS

The file lives on a read-only file system.

Each of the three time stamps has a corresponding microsecond part, which extends its resolution.  These fields are called st_atime_usec, st_mtime_usec, and st_ctime_usec; each has a value between 0 and 999,999, which indicates the time in microseconds.  They correspond to the tv_usec field of a timeval structure; see See High-Resolution Calendar.

The utimes function is like utime, but also lets you specify the fractional part of the file times.  The prototype for this function is in the header file 'sys/time.h'.  K

 - Function: int utimes (const char *filename, struct timeval tvp[2])

This function sets the file access and modification times for the file named by filename.  The new file access time is specified by tvp[0], and the new modification time by tvp[1].  This function comes from BSD.

The return values and error conditions are the same as for the utime function.


Making Special Files
Node: Making Special Files, Next: Temporary Files, Prev: File Attributes, Up: File System Interface

Making Special Files

The mknod function is the primitive for making special files, such as files that correspond to devices.  The GNU library includes this function for compatibility with BSD.

The prototype for mknod is declared in 'sys/stat.h'.  K

 - Function: int mknod (const char *filename, int mode, int dev)

The mknod function makes a special file with name filename.  The mode specifies the mode of the file, and may include the various special file bits, such as S_IFCHR (for a character special file) or S_IFBLK (for a block special file).  See Testing File Type.

The dev argument specifies which device the special file refers to.  Its exact interpretation depends on the kind of special file being created.

The return value is 0 on success and -1 on error.  In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for this function:

EPERM

The calling process is not privileged.  Only the superuser can create special files.

ENOSPC

The directory or file system that would contain the new file is "full" and cannot be extended.

EROFS

The directory containing the new file can't be modified because it's on a read-only file system.

EEXIST

There is already a file named filename.  If you want to replace this file, you must remove the old file explicitly first.


Temporary Files
Node: Temporary Files, Next: , Prev: Making Special Files, Up: File System Interface

Temporary Files

If you need to use a temporary file in your program, you can use the tmpfile function to open it.  Or you can use the tmpnam function make a name for a temporary file and then open it in the usual way with fopen.

The tempnam function is like tmpnam but lets you choose what directory temporary files will go in, and something about what their file names will look like.

These facilities are declared in the header file 'stdio.h'.  K

 - Function: FILE * tmpfile (void)

This function creates a temporary binary file for update mode, as if by calling fopen with mode "wb+".  The file is deleted automatically when it is closed or when the program terminates.  (On some other ANSI C systems the file may fail to be deleted if the program terminates abnormally).

 - Function: char * tmpnam (char *result)

This function constructs and returns a file name that is a valid file name and that does not name any existing file.  If the result argument is a null pointer, the return value is a pointer to an internal static string, which might be modified by subsequent calls.  Otherwise, the result argument should be a pointer to an array of at least L_tmpnam characters, and the result is written into that array.

It is possible for tmpnam to fail if you call it too many times.  This is because the fixed length of a temporary file name gives room for only a finite number of different names.  If tmpnam fails, it returns a null pointer.

 - Macro: int L_tmpnam

The value of this macro is an integer constant expression that represents the minimum allocation size of a string large enough to hold the file name generated by the tmpnam function.

 - Macro: int TMP_MAX

The macro TMP_MAX is a lower bound for how many temporary names you can create with tmpnam.  You can rely on being able to call tmpnam at least this many times before it might fail saying you have made too many temporary file names.

With the GNU library, you can create a very large number of temporary file names--if you actually create the files, you will probably run out of disk space before you run out of names.  Some other systems have a fixed, small limit on the number of temporary files.  The limit is never less than 25.

 - Function: char * tempnam (const char *dir, const char *prefix)

This function generates a unique temporary filename.  If prefix is not a null pointer, up to five characters of this string are used as a prefix for the file name.

The directory prefix for the temporary file name is determined by testing each of the following, in sequence.  The directory must exist and be writable.

•	The environment variable TMPDIR, if it is defined.

•	The dir argument, if it is not a null pointer.

•	The value of the P_tmpdir macro.

•	The directory '/tmp'.

This function is defined for SVID compatibility.

 - SVID Macro: char * P_tmpdir

This macro is the name of the default directory for temporary files.

Older Unix systems did not have the functions just described.  Instead they used mktemp and mkstemp.  Both of these functions work by modifying a file name template string you pass.  The last six characters of this string must be 'XXXXXX'.  These six 'X's are replaced with six characters which make the whole string a unique file name.  Usually the template string is something like '/tmp/prefixXXXXXX', and each program uses a unique prefix.

Note: Because mktemp and mkstemp modify the template string, you must not pass string constants to them.  String constants are normally in read-only storage, so your program would crash when mktemp or mkstemp tried to modify the string.

 - Function: char * mktemp (char *template)

The mktemp function generates a unique file name by modifying template as described above.  If successful, it returns template as modified.  If mktemp cannot find a unique file name, it makes template an empty string and returns that.  If template does not end with 'XXXXXX', mktemp returns a null pointer.

 - Function: int mkstemp (char *template)

The mkstemp function generates a unique file name just as mktemp does, but it also opens the file for you with open (see Opening and Closing Files).  If successful, it modifies template in place and returns a file descriptor open on that file for reading and writing.  If mkstemp cannot create a uniquely-named file, it makes template an empty string and returns -1.  If template does not end with 'XXXXXX', mkstemp returns -1 and does not modify template.

Unlike mktemp, mkstemp is actually guaranteed to create a unique file that cannot possibly clash with any other program trying to create a temporary file.  This is because it works by calling open with the O_EXCL flag bit, which says you want to always create a new file, and get an error if the file already exists.


Pipes and FIFOs
Node: Pipes and FIFOs, Next: Sockets, Prev: File System Interface, Up: Top

Pipes and FIFOs

A "pipe" is a mechanism for interprocess communication; data written to the pipe by one process can be read by another process.  The data is handled in a first-in, first-out (FIFO) order.  The pipe has no name; it is created for one use and both ends must be inherited from the single process which created the pipe.

A "FIFO special file" is similar to a pipe, but instead of being an anonymous, temporary connection, a FIFO has a name or names like any other file.  Processes open the FIFO by name in order to communicate through it.

A pipe or FIFO has to be open at both ends simultaneously.  If you read from a pipe or FIFO file that doesn't have any processes writing to it (perhaps because they have all closed the file, or exited), the read returns end-of-file.  Writing to a pipe or FIFO that doesn't have a reading process is treated as an error condition; it generates a SIGPIPE signal, and fails with error code EPIPE if the signal is handled or blocked.

Neither pipes nor FIFO special files allow file positioning.  Both reading and writing operations happen sequentially; reading from the beginning of the file and writing at the end.

* Menu:

Creating a Pipe	Making a pipe with the pipe function.

Pipe to a Subprocess	Using a pipe to communicate with a child process.

FIFO Special Files	Making a FIFO special file.

Pipe Atomicity	When pipe (or FIFO) I/O is atomic.


Creating a Pipe
Node: Creating a Pipe, Next: Pipe to a Subprocess, Prev: , Up: Pipes and FIFOs

Creating a Pipe

The primitive for creating a pipe is the pipe function.  This creates both the reading and writing ends of the pipe.  It is not very useful for a single process to use a pipe to talk to itself.  In typical use, a process creates a pipe just before it forks one or more child processes (see Creating a Process).  The pipe is then used for communication either between the parent or child processes, or between two sibling processes.

The pipe function is declared in the header file 'unistd.h'.  K

 - Function: int pipe (int filedes[2])

The pipe function creates a pipe and puts the file descriptors for the reading and writing ends of the pipe (respectively) into filedes[0] and filedes[1].

An easy way to remember that the input end comes first is that file descriptor 0 is standard input, and file descriptor 1 is standard output.

If successful, pipe returns a value of 0.  On failure, -1 is returned.  The following errno error conditions are defined for this function:

EMFILE

The process has too many files open.

ENFILE

There are too many open files in the entire system.  See Error Codes, for more information about ENFILE.

Here is an example of a simple program that creates a pipe.  This program uses the fork function (see Creating a Process) to create a child process.  The parent process writes data to the pipe, which is read by the child process.

#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

/* Read characters from the pipe and echo them to stdout. */

void
read_from_pipe (int file)
{
  FILE *stream;
  int c;
  stream = fdopen (file, "r");
  while ((c = fgetc (stream)) != EOF)
    putchar (c);
  fclose (stream);
}

/* Write some random text to the pipe. */

void
write_to_pipe (int file)
{
  FILE *stream;
  stream = fdopen (file, "w");
  fprintf (stream, "hello, world!\n");
  fprintf (stream, "goodbye, world!\n");
  fclose (stream);
}

int
main (void)
{
  pid_t pid;
  int mypipe[2];

/* Create the pipe. */
  if (pipe (mypipe))
    {
      fprintf (stderr, "Pipe failed.\n");
      return EXIT_FAILURE;
    }

  /* Create the child process. */
  pid = fork ();
  if (pid == (pid_t) 0)
    {
      /* This is the child process. */
      read_from_pipe (mypipe[0]);
      return EXIT_SUCCESS;
    }
  else if (pid < (pid_t) 0)
    {
      /* The fork failed. */
      fprintf (stderr, "Fork failed.\n");
      return EXIT_FAILURE;
    }
  else
    {
      /* This is the parent process. */
      write_to_pipe (mypipe[1]);
      return EXIT_SUCCESS;
    }
}


Pipe to a Subprocess
Node: Pipe to a Subprocess, Next: FIFO Special Files, Prev: Creating a Pipe, Up: Pipes and FIFOs

Pipe to a Subprocess

A common use of pipes is to send data to or receive data from a program being run as subprocess.  One way of doing this is by using a combination of pipe (to create the pipe), fork (to create the subprocess), dup2 (to force the subprocess to use the pipe as its standard input or output channel), and exec (to execute the new program).  Or, you can use popen and pclose.

The advantage of using popen and pclose is that the interface is much simpler and easier to use.  But it doesn't offer as much flexibility as using the low-level functions directly.

 - Function: FILE * popen (const char *command, const char *mode)

The popen function is closely related to the system function; see See Running a Command.  It executes the shell command command as a subprocess.  However, instead of waiting for the command to complete, it creates a pipe to the subprocess and returns a stream that corresponds to that pipe.

If you specify a mode argument of "r", you can read from the stream to retrieve data from the standard output channel of the subprocess.  The subprocess inherits its standard input channel from the parent process.

Similarly, if you specify a mode argument of "w", you can write to the stream to send data to the standard input channel of the subprocess.  The subprocess inherits its standard output channel from the parent process.

In the event of an error, popen returns a null pointer.  This might happen if the pipe or stream cannot be created, if the subprocess cannot be forked, or if the program cannot be executed.

 - Function: int pclose (FILE *stream)

The pclose function is used to close a stream created by popen.  It waits for the child process to terminate and returns its status value, as for the system function.

Here is an example showing how to use popen and pclose to filter output through another program, in this case the paging program more.

#include <stdio.h>
#include <stdlib.h>

void
write_data (FILE * stream)
{
  int i;
  for (i = 0; i < 100; i++)
    fprintf (stream, "%d\n", i);
  if (ferror (stream))
    {
      fprintf (stderr, "Output to stream failed.\n");
      exit (EXIT_FAILURE);
    }
}

int
main (void)
{
  FILE *output;

  output = popen ("more", "w");
  if (!output)
    {
      fprintf (stderr, "Could not run more.\n");
      return EXIT_FAILURE;
    }
  write_data (output);
  pclose (output);
  return EXIT_SUCCESS;
}


FIFO Special Files
Node: FIFO Special Files, Next: Pipe Atomicity, Prev: Pipe to a Subprocess, Up: Pipes and FIFOs

FIFO Special Files

A FIFO special file is similar to a pipe, except that it is created in a different way.  Instead of being an anonymous communications channel, a FIFO special file is entered into the file system by calling mkfifo.

Once you have created a FIFO special file in this way, any process can open it for reading or writing, in the same way as an ordinary file.  However, it has to be open at both ends simultaneously before you can proceed to do any input or output operations on it.  Opening a FIFO for reading normally blocks until some other process opens the same FIFO for writing, and vice versa.

The mkfifo function is declared in the header file 'sys/stat.h'.  K

 - Function: int mkfifo (const char *filename, mode_t mode)

The mkfifo function makes a FIFO special file with name filename.  The mode argument is used to set the file's permissions; see See Setting Permissions.

The normal, successful return value from mkfifo is 0.  In the case of an error, -1 is returned.  In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for this function:

EEXIST

The named file already exists.

ENOSPC

The directory or file system cannot be extended.

EROFS

The directory that would contain the file resides on a read-only file system.


Atomicity of Pipe I/O
Node: Pipe Atomicity, Next: , Prev: FIFO Special Files, Up: Pipes and FIFOs

Atomicity of Pipe I/O

Reading or writing pipe data is "atomic" if the size of data written is less than PIPE_BUF.  This means that the data transfer seems to be an instantaneous unit, in that nothing else in the system can observe a state in which it is partially complete.  Atomic I/O may not begin right away (it may need to wait for buffer space or for data), but once it does begin, it finishes immediately.

Reading or writing a larger amount of data may not be atomic; for example, output data from other processes sharing the descriptor may be interspersed.

See Limits for Files, for information about the PIPE_BUF parameter.


Sockets
Node: Sockets, Next: Low-Level Terminal Interface, Prev: Pipes and FIFOs, Up: Top

Sockets

This chapter describes the GNU facilities for interprocess communication using sockets.

A "socket" is a generalized interprocess communication channel.  Like a pipe, a socket is represented as a file descriptor.  But, unlike pipes, sockets support communication between unrelated processes, and even between processes running on different machines that communicate over a network.  Sockets are the primary means of communicating with other machines; telnet, rlogin, ftp, talk, and the other familiar network programs use sockets.

Not all operating systems support sockets.  In the GNU library, the header file 'sys/socket.h' exists regardless of the operating system, and the socket functions always exist, but if the system does not really support sockets, these functions always fail.

Incomplete: We do not currently document the facilities for broadcast messages or for configuring Internet interfaces.

* Menu:

Socket Concepts	Basic concepts you need to know about.

Communication Styles	Stream communication, datagrams, and other styles.

Socket Addresses	How socket names ("addresses") work.

File Namespace	Details about the file namespace.

Internet Namespace	Details about the Internet namespace.

Misc Namespaces	Other namespaces not documented fully here.

Open/Close Sockets	Creating sockets and destroying them.

Connections	Operations on sockets with connection state.

Datagrams	Operations on datagram sockets.

Inetd	Inetd is a daemon that starts servers on request. The most convenient way to write a server is to make it work with Inetd.

Socket Options	Miscellaneous low-level socket options.

Networks Database	Accessing the database of network names.


Socket Concepts
Node: Socket Concepts, Next: Communication Styles, Prev: , Up: Sockets

Socket Concepts

When you create a socket, you must specify the style of communication you want to use and the type of protocol that should implement it.  The "communication style" of a socket defines the user-level semantics of sending and receiving data on the socket.  Choosing a communication style specifies the answers to questions such as these:

•	What are the units of data transmission?  Some communication styles regard the data as a sequence of bytes, with no larger structure; others group the bytes into records (which are known in this context as "packets").

•	Can data be lost during normal operation?  Some communication styles guarantee that all the data sent arrives in the order it was sent (barring system or network crashes); other styles occasionally lose data as a normal part of operation, and may sometimes deliver packets more than once or in the wrong order.

Designing a program to use unreliable communication styles usually involves taking precautions to detect lost or misordered packets and to retransmit data as needed.

•	Is communication entirely with one partner?  Some communication styles are like a telephone call--you make a "connection" with one remote socket, and then exchange data freely.  Other styles are like mailing letters--you specify a destination address for each message you send.

You must also choose a "namespace" for naming the socket.  A socket name ("address") is meaningful only in the context of a particular namespace.  In fact, even the data type to use for a socket name may depend on the namespace.  Namespaces are also called "domains", but we avoid that word as it can be confused with other usage of the same term.  Each namespace has a symbolic name that starts with 'PF_'.  A corresponding symbolic name starting with 'AF_' designates the address format for that namespace.

Finally you must choose the "protocol" to carry out the communication.  The protocol determines what low-level mechanism is used to transmit and receive data.  Each protocol is valid for a particular namespace and communication style; a namespace is sometimes called a "protocol family" because of this, which is why the namespace names start with 'PF_'.

The rules of a protocol apply to the data passing between two programs, perhaps on different computers; most of these rules are handled by the operating system, and you need not know about them.  What you do need to know about protocols is this:

•	In order to have communication between two sockets, they must specify the same protocol.

•	Each protocol is meaningful with particular style/namespace combinations and cannot be used with inappropriate combinations.  For example, the TCP protocol fits only the byte stream style of communication and the Internet namespace.

•	For each combination of style and namespace, there is a "default protocol" which you can request by specifying 0 as the protocol number.  And that's what you should normally do--use the default.


Communication Styles
Node: Communication Styles, Next: Socket Addresses, Prev: Socket Concepts, Up: Sockets

Communication Styles

The GNU library includes support for several different kinds of sockets, each with different characteristics.  This section describes the supported socket types.  The symbolic constants listed here are defined in 'sys/socket.h'.  K

 - Macro: int SOCK_STREAM

The SOCK_STREAM style is like a pipe (see Pipes and FIFOs); it operates over a connection with a particular remote socket, and transmits data reliably as a stream of bytes.

Use of this style is covered in detail in See Connections.

 - Macro: int SOCK_DGRAM

The SOCK_DGRAM style is used for sending individually-addressed packets, unreliably.  It is the diametrical opposite of SOCK_STREAM.

Each time you write data to a socket of this kind, that data becomes one packet.  Since SOCK_DGRAM sockets do not have connections, you must specify the recipient address with each packet.

The only guarantee that the system makes about your requests to transmit data is that it will try its best to deliver each packet you send.  It may succeed with the sixth packet after failing with the fourth and fifth packets; the seventh packet may arrive before the sixth, and may arrive a second time after the sixth.

The typical use for SOCK_DGRAM is in situations where it is acceptable to simply resend a packet if no response is seen in a reasonable amount of time.

See Datagrams, for detailed information about how to use datagram sockets.

 - Macro: int SOCK_RAW

This style provides access to low-level network protocols and interfaces.  Ordinary user programs usually have no need to use this style.


Socket Addresses
Node: Socket Addresses, Next: File Namespace, Prev: Communication Styles, Up: Sockets

Socket Addresses

The name of a socket is normally called an "address".  The functions and symbols for dealing with socket addresses were named inconsistently, sometimes using the term "name" and sometimes using "address".  You can regard these terms as synonymous where sockets are concerned.

A socket newly created with the socket function has no address.  Other processes can find it for communication only if you give it an address.  We call this "binding" the address to the socket, and the way to do it is with the bind function.

You need be concerned with the address of a socket if other processes are to find it and start communicating with it.  You can specify an address for other sockets, but this is usually pointless; the first time you send data from a socket, or use it to initiate a connection, the system assigns an address automatically if you have not specified one.

Occasionally a client needs to specify an address because the server discriminates based on addresses; for example, the rsh and rlogin protocols look at the client's socket address and don't bypass password checking unless it is less than IPPORT_RESERVED (see Ports).

The details of socket addresses vary depending on what namespace you are using.  See File Namespace, or See Internet Namespace, for specific information.

Regardless of the namespace, you use the same functions bind and getsockname to set and examine a socket's address.  These functions use a phony data type, struct sockaddr *, to accept the address.  In practice, the address lives in a structure of some other data type appropriate to the address format you are using, but you cast its address to struct sockaddr * when you pass it to bind.

* Menu:

Address Formats	About struct sockaddr.

Setting Address	Binding an address to a socket.

Reading Address	Reading the address of a socket.


Address Formats
Node: Address Formats, Next: Setting Address, Prev: , Up: Socket Addresses

Address Formats

The functions bind and getsockname use the generic data type struct sockaddr * to represent a pointer to a socket address.  You can't use this data type effectively to interpret an address or construct one; for that, you must use the proper data type for the socket's namespace.

Thus, the usual practice is to construct an address in the proper namespace-specific type, then cast a pointer to struct sockaddr * when you call bind or getsockname.

The one piece of information that you can get from the struct sockaddr data type is the "address format" designator which tells you which data type to use to understand the address fully.

The symbols in this section are defined in the header file 'sys/socket.h'.

 - Date Type: struct sockaddr

The struct sockaddr type itself has the following members:

short int sa_family

This is the code for the address format of this address.  It identifies the format of the data which follows.

char sa_data[14]

This is the actual socket address data, which is format-dependent.  Its length also depends on the format, and may well be more than 14.  The length 14 of sa_data is essentially arbitrary.

Each address format has a symbolic name which starts with 'AF_'.  Each of them corresponds to a 'PF_' symbol which designates the corresponding namespace.  Here is a list of address format names:

AF_FILE

This designates the address format that goes with the file namespace.  (PF_FILE is the name of that namespace.)  See File Namespace Details, for information about this address format.

AF_UNIX

This is a synonym for AF_FILE, for compatibility.  (PF_UNIX is likewise a synonym for PF_FILE.)

AF_INET

This designates the address format that goes with the Internet namespace.  (PF_INET is the name of that namespace.) See Internet Address Format.

AF_UNSPEC

This designates no particular address format.  It is used only in rare cases, such as to clear out the default destination address of a "connected" datagram socket.  See Sending Datagrams.

The corresponding namespace designator symbol PF_UNSPEC exists for completeness, but there is no reason to use it in a program.

'sys/socket.h' defines symbols starting with 'AF_' for many different kinds of networks, all or most of which are not actually implemented.  We will document those that really work, as we receive information about how to use them.


Setting the Address of a Socket
Node: Setting Address, Next: Reading Address, Prev: Address Formats, Up: Socket Addresses

Setting the Address of a Socket

Use the bind function to assign an address to a socket.  The prototype for bind is in the header file 'sys/socket.h'.  For examples of use, see See File Namespace, or see See Inet Example.

 - Function: int bind (int socket, struct sockaddr *addr, size_t length)

The bind function assigns an address to the socket socket.  The addr and length arguments specify the address; the detailed format of the address depends on the namespace.  The first part of the address is always the format designator, which specifies a namespace, and says that the address is in the format for that namespace.

The return value is 0 on success and -1 on failure.  The following errno error conditions are defined for this function:

EBADF

The socket argument is not a valid file descriptor.

ENOTSOCK

The descriptor socket is not a socket.

EADDRNOTAVAIL

The specified address is not available on this machine.

EADDRINUSE

Some other socket is already using the specified address.

EINVAL

The socket socket already has an address.

EACCESS

You do not have permission to access the requested address.  (In the Internet domain, only the super-user is allowed to specify a port number in the range 0 through IPPORT_RESERVED minus one; see See Ports.)

Additional conditions may be possible depending on the particular namespace of the socket.


Reading the Address of a Socket
Node: Reading Address, Next: , Prev: Setting Address, Up: Socket Addresses

Reading the Address of a Socket

Use the function getsockname to examine the address of an Internet socket.  The prototype for this function is in the header file 'sys/socket.h'.

 - Function: int getsockname (int socket, struct sockaddr *addr, size_t *length_ptr)

The getsockname function returns information about the address of the socket socket in the locations specified by the addr and length_ptr arguments.  Note that the length_ptr is a pointer; you should initialize it to be the allocation size of addr, and on return it contains the actual size of the address data.

The format of the address data depends on the socket namespace.  The length of the information is usually fixed for a given namespace, so normally you can know exactly how much space is needed and can provide that much.  The usual practice is to allocate a place for the value using the proper data type for the socket's namespace, then cast its address to struct sockaddr * to pass it to getsockname.

The return value is 0 on success and -1 on error.  The following errno error conditions are defined for this function:

EBADF

The socket argument is not a valid file descriptor.

ENOTSOCK

The descriptor socket is not a socket.

ENOBUFS

There are not enough internal buffers available for the operation.

You can't read the address of a socket in the file namespace.  This is consistent with the rest of the system; in general, there's no way to find a file's name from a descriptor for that file.


The File Namespace
Node: File Namespace, Next: Internet Namespace, Prev: Socket Addresses, Up: Sockets

The File Namespace

This section describes the details of the file namespace, whose symbolic name (required when you create a socket) is PF_FILE.

* Menu:

Concepts	What you need to understand.

Details	Address format, symbolic names, etc.

Example	Example of creating a socket.


File Namespace Concepts
Node: File Namespace Concepts, Next: File Namespace Details, Prev: , Up: File Namespace

File Namespace Concepts

In the file namespace, socket addresses are file names.  You can specify any file name you want as the address of the socket, but you must have write permission on the directory containing it.  In order to connect to a socket, you must have read permission for it.  It's common to put these files in the '/tmp' directory.

One peculiarity of the file namespace is that the name is only used when opening the connection; once that is over with, the address is not meaningful and may not exist.

Another peculiarity is that you cannot connect to such a socket from another machine-not even if the other machine shares the file system which contains the name of the socket.  You can see the socket in a directory listing, but connecting to it never succeeds.  Some programs take advantage of this, such as by asking the client to send its own process ID, and using the process IDs to distinguish between clients.  However, we recommend you not use this method in protocols you design, as we might someday permit connections from other machines that mount the same file systems.  Instead, send each new client an identifying number if you want it to have one.

After you close a socket in the file namespace, you should delete the file name from the file system.  Use unlink or remove to do this; see See Deleting Files.

The file namespace supports just one protocol for any communication style; it is protocol number 0.


Details of File Namespace
Node: File Namespace Details, Next: File Socket Example, Prev: File Namespace Concepts, Up: File Namespace

Details of File Namespace

To create a socket in the file namespace, use the constant PF_FILE as the namespace argument to socket or socketpair.  This constant is defined in 'sys/socket.h'.

 - Macro: int PF_FILE

This designates the file namespace, in which socket addresses are file names, and its associated family of protocols.

 - Macro: int PF_UNIX

This is a synonym for PF_FILE, for compatibility's sake.

The structure for specifying socket names in the file namespace is defined in the header file 'sys/un.h': K

 - Data Type: struct sockaddr_un

This structure is used to specify file namespace socket addresses.  It has the following members:

short int sun_family

This identifies the address family or format of the socket address.  You should store the value AF_FILE to designate the file namespace.  See Socket Addresses.

char sun_path[108]

This is the file name to use.

Incomplete:  Why is 108 a magic number?  RMS suggests making this a zero-length array and tweaking the example following to use alloca to allocate an appropriate amount of storage based on the length of the filename.

You should compute the length parameter for a socket address in the file namespace as the sum of the size of the sun_family component and the string length (not the allocation size!) of the file name string.


Example of File-Namespace Sockets
Node: File Socket Example, Next: , Prev: File Namespace Details, Up: File Namespace

Example of File-Namespace Sockets

Here is an example showing how to create and name a socket in the file namespace.

#include <stddef.h>
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/un.h>

int
make_named_socket (const char *filename)
{
  struct sockaddr_un name;
  int sock;
  size_t size;

  /* Create the socket. */

  sock = socket (PF_UNIX, SOCK_DGRAM, 0);
  if (sock < 0)
    {
      perror ("socket");
      exit (EXIT_FAILURE);
    }

  /* Bind a name to the socket. */

  name.sun_family = AF_FILE;
  strcpy (name.sun_path, filename);

  /* The size of the address is
     the offset of the start of the filename,
     plus its length,
     plus one for the terminating null byte. */
  size = (offsetof (struct sockaddr_un, sun_path)
          + strlen (name.sun_path) + 1);

  if (bind (sock, (struct sockaddr *) &name, size) < 0)
    {
      perror ("bind");
      exit (EXIT_FAILURE);
    }

  return sock;
}


The Internet Namespace
Node: Internet Namespace, Next: Misc Namespaces, Prev: File Namespace, Up: Sockets

The Internet Namespace

This section describes the details the protocols and socket naming conventions used in the Internet namespace.

To create a socket in the Internet namespace, use the symbolic name PF_INET of this namespace as the namespace argument to socket or socketpair.  This macro is defined in 'sys/socket.h'.  K

 - Macro: int PF_INET

This designates the Internet namespace and associated family of protocols.

A socket address for the Internet namespace includes the following components:

•	The address of the machine you want to connect to.  Internet addresses can be specified in several ways; these are discussed in See Internet Address Format, See Host Addresses, and See Host Names.

•	A port number for that machine.  See Ports.

You must ensure that the address and port number are represented in a canonical format called "network byte order".  See Byte Order, for information about this.

* Menu:

Internet Address Format	How socket addresses are specified in the Internet namespace.

Host Addresses	All about host addresses of internet host.

Protocols Database	Referring to protocols by name.

Ports	Internet port numbers.

Services Database	Ports may have symbolic names.

Byte Order	Different hosts may use different byte ordering conventions; you need to canonicalize host address and port number.

Inet Example	Putting it all together.


Internet Socket Address Format
Node: Internet Address Format, Next: Host Addresses, Prev: , Up: Internet Namespace

Internet Socket Address Format

In the Internet namespace, a socket address consists of a host address and a port on that host.  In addition, the protocol you choose serves effectively as a part of the address because local port numbers are meaningful only within a particular protocol.

The data type for representing socket addresses in the Internet namespace is defined in the header file 'netinet/in.h'.  K

 - Data Type: struct sockaddr_in

This is the data type used to represent socket addresses in the Internet namespace.  It has the following members:

short int sin_family

This identifies the address family or format of the socket address.  You should store the value of AF_INET in this member.  See Socket Addresses.

struct in_addr sin_addr

This is the Internet address of the host machine.  See Host Addresses, and See Host Names, for how to get a value to store here.

unsigned short int sin_port

This is the port number.  See Ports.

When you call bind or getsockname, you should specify sizeof (struct sockaddr_in) as the length parameter if you are using an Internet namespace socket address.


Host Addresses
Node: Host Addresses, Next: Protocols Database, Prev: Internet Address Format, Up: Internet Namespace

Host Addresses

Each computer on the Internet has one or more "Internet addresses", numbers which identify that computer among all those on the Internet.  Users typically write numeric host addresses as sequences of four numbers, separated by periods, as in '128.52.46.32'.

Each computer also has one or more "host names", which are strings of words separated by periods, as in 'churchy.gnu.ai.mit.edu'.

Programs that let the user specify a host typically accept both numeric addresses and host names.  But the program needs a numeric address to open a connection; to use a host name, you must convert it to the numeric address it stands for.

* Menu:

Abstract Host Addresses	What a host number consists of.

Data type	Data type for a host number.

Functions	Functions to operate on them.

Names	Translating host names to host numbers.


Internet Host Addresses
Node: Abstract Host Addresses, Next: Host Address Data Type, Prev: , Up: Host Addresses

Internet Host Addresses

Each computer on the Internet has one or more Internet addresses, numbers which identify that computer among all those on the Internet.

An Internet host address is a number containing four bytes of data.  These are divided into two parts, a "network number" and a "local network address number" within that network.  The network number consists of the first one, two or three bytes; the rest of the bytes are the local address.

Network numbers are registered with the Network Information Center (NIC), and are divided into three classes--A, B, and C.  The local network address numbers of individual machines are registered with the administrator of the particular network.

Class A networks have single-byte numbers in the range 0 to 127.  There are only a small number of Class A networks, but they can each support a very large number of hosts.  Medium-sized Class B networks have two-byte network numbers, with the first byte in the range 128 to 191.  Class C networks are the smallest; they have three-byte network numbers, with the first byte in the range 192-255.  Thus, the first 1, 2, or 3 bytes of an Internet address specifies a network.  The remaining bytes of the Internet address specify the address within that network.

The Class A network 0 is reserved for broadcast to all networks.  In addition, the host number 0 within each network is reserved for broadcast to all hosts in that network.

The Class A network 127 is reserved for loopback; you can always use the Internet address '127.0.0.1' to refer to the host machine.

Since a single machine can be a member of multiple networks, it can have multiple Internet host addresses.  However, there is never supposed to be more than one machine with the same host address.

There are four forms of the "standard numbers-and-dots notation" for Internet addresses:

a.b.c.d

This specifies all four bytes of the address individually.

a.b.c

The last part of the address, c, is interpreted as a 2-byte quantity.  This is useful for specifying host addresses in a Class B network with network address number a.b.

a.b

The last part of the address, c, is interpreted as a 3-byte quantity.  This is useful for specifying host addresses in a Class A network with network address number a.

a

If only one part is given, this corresponds directly to the host address number.

Within each part of the address, the usual C conventions for specifying the radix apply.  In other words, a leading '0x' or '0X' implies hexadecimal radix; a leading '0' implies octal; and otherwise decimal radix is assumed.


Host Address Data Type
Node: Host Address Data Type, Next: Host Address Functions, Prev: Abstract Host Addresses, Up: Host Addresses

Host Address Data Type

Internet host addresses are represented in some contexts as integers (type unsigned long int).  In other contexts, the integer is packaged inside a structure of type struct in_addr.  It would be better if the usage were made consistent, but it is not hard to extract the integer from the structure or put the integer into a structure.

The following basic definitions for Internet addresses appear in the header file 'netinet/in.h': K

 - Data Type: struct in_addr

This data type is used in certain contexts to contain an Internet host address.  It has just one field, named s_addr, which records the host address number as an unsigned long int.

 - Macro: unsigned long int INADDR_LOOPBACK

You can use this constant to stand for "the address of this machine," instead of finding its actual address.  It is the Internet address '127.0.0.1', which is usually called 'localhost'.  This special constant saves you the trouble of looking up the address of your own machine.  Also, the system usually implements INADDR_LOOPBACK specially, avoiding any network traffic for the case of one machine talking to itself.

 - Macro: unsigned long int INADDR_ANY

You can use this constant to stand for "any incoming address," when binding to an address.  See Setting Address.  This is the usual address to give in the sin_addr member of struct sockaddr_in when you want to accept Internet connections.

 - Macro: unsigned long int INADDR_BROADCAST

This constant is the address you use to send a broadcast message.

 - Macro: unsigned long int INADDR_NONE

This constant is returned by some functions to indicate an error.


Host Address Functions
Node: Host Address Functions, Next: Host Names, Prev: Host Address Data Type, Up: Host Addresses

Host Address Functions

These additional functions for manipulating Internet addresses are declared in 'arpa/inet.h'.  They represent Internet addresses in network byte order; they represent network numbers and local-address-within-network numbers in host byte order.  See Byte Order, for an explanation of network and host byte order.

 - Function: int inet_aton (const char *name, struct in_addr *addr)

This function converts the Internet host address name from the standard numbers-and-dots notation into binary data and stores it in the struct in_addr that addr points to.  inet_aton returns nonzero if the address is valid, zero if not.

 - Function: unsigned long int inet_addr (const char *name)

This function converts the Internet host address name from the standard numbers-and-dots notation into binary data.  If the input is not valid, inet_addr returns INADDR_NONE.  This is an obsolete interface to inet_aton, described immediately above; it is obsolete because INADDR_NONE is a valid address (255.255.255.255), and inet_aton provides a cleaner way to indicate error return.

 - Function: unsigned long int inet_network (const char *name)

This function extracts the network number from the address name, given in the standard numbers-and-dots notation.  If the input is not valid, inet_network returns -1.

 - Function: char * inet_ntoa (struct in_addr addr)

This function converts the Internet host address addr to a string in the standard numbers-and-dots notation.  The return value is a pointer into a statically-allocated buffer.  Subsequent calls will overwrite the same buffer, so you should copy the string if you need to save it.

 - Function: struct in_addr inet_makeaddr (int net, int local)

This function makes an Internet host address by combining the network number net with the local-address-within-network number local.

 - Function: int inet_lnaof (struct in_addr addr)

This function returns the local-address-within-network part of the Internet host address addr.

 - Function: int inet_netof (struct in_addr addr)

This function returns the network number part of the Internet host address addr.


Host Names
Node: Host Names, Next: , Prev: Host Address Functions, Up: Host Addresses

Host Names

Besides the standard numbers-and-dots notation for Internet addresses, you can also refer to a host by a symbolic name.  The advantage of a symbolic name is that it is usually easier to remember.  For example, the machine with Internet address '128.52.46.32' is also known as 'churchy.gnu.ai.mit.edu'; and other machines in the 'gnu.ai.mit.edu' domain can refer to it simply as 'churchy'.

Internally, the system uses a database to keep track of the mapping between host names and host numbers.  This database is usually either the file '/etc/hosts' or an equivalent provided by a name server.  The functions and other symbols for accessing this database are declared in 'netdb.h'.  They are BSD features, defined unconditionally if you include 'netdb.h'.

 - Data Type: struct hostent

This data type is used to represent an entry in the hosts database.  It has the following members:

char *h_name

This is the "official" name of the host.

char **h_aliases

These are alternative names for the host, represented as a null-terminated vector of strings.

int h_addrtype

This is the host address type; in practice, its value is always AF_INET.  In principle other kinds of addresses could be represented in the data base as well as Internet addresses; if this were done, you might find a value in this field other than AF_INET.  See Socket Addresses.

int h_length

This is the length, in bytes, of each address.

char **h_addr_list

This is the vector of addresses for the host.  (Recall that the host might be connected to multiple networks and have different addresses on each one.)  The vector is terminated by a null pointer.

char *h_addr

This is a synonym for h_addr_list[0]; in other words, it is the first host address.

As far as the host database is concerned, each address is just a block of memory h_length bytes long.  But in other contexts there is an implicit assumption that you can convert this to a struct in_addr or an unsigned long int.  Host addresses in a struct hostent structure are always given in network byte order; see See Byte Order.

You can use gethostbyname or gethostbyaddr to search the hosts database for information about a particular host.  The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls.

 - Function: struct hostent * gethostbyname (const char *name)

The gethostbyname function returns information about the host named name.  If the lookup fails, it returns a null pointer.

 - Function: struct hostent * gethostbyaddr (const char *addr, int length, int format)

The gethostbyaddr function returns information about the host with Internet address addr.  The length argument is the size (in bytes) of the address at addr.  format specifies the address format; for an Internet address, specify a value of AF_INET.

If the lookup fails, gethostbyaddr returns a null pointer.

If the name lookup by gethostbyname or gethostbyaddr fails, you can find out the reason by looking at the value of the variable h_errno.  (It would be cleaner design for these functions to set errno, but use of h_errno is compatible with other systems.)  Before using h_errno, you must declare it like this:

extern int h_errno;

Here are the error codes that you may find in h_errno:

HOST_NOT_FOUND

No such host is known in the data base.

TRY_AGAIN

This condition happens when the name server could not be contacted.  If you try again later, you may succeed then.

NO_RECOVERY

A non-recoverable error occurred.

NO_ADDRESS

The host database contains an entry for the name, but it doesn't have an associated Internet address.

You can also scan the entire hosts database one entry at a time using sethostent, gethostent, and endhostent.  Be careful in using these functions, because they are not reentrant.

 - Function: void sethostent (int stayopen)

This function opens the hosts database to begin scanning it.  You can then call gethostent to read the entries.

If the stayopen argument is nonzero, this sets a flag so that subsequent calls to gethostbyname or gethostbyaddr will not close the database (as they usually would).  This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.

 - Function: struct hostent * gethostent ()

This function returns the next entry in the hosts database.  It returns a null pointer if there are no more entries.

 - Function: void endhostent ()

This function closes the hosts database.


Internet Ports
Node: Ports, Next: Services Database, Prev: Protocols Database, Up: Internet Namespace

Internet Ports

A socket address in the Internet namespace consists of a machine's Internet address plus a "port number" which distinguishes the sockets on a given machine (for a given protocol).  Port numbers range from 0 to 65,535.

Port numbers less than IPPORT_RESERVED are reserved for standard servers, such as finger and telnet.  There is a database that keeps track of these, and you can use the getservbyname function to map a service name onto a port number; see See Services Database.

If you write a server that is not one of the standard ones defined in the database, you must choose a port number for it.  Use a number greater than IPPORT_USERRESERVED; such numbers are reserved for servers and won't ever be generated automatically by the system.  Avoiding conflicts with servers being run by other users is up to you.

When you use a socket without specifying its address, the system generates a port number for it.  This number is between IPPORT_RESERVED and IPPORT_USERRESERVED.

On the Internet, it is actually legitimate to have two different sockets with the same port number, as long as they never both try to communicate with the same socket address (host address plus port number).  You shouldn't duplicate a port number except in special circumstances where a higher-level protocol requires it.  Normally, the system won't let you do it; bind normally insists on distinct port numbers.  To reuse a port number, you must set the socket option SO_REUSEADDR.  See Socket-Level Options.

These macros are defined in the header file 'netinet/in.h'.

 - Macro: int IPPORT_RESERVED

Port numbers less than IPPORT_RESERVED are reserved for superuser use.

 - Macro: int IPPORT_USERRESERVED

Port numbers greater than or equal to IPPORT_USERRESERVED are reserved for explicit use; they will never be allocated automatically.


The Services Database
Node: Services Database, Next: Byte Order, Prev: Ports, Up: Internet Namespace

The Services Database

The database that keeps track of "well-known" services is usually either the file '/etc/services' or an equivalent from a name server.  You can use these utilities, declared in 'netdb.h', to access the services database.  K

 - Data Type: struct servent

This data type holds information about entries from the services database.  It has the following members:

char *s_name

This is the "official" name of the service.

char **s_aliases

These are alternate names for the service, represented as an array of strings.  A null pointer terminates the array.

int s_port

This is the port number for the service.  Port numbers are given in network byte order; see See Byte Order.

char *s_proto

This is the name of the protocol to use with this service.  See Protocols Database.

To get information about a particular service, use the getservbyname or getservbyport functions.  The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls.

 - Function: struct servent * getservbyname (const char *name, const char *proto)

The getservbyname function returns information about the service named name using protocol proto.  If it can't find such a service, it returns a null pointer.

This function is useful for servers as well as for clients; servers use it to determine which port they should listen on (see Listening).

 - Function: struct servent * getservbyport (int port, const char *proto)

The getservbyport function returns information about the service at port port using protocol proto.  If it can't find such a service, it returns a null pointer.

You can also scan the services database using setservent, getservent, and endservent.  Be careful in using these functions, because they are not reentrant.

 - Function: void setservent (int stayopen)

This function opens the services database to begin scanning it.

If the stayopen argument is nonzero, this sets a flag so that subsequent calls to getservbyname or getservbyport will not close the database (as they usually would).  This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.

 - Function: struct servent * getservent (void)

This function returns the next entry in the services database.  If there are no more entries, it returns a null pointer.

 - Function: void endservent (void)

This function closes the services database.


Byte Order Conversion
Node: Byte Order, Next: Inet Example, Prev: Services Database, Up: Internet Namespace

Byte Order Conversion

Different kinds of computers use different conventions for the ordering of bytes within a word.  Some computers put the most significant byte within a word first (this is called "big-endian" order), and others put it last ("little-endian" order).

So that machines with different byte order conventions can communicate, the Internet protocols specify a canonical byte order convention for data transmitted over the network.  This is known as the "network byte order".

When establishing an Internet socket connection, you must make sure that the data in the sin_port and sin_addr members of the sockaddr_in structure are represented in the network byte order.  If you are encoding integer data in the messages sent through the socket, you should convert this to network byte order too.  If you don't do this, your program may fail when running on or talking to other kinds of machines.

If you use getservbyname and gethostbyname or inet_addr to get the port number and host address, the values are already in the network byte order, and you can copy them directly into the sockaddr_in structure.

Otherwise, you have to convert the values explicitly.  Use htons and ntohs to convert values for the sin_port member.  Use htonl and ntohl to convert values for the sin_addr member.  (Remember, struct in_addr is equivalent to unsigned long int.)  These functions are declared in 'netinet/in.h'.  K

 - Function: unsigned short int htons (unsigned short int hostshort)

This function converts the short integer hostshort from host byte order to network byte order.

 - Function: unsigned short int ntohs (unsigned short int netshort)

This function converts the short integer netshort from network byte order to host byte order.

 - Function: unsigned long int htonl (unsigned long int hostlong)

This function converts the long integer hostlong from host byte order to network byte order.

 - Function: unsigned long int ntohl (unsigned long int netlong)

This function converts the long integer netlong from network byte order to host byte order.


Protocols Database
Node: Protocols Database, Next: Ports, Prev: Host Addresses, Up: Internet Namespace

Protocols Database

The communications protocol used with a socket controls low-level details of how data is exchanged.  For example, the protocol implements things like checksums to detect errors in transmissions, and routing instructions for messages.  Normal user programs have little reason to mess with these details directly.

The default communications protocol for the Internet namespace depends on the communication style.  For stream communication, the default is TCP ("transmission control protocol").  For datagram communication, the default is UDP ("user datagram protocol").  For reliable datagram communication, the default is RDP ("reliable datagram protocol").  You should nearly always use the default.

Internet protocols are generally specified by a name instead of a number.  The network protocols that a host knows about are stored in a database.  This is usually either derived from the file '/etc/protocols', or it may be an equivalent provided by a name server.  You look up the protocol number associated with a named protocol in the database using the getprotobyname function.

Here are detailed descriptions of the utilities for accessing the protocols database.  These are declared in 'netdb.h'.  K

 - Data Type: struct protoent

This data type is used to represent entries in the network protocols database.  It has the following members:

char *p_name

This is the official name of the protocol.

char **p_aliases

These are alternate names for the protocol, specified as an array of strings.  The last element of the array is a null pointer.

int p_proto

This is the protocol number (in host byte order); use this member as the protocol argument to socket.

You can use getprotobyname and getprotobynumber to search the protocols database for a specific protocol.  The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls.

 - Function: struct protoent * getprotobyname (const char *name)

The getprotobyname function returns information about the network protocol named name.  If there is no such protocol, it returns a null pointer.

 - Function: struct protoent * getprotobynumber (int protocol)

The getprotobynumber function returns information about the network protocol with number protocol.  If there is no such protocol, it returns a null pointer.

You can also scan the whole protocols database one protocol at a time by using setprotoent, getprotoent, and endprotoent.  Be careful in using these functions, because they are not reentrant.

 - Function: void setprotoent (int stayopen)

This function opens the protocols database to begin scanning it.

If the stayopen argument is nonzero, this sets a flag so that subsequent calls to getprotobyname or getprotobynumber will not close the database (as they usually would).  This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.

 - Function: struct protoent * getprotoent (void)

This function returns the next entry in the protocols database.  It returns a null pointer if there are no more entries.

 - Function: void endprotoent (void)

This function closes the protocols database.


Internet Socket Example
Node: Inet Example, Next: , Prev: Byte Order, Up: Internet Namespace

Internet Socket Example

Here is an example showing how to create and name a socket in the Internet namespace.  The newly created socket exists on the machine that the program is running on.  Rather than finding and using the machine's Internet address, this example specifies INADDR_ANY as the host address; the system replaces that with the machine's actual address.

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>

int
make_socket (unsigned short int port)
{
  int sock;
  struct sockaddr_in name;

  /* Create the socket. */
  sock = socket (PF_INET, SOCK_STREAM, 0);
  if (sock < 0)
    {
      perror ("socket");
      exit (EXIT_FAILURE);
    }

  /* Give the socket a name. */
  name.sin_family = AF_INET;
  name.sin_port = htons (port);
  name.sin_addr.s_addr = htonl (INADDR_ANY);
  if (bind (sock, (struct sockaddr *) &name, sizeof (name)) < 0)
    {
      perror ("bind");
      exit (EXIT_FAILURE);
    }

  return sock;
}

Here is another example, showing how you can fill in a sockaddr_in structure, given a host name string and a port number:

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>

void
init_sockaddr (struct sockaddr_in *name,
               const char *hostname,
               unsigned short int port)
{
  struct hostent *hostinfo;

  name->sin_family = AF_INET;
  name->sin_port = htons (port);
  hostinfo = gethostbyname (hostname);
  if (hostinfo == NULL)
    {
      fprintf (stderr, "Unknown host %s.\n", hostname);
      exit (EXIT_FAILURE);
    }
  name->sin_addr = *(struct in_addr *) hostinfo->h_addr;
}


Other Namespaces
Node: Misc Namespaces, Next: Open/Close Sockets, Prev: Internet Namespace, Up: Sockets

Other Namespaces

Certain other namespaces and associated protocol families are supported but not documented yet because they are not often used.  PF_NS refers to the Xerox Network Software protocols.  PF_ISO stands for Open Systems Interconnect.  PF_CCITT refers to protocols from CCITT.  'socket.h' defines these symbols and others naming protocols not actually implemented.

PF_IMPLINK is used for communicating between hosts and Internet Message Processors.  For information on this, and on PF_ROUTE, an occasionally-used local area routing protocol, see the GNU Hurd Manual (to appear in the future).


Opening and Closing Sockets
Node: Open/Close Sockets, Next: Connections, Prev: Misc Namespaces, Up: Sockets

Opening and Closing Sockets

This section describes the actual library functions for opening and closing sockets.  The same functions work for all namespaces and connection styles.

* Menu:

Creating a Socket	How to open a socket.

Closing a Socket	How to close a socket.

Socket Pairs	These are created like pipes.


Creating a Socket
Node: Creating a Socket, Next: Closing a Socket, Prev: , Up: Open/Close Sockets

Creating a Socket

The primitive for creating a socket is the socket function, declared in 'sys/socket.h'.  K

 - Function: int socket (int namespace, int style, int protocol)

This function creates a socket and specifies communication style style, which should be one of the socket styles listed in See Communication Styles.  The namespace argument specifies the namespace; it must be PF_FILE (see File Namespace) or PF_INET (see Internet Namespace).  protocol designates the specific protocol (see Socket Concepts); zero is usually right for protocol.

The return value from socket is the file descriptor for the new socket, or -1 in case of error.  The following errno error conditions are defined for this function:

EPROTONOSUPPORT

The protocol or style is not supported by the namespace specified.

EMFILE

The process already has too many file descriptors open.

ENFILE

The system already has too many file descriptors open.

EACCESS

The process does not have privilege to create a socket of the specified style or protocol.

ENOBUFS

The system ran out of internal buffer space.

The file descriptor returned by the socket function supports both read and write operations.  But, like pipes, sockets do not support file positioning operations.

For examples of how to call the socket function, see See File Namespace, or See Inet Example.


Closing a Socket
Node: Closing a Socket, Next: Socket Pairs, Prev: Creating a Socket, Up: Open/Close Sockets

Closing a Socket

When you are finished using a socket, you can simply close its file descriptor with close; see See Opening and Closing Files.  If there is still data waiting to be transmitted over the connection, normally close tries to complete this transmission.  You can control this behavior using the SO_LINGER socket option to specify a timeout period; see See Socket Options.

You can also shut down only reception or only transmission on a connection by calling shutdown, which is declared in 'sys/socket.h'.

 - Function: int shutdown (int socket, int how)

The shutdown function shuts down the connection of socket socket.  The argument how specifies what action to perform:

0

Stop receiving data for this socket.  If further data arrives, reject it.

1

Stop trying to transmit data from this socket.  Discard any data waiting to be sent.  Stop looking for acknowledgement of data already sent; don't retransmit it if it is lost.

2

Stop both reception and transmission.

The return value is 0 on success and -1 on failure.  The following errno error conditions are defined for this function:

EBADF

socket is not a valid file descriptor.

ENOTSOCK

socket is not a socket.

ENOTCONN

socket is not connected.


Socket Pairs
Node: Socket Pairs, Next: , Prev: Closing a Socket, Up: Open/Close Sockets

Socket Pairs

A "socket pair" consists of a pair of connected (but unnamed) sockets.  It is very similar to a pipe and is used in much the same way.  Socket pairs are created with the socketpair function, declared in 'sys/socket.h'.  A socket pair is much like a pipe; the main difference is that the socket pair is bidirectional, whereas the pipe has one input-only end and one output-only end (see Pipes and FIFOs).

 - Function: int socketpair (int namespace, int style, int protocol, int filedes[2])

This function creates a socket pair, returning the file descriptors in filedes[0] and filedes[1].  The socket pair is a full-duplex communications channel, so that both reading and writing may be performed at either end.

The namespace, style, and protocol arguments are interpreted as for the socket function.  style should be one of the communication styles listed in See Communication Styles.  The namespace argument specifies the namespace, which must be AF_FILE (see File Namespace); protocol specifies the communications protocol, but zero is the only meaningful value.

If style specifies a connectionless communication style, then the two sockets you get are not connected, strictly speaking, but each of them knows the other as the default destination address, so they can send packets to each other.

The socketpair function returns 0 on success and -1 on failure.  The following errno error conditions are defined for this function:

EMFILE

The process has too many file descriptors open.

EAFNOSUPPORT

The specified namespace is not supported.

EPROTONOSUPPORT

The specified protocol is not supported.

EOPNOTSUPP

The specified protocol does not support the creation of socket pairs.


Using Sockets with Connections
Node: Connections, Next: Datagrams, Prev: Open/Close Sockets, Up: Sockets

Using Sockets with Connections

The most common communication styles involve making a connection to a particular other socket, and then exchanging data with that socket over and over.  Making a connection is asymmetric; one side (the "client") acts to request a connection, while the other side (the "server") makes a socket and waits for the connection request.

* Menu:

Connecting	What the client program must do.

Listening	How a server program waits for requests.

Accepting Connections	What the server does when it gets a request.

Who is Connected	Getting the address of the other side of a connection.

Transferring Data	How to send and receive data.

Byte Stream Example	An example program: a client for communicating over a byte stream socket in the Internet namespace.

Server Example	A corresponding server program.

Out-of-Band Data	This is an advanced feature.


Making a Connection
Node: Connecting, Next: Listening, Prev: , Up: Connections

Making a Connection

In making a connection, the client makes a connection while the server waits for and accepts the connection.  Here we discuss what the client program must do, using the connect function, which is declared in 'sys/socket.h'.

 - Function: int connect (int socket, struct sockaddr *addr, size_t length)

The connect function initiates a connection from the socket with file descriptor socket to the socket whose address is specified by the addr and length arguments.  (This socket is typically on another machine, and it must be already set up as a server.)  See Socket Addresses, for information about how these arguments are interpreted.

Normally, connect waits until the server responds to the request before it returns.  You can set nonblocking mode on the socket socket to make connect return immediately without waiting for the response.  See File Status Flags, for information about nonblocking mode.

The normal return value from connect is 0.  If an error occurs, connect returns -1.  The following errno error conditions are defined for this function:

EBADF

The socket socket is not a valid file descriptor.

ENOTSOCK

The socket socket is not a socket.

EADDRNOTAVAIL

The specified address is not available on the remote machine.

EAFNOSUPPORT

The namespace of the addr is not supported by this socket.

EISCONN

The socket socket is already connected.

ETIMEDOUT

The attempt to establish the connection timed out.

ECONNREFUSED

The server has actively refused to establish the connection.

ENETUNREACH

The network of the given addr isn't reachable from this host.

EADDRINUSE

The socket address of the given addr is already in use.

EINPROGRESS

The socket socket is non-blocking and the connection could not be established immediately.

EALREADY

The socket socket is non-blocking and already has a pending connection in progress.


Listening for Connections
Node: Listening, Next: Accepting Connections, Prev: Connecting, Up: Connections

Listening for Connections

Now let us consider what the server process must do to accept connections on a socket.  This involves the use of the listen function to enable connection requests on the socket, and later using the accept function (see Accepting Connections) to act on a request.  The listen function is not allowed for sockets using connectionless communication styles.

You can write a network server that does not even start running until a connection to it is requested.  See Inetd Servers.

In the Internet namespace, there are no special protection mechanisms for controlling access to connect to a port; any process on any machine can make a connection to your server.  If you want to restrict access to your server, make it examine the addresses associated with connection requests or implement some other handshaking or identification protocol.

In the File namespace, the ordinary file protection bits control who has access to connect to the socket.

 - Function: int listen (int socket, unsigned int n)

The listen function enables the socket socket to accept connections, thus making it a server socket.

The argument n specifies the length of the queue for pending connections.

The listen function returns 0 on success and -1 on failure.  The following errno error conditions are defined for this function:

EBADF

The argument socket is not a valid file descriptor.

ENOTSOCK

The argument socket is not a socket.

EOPNOTSUPP

The socket socket does not support this operation.


Accepting Connections
Node: Accepting Connections, Next: Who is Connected, Prev: Listening, Up: Connections

Accepting Connections

When a server receives a connection request, it can complete the connection by accepting the request.  Use the function accept to do this.

A socket that has been established as a server can accept connection requests from multiple clients.  The server's original socket does not become part of the connection; instead, accept makes a new socket which participates in the connection.  accept returns the descriptor for this socket.  The server's original socket remains available for listening for further connection requests.

The number of pending connection requests on a server socket is finite.  If connection requests arrive from clients faster than the server can act upon them, the queue can fill up and additional requests are refused with a ECONNREFUSED error.  You can specify the maximum length of this queue as an argument to the listen function, although the system may also impose its own internal limit on the length of this queue.

 - Function: int accept (int socket, struct sockaddr *addr, size_t *length_ptr)

This function is used to accept a connection request on the server socket socket.

The accept function waits if there are no connections pending, unless the socket socket has nonblocking mode set.  (You can use select to wait for a pending connection, with a nonblocking socket.)  See File Status Flags, for information about nonblocking mode.

The addr and length_ptr arguments are used to return information about the name of the client socket that initiated the connection.  See Socket Addresses, for information about the format of the information.

Accepting a connection does not make socket part of the connection.  Instead, it creates a new socket which becomes connected.  The normal return value of accept is the file descriptor for the new socket.

After accept, the original socket socket remains open and unconnected, and continues listening until you close it.  You can accept further connections with socket by calling accept again.

If an error occurs, accept returns -1.  The following errno error conditions are defined for this function:

EBADF

The socket argument is not a valid file descriptor.

ENOTSOCK

The descriptor socket argument is not a socket.

EOPNOTSUPP

The descriptor socket does not support this operation.

EWOULDBLOCK

socket has nonblocking mode set, and there are no pending connections immediately available.

The accept function is not allowed for sockets using connectionless communication styles.


Who is Connected to Me?
Node: Who is Connected, Next: Transferring Data, Prev: Accepting Connections, Up: Connections

Who is Connected to Me?

 - Function: int getpeername (int socket, struct sockaddr *addr, size_t *length_ptr)

The getpeername function returns the address of the socket that socket is connected to; it stores the address in the memory space specified by addr and length_ptr.  It stores the length of the address in *length_ptr.

See Socket Addresses, for information about the format of the address.  In some operating systems, getpeername works only for sockets in the Internet domain.

The return value is 0 on success and -1 on error.  The following errno error conditions are defined for this function:

EBADF

The argument socket is not a valid file descriptor.

ENOTSOCK

The descriptor socket is not a socket.

ENOTCONN

The socket socket is not connected.

ENOBUFS

There are not enough internal buffers available.


Transferring Data
Node: Transferring Data, Next: Byte Stream Example, Prev: Who is Connected, Up: Connections

Transferring Data

Once a socket has been connected to a peer, you can use the ordinary read and write operations (see I/O Primitives) to transfer data.  A socket is a two-way communications channel, so read and write operations can be performed at either end.

There are also some I/O modes that are specific to socket operations.  In order to specify these modes, you must use the recv and send functions instead of the more generic read and write functions.  The recv and send functions take an additional argument which you can use to specify various flags to control the special I/O modes.  For example, you can specify the MSG_OOB flag to read or write out-of-band data, the MSG_PEEK flag to peek at input, or the MSG_DONTROUTE flag to control inclusion of routing information on output.

* Menu:

Sending Data	Sending data with send.

Receiving Data	Reading data with recv.

Socket Data Options	Using send and recv.


Sending Data
Node: Sending Data, Next: Receiving Data, Prev: , Up: Transferring Data

Sending Data

The send function is declared in the header file 'sys/socket.h'.  If your flags argument is zero, you can just as well use write instead of send; see See I/O Primitives.  If the socket was connected but the connection has broken, you get a SIGPIPE signal for any use of send or write (see Miscellaneous Signals).

 - Function: int send (int socket, void *buffer, size_t size, int flags)

The send function is like write, but with the additional flags flags.  The possible values of flags are described in See Socket Data Options.

This function returns the number of bytes transmitted, or -1 on failure.  If the socket is nonblocking, then send (like write) can return after sending just part of the data.  See File Status Flags, for information about nonblocking mode.

Note, however, that a successful return value merely indicates that the message has been sent without error, not necessarily that it has been received without error.

The following errno error conditions are defined for this function:

EBADF

The socket argument is not a valid file descriptor.

EINTR

The operation was interrupted by a signal before any data was sent.  See Interrupted Primitives.

ENOTSOCK

The descriptor socket is not a socket.

EMSGSIZE

The socket type requires that the message be sent atomically, but the message is too large for this to be possible.

EWOULDBLOCK

Nonblocking mode has been set on the socket, and the write operation would block.  (Normally send blocks until the operation can be completed.)

ENOBUFS

There is not enough internal buffer space available.

ENOTCONN

You never connected this socket.

EPIPE

This socket was connected but the connection is now broken.  In this case, send generates a SIGPIPE signal first; if that signal is ignored or blocked, or if its handler returns, then send fails with EPIPE.


Receiving Data
Node: Receiving Data, Next: Socket Data Options, Prev: Sending Data, Up: Transferring Data

Receiving Data

The recv function is declared in the header file 'sys/socket.h'.  If your flags argument is zero, you can just as well use read instead of recv; see See I/O Primitives.

 - Function: int recv (int socket, void *buffer, size_t size, int flags)

The recv function is like read, but with the additional flags flags.  The possible values of flags are described In See Socket Data Options.

If nonblocking mode is set for socket, and no data is available to be read, recv fails immediately rather than waiting.  See File Status Flags, for information about nonblocking mode.

This function returns the number of bytes received, or -1 on failure.  The following errno error conditions are defined for this function:

EBADF

The socket argument is not a valid file descriptor.

ENOTSOCK

The descriptor socket is not a socket.

EWOULDBLOCK

Nonblocking mode has been set on the socket, and the read operation would block.  (Normally, recv blocks until there is input available to be read.)

EINTR

The operation was interrupted by a signal before any data was read.  See Interrupted Primitives.

ENOTCONN

You never connected this socket.


Socket Data Options
Node: Socket Data Options, Next: , Prev: Receiving Data, Up: Transferring Data

Socket Data Options

The flags argument to send and recv is a bit mask.  You can bitwise-OR the values of the following macros together to obtain a value for this argument.  All are defined in the header file 'sys/socket.h'.

 - Macro: int MSG_OOB

Send or receive out-of-band data.  See Out-of-Band Data.

 - Macro: int MSG_PEEK

Look at the data but don't remove it from the input queue.  This is only meaningful with input functions such as recv, not with send.

 - Macro: int MSG_DONTROUTE

Don't include routing information in the message.  This is only meaningful with output operations, and is usually only of interest for diagnostic or routing programs.  We don't try to explain it here.


Byte Stream Socket Example
Node: Byte Stream Example, Next: Server Example, Prev: Transferring Data, Up: Connections

Byte Stream Socket Example

Here is an example client program that makes a connection for a byte stream socket in the Internet namespace.  It doesn't do anything particularly interesting once it has connected to the server; it just sends a text string to the server and exits.

#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>

#define PORT            5555
#define MESSAGE         "Yow!!! Are we having fun yet?!?"
#define SERVERHOST      "churchy.gnu.ai.mit.edu"

void
write_to_server (int filedes)
{
  int nbytes;

  nbytes = write (filedes, MESSAGE, strlen (MESSAGE) + 1);
  if (nbytes < 0)
    {
      perror ("write");
      exit (EXIT_FAILURE);
    }
}

int
main (void)
{
  extern void init_sockaddr (struct sockaddr_in *name,
                             const char *hostname,
                             unsigned short int port);
  int sock;
  struct sockaddr_in servername;

  /* Create the socket. */
  sock = socket (PF_INET, SOCK_STREAM, 0);
  if (sock < 0)
    {
      perror ("socket (client)");
      exit (EXIT_FAILURE);
    }

  /* Connect to the server. */
  init_sockaddr (&servername, SERVERHOST, PORT);
  if (0 > connect (sock,
                   (struct sockaddr *) &servername,
                   sizeof (servername)))
    {
      perror ("connect (client)");
      exit (EXIT_FAILURE);
    }

  /* Send data to the server. */
  write_to_server (sock);
  close (sock);
  exit (EXIT_SUCCESS);
}


Byte Stream Connection Server Example
Node: Server Example, Next: Out-of-Band Data, Prev: Byte Stream Example, Up: Connections

Byte Stream Connection Server Example

The server end is much more complicated.  Since we want to allow multiple clients to be connected to the server at the same time, it would be incorrect to wait for input from a single client by simply calling read or recv.  Instead, the right thing to do is to use select (see Waiting for I/O) to wait for input on all of the open sockets.  This also allows the server to deal with additional connection requests.

This particular server doesn't do anything interesting once it has gotten a message from a client.  It does close the socket for that client when it detects an end-of-file condition (resulting from the client shutting down its end of the connection).

This program uses make_socket and init_sockaddr to set up the socket address; see See Inet Example.

#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>

#define PORT    5555
#define MAXMSG  512

int
read_from_client (int filedes)
{
  char buffer[MAXMSG];
  int nbytes;

  nbytes = read (filedes, buffer, MAXMSG);
  if (nbytes < 0)
    {
      /* Read error. */
      perror ("read");
      exit (EXIT_FAILURE);
    }
  else if (nbytes == 0)
    /* End-of-file. */
    return -1;
  else
    {
      /* Data read. */
      fprintf (stderr, "Server: got message: `%s'\n", buffer);
      return 0;
    }
}

int
main (void)
{
  extern int make_socket (unsigned short int port);
  int sock;
  fd_set active_fd_set, read_fd_set;
  int i;
  struct sockaddr_in clientname;
  size_t size;

  /* Create the socket and set it up to accept connections. */
  sock = make_socket (PORT);
  if (listen (sock, 1) < 0)
    {
      perror ("listen");
      exit (EXIT_FAILURE);
    }

  /* Initialize the set of active sockets. */
  FD_ZERO (&active_fd_set);
  FD_SET (sock, &active_fd_set);

  while (1)
    {
      /* Block until input arrives on one or more active sockets. */
      read_fd_set = active_fd_set;
      if (select (FD_SETSIZE, &read_fd_set, NULL, NULL, NULL) < 0)
        {
          perror ("select");
          exit (EXIT_FAILURE);
        }

      /* Service all the sockets with input pending. */
      for (i = 0; i < FD_SETSIZE; ++i)
        if (FD_ISSET (i, &read_fd_set))
          {
            if (i == sock)
              {
                /* Connection request on original socket. */
                int new;
                size = sizeof (clientname);
                new = accept (sock, (struct sockaddr *) &clientname, &size);
                if (new < 0)
                  {
                    perror ("accept");
                    exit (EXIT_FAILURE);
                  }
                fprintf (stderr,
                         "Server: connect from host %s, port %hd.\n",
                         inet_ntoa (clientname.sin_addr),
                         ntohs (clientname.sin_port));
                FD_SET (new, &active_fd_set);
              }
            else
              {
                /* Data arriving on an already-connected socket. */
                if (read_from_client (i) < 0)
                  {
                    close (i);
                    FD_CLR (i, &active_fd_set);
                  }
              }
          }
    }
}


Out-of-Band Data
Node: Out-of-Band Data, Next: , Prev: Server Example, Up: Connections

Out-of-Band Data

Streams with connections permit "out-of-band" data that is delivered with higher priority than ordinary data.  Typically the reason for sending out-of-band data is to send notice of an exceptional condition.  The way to send out-of-band data is using send, specifying the flag MSG_OOB (see Sending Data).

Out-of-band data is received with higher priority because the receiving process need not read it in sequence; to read the next available out-of-band data, use recv with the MSG_OOB flag (see Receiving Data).  Ordinary read operations do not read out-of-band data; they read only the ordinary data.

When a socket finds that out-of-band data is on its way, it sends a SIGURG signal to the owner process or process group of the socket.  You can specify the owner using the F_SETOWN command to the fcntl function; see See Interrupt Input.  You must also establish a handler for this signal, as described in See Signal Handling, in order to take appropriate action such as reading the out-of-band data.

Alternatively, you can test for pending out-of-band data, or wait until there is out-of-band data, using the select function; it can wait for an exceptional condition on the socket.  See Waiting for I/O, for more information about select.

Notification of out-of-band data (whether with SIGURG or with select) indicates that out-of-band data is on the way; the data may not actually arrive until later.  If you try to read the out-of-band data before it arrives, recv fails with an EWOULDBLOCK error.

Sending out-of-band data automatically places a "mark" in the stream of ordinary data, showing where in the sequence the out-of-band data "would have been".  This is useful when the meaning of out-of-band data is "cancel everything sent so far".  Here is how you can test, in the receiving process, whether any ordinary data was sent before the mark:

success = ioctl (socket, SIOCATMARK, &result);

Here's a function to discard any ordinary data preceding the out-of-band mark:

int
discard_until_mark (int socket)
{
  while (1)
    {
      /* This is not an arbitrary limit; any size will do.  */
      char buffer[1024];
      int result, success;

      /* If we have reached the mark, return.  */
      success = ioctl (socket, SIOCATMARK, &result);
      if (success < 0)
        perror ("ioctl");
      if (result)
        return;

      /* Otherwise, read a bunch of ordinary data and discard it.
         This is guaranteed not to read past the mark
         if it starts before the mark.  */
      success = read (socket, buffer, sizeof buffer);
      if (success < 0)
        perror ("read");
    }
}

If you don't want to discard the ordinary data preceding the mark, you may need to read some of it anyway, to make room in internal system buffers for the out-of-band data.  If you try to read out-of-band data and get an EWOULDBLOCK error, try reading some ordinary data (saving it so that you can use it when you want it) and see if that makes room.  Here is an example:

struct buffer
{
  char *buffer;
  int size;
  struct buffer *next;
};

/* Read the out-of-band data from SOCKET and return it
   as a `struct buffer', which records the address of the data
   and its size.

   It may be necessary to read some ordinary data
   in order to make room for the out-of-band data.
   If so, the ordinary data is saved as a chain of buffers
   found in the `next' field of the value.  */

struct buffer *
read_oob (int socket)
{
  struct buffer *tail = 0;
  struct buffer *list = 0;

  while (1)
    {
      /* This is an arbitrary limit.
         Does anyone know how to do this without a limit?  */
      char *buffer = (char *) xmalloc (1024);
      struct buffer *link;
      int success;
      int result;

      /* Try again to read the out-of-band data.  */
      success = recv (socket, buffer, sizeof buffer, MSG_OOB);
      if (success >= 0)
        {
          /* We got it, so return it.  */
          struct buffer *link
            = (struct buffer *) xmalloc (sizeof (struct buffer));
          link->buffer = buffer;
          link->size = success;
          link->next = list;
          return link;
        }

      /* If we fail, see if we are at the mark.  */
      success = ioctl (socket, SIOCATMARK, &result);
      if (success < 0)
        perror ("ioctl");
      if (result)
        {
          /* At the mark; skipping past more ordinary data cannot help.
             So just wait a while.  */
          sleep (1);
          continue;
        }

      /* Otherwise, read a bunch of ordinary data and save it.
         This is guaranteed not to read past the mark
         if it starts before the mark.  */
      success = read (socket, buffer, sizeof buffer);
      if (success < 0)
        perror ("read");

      /* Save this data in the buffer list.  */
      {
        struct buffer *link
          = (struct buffer *) xmalloc (sizeof (struct buffer));
        link->buffer = buffer;
        link->size = success;

        /* Add the new link to the end of the list.  */
        if (tail)
          tail->next = link;
        else
          list = link;
        tail = link;
      }
    }
}


Datagram Socket Operations
Node: Datagrams, Next: Inetd, Prev: Connections, Up: Sockets

Datagram Socket Operations

This section describes how to use communication styles that don't use connections (styles SOCK_DGRAM and SOCK_RDM).  Using these styles, you group data into packets and each packet is an independent communication.  You specify the destination for each packet individually.

Datagram packets are like letters: you send each one independently, with its own destination address, and they may arrive in the wrong order or not at all.

The listen and accept functions are not allowed for sockets using connectionless communication styles.

* Menu:

Sending Datagrams	Sending packets on a datagram socket.

Receiving Datagrams	Receiving packets on a datagram socket.

Datagram Example	An example program: packets sent over a datagram socket in the file namespace.

Example Receiver	Another program, that receives those packets.


Sending Datagrams
Node: Sending Datagrams, Next: Receiving Datagrams, Prev: , Up: Datagrams

Sending Datagrams

The normal way of sending data on a datagram socket is by using the sendto function, declared in 'sys/socket.h'.

You can call connect on a datagram socket, but this only specifies a default destination for further data transmission on the socket.  When a socket has a default destination, then you can use send (see Sending Data) or even write (see I/O Primitives) to send a packet there.  You can cancel the default destination by calling connect using an address format of AF_UNSPEC in the addr argument.  See Connecting, for more information about the connect function.

 - Function: int sendto (int socket, void *buffer. size_t size, int flags, struct sockaddr *addr, size_t length)

The sendto function transmits the data in the buffer through the socket socket to the destination address specified by the addr and length arguments.  The size argument specifies the number of bytes to be transmitted.

The flags are interpreted the same way as for send; see See Socket Data Options.

The return value and error conditions are also the same as for send, but you cannot rely on the system to detect errors and report them; the most common error is that the packet is lost or there is no one at the specified address to receive it, and the operating system on your machine usually does not know this.

It is also possible for one call to sendto to report an error due to a problem related to a previous call.


Receiving Datagrams
Node: Receiving Datagrams, Next: Datagram Example, Prev: Sending Datagrams, Up: Datagrams

Receiving Datagrams

The recvfrom function reads a packet from a datagram socket and also tells you where it was sent from.  This function is declared in 'sys/socket.h'.

 - Function: int recvfrom (int socket, void *buffer, size_t size, int flags, struct sockaddr *addr, size_t *length_ptr)

The recvfrom function reads one packet from the socket socket into the buffer buffer.  The size argument specifies the maximum number of bytes to be read.

If the packet is longer than size bytes, then you get the first size bytes of the packet, and the rest of the packet is lost.  There's no way to read the rest of the packet.  Thus, when you use a packet protocol, you must always know how long a packet to expect.

The addr and length_ptr arguments are used to return the address where the packet came from.  See Socket Addresses.  For a socket in the file domain, the address information won't be meaningful, since you can't read the address of such a socket (see File Namespace).  You can specify a null pointer as the addr argument if you are not interested in this information.

The flags are interpreted the same way as for recv (see Socket Data Options).  The return value and error conditions are also the same as for recv.

You can use plain recv (see Receiving Data) instead of recvfrom if you know don't need to find out who sent the packet (either because you know where it should come from or because you treat all possible senders alike).  Even read can be used if you don't want to specify flags (see I/O Primitives).


Datagram Socket Example
Node: Datagram Example, Next: Example Receiver, Prev: Receiving Datagrams, Up: Datagrams

Datagram Socket Example

Here is a set of example programs that send messages over a datagram stream in the file namespace.  Both the client and server programs use the make_named_socket function that was presented in See File Namespace, to create and name their sockets.

First, here is the server program.  It sits in a loop waiting for messages to arrive, bouncing each message back to the sender.  Obviously, this isn't a particularly useful program, but it does show the general ideas involved.

#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/un.h>

#define SERVER  "/tmp/serversocket"
#define MAXMSG  512

int
main (void)
{
  int sock;
  char message[MAXMSG];
  struct sockaddr_un name;
  size_t size;
  int nbytes;

  /* Make the socket, then loop endlessly. */

  sock = make_named_socket (SERVER);
  while (1)
    {
      /* Wait for a datagram. */
      size = sizeof (name);
      nbytes = recvfrom (sock, message, MAXMSG, 0,
                         (struct sockaddr *) & name, &size);
      if (nbytes < 0)
        {
          perror ("recfrom (server)");
          exit (EXIT_FAILURE);
        }

      /* Give a diagnostic message. */
      fprintf (stderr, "Server: got message: %s\n", message);

      /* Bounce the message back to the sender. */
      nbytes = sendto (sock, message, nbytes, 0,
                       (struct sockaddr *) & name, size);
      if (nbytes < 0)
        {
          perror ("sendto (server)");
          exit (EXIT_FAILURE);
        }
    }
}


Example of Reading Datagrams
Node: Example Receiver, Next: , Prev: Datagram Example, Up: Datagrams

Example of Reading Datagrams

Here is the client program corresponding to the server above.

It sends a datagram to the server and then waits for a reply.  Notice that the socket for the client (as well as for the server) in this example has to be given a name.  This is so that the server can direct a message back to the client.  Since the socket has no associated connection state, the only way the server can do this is by referencing the name of the client.

#include <stdio.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/un.h>

#define SERVER  "/tmp/serversocket"
#define CLIENT  "/tmp/mysocket"
#define MAXMSG  512
#define MESSAGE "Yow!!! Are we having fun yet?!?"

int
main (void)
{
  extern int make_named_socket (const char *name);
  int sock;
  char message[MAXMSG];
  struct sockaddr_un name;
  size_t size;
  int nbytes;

  /* Make the socket. */
  sock = make_named_socket (CLIENT);

  /* Initialize the server socket address. */
  name.sun_family = AF_UNIX;
  strcpy (name.sun_path, SERVER);
  size = strlen (name.sun_path) + sizeof (name.sun_family);

  /* Send the datagram. */
  nbytes = sendto (sock, MESSAGE, strlen (MESSAGE) + 1, 0,
                   (struct sockaddr *) & name, size);
  if (nbytes < 0)
    {
      perror ("sendto (client)");
      exit (EXIT_FAILURE);
    }

  /* Wait for a reply. */
  nbytes = recvfrom (sock, message, MAXMSG, 0, NULL, 0);
  if (nbytes < 0)
    {
      perror ("recfrom (client)");
      exit (EXIT_FAILURE);
    }

  /* Print a diagnostic message. */
  fprintf (stderr, "Client: got message: %s\n", message);

  /* Clean up. */
  remove (CLIENT);
  close (sock);
}

Keep in mind that datagram socket communications are unreliable.  In this example, the client program waits indefinitely if the message never reaches the server or if the server's response never comes back.  It's up to the user running the program to kill it and restart it, if desired.  A more automatic solution could be to use select (see Waiting for I/O) to establish a timeout period for the reply, and in case of timeout either resend the message or shut down the socket and exit.


The inetd Daemon
Node: Inetd, Next: Socket Options, Prev: Datagrams, Up: Sockets

The inetd Daemon

We've explained above how to write a server program that does its own listening.  Such a server must already be running in order for anyone to connect to it.

Another way to provide service for an Internet port is to let the daemon program inetd do the listening.  inetd is a program that runs all the time and waits (using select) for messages on a specified set of ports.  When it receives a message, it accepts the connection (if the socket style calls for connections) and then forks a child process to run the corresponding server program.  You specify the ports and their programs in the file '/etc/inetd.conf'.

* Menu:

Inetd Servers

Configuring Inetd


inetd Servers
Node: Inetd Servers, Next: Configuring Inetd, Prev: , Up: Inetd

inetd Servers

Writing a server program to be run by inetd is very simple.  Each time someone requests a connection to the appropriate port, a new server process starts.  The connection already exists at this time; the socket is available as the standard input descriptor and as the standard output descriptor (descriptors 0 and 1) in the server process.  So the server program can begin reading and writing data right away.  Often the program needs only the ordinary I/O facilities; in fact, a general-purpose filter program that knows nothing about sockets can work as a byte stream server run by inetd.

You can also use inetd for servers that use connectionless communication styles.  For these servers, inetd does not try to accept a connection, since no connection is possible.  It just starts the server program, which can read the incoming datagram packet from descriptor 0.  The server program can handle one request and then exit, or you can choose to write it to keep reading more requests until no more arrive, and then exit.  You must specify which of these two techniques the server uses, when you configure inetd.


Configuring inetd
Node: Configuring Inetd, Next: , Prev: Inetd Servers, Up: Inetd

Configuring inetd

The file '/etc/inetd.conf' tells inetd which ports to listen to and what server programs to run for them.  Normally each entry in the file is one line, but you can split it onto multiple lines provided all but the first line of the entry start with whitespace.  Lines that start with '#' are comments.

Here are two standard entries in '/etc/inetd.conf':

ftp	stream	tcp	nowait	root	/libexec/ftpd	ftpd
talk	dgram	udp	wait	root	/libexec/talkd	talkd

An entry has this format:

service style protocol wait username program arguments

The service field says which service this program provides.  It should be the name of a service defined in '/etc/services'.  inetd uses service to decide which port to listen on for this entry.

The fields style and protocol specify the communication style and the protocol to use for the listening socket.  The style should be the name of a communication style, converted to lower case and with 'SOCK_' deleted--for example, 'stream' or 'dgram'.  protocol should be one of the protocols listed in '/etc/protocols'.  The typical protocol names are 'tcp' for byte stream connections and 'udp' for unreliable datagrams.

The wait field should be either 'wait' or 'nowait'.  Use 'wait' if style is a connectionless style and the server, once started, handles multiple requests, as many as come in.  Use 'nowait' if inetd should start a new process for each message or request that comes in.  If style uses connections, then wait must be 'nowait'.

user is the user name that the server should run as.  inetd runs as root, so it can set the user ID of its children arbitrarily.  It's best to avoid using 'root' for user if you can; but some servers, such as Telnet and FTP, read a username and password themselves.  These servers need to be root initially so they can log in as commanded by the data coming over the network.

program together with arguments specifies the command to run to start the server.  program should be an absolute file name specifying the executable file to run.  arguments consists of any number of whitespace-separated words, which become the command-line arguments of program.  The first word in arguments is argument zero, which should by convention be the program name itself (sans directories).

If you edit '/etc/inetd.conf', you can tell inetd to reread the file and obey its new contents by sending the inetd process the SIGHUP signal.  You'll have to use ps to determine the process ID of the inetd process, as it is not fixed.


Socket Options
Node: Socket Options, Next: Networks Database, Prev: Inetd, Up: Sockets

Socket Options

This section describes how to read or set various options that modify the behavior of sockets and their underlying communications protocols.

When you are manipulating a socket option, you must specify which "level" the option pertains to.  This describes whether the option applies to the socket interface, or to a lower-level communications protocol interface.

* Menu:

Socket Option Functions	The basic functions for setting and getting socket options.

Socket-Level Options	Details of the options at the socket level.


Socket Option Functions
Node: Socket Option Functions, Next: Socket-Level Options, Prev: , Up: Socket Options

Socket Option Functions

Here are the functions for examining and modifying socket options.  They are declared in 'sys/socket.h'.

 - Function: int getsockopt (int socket, int level, int optname, void *optval, size_t *optlen_ptr)

The getsockopt function gets information about the value of option optname at level level for socket socket.

The option value is stored in a buffer that optval points to.  Before the call, you should supply in *optlen_ptr the size of this buffer; on return, it contains the number of bytes of information actually stored in the buffer.

Most options interpret the optval buffer as a single int value.

The actual return value of getsockopt is 0 on success and -1 on failure.  The following errno error conditions are defined:

EBADF

The socket argument is not a valid file descriptor.

ENOTSOCK

The descriptor socket is not a socket.

ENOPROTOOPT

The optname doesn't make sense for the given level.

 - Function: int setsockopt (int socket, int level, int optname, void *optval, size_t optlen)

This function is used to set the socket option optname at level level for socket socket.  The value of the option is passed in the buffer optval, which has size optlen.

The return value and error codes for setsockopt are the same as for getsockopt.


Socket-Level Options
Node: Socket-Level Options, Next: , Prev: Socket Option Functions, Up: Socket Options

Socket-Level Options

 - Constant: int SOL_SOCKET

Use this constant as the level argument to getsockopt or setsockopt to manipulate the socket-level options described in this section.

Here is a table of socket-level option names; all are defined in the header file 'sys/socket.h'.

SO_DEBUG

This option toggles recording of debugging information in the underlying protocol modules.  The value has type int; a nonzero value means "yes".

SO_REUSEADDR

This option controls whether bind (see Setting Address) should permit reuse of local addresses for this socket.  If you enable this option, you can actually have two sockets with the same Internet port number; but the system won't allow you to use the two identically-named sockets in a way that would confuse the Internet.  The reason for this option is that some higher-level Internet protocols, including FTP, require you to keep reusing the same socket number.

The value has type int; a nonzero value means "yes".

SO_KEEPALIVE

This option controls whether the underlying protocol should periodically transmit messages on a connected socket.  If the peer fails to respond to these messages, the connection is considered broken.  The value has type int; a nonzero value means "yes".

SO_DONTROUTE

This option controls whether outgoing messages bypass the normal message routing facilities.  If set, messages are sent directly to the network interface instead.  The value has type int; a nonzero value means "yes".

SO_LINGER

This option specifies what should happen when the socket of a type that promises reliable delivery still has untransmitted messages when it is closed; see See Closing a Socket.  The value has type struct linger.

 - Data Type: struct linger

This structure type has the following members:

int l_onoff

This field is interpreted as a boolean.  If nonzero, close blocks until the data is transmitted or the timeout period has expired.

int l_linger

This specifies the timeout period, in seconds.

SO_BROADCAST

This option controls whether datagrams may be broadcast from the socket.  The value has type int; a nonzero value means "yes".

SO_OOBINLINE

If this option is set, out-of-band data received on the socket is placed in the normal input queue.  This permits it to be read using read or recv without specifying the MSG_OOB flag.  See Out-of-Band Data.  The value has type int; a nonzero value means "yes".

SO_SNDBUF

This option gets or sets the size of the output buffer.  The value is a size_t, which is the size in bytes.

SO_RCVBUF

This option gets or sets the size of the input buffer.  The value is a size_t, which is the size in bytes.

SO_STYLE

SO_TYPE

This option can be used with getsockopt only.  It is used to get the socket's communication style.  SO_TYPE is the historical name, and SO_STYLE is the preferred name in GNU.  The value has type int and its value designates a communication style; see See Communication Styles.

SO_ERROR

This option can be used with getsockopt only.  It is used to reset the error status of the socket.  The value is an int, which represents the previous error status.


Networks Database
Node: Networks Database, Next: , Prev: Socket Options, Up: Sockets

Networks Database

Many systems come with a database that records a list of networks known to the system developer.  This is usually kept either in the file '/etc/networks' or in an equivalent from a name server.  This data base is useful for routing programs such as route, but it is not useful for programs that simply communicate over the network.  We provide functions to access this data base, which are declared in 'netdb.h'.

 - Data Type: struct netent

This data type is used to represent information about entries in the networks database.  It has the following members:

char *n_name

This is the "official" name of the network.

char **n_aliases

These are alternative names for the network, represented as a vector of strings.  A null pointer terminates the array.

int n_addrtype

This is the type of the network number; this is always equal to AF_INET for Internet networks.

unsigned long int n_net

This is the network number.  Network numbers are returned in host byte order; see See Byte Order.

Use the getnetbyname or getnetbyaddr functions to search the networks database for information about a specific network.  The information is returned in a statically-allocated structure; you must copy the information if you need to save it.

 - Function: struct netent * getnetbyname (const char *name)

The getnetbyname function returns information about the network named name.  It returns a null pointer if there is no such network.

 - Function: struct netent * getnetbyaddr (long net, int type)

The getnetbyaddr function returns information about the network of type type with number net.  You should specify a value of AF_INET for the type argument for Internet networks.

getnetbyaddr returns a null pointer if there is no such network.

You can also scan the networks database using setnetent, getnetent, and endnetent.  Be careful in using these functions, because they are not reentrant.

 - Function: void setnetent (int stayopen)

This function opens and rewinds the networks database.

If the stayopen argument is nonzero, this sets a flag so that subsequent calls to getnetbyname or getnetbyaddr will not close the database (as they usually would).  This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.

 - Function: struct netent * getnetent (void)

This function returns the next entry in the networks database.  It returns a null pointer if there are no more entries.

 - Function: void endnetent (void)

This function closes the networks database.


Low-Level Terminal Interface
Node: Low-Level Terminal Interface, Next: Mathematics, Prev: Sockets, Up: Top

Low-Level Terminal Interface

This chapter describes functions that are specific to terminal devices.  You can use these functions to do things like turn off input echoing; set serial line characteristics such as line speed and flow control; and change which characters are used for end-of-file, command-line editing, sending signals, and similar control functions.

Most of the functions in this chapter operate on file descriptors.  See Low-Level I/O, for more information about what a file descriptor is and how to open a file descriptor for a terminal device.

* Menu:

Is It a Terminal	How to determine if a file is a terminal device, and what its name is.

I/O Queues	About flow control and typeahead.

Canonical or Not	Two basic styles of input processing.

Terminal Modes	How to examine and modify flags controlling details of terminal I/O: echoing, signals, editing.

Line Control	Sending break sequences, clearing terminal buffers ...

Noncanon Example	How to read single characters without echo.


Identifying Terminals
Node: Is It a Terminal, Next: I/O Queues, Prev: , Up: Low-Level Terminal Interface

Identifying Terminals

The functions described in this chapter only work on files that correspond to terminal devices.  You can find out whether a file descriptor is associated with a terminal by using the isatty function.

Prototypes for both isatty and ttyname are declared in the header file 'unistd.h'.

 - Function: int isatty (int filedes)

This function returns 1 if filedes is a file descriptor associated with an open terminal device, and 0 otherwise.

If a file descriptor is associated with a terminal, you can get its associated file name using the ttyname function.  See also the ctermid function, described in See Identifying the Terminal.

 - Function: char * ttyname (int filedes)

If the file descriptor filedes is associated with a terminal device, the ttyname function returns a pointer to a statically-allocated, null-terminated string containing the file name of the terminal file.  The value is a null pointer if the file descriptor isn't associated with a terminal, or the file name cannot be determined.


I/O Queues
Node: I/O Queues, Next: Canonical or Not, Prev: Is It a Terminal, Up: Low-Level Terminal Interface

I/O Queues

Many of the remaining functions in this section refer to the input and output queues of a terminal device.  These queues implement a form of buffering within the kernel independent of the buffering implemented by I/O streams (see I/O on Streams).

The "terminal input queue" is also sometimes referred to as its "typeahead buffer".  It holds the characters that have been received from the terminal but not yet read by any process.

The size of the terminal's input queue is described by the _POSIX_MAX_INPUT and MAX_INPUT parameters; see See Limits for Files.  If input flow control is enabled by setting the IXOFF input mode bit (see Input Modes), the terminal driver transmits STOP and START characters to the terminal when necessary to prevent the queue from overflowing.  Otherwise, input may be lost if it comes in too fast from the terminal.  (This is unlikely if you are typing the input by hand!)

The "terminal output queue" is like the input queue, but for output; it contains characters that have been written by processes, but not yet transmitted to the terminal.  If output flow control is enabled by setting the IXON input mode bit (see Input Modes), the terminal driver obeys STOP and STOP characters sent by the terminal to stop and restart transmission of output.

"Clearing" the terminal input queue means discarding any characters that have been received but not yet read.  Similarly, clearing the terminal output queue means discarding any characters that have been written but not yet transmitted.


Two Styles of Input: Canonical or Not
Node: Canonical or Not, Next: Terminal Modes, Prev: I/O Queues, Up: Low-Level Terminal Interface

Two Styles of Input: Canonical or Not

POSIX systems support two basic modes of input: canonical and noncanonical.

In "canonical input processing" mode, terminal input is processed in lines terminated by newline ('\n'), EOF, or EOL characters.  No input can be read until an entire line has been typed by the user, and the read function (see I/O Primitives) returns at most a single line of input, no matter how many bytes are requested.

In canonical input mode, the operating system provides input editing facilities: the ERASE and KILL characters are interpreted specially to perform editing operations within the current line of text.  See Editing Characters.

The constants _POSIX_MAX_CANON and MAX_CANON parameterize the maximum number of bytes which may appear in a single line of canonical input.  See Limits for Files.

In "noncanonical input processing" mode, characters are not grouped into lines, and ERASE and KILL processing is not performed.  The granularity with which bytes are read in noncanonical input mode is controlled by the MIN and TIME settings.  See Noncanonical Input.

Most programs use canonical input mode, because this gives the user a way to edit input line by line.  The usual reason to use noncanonical mode is when the program accepts single-character commands or provides its own editing facilities.

The choice of canonical or noncanonical input is controlled by the ICANON flag in the c_lflag member of struct termios.  See Local Modes.


Terminal Modes
Node: Terminal Modes, Next: Line Control, Prev: Canonical or Not, Up: Low-Level Terminal Interface

Terminal Modes

This section describes the various terminal attributes that control how input and output are done.  The functions, data structures, and symbolic constants are all declared in the header file 'termios.h'.

* Menu:

Mode Data Types	The data type struct termios and related types.

Mode Functions	Functions to read and set the terminal attributes.

Setting Modes	The right way to set terminal attributes reliably.

Input Modes	Flags controlling low-level input handling.

Output Modes	Flags controlling low-level output handling.

Control Modes	Flags controlling serial port behavior.

Local Modes	Flags controlling high-level input handling.

Line Speed	How to read and set the terminal line speed.

Special Characters	Characters that have special effects, and how to change them.

Noncanonical Input	Controlling how long to wait for input.


Terminal Mode Data Types
Node: Mode Data Types, Next: Mode Functions, Prev: , Up: Terminal Modes

Terminal Mode Data Types

The entire collection of attributes of a terminal is stored in a structure of type struct termios.  This structure is used with the functions tcgetattr and tcsetattr to read and set the attributes.

 - Data Type: struct termios

Structure that records all the I/O attributes of a terminal.  The structure includes at least the following members:

tcflag_t c_iflag

A bit mask specifying flags for input modes; see See Input Modes.

tcflag_t c_oflag

A bit mask specifying flags for output modes; see See Output Modes.

tcflag_t c_cflag

A bit mask specifying flags for control modes; see See Control Modes.

tcflag_t c_lflag

A bit mask specifying flags for local modes; see See Local Modes.

cc_t c_cc[NCCS]

An array specifying which characters are associated with various control functions; see See Special Characters.

The struct termios structure also contains members which encode input and output transmission speeds, but the representation is not specified.  See Line Speed, for how to examine and store the speed values.

The following sections describe the details of the members of the struct termios structure.

 - Data Type: tcflag_t

This is an unsigned integer type used to represent the various bit masks for terminal flags.

 - Data Type: cc_t

This is an unsigned integer type used to represent characters associated with various terminal control functions.

 - Macro: int NCCS

The value of this macro is the number of elements in the c_cc array.


Terminal Mode Functions
Node: Mode Functions, Next: Setting Modes, Prev: Mode Data Types, Up: Terminal Modes

Terminal Mode Functions

 - Function: int tcgetattr (int filedes, struct termios *termios_p)

This function is used to examine the attributes of the terminal device with file descriptor filedes.  The attributes are returned in the structure that termios_p points to.

If successful, tcgetattr returns 0.  A return value of -1 indicates an error.  The following errno error conditions are defined for this function:

EBADF

The filedes argument is not a valid file descriptor.

ENOTTY

The filedes is not associated with a terminal.

 - Function: int tcsetattr (int filedes, int when, const struct termios *termios_p)

This function sets the attributes of the terminal device with file descriptor filedes.  The new attributes are taken from the structure that termios_p points to.

The when argument specifies how to deal with input and output already queued.  It can be one of the following values:

TCSANOW

Make the change immediately.

TCSADRAIN

Make the change after waiting until all queued output has been written.  You should usually use this option when changing parameters that affect output.

TCSAFLUSH

This is like TCSADRAIN, but also discards any queued input.

TCSASOFT

This is a flag bit that you can add to any of the above alternatives.  Its meaning is to inhibit alteration of the state of the terminal hardware.  It is a BSD extension; it has no effect on non-BSD systems.

If this function is called from a background process on its controlling terminal, normally all processes in the process group are sent a SIGTTOU signal, in the same way as if the process were trying to write to the terminal.  The exception is if the calling process itself is ignoring or blocking SIGTTOU signals, in which case the operation is performed and no signal is sent.  See Job Control.

If successful, tcsetattr returns 0.  A return value of -1 indicates an error.  The following errno error conditions are defined for this function:

EBADF

The filedes argument is not a valid file descriptor.

ENOTTY

The filedes is not associated with a terminal.

EINVAL

Either the value of the when argument is not valid, or there is something wrong with the data in the termios_p argument.

Although tcgetattr and tcsetattr specify the terminal device with a file descriptor, the attributes are those of the terminal device itself and not of the file descriptor.  This means that the effects of changing terminal attributes are persistent; if another process opens the terminal file later on, it will see the changed attributes even though it doesn't have anything to do with the open file descriptor you originally specified in changing the attributes.

Similarly, if a single process has multiple or duplicated file descriptors for the same terminal device, changing the terminal attributes affects input and output to all of these file descriptors.  This means, for example, that you can't open one file descriptor or stream to read from a terminal in the normal line-buffered, echoed mode; and simultaneously have another file descriptor for the same terminal that you use to read from it in single-character, non-echoed mode.  Instead, you have to explicitly switch the terminal back and forth between the two modes.


Setting Terminal Modes Properly
Node: Setting Modes, Next: Input Modes, Prev: Mode Functions, Up: Terminal Modes

Setting Terminal Modes Properly

When you set terminal modes, you should call tcgetattr first to get the current modes of the particular terminal device, modify only those modes that you are really interested in, and store the result with tcsetattr.

It's a bad idea to simply initialize a struct termios structure to a chosen set of attributes and pass it directly to tcsetattr.  Your program may be run years from now, on systems that support members not documented in this manual.  The way to avoid setting these members to unreasonable values is to avoid changing them.

What's more, different terminal devices may require different mode settings in order to function properly.  So you should avoid blindly copying attributes from one terminal device to another.

When a member contains a collection of independent flags, as the c_iflag, c_oflag and c_cflag members do, even setting the entire member is a bad idea, because particular operating systems have their own flags.  Instead, you should start with the current value of the member and alter only the flags whose values matter in your program, leaving any other flags unchanged.

Here is an example of how to set one flag (ISTRIP) in the struct termios structure while properly preserving all the other data in the structure:

int
set_istrip (int desc, int value)
{
  struct termios settings;
  int result;

  result = tcgetattr (desc, &settings);
  if (result < 0)
    {
      perror ("error in tcgetattr");
      return 0;
    }
  settings.c_iflag &= ~ISTRIP;
  if (value)
    settings.c_iflag |= ISTRIP;
  result = tcgetattr (desc, &settings);
  if (result < 0)
    {
      perror ("error in tcgetattr");
      return;
   }
  return 1;
}


Input Modes
Node: Input Modes, Next: Output Modes, Prev: Setting Modes, Up: Terminal Modes

Input Modes

This section describes the terminal attribute flags that control fairly low-level aspects of input processing: handling of parity errors, break signals, flow control, and RET and LFD characters.

All of these flags are bits in the c_iflag member of the struct termios structure.  The member is an integer, and you change flags using the operators &, | and ^.  Don't try to specify the entire value for c_iflag--instead, change only specific flags and leave the rest untouched (see Setting Modes).

INPCK

If this bit is set, input parity checking is enabled.  If it is not set, no checking at all is done for parity errors on input; the characters are simply passed through to the application.

Parity checking on input processing is independent of whether parity detection and generation on the underlying terminal hardware is enabled; see See Control Modes.  For example, you could clear the INPCK input mode flag and set the PARENB control mode flag to ignore parity errors on input, but still generate parity on output.

If this bit is set, what happens when a parity error is detected depends on whether the IGNPAR or PARMRK bits are set.  If neither of these bits are set, a byte with a parity error is passed to the application as a '\0' character.

IGNPAR

If this bit is set, any byte with a framing or parity error is ignored.  This is only useful if INPCK is also set.

PARMRK

If this bit is set, input bytes with parity or framing errors are marked when passed to the program.  This bit is meaningful only when INPCK is set and IGNPAR is not set.

The way erroneous bytes are marked is with two preceding bytes, 377 and 0.  Thus, the program actually reads three bytes for one erroneous byte received from the terminal.

If a valid byte has the value 0377, and ISTRIP (see below) is not set, the program might confuse it with the prefix that marks a parity error.  So a valid byte 0377 is passed to the program as two bytes, 0377 0377, in this case.

ISTRIP

If this bit is set, valid input bytes are stripped to seven bits; otherwise, all eight bits are available for programs to read.

IGNBRK

If this bit is set, break conditions are ignored.

A "break condition" is defined in the context of asynchronous serial data transmission as a series of zero-value bits longer than a single byte.

BRKINT

If this bit is set and IGNBRK is not set, a break condition clears the terminal input and output queues and raises a SIGINT signal for the foreground process group associated with the terminal.

If neither BRKINT nor IGNBRK are set, a break condition is passed to the application as a single '\0' character if PARMRK is not set, or otherwise as a three-character sequence '\377', '\0', '\0'.

IGNCR

If this bit is set, carriage return characters ('\r') are discarded on input.  Discarding carriage return may be useful on terminals that send both carriage return and linefeed when you type the RET key.

ICRNL

If this bit is set and IGNCR is not set, carriage return characters ('\r') received as input are passed to the application as newline characters ('\n').

INLCR

If this bit is set, newline characters ('\n') received as input are passed to the application as carriage return characters ('\r').

IXOFF

If this bit is set, start/stop control on input is enabled.  In other words, the computer sends STOP and START characters as necessary to prevent input from coming in faster than programs are reading it.  The idea is that the actual terminal hardware that is generating the input data responds to a STOP character by suspending transmission, and to a START character by resuming transmission.  See Start/Stop Characters.

IXON

If this bit is set, start/stop control on output is enabled.  In other words, if the computer receives a STOP character, it suspends output until a START character is received.  In this case, the STOP and START characters are never passed to the application program.  If this bit is not set, then START and STOP can be read as ordinary characters.  See Start/Stop Characters.

IXANY

If this bit is set, any input character restarts output when output has been suspended with the STOP character.  Otherwise, only the START character restarts output.

IMAXBEL

If this bit is set, then filling up the terminal input buffer sends a BEL character (code 007) to the terminal to ring the bell.


Output Modes
Node: Output Modes, Next: Control Modes, Prev: Input Modes, Up: Terminal Modes

Output Modes

This section describes the terminal flags and fields that control how output characters are translated and padded for display.  All of these are contained in the c_oflag member of the struct termios structure.

The c_oflag member itself is an integer, and you change the flags and fields using the operators &, |, and ^.  Don't try to specify the entire value for c_oflag--instead, change only specific flags and leave the rest untouched (see Setting Modes).

 - Macro: int OPOST

If this bit is set, output data is processed in some unspecified way so that it is displayed appropriately on the terminal device.  This typically includes mapping newline characters ('\n') onto carriage return and linefeed pairs.

If this bit isn't set, the characters are transmitted as-is.

The following three bits are BSD features, and they have no effect on non-BSD systems.  On all systems, they are effective only if OPOST is set.

 - Macro: int ONLCR

If this bit is set, convert the newline character on output into a pair of characters, carriage return followed by linefeed.

 - Macro: int OXTABS

If this bit is set, convert tab characters on output into the appropriate number of spaces to emulate a tab stop every eight columns.

 - Macro: int ONOEOT

If this bit is set, discard ^C-d characters (code 004) on output.  These characters cause many dial-up terminals to disconnect.


Control Modes
Node: Control Modes, Next: Local Modes, Prev: Output Modes, Up: Terminal Modes

Control Modes

This section describes the terminal flags and fields that control parameters usually associated with asynchronous serial data transmission.  These flags may not make sense for other kinds of terminal ports (such as a network connection pseudo-terminal).  All of these are contained in the c_cflag member of the struct termios structure.

The c_cflag member itself is an integer, and you change the flags and fields using the operators &, |, and ^.  Don't try to specify the entire value for c_cflag--instead, change only specific flags and leave the rest untouched (see Setting Modes).

CLOCAL

If this bit is set, it indicates that the terminal is connected "locally" and that the modem status lines (such as carrier detect) should be ignored.  KIf this bit is not set and you call open without the O_NONBLOCK flag set, open blocks until a modem connection is established.

If this bit is not set and a modem disconnect is detected, a SIGHUP signal is sent to the controlling process group for the terminal (if it has one).  Normally, this causes the process to exit; see See Signal Handling.  Reading from the terminal after a disconnect causes an end-of-file condition, and writing causes an EIO error to be returned.  The terminal device must be closed and reopened to clear the condition.  K

HUPCL

If this bit is set, a modem disconnect is generated when all processes that have the terminal device open have either closed the file or exited.

CREAD

If this bit is set, input can be read from the terminal.  Otherwise, input is discarded when it arrives.

CSTOPB

If this bit is set, two stop bits are used.  Otherwise, only one stop bit is used.

PARENB

If this bit is set, generation and detection of a parity bit are enabled.  See Input Modes, for information on how input parity errors are handled.

If this bit is not set, no parity bit is added to output characters, and input characters are not checked for correct parity.

PARODD

This bit is only useful if PARENB is set.  If PARODD is set, odd parity is used, otherwise even parity is used.

The control mode flags also includes a field for the number of bits per character.  You can use the CSIZE macro as a mask to extract the value, like this: settings.c_cflag & CSIZE.

CSIZE

This is a mask for the number of bits per character.

CS5

This specifies five bits per byte.

CS6

This specifies six bits per byte.

CS7

This specifies seven bits per byte.

CS8

This specifies eight bits per byte.

CCTS_OFLOW

If this bit is set, enable flow control of output based on the CTS wire (RS232 protocol).

CRTS_IFLOW

If this bit is set, enable flow control of input based on the RTS wire (RS232 protocol).

MDMBUF

If this bit is set, enable carrier-based flow control of output.


Local Modes
Node: Local Modes, Next: Line Speed, Prev: Control Modes, Up: Terminal Modes

Local Modes

This section describes the flags for the c_lflag member of the struct termios structure.  These flags generally control higher-level aspects of input processing than the input modes flags described in See Input Modes, such as echoing, signals, and the choice of canonical or noncanonical input.

The c_lflag member itself is an integer, and you change the flags and fields using the operators &, |, and ^.  Don't try to specify the entire value for c_lflag--instead, change only specific flags and leave the rest untouched (see Setting Modes).

ICANON

This bit, if set, enables canonical input processing mode.  Otherwise, input is processed in noncanonical mode.  See Canonical or Not.

ECHO

If this bit is set, echoing of input characters back to the terminal is enabled.  K

ECHOE

If this bit is set, echoing indicates erasure of input with the ERASE character by erasing the last character in the current line from the screen.  Otherwise, the character erased is re-echoed to show what has happened (suitable for a printing terminal).

This bit only controls the display behavior; the ICANON bit by itself controls actual recognition of the ERASE character and erasure of input, without which ECHOE is simply irrelevant.

ECHOK

This bit enables special display of the KILL character.  There are two ways this can be done.  The better way is by erasing on the screen the entire line that has been killed.  The worse way is by moving to a new line after echoing the KILL character normally.  Some systems do one, some systems do the other, and some let you choose either way.

If this bit is not set, the KILL character echoes just as it would if it were not the KILL character.  Then it is up to the user to remember that the KILL character has erased the preceding input; there is no indication of this on the screen.

This bit only controls the display behavior; the ICANON bit by itself controls actual recognition of the KILL character and erasure of input, without which ECHOK is simply irrelevant.

ECHONL

If this bit is set and the ICANON bit is also set, then the newline ('\n') character is echoed even if the ECHO bit is not set.

ISIG

This bit controls whether the INTR, QUIT, and SUSP characters are recognized.  The functions associated with these characters are performed if and only if this bit is set.  Being in canonical or noncanonical input mode has no affect on the interpretation of these characters.

You should use caution when disabling recognition of these characters.  Programs that cannot be interrupted interactively are very user-unfriendly.  If you clear this bit, your program should provide some alternate interface that allows the user to interactively send the signals associated with these characters, or to escape from the program.  KSee Signal Characters.

IEXTEN

This bit is similar to ISIG, but controls implementation-defined special characters.  If it is set, it might override the default behavior for the ICANON and ISIG local mode flags, and the IXON and IXOFF input mode flags.

NOFLSH

Normally, the INTR, QUIT, and SUSP characters cause input and output queues for the terminal to be cleared.  If this bit is set, the queues are not cleared.

TOSTOP

If this bit is set and the system supports job control, then SIGTTOU signals are generated by background processes that attempt to write to the terminal.  See Access to the Terminal.

The following bits are BSD extensions; the GNU library defines these symbols on any system if you ask for them, but the settings of the bits have no effect except on BSD systems and on the GNU system.

ECHOKE

On BSD systems, this bit selects between the two alternative ways of displaying the KILL character, when ECHOK is set.  If ECHOKE is set, then the KILL character erases the whole screen line; otherwise, the KILL character moves to the next screen line.  The setting of ECHOKE has no effect when ECHOK is clear.

ECHOPRT

This bit enables display of the ERASE character in a way that is geared to a hardcopy terminal.

ECHOCTL

If this bit is set, echo control characters with '^' followed by the corresponding text character.  Thus, control-A echoes as '^A'.

ALTWERASE

This bit determines how far the WERASE character should erase.  The WERASE character erases back to the beginning of a word; the question is, where do words begin?

If this bit is clear, then the beginning of a word is a nonwhitespace character following a whitespace character.  If the bit is set, then the beginning of a word is an alphanumeric character or underscore following a character which is none of those.

FLUSHO

This is the bit that toggles when the user types the DISCARD character.  While this bit is set, all output is discarded.  See Other Special.

NOKERNINFO

Setting this bit disables handling of the STATUS character.  See Other Special.

PENDIN

If this bit is set, it indicates that there is a line of input that needs to be reprinted.  Typing the REPRINT character sets this bit; the bit remains set until reprinting is finished.  See BSD Editing.


Line Speed
Node: Line Speed, Next: Special Characters, Prev: Local Modes, Up: Terminal Modes

Line Speed

The terminal line speed tells the computer how fast to read and write data on the terminal.

If the terminal is connected to a real serial line, the terminal speed you specify actually controls the line--if it doesn't match the terminal's own idea of the speed, communication does not work.  Real serial ports accept only certain standard speeds.  Also, particular hardware may not support even all the standard speeds.  Specifying a speed of zero hangs up a dialup connection and turns off modem control signals.

If the terminal is not a real serial line (for example, if it is a network connection), then the line speed won't really affect data transmission speed, but some programs will use it to determine the amount of padding needed.  It's best to specify a line speed value that matches the actual speed of the actual terminal, but you can safely experiment with different values to vary the amount of padding.

There are actually two line speeds for each terminal, one for input and one for output.  You can set them independently, but most often terminals use the same speed for both directions.

The speed values are stored in the struct termios structure, but don't try to access them in the struct termios structure directly.  Instead, you should use the following functions to read and store them:

 - Function: speed_t cfgetospeed (const struct termios *termios_p)

This function returns the output line speed stored in the structure *termios_p.

 - Function: speed_t cfgetispeed (const struct termios *termios_p)

This function returns the input line speed stored in the structure *termios_p.

 - Function: int cfsetospeed (struct termios *termios_p, speed_t speed)

This function stores speed in *termios_p as the output speed.  The normal return value is 0; a value of -1 indicates an error.  If speed is not a speed, cfsetospeed returns -1.

 - Function: int cfsetispeed (struct termios *termios_p, speed_t speed)

This function stores speed in *termios_p as the input speed.  The normal return value is 0; a value of -1 indicates an error.  If speed is not a speed, cfsetospeed returns -1.

 - Function: int cfsetspeed (struct termios *termios_p, speed_t speed)

This function stores speed in *termios_p as both the input and output speeds.  The normal return value is 0; a value of -1 indicates an error.  If speed is not a speed, cfsetspeed returns -1.  This function is an extension in 4.4 BSD.

 - Data Type: speed_t

The speed_t type is an unsigned integer data type used to represent line speeds.

The functions cfsetospeed and cfsetispeed report errors only for speed values that the system simply cannot handle.  If you specify a speed value that is basically acceptable, then those functions will succeed.  But they do not check that a particular hardware device can actually support the specified speeds--in fact, they don't know which device you plan to set the speed for.  If you use tcsetattr to set the speed of a particular device to a value that it cannot handle, tcsetattr returns -1.

Portability note: In the GNU library, the functions above accept speeds measured in bits per second as input, and return speed values measured in bits per second.  Other libraries require speeds to be indicated by special codes.  For POSIX.1 portability, you must use one of the following symbols to represent the speed; their precise numeric values are system-dependent, but each name has a fixed meaning: B110 stands for 110 bps, B300 for 300 bps, and so on.  There is no portable way to represent any speed but these, but these are the only speeds that typical serial lines can support.

B0  B50  B75  B110  B134  B150  B200
B300  B600  B1200  B1800  B2400  B4800
B9600  B19200  B38400

BSD defines two additional speed symbols as aliases: EXTA is an alias for B19200 and EXTB is an alias for B38400.  These aliases are obsolete.

 - Function: int cfmakeraw (struct termios *termios_p)

This function provides an easy way to set up *termios_p for what has traditionally been called "raw mode" in BSD.  It does exactly this:

  termios_p->c_iflag &= ~(IGNBRK|BRKINT|PARMRK|ISTRIP
                  |INLCR|IGNCR|ICRNL|IXON);
  termios_p->c_oflag &= ~OPOST;
  termios_p->c_lflag &= ~(ECHO|ECHONL|ICANON|ISIG|IEXTEN);
  termios_p->c_cflag &= ~(CSIZE|PARENB);
  termios_p->c_cflag |= CS8;


Special Characters
Node: Special Characters, Next: Noncanonical Input, Prev: Line Speed, Up: Terminal Modes

Special Characters

In canonical input, the terminal driver recognizes a number of special characters which perform various control functions.  These include the ERASE character (usually DEL) for editing input, and other editing characters.  The INTR character (normally ^C-c) for sending a SIGINT signal, and other signal-raising characters, may be available in either canonical or noncanonical input mode.  All these characters are described in this section.

The particular characters used are specified in the c_cc member of the struct termios structure.  This member is an array; each element specifies the character for a particular role.  Each element has a symbolic constant that stands for the index of that element--for example, INTR is the index of the element that specifies the INTR character, so storing '=' in termios.c_cc[INTR] specifies '=' as the INTR character.

On some systems, you can disable a particular special character function by specifying the value _POSIX_VDISABLE for that role.  This value is unequal to any possible character code.  See Options for Files, for more information about how to tell whether the operating system you are using supports _POSIX_VDISABLE.

* Menu:

Editing Characters	Special characters that terminate lines and delete text, and other editing functions.

BSD Editing	Special editing characters for BSD systems.

Signal Characters	Special characters that send or raise signals to or for certain classes of processes.

Start/Stop Characters	Special characters that suspend or resume suspended output.

Other Special	Other special characters for BSD systems: they can discard output, and print status.


Characters for Input Editing
Node: Editing Characters, Next: BSD Editing, Prev: , Up: Special Characters

Characters for Input Editing

These special characters are active only in canonical input mode.  See Canonical or Not.

 - Macro: int VEOF

This is the subscript for the EOF character in the special control character array.  termios.c_cc[VEOF] holds the character itself.

The EOF character is recognized only in canonical input mode.  It acts as a line terminator in the same way as a newline character, but if the EOF character is typed at the beginning of a line it causes read to return a byte count of zero, indicating end-of-file.  The EOF character itself is discarded.

Usually, the EOF character is ^C-d.

 - Macro: int VEOL

This is the subscript for the EOL character in the special control character array.  termios.c_cc[VEOL] holds the character itself.

The EOL character is recognized only in canonical input mode.  It acts as a line terminator, just like a newline character.  The EOL character is not discarded; it is read as the last character in the input line.

You don't need to use the EOL character to make RET end a line.  Just set the ICRNL flag.  In fact, this is the default state of affairs.

 - Macro: int VERASE

This is the subscript for the ERASE character in the special control character array.  termios.c_cc[VERASE] holds the character itself.

The ERASE character is recognized only in canonical input mode.  When the user types the erase character, the previous character typed is discarded.  (If the terminal generates multibyte character sequences, this may cause more than one byte of input to be discarded.)  This cannot be used to erase past the beginning of the current line of text.  The ERASE character itself is discarded.

Usually, the ERASE character is DEL.

 - Macro: int VKILL

This is the subscript for the KILL character in the special control character array.  termios.c_cc[VKILL] holds the character itself.

The KILL character is recognized only in canonical input mode.  When the user types the kill character, the entire contents of the current line of input are discarded.  The kill character itself is discarded too.

The KILL character is usually ^C-u.


BSD Extensions to Editing Characters
Node: BSD Editing, Next: Signal Characters, Prev: Editing Characters, Up: Special Characters

BSD Extensions to Editing Characters

These special characters are active only in canonical input mode.  See Canonical or Not.  They are BSD extensions; the GNU library defines the symbols on any system if you ask for them, but the characters you specify don't actually do anything except on a BSD system.

 - Macro: int VEOL2

This is the subscript for the EOL2 character in the special control character array.  termios.c_cc[VEOL2] holds the character itself.

The EOL2 character works just like the EOL character (see above), but it can be a different character.  Thus, you can specify two characters to terminate an input line, but setting EOL to one of them and EOL2 to the other.

 - Macro: int VWERASE

This is the subscript for the WERASE character in the special control character array.  termios.c_cc[VWERASE] holds the character itself.

The WERASE character is recognized only in canonical input mode.  It erases an entire word of prior input.

 - Macro: int VREPRINT

This is the subscript for the REPRINT character in the special control character array.  termios.c_cc[VREPRINT] holds the character itself.

The REPRINT character is recognized only in canonical input mode.  It reprints the current input line.

 - Macro: int VLNEXT

This is the subscript for the LNEXT character in the special control character array.  termios.c_cc[VLNEXT] holds the character itself.

The LNEXT character is recognized only when IEXTEN is set.  It disables the editing significance of the next character the user types.  It is the analogue of the ^C-q command in Emacs.  "LNEXT" stands for "literal next."

The LNEXT character is usually ^C-v.


Characters that Cause Signals
Node: Signal Characters, Next: Start/Stop Characters, Prev: BSD Editing, Up: Special Characters

Characters that Cause Signals

These special characters may be active in either canonical or noncanonical input mode, but only when the ISIG flag is set (see Local Modes).

 - Macro: int VINTR

This is the subscript for the INTR character in the special control character array.  termios.c_cc[VINTR] holds the character itself.

The INTR (interrupt) character raises a SIGINT signal for all processes in the foreground job associated with the terminal.  The INTR character itself is then discarded.  See Signal Handling, for more information about signals.

Typically, the INTR character is ^C-c.

 - Macro: int VQUIT

This is the subscript for the QUIT character in the special control character array.  termios.c_cc[VQUIT] holds the character itself.

The QUIT character raises a SIGQUIT signal for all processes in the foreground job associated with the terminal.  The QUIT character itself is then discarded.  See Signal Handling, for more information about signals.

Typically, the QUIT character is ^C-\.

 - Macro: int VSUSP

This is the subscript for the SUSP character in the special control character array.  termios.c_cc[VSUSP] holds the character itself.

The SUSP (suspend) character is recognized only if the implementation supports job control (see Job Control).  It causes a SIGTSTP signal to be sent to all processes in the foreground job associated with the terminal.  The SUSP character itself is then discarded.  See Signal Handling, for more information about signals.

Typically, the SUSP character is ^C-z.

Few applications disable the normal interpretation of the SUSP character.  If your program does this, it should provide some other mechanism for the user to stop the job.  When the user invokes this mechanism, the program should send a SIGTSTP signal to the process group of the process, not just to the process itself.  See Signaling Another Process.

 - Macro: int VDSUSP

This is the subscript for the DSUSP character in the special control character array.  termios.c_cc[VDSUSP] holds the character itself.

The DSUSP (suspend) character is recognized only if the implementation supports job control (see Job Control).  It sends a SIGTSTP signal, like the SUSP character, but not right away--only when the program tries to read it as input.  Not all systems with job control support DSUSP; only BSD systems.

See Signal Handling, for more information about signals.

Typically, the DSUSP character is ^C-y.


Special Characters for Flow Control
Node: Start/Stop Characters, Next: Other Special, Prev: Signal Characters, Up: Special Characters

Special Characters for Flow Control

These special characters may be active in either canonical or noncanonical input mode, but their use is controlled by the flags IXON and IXOFF (see Input Modes).

 - Macro: int VSTART

This is the subscript for the START character in the special control character array.  termios.c_cc[VSTART] holds the character itself.

The START character is used to support the IXON and IXOFF input modes.  If IXON is set, receiving a START character resumes suspended output; the START character itself is discarded.  If IXOFF is set, the system may also transmit START characters to the terminal.

The usual value for the START character is ^C-q.  You may not be able to change this value--the hardware may insist on using ^C-q regardless of what you specify.

 - Macro: int VSTOP

This is the subscript for the STOP character in the special control character array.  termios.c_cc[VSTOP] holds the character itself.

The STOP character is used to support the IXON and IXOFF input modes.  If IXON is set, receiving a STOP character causes output to be suspended; the STOP character itself is discarded.  If IXOFF is set, the system may also transmit STOP characters to the terminal, to prevent the input queue from overflowing.

The usual value for the STOP character is ^C-s.  You may not be able to change this value--the hardware may insist on using ^C-s regardless of what you specify.


Other Special Characters
Node: Other Special, Next: , Prev: Start/Stop Characters, Up: Special Characters

Other Special Characters

Here are two additional special characters that are meaningful on BSD systems.

 - Macro: int VDISCARD

This is the subscript for the DISCARD character in the special control character array.  termios.c_cc[VDISCARD] holds the character itself.

The DISCARD character is recognized only when IEXTEN is set.  Its effect is to toggle the discard-output flag.  When this flag is set, all program output is discarded.  Setting the flag also discards all output currently in the output buffer.

 - Macro: int VSTATUS

This is the subscript for the STATUS character in the special control character array.  termios.c_cc[VSTATUS] holds the character itself.

The STATUS character's effect is to print out a status message about how the current process is running.

The STATUS character is recognized only when canonical mode.  This is a peculiar design decision, since the STATUS character's meaning has nothing to do with input, but that's the way it was done.


Noncanonical Input
Node: Noncanonical Input, Next: , Prev: Special Characters, Up: Terminal Modes

Noncanonical Input

In noncanonical input mode, the special editing characters such as ERASE and KILL are ignored.  The system facilities for the user to edit input are disabled in noncanonical mode, so that all input characters (unless they are special for signal or flow-control purposes) are passed to the application program exactly as typed.  It is up to the application program to give the user ways to edit the input, if appropriate.

Noncanonical mode offers special parameters called MIN and TIME for controlling whether and how long to wait for input to be available.  You can even use them to avoid ever waiting--to return immediately with whatever input is available, or with no input.

The MIN and TIME are stored in elements of the c_cc array, which is a member of the struct termios structure.  Each element of this array has a particular role, and each element has a symbolic constant that stands for the index of that element.  VMIN and VMAX are the names for the indices in the array of the MIN and TIME slots.

 - Macro: int VMIN

This is the subscript for the MIN slot in the c_cc array.  Thus, termios.c_cc[VMIN] is the value itself.

The MIN slot is only meaningful in noncanonical input mode; it specifies the minimum number of bytes that must be available in the input queue in order for read to return.

 - Macro: int VTIME

This is the subscript for the TIME slot in the c_cc array.  Thus, termios.c_cc[VTIME] is the value itself.

The TIME slot is only meaningful in noncanonical input mode; it specifies how long to wait for input before returning, in units of 0.1 seconds.

The MIN and TIME values interact to determine the criterion for when read should return; their precise meanings depend on which of them are nonzero.  There are four possible cases:

•	Both MIN and TIME are zero.

In this case, read always returns immediately with as many characters as are available in the queue, up to the number requested.  If no input is immediately available, read returns a value of zero.

•	MIN is zero but TIME has a nonzero value.

In this case, read waits for time TIME for input to become available; the availability of a single byte is enough to satisfy the read request and cause read to return.  When it returns, it returns as many characters as are available, up to the number requested.  If no input is available before the timer expires, read returns a value of zero.

•	TIME is zero but MIN has a nonzero value.

In this case, read waits until at least MIN bytes are available in the queue.  At that time, read returns as many characters as are available, up to the number requested.  read can return more than MIN characters if more than MIN happen to be in the queue.

•	Both TIME and MIN are nonzero.

In this case, TIME specifies how long to wait after each input character to see if more input arrives.  read keeps waiting until either MIN bytes have arrived, or TIME elapses with no further input.

read can return no input if TIME elapses before the first input character arrives.  read can return more than MIN characters if more than MIN happen to be in the queue.

What happens if MIN is 50 and you ask to read just 10 bytes?  Normally, read waits until there are 50 bytes in the buffer (or, more generally, the wait condition described above is satisfied), and then reads 10 of them, leaving the other 40 buffered in the operating system for a subsequent call to read.

Portability note: On some systems, the MIN and TIME slots are actually the same as the EOF and EOL slots.  This causes no serious problem because the MIN and TIME slots are used only in noncanonical input and the EOF and EOL slots are used only in canonical input, but it isn't very clean.  The GNU library allocates separate slots for these uses.


Line Control Functions
Node: Line Control, Next: Noncanon Example, Prev: Terminal Modes, Up: Low-Level Terminal Interface

Line Control Functions

These functions perform miscellaneous control actions on terminal devices.  As regards terminal access, they are treated like doing output: if any of these functions is used by a background process on its controlling terminal, normally all processes in the process group are sent a SIGTTOU signal.  The exception is if the calling process itself is ignoring or blocking SIGTTOU signals, in which case the operation is performed and no signal is sent.  See Job Control.

- Function: int tcsendbreak (int filedes, int duration)

This function generates a break condition by transmitting a stream of zero bits on the terminal associated with the file descriptor filedes.  The duration of the break is controlled by the duration argument.  If zero, the duration is between 0.25 and 0.5 seconds.  The meaning of a nonzero value depends on the operating system.

This function does nothing if the terminal is not an asynchronous serial data port.

The return value is normally zero.  In the event of an error, a value of -1 is returned.  The following errno error conditions are defined for this function:

EBADF

The filedes is not a valid file descriptor.

ENOTTY

The filedes is not associated with a terminal device.

- Function: int tcdrain (int filedes)

The tcdrain function waits until all queued output to the terminal filedes has been transmitted.

The return value is normally zero.  In the event of an error, a value of -1 is returned.  The following errno error conditions are defined for this function:

EBADF

The filedes is not a valid file descriptor.

ENOTTY

The filedes is not associated with a terminal device.

EINTR

The operation was interrupted by delivery of a signal.  See Interrupted Primitives.

- Function: int tcflush (int filedes, int queue)

The tcflush function is used to clear the input and/or output queues associated with the terminal file filedes.  The queue argument specifies which queue(s) to clear, and can be one of the following values:

TCIFLUSH

Clear any input data received, but not yet read.

TCOFLUSH

Clear any output data written, but not yet transmitted.

TCIOFLUSH

Clear both queued input and output.

The return value is normally zero.  In the event of an error, a value of -1 is returned.  The following errno error conditions are defined for this function:

EBADF

The filedes is not a valid file descriptor.

ENOTTY

The filedes is not associated with a terminal device.

EINVAL

A bad value was supplied as the queue argument.

It is unfortunate that this function is named tcflush, because the term "flush" is normally used for quite another operation--waiting until all output is transmitted--and using it for discarding input or output would be confusing.  Unfortunately, the name tcflush comes from POSIX and we cannot change it.

- Function: int tcflow (int filedes, int action)

The tcflow function is used to perform operations relating to XON/XOFF flow control on the terminal file specified by filedes.

The action argument specifies what operation to perform, and can be one of the following values:

TCOOFF

Suspend transmission of output.

TCOON

Restart transmission of output.

TCIOFF

Transmit a STOP character.

TCION

Transmit a START character.

For more information about the STOP and START characters, see See Special Characters.

The return value is normally zero.  In the event of an error, a value of -1 is returned.  The following errno error conditions are defined for this function:

EBADF

The filedes is not a valid file descriptor.

ENOTTY

The filedes is not associated with a terminal device.

EINVAL

A bad value was supplied as the action argument.


Noncanonical Mode Example
Node: Noncanon Example, Next: , Prev: Line Control, Up: Low-Level Terminal Interface

Noncanonical Mode Example

Here is an example program that shows how you can set up a terminal device to read single characters in noncanonical input mode, without echo.

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <termios.h>

/* Use this variable to remember original terminal attributes. */

struct termios saved_attributes;

void
reset_input_mode (void)
{
  tcsetattr (STDIN_FILENO, TCSANOW, &saved_attributes);
}

void
set_input_mode (void)
{
  struct termios tattr;
  char *name;

  /* Make sure stdin is a terminal. */
  if (!isatty (STDIN_FILENO))
    {
      fprintf (stderr, "Not a terminal.\n");
      exit (EXIT_FAILURE);
    }

  /* Save the terminal attributes so we can restore them later. */
  tcgetattr (STDIN_FILENO, &saved_attributes);
  atexit (reset_input_mode);

/* Set the funny terminal modes. */
  tcgetattr (STDIN_FILENO, &tattr);
  tattr.c_lflag &= ~(ICANON|ECHO); /* Clear ICANON and ECHO. */
  tattr.c_cc[VMIN] = 1;
  tattr.c_cc[VTIME] = 0;
  tcsetattr (STDIN_FILENO, TCSAFLUSH, &tattr);
}

int
main (void)
{
  char c;

  set_input_mode ();

  while (1)
    {
      read (STDIN_FILENO, &c, 1);
      if (c == '\004')          /* ^C-d */
        break;
      else
        putchar (c);
    }

  return EXIT_SUCCESS;
}

This program is careful to restore the original terminal modes before exiting or terminating with a signal.  It uses the atexit function (see Cleanups on Exit) to make sure this is done by exit.

The shell is supposed to take care of resetting the terminal modes when a process is stopped or continued; see See Job Control.  But some existing shells do not actually do this, so you may wish to establish handlers for job control signals that reset terminal modes.  The above example does so.


Mathematics
Node: Mathematics, Next: Arithmetic, Prev: Low-Level Terminal Interface, Up: Top

Mathematics

This chapter contains information about functions for performing mathematical computations, such as trigonometric functions.  Most of these functions have prototypes declared in the header file 'math.h'.  KAll of the functions that operate on floating-point numbers accept arguments and return results of type double.  In the future, there may be additional functions that operate on float and long double values.  For example, cosf and cosl would be versions of the cos function that operate on float and long double arguments, respectively.  In the meantime, you should avoid using these names yourself.  See Reserved Names.

* Menu:

Domain and Range Errors	Detecting overflow conditions and the like.

Trig Functions	Sine, cosine, and tangent.

Inverse Trig Functions	Arc sine, arc cosine, and arc tangent.

Exponents and Logarithms	Also includes square root.

Hyperbolic Functions	Hyperbolic sine and friends.

Pseudo-Random Numbers	Functions for generating pseudo-random numbers.


Domain and Range Errors
Node: Domain and Range Errors, Next: Trig Functions, Prev: , Up: Mathematics

Domain and Range Errors

Many of the functions listed in this chapter are defined mathematically over a domain that is only a subset of real numbers.  For example, the acos function is defined over the domain between -1 and 1.  If you pass an argument to one of these functions that is outside the domain over which it is defined, the function sets errno to EDOM to indicate a "domain error".  On machines that support IEEE floating point, functions reporting error EDOM also return a NaN.

Some of these functions are defined mathematically to result in a complex value over parts of their domains.  The most familiar example of this is taking the square root of a negative number.  The functions in this chapter take only real arguments and return only real values; therefore, if the value ought to be nonreal, this is treated as a domain error.

A related problem is that the mathematical result of a function may not be representable as a floating point number.  If magnitude of the correct result is too large to be represented, the function sets errno to ERANGE to indicate a "range error", and returns a particular very large value (named by the macro HUGE_VAL) or its negation (- HUGE_VAL).

If the magnitude of the result is too small, a value of zero is returned instead.  In this case, errno might or might not be set to ERANGE.

The only completely reliable way to check for domain and range errors is to set errno to 0 before you call the mathematical function and test errno afterward.  As a consequence of this use of errno, use of the mathematical functions is not reentrant if you check for errors.

None of the mathematical functions ever generates signals as a result of domain or range errors.  In particular, this means that you won't see SIGFPE signals generated within these functions.  (See Signal Handling, for more information about signals.)

 - Macro: double HUGE_VAL

An expression representing a particular very large number.  On machines that use IEEE floating point format, the value is "infinity".  On other machines, it's typically the largest positive number that can be represented.

The value of this macro is used as the return value from various mathematical functions in overflow situations.

For more information about floating-point representations and limits, see See Floating Point Parameters.  In particular, the macro DBL_MAX might be more appropriate than HUGE_VAL for many uses other than testing for an error in a mathematical function.


Trigonometric Functions
Node: Trig Functions, Next: Inverse Trig Functions, Prev: Domain and Range Errors, Up: Mathematics

Trigonometric Functions

These are the familiar sin, cos, and tan functions.  The arguments to all of these functions are in units of radians; recall that pi radians equals 180 degrees.

The math library doesn't define a symbolic constant for pi, but you can define your own if you need one:

#define PI 3.14159265358979323846264338327

You can also compute the value of pi with the expression acos (-1.0).

 - Function: double sin (double x)

This function returns the sine of x, where x is given in radians.  The return value is in the range -1 to 1.

 - Function: double cos (double x)

This function returns the cosine of x, where x is given in radians.  The return value is in the range -1 to 1.

 - Function: double tan (double x)

This function returns the tangent of x, where x is given in radians.

The following errno error conditions are defined for this function:

ERANGE

Mathematically, the tangent function has singularities at odd multiples of pi/2.  If the argument x is too close to one of these singularities, tan sets errno to ERANGE and returns either positive or negative HUGE_VAL.


Inverse Trigonometric Functions
Node: Inverse Trig Functions, Next: Exponents and Logarithms, Prev: Trig Functions, Up: Mathematics

Inverse Trigonometric Functions

These are the usual arc sine, arc cosine and arc tangent functions, which are the inverses of the sine, cosine and tangent functions, respectively.

 - Function: double asin (double x)

This function computes the arc sine of x--that is, the value whose sine is x.  The value is in units of radians.  Mathematically, there are infinitely many such values; the one actually returned is the one between -pi/2 and pi/2 (inclusive).

asin fails, and sets errno to EDOM, if x is out of range.  The arc sine function is defined mathematically only over the domain -1 to 1.

 - Function: double acos (double x)

This function computes the arc cosine of x--that is, the value whose cosine is x.  The value is in units of radians.  Mathematically, there are infinitely many such values; the one actually returned is the one between 0 and pi (inclusive).

acos fails, and sets errno to EDOM, if x is out of range.  The arc cosine function is defined mathematically only over the domain -1 to 1.

 - Function: double atan (double x)

This function computes the arc tangent of x--that is, the value whose tangent is x.  The value is in units of radians.  Mathematically, there are infinitely many such values; the one actually returned is the one between -pi/2 and pi/2 (inclusive).

 - Function: double atan2 (double y, double x)

This is the two argument arc tangent function.  It is similar to computing the arc tangent of y/x, except that the signs of both arguments are used to determine the quadrant of the result, and x is permitted to be zero.  The return value is given in radians and is in the range -pi to pi, inclusive.

If x and y are coordinates of a point in the plane, atan2 returns the signed angle between the line from the origin to that point and the x-axis.  Thus, atan2 is useful for converting Cartesian coordinates to polar coordinates.  (To compute the radial coordinate, use hypot; see See Exponents and Logarithms.)

The function atan2 sets errno to EDOM if both x and y are zero; the return value is not defined in this case.


Exponentiation and Logarithms
Node: Exponents and Logarithms, Next: Hyperbolic Functions, Prev: Inverse Trig Functions, Up: Mathematics

Exponentiation and Logarithms

 - Function: double exp (double x)

The exp function returns the value of e (the base of natural logarithms) raised to power x.

The function fails, and sets errno to ERANGE, if the magnitude of the result is too large to be representable.

 - Function: double log (double x)

This function returns the natural logarithm of x.  exp (log (x)) equals x, exactly in mathematics and approximately in C.

The following errno error conditions are defined for this function:

EDOM

The argument x is negative.  The log function is defined mathematically to return a real result only on positive arguments.

ERANGE

The argument is zero.  The log of zero is not defined.

 - Function: double log10 (double x)

This function returns the base-10 logarithm of x.  Except for the different base, it is similar to the log function.  In fact, log10 (x) equals log (x) / log (10).

 - Function: double pow (double base, double power)

This is a general exponentiation function, returning base raised to power.

The following errno error conditions are defined for this function:

EDOM

The argument base is negative and power is not an integral value.  Mathematically, the result would be a complex number in this case.

ERANGE

An underflow or overflow condition was detected in the result.

- Function: double sqrt (double x)

This function returns the nonnegative square root of x.

The sqrt function fails, and sets errno to EDOM, if x is negative.  Mathematically, the square root would be a complex number.

- Function: double cbrt (double x)

This function returns the cube root of x.  This function cannot fail; every representable real value has a representable real cube root.

 - Function: double hypot (double x, double y)

The hypot function returns sqrt (x*x + y*y).  (This is the length of the hypotenuse of a right triangle with sides of length x and y, or the distance of the point (x, y) from the origin.)  See also the function cabs in See Absolute Value.

 - Function: double expm1 (double x)

This function returns a value equivalent to exp (x) - 1.  It is computed in a way that is accurate even if the value of x is near zero--a case where exp (x) - 1 would be inaccurate due to subtraction of two numbers that are nearly equal.

 - Function: double log1p (double x)

This function returns a value equivalent to log (1 + x).  It is computed in a way that is accurate even if the value of x is near zero.


Hyperbolic Functions
Node: Hyperbolic Functions, Next: Pseudo-Random Numbers, Prev: Exponents and Logarithms, Up: Mathematics

Hyperbolic Functions

The functions in this section are related to the exponential functions; see See Exponents and Logarithms.

 - Function: double sinh (double x)

The sinh function returns the hyperbolic sine of x, defined mathematically as exp (x) - exp (-x) / 2.  The function fails, and sets errno to ERANGE, if the value of x is too large; that is, if overflow occurs.

 - Function: double cosh (double x)

The cosh function returns the hyperbolic cosine of x, defined mathematically as exp (x) + exp (-x) / 2.  The function fails, and sets errno to ERANGE, if the value of x is too large; that is, if overflow occurs.

 - Function: double tanh (double x)

This function returns the hyperbolic tangent of x, whose mathematical definition is sinh (x) / cosh (x).

 - Function: double asinh (double x)

This function returns the inverse hyperbolic sine of x--the value whose hyperbolic sine is x.

 - Function: double acosh (double x)

This function returns the inverse hyperbolic cosine of x--the value whose hyperbolic cosine is x.  If x is less than 1, acosh returns HUGE_VAL.

 - Function: double atanh (double x)

This function returns the inverse hyperbolic tangent of x--the value whose hyperbolic tangent is x.  If the absolute value of x is greater than or equal to 1, atanh returns HUGE_VAL.


Pseudo-Random Numbers
Node: Pseudo-Random Numbers, Next: , Prev: Hyperbolic Functions, Up: Mathematics

Pseudo-Random Numbers

This section describes the GNU facilities for generating a series of pseudo-random numbers.  The numbers generated are not truly random; typically, they form a sequence that repeats periodically, with a period so large that you can ignore it for ordinary purposes.  The random number generator works by remembering at all times a "seed" value which it uses to compute the next random number and also to compute a new seed.

Although the generated numbers look unpredictable within one run of a program, the sequence of numbers is exactly the same from one run to the next.  This is because the initial seed is always the same.  This is convenient when you are debugging a program, but it is unhelpful if you want the program to behave unpredictably.  If you want truly random numbers, not just pseudo-random, specify a seed based on the current time.

You can get repeatable sequences of numbers on a particular machine type by specifying the same initial seed value for the random number generator.  There is no standard meaning for a particular seed value; the same seed, used in different C libraries or on different CPU types, will give you different random numbers.

The GNU library supports the standard ANSI C random number functions plus another set derived from BSD.  We recommend you use the standard ones, rand and srand.

* Menu:

ANSI Random	rand and friends.

BSD Random	random and friends.


ANSI C Random Number Functions
Node: ANSI Random, Next: BSD Random, Prev: , Up: Pseudo-Random Numbers

ANSI C Random Number Functions

This section describes the random number functions that are part of the ANSI C standard.

To use these facilities, you should include the header file 'stdlib.h' in your program.  K

 - Macro: int RAND_MAX

The value of this macro is an integer constant expression that represents the maximum possible value returned by the rand function.  In the GNU library, it is 037777777, which is the largest signed integer representable in 32 bits.  In other libraries, it may be as low as 32767.

 - Function: int rand ()

The rand function returns the next pseudo-random number in the series.  The value is in the range from 0 to RAND_MAX.

 - Function: void srand (unsigned int seed)

This function establishes seed as the seed for a new series of pseudo-random numbers.  If you call rand before a seed has been established with srand, it uses the value 1 as a default seed.

To produce truly random numbers (not just pseudo-random), do srand (time (0)).


BSD Random Number Functions
Node: BSD Random, Next: , Prev: ANSI Random, Up: Pseudo-Random Numbers

BSD Random Number Functions

This section describes a set of random number generation functions that are derived from BSD.  There is no advantage to using these functions with the GNU C library; we support them for BSD compatibility only.

The prototypes for these functions are in 'stdlib.h'.  K

 - Function: long int random ()

This function returns the next pseudo-random number in the sequence.  The range of values returned is from 0 to RAND_MAX.

 - Function: void srandom (unsigned int seed)

The srandom function sets the seed for the current random number state based on the integer seed.  If you supply a seed value of 1, this will cause random to reproduce the default set of random numbers.

To produce truly random numbers (not just pseudo-random), do srandom (time (0)).

 - Function: void * initstate (unsigned int seed, void *state, size_t size)

The initstate function is used to initialize the random number generator state.  The argument state is an array of size bytes, used to hold the state information.  The size must be at least 8 bytes, and optimal sizes are 8, 16, 32, 64, 128, and 256.  The bigger the state array, the better.

The return value is the previous value of the state information array.  You can use this value later as an argument to setstate to restore that state.

 - Function: void * setstate (void *state)

The setstate function restores the random number state information state.  The argument must have been the result of a previous call to initstate or setstate.

The return value is the previous value of the state information array.  You can use thise value later as an argument to setstate to restore that state.


Low-Level Arithmetic Functions
Node: Arithmetic, Next: Date and Time, Prev: Mathematics, Up: Top

Low-Level Arithmetic Functions

This chapter contains information about functions for doing basic arithmetic operations, such as splitting a float into its integer and fractional parts.  These functions are declared in the header file 'math.h'.

* Menu:

Not a Number	Making NaNs and testing for NaNs.

Predicates on Floats	Testing for infinity and for NaNs.

Absolute Value	Absolute value functions.

Normalization Functions	Hacks for radix-2 representations.

Rounding and Remainders	Determinining the integer and fractional parts of a float.

Integer Division	Functions for performing integer division.

Parsing of Numbers	Functions for "reading" numbers from strings.


"Not a Number" Values
Node: Not a Number, Next: Predicates on Floats, Prev: , Up: Arithmetic

"Not a Number" Values

The IEEE floating point format used by most modern computers supports values that are "not a number".  These values are called "NaNs".  "Not a number" values result from certain operations which have no meaningful numeric result, such as zero divided by zero or infinity divided by infinity.

One noteworthy property of NaNs is that they are not equal to themselves.  Thus, x == x can be 0 if the value of x is a NaN.  You can use this to test whether a value is a NaN or not: if it is not equal to itself, then it is a NaN.  But the recommended way to test for a NaN is with the isnan function (see Predicates on Floats).

Almost any arithmetic operation in which one argument is a NaN returns a NaN.

 - Macro: double NAN

An expression representing a value which is "not a number".  This macro is a GNU extension, available only on machines that support "not a number" values--that is to say, on all machines that support IEEE floating point.

You can use '#ifdef NAN' to test whether the machine supports NaNs.  (Of course, you must arrange for GNU extensions to be visible, such as by defining _GNU_SOURCE, and then you must include 'math.h'.)


Predicates on Floats
Node: Predicates on Floats, Next: Absolute Value, Prev: Not a Number, Up: Arithmetic

Predicates on Floats

This section describes some miscellaneous test functions on doubles.  Prototypes for these functions appear in 'math.h'.  These are BSD functions, and thus are available if you define _BSD_SOURCE or _GNU_SOURCE.

 - Function: int isinf (double x)

This function returns -1 if x represents negative infinity, 1 if x represents positive infinity, and 0 otherwise.

 - Function: int isnan (double x)

This function returns a nonzero value if x is a "not a number" value, and zero otherwise.  (You can just as well use x != x to get the same result).

 - Function: int finite (double x)

This function returns a nonzero value if x is finite or a "not a number" value, and zero otherwise.

 - Function: double infnan (int error)

This function is provided for compatibility with BSD.  The other mathematical functions use infnan to decide what to return on occasion of an error.  Its argument is an error code, EDOM or ERANGE; infnan returns a suitable value to indicate this with.  -ERANGE is also acceptable as an argument, and corresponds to -HUGE_VAL as a value.

In the BSD library, on certain machines, infnan raises a fatal signal in all cases.  The GNU library does not do likewise, because that does not fit the ANSI C specification.

Portability Note: The functions listed in this section are BSD extensions.


Absolute Value
Node: Absolute Value, Next: Normalization Functions, Prev: Predicates on Floats, Up: Arithmetic

Absolute Value

These functions are provided for obtaining the "absolute value" (or "magnitude") of a number.  The absolute value of a real number x is x is x is positive, -x if x is negative.  For a complex number z, whose real part is x and whose imaginary part is y, the absolute value is sqrt (x*x + y*y).

Prototypes for abs and labs are in 'stdlib.h'; fabs and cabs are declared in 'math.h'.

 - Function: int abs (int number)

This function returns the absolute value of number.

Most computers use a two's complement integer representation, in which the absolute value of INT_MIN (the smallest possible int) cannot be represented; thus, abs (INT_MIN) is not defined.

 - Function: long int labs (long int number)

This is similar to abs, except that both the argument and result are of type long int rather than int.

 - Function: double fabs (double number)

This function returns the absolute value of the floating-point number number.

 - Function: double cabs (struct { double real, imag; } z)

The cabs function returns the absolute value of the complex number z, whose real part is z.real and whose imaginary part is z.imag.  (See also the function hypot in See Exponents and Logarithms.)  The value is:

sqrt (z.real*z.real + z.imag*z.imag)


Normalization Functions
Node: Normalization Functions, Next: Rounding and Remainders, Prev: Absolute Value, Up: Arithmetic

Normalization Functions

The functions described in this section are primarily provided as a way to efficiently perform certain low-level manipulations on floating point numbers that are represented internally using a binary radix; see See Floating Point Concepts.  These functions are required to have equivalent behavior even if the representation does not use a radix of 2, but of course they are unlikely to be particularly efficient in those cases.

All these functions are declared in 'math.h'.

 - Function: double frexp (double value, int *exponent)

The frexp function is used to split the number value into a normalized fraction and an exponent.

If the argument value is not zero, the return value is value times a power of two, and is always in the range 1/2 (inclusive) to 1 (exclusive).  The corresponding exponent is stored in *exponent; the return value multiplied by 2 raised to this exponent equals the original number value.

For example, frexp (12.8, &exponent) returns 0.8 and stores 4 in exponent.

If value is zero, then the return value is zero and zero is stored in *exponent.

 - Function: double ldexp (double value, int exponent)

This function returns the result of multiplying the floating-point number value by 2 raised to the power exponent.  (It can be used to reassemble floating-point numbers that were taken apart by frexp.)

For example, ldexp (0.8, 4) returns 12.8.

The following functions which come from BSD provide facilities equivalent to those of ldexp and frexp:

 - Function: double scalb (double value, int exponent)

The scalb function is the BSD name for ldexp.

 - Function: double logb (double x)

This BSD function returns the integer part of the base-2 logarithm of x, an integer value represented in type double.  This is the highest integer power of 2 contained in x.  The sign of x is ignored.  For example, logb (3.5) is 1.0 and logb (4.0) is 2.0.

When 2 raised to this power is divided into x, it gives a quotient between 1 (inclusive) and 2 (exclusive).

If x is zero, the value is minus infinity (if the machine supports such a value), or else a very small number.  If x is infinity, the value is infinity.

The value returned by logb is one less than the value that frexp would store into *exponent.

 - Function: double copysign (double value, double sign)

The copysign function returns a value whose absolute value is the same as that of value, and whose sign matches that of sign.  This is a BSD function.


Rounding and Remainder Functions
Node: Rounding and Remainders, Next: Integer Division, Prev: Normalization Functions, Up: Arithmetic

Rounding and Remainder Functions

The functions listed here perform operations such as rounding, truncation, and remainder in division of floating point numbers.  Some of these functions convert floating point numbers to integer values.  They are all declared in 'math.h'.

You can also convert floating-point numbers to integers simply by casting them to int.  This discards the fractional part, effectively rounding towards zero.  However, this only works if the result can actually be represented as an int--for very large numbers, this is impossible.  The functions listed here return the result as a double instead to get around this problem.

 - Function: double ceil (double x)

The ceil function rounds x upwards to the nearest integer, returning that value as a double.  Thus, ceil (1.5) is 2.0.

 - Function: double floor (double x)

The ceil function rounds x downwards to the nearest integer, returning that value as a double.  Thus, floor (1.5) is 1.0 and floor (-1.5) is -2.0.

 - Function: double rint (double x)

This function rounds x to an integer value according to the current rounding mode.  See Floating Point Parameters, for information about the various rounding modes.  The default rounding mode is to round to the nearest integer; some machines support other modes, but round-to-nearest is always used unless you explicit select another.

 - Function: double modf (double value, double *integer_part)

This function breaks the argument value into an integer part and a fractional part (between -1 and 1, exclusive).  Their sum equals value.  Each of the parts has the same sign as value, so the rounding of the integer part is towards zero.

modf stores the integer part in *integer_part, and returns the fractional part.  For example, modf (2.5, &intpart) returns 0.5 and stores 2.0 into intpart.

 - Function: double fmod (double numerator, double denominator)

This function computes the remainder from the division of numerator by denominator.  Specifically, the return value is numerator - n * denominator, where n is the quotient of numerator divided by denominator, rounded towards zero to an integer.  Thus, fmod (6.5, 2.3) returns 1.9, which is 6.5 minus 4.6.

The result has the same sign as the numerator and has magnitude less than the magnitude of the denominator.

If denominator is zero, fmod fails and sets errno to EDOM.

 - Function: double drem (double numerator, double denominator)

The function drem is like fmod except that it rounds the internal quotient n to the nearest integer instead of towards zero to an integer.  For example, drem (6.5, 2.3) returns -0.4, which is 6.5 minus 6.9.

The absolute value of the result is less than or equal to half the absolute value of the denominator.  The difference between fmod (numerator, denominator) and drem (numerator, denominator) is always either denominator, minus denominator, or zero.

If denominator is zero, drem fails and sets errno to EDOM.


Integer Division
Node: Integer Division, Next: Parsing of Numbers, Prev: Rounding and Remainders, Up: Arithmetic

Integer Division

This section describes functions for performing integer division.  These functions are redundant in the GNU C library, since in GNU C the '/' operator always rounds towards zero.  But in other C implementations, '/' may round differently with negative arguments.  div and ldiv are useful because they specify how to round the quotient: towards zero.  The remainder has the same sign as the numerator.

These functions are specified to return a result r such that the value r.quot*denominator + r.rem equals numerator.

To use these facilities, you should include the header file 'stdlib.h' in your program.

 - Data Type: div_t

This is a structure type used to hold the result returned by the div function.  It has the following members:

int quot

The quotient from the division.

int rem

The remainder from the division.

 - Function: div_t div (int numerator, int denominator)

This function div computes the quotient and remainder from the division of numerator by denominator, returning the result in a structure of type div_t.

If the result cannot be represented (as in a division by zero), the behavior is undefined.

Here is an example, albeit not a very useful one.

div_t result;
result = div (20, -6);

Now result.quot is -3 and result.rem is 2.

 - Data Type: ldiv_t

This is a structure type used to hold the result returned by the ldiv function.  It has the following members:

long int quot

The quotient from the division.

long int rem

The remainder from the division.

(This is identical to div_t except that the components are of type long int rather than int.)

 - Function: ldiv_t ldiv (long int numerator, long int denominator)

The ldiv function is similar to div, except that the arguments are of type long int and the result is returned as a structure of type ldiv.


Parsing of Numbers
Node: Parsing of Numbers, Next: , Prev: Integer Division, Up: Arithmetic

Parsing of Numbers

This section describes functions for "reading" integer and floating-point numbers from a string.  It may be more convenient in some cases to use sscanf or one of the related functions; see See Formatted Input.  But often you can make a program more robust by finding the tokens in the string by hand, then converting the numbers one by one.

* Menu:

Parsing of Integers	Functions for conversion of integer values.

Parsing of Floats	Functions for conversion of floating-point values.


Parsing of Integers
Node: Parsing of Integers, Next: Parsing of Floats, Prev: , Up: Parsing of Numbers

Parsing of Integers

These functions are declared in 'stdlib.h'.

 - Function: long int strtol (const char *string, char **tailptr, int base)

The strtol ("string-to-long") function converts the initial part of string to a signed integer, which is returned as a value of type long int.

This function attempts to decompose string as follows:

•	A (possibly empty) sequence of whitespace characters.  Which characters are whitespace is determined by the isspace function (see Classification of Characters).  These are discarded.

•	An optional plus or minus sign ('+' or '-').

•	A nonempty sequence of digits in the radix specified by base.

If base is zero, decimal radix is assumed unless the series of digits begins with '0' (specifying octal radix), or '0x' or '0X' (specifying hexadecimal radix); in other words, the same syntax used for integer constants in C.

Otherwise base must have a value between 2 and 35.  If base is 16, the digits may optionally be preceded by '0x' or '0X'.

•	Any remaining characters in the string.  If tailptr is not a null pointer, strtol stores a pointer to this tail in *tailptr.

If the string is empty, contains only whitespace, or does not contain an initial substring that has the expected syntax for an integer in the specified base, no conversion is performed.  In this case, strtol returns a value of zero and the value stored in *tailptr is the value of string.

In a locale other than the standard "C" locale, this function may recognize additional implementation-dependent syntax.

If the string has valid syntax for an integer but the value is not representable because of overflow, strtol returns either LONG_MAX or LONG_MIN (see Range of Type), as appropriate for the sign of the value.  It also sets errno to ERANGE to indicate there was overflow.

There is an example at the end of this section.

 - Function: unsigned long int strtoul (const char *string, char **tailptr, int base)

The strtoul ("string-to-unsigned-long") function is like strtol except that it returns its value with type unsigned long int.  The value returned in case of overflow is ULONG_MAX (see Range of Type).

 - Function: long int atol (const char *string)

This function is similar to the strtol function with a base argument of 10, except that it need not detect overflow errors.  The atol function is provided mostly for compatibility with existing code; using strtol is more robust.

 - Function: int atoi (const char *string)

This function is like atol, except that it returns an int value rather than long int.  The atoi function is also considered obsolete; use strtol instead.

Here is a function which parses a string as a sequence of integers and returns the sum of them:

int
sum_ints_from_string (char *string)
{
  int sum = 0;

  while (1) {
    char *tail;
    int next;

    /* Skip whitespace by hand, to detect the end.  */
    while (isspace (*string)) string++;
    if (*string == 0)
      break;

    /* There is more nonwhitespace,  */
    /* so it ought to be another number.  */
    errno = 0;
    /* Parse it.  */
    next = strtol (string, &tail, 0);
    /* Add it in, if not overflow.  */
    if (errno)
      printf ("Overflow\n");
    else
      sum += next;
    /* Advance past it.  */
    string = tail;
  }

  return sum;
}


Parsing of Floats
Node: Parsing of Floats, Next: , Prev: Parsing of Integers, Up: Parsing of Numbers

Parsing of Floats

These functions are declared in 'stdlib.h'.

 - Function: double strtod (const char *string, char **tailptr)

The strtod ("string-to-double") function converts the initial part of string to a floating-point number, which is returned as a value of type double.

This function attempts to decompose string as follows:

•	A (possibly empty) sequence of whitespace characters.  Which characters are whitespace is determined by the isspace function (see Classification of Characters).  These are discarded.

•	An optional plus or minus sign ('+' or '-').

•	A nonempty sequence of digits optionally containing a decimal-point character--normally '.', but it depends on the locale (see Numeric Formatting).

•	An optional exponent part, consisting of a character 'e' or 'E', an optional sign, and a sequence of digits.

•	Any remaining characters in the string.  If tailptr is not a null pointer, a pointer to this tail of the string is stored in *tailptr.

If the string is empty, contains only whitespace, or does not contain an initial substring that has the expected syntax for a floating-point number, no conversion is performed.  In this case, strtod returns a value of zero and the value returned in *tailptr is the value of string.

In a locale other than the standard "C" locale, this function may recognize additional locale-dependent syntax.

If the string has valid syntax for a floating-point number but the value is not representable because of overflow, strtod returns either positive or negative HUGE_VAL (see Mathematics), depending on the sign of the value.  Similarly, if the value is not representable because of underflow, strtod returns zero.  It also sets errno to ERANGE if there was overflow or underflow.

 - Function: double atof (const char *string)

This function is similar to the strtod function, except that it need not detect overflow and underflow errors.  The atof function is provided mostly for compatibility with existing code; using strtod is more robust.


Searching and Sorting
Node: Searching and Sorting, Next: Pattern Matching, Prev: Locales, Up: Top

Searching and Sorting

This chapter describes functions for searching and sorting arrays of arbitrary objects.  You pass the appropriate comparison function to be applied as an argument, along with the size of the objects in the array and the total number of elements.

* Menu:

Comparison Functions	Defining how to compare two objects. Since the sort and search facilities are general, you have to specify the ordering.

Array Search Function	The bsearch function.

Array Sort Function	The qsort function.

Search/Sort Example	An example program.


Defining the Comparison Function
Node: Comparison Functions, Next: Array Search Function, Prev: , Up: Searching and Sorting

Defining the Comparison Function

In order to use the sorted array library functions, you have to describe how to compare the elements of the array.

To do this, you supply a comparison function to compare two elements of the array.  The library will call this function, passing as arguments pointers to two array elements to be compared.  Your comparison function should return a value the way strcmp (see String/Array Comparison) does: negative if the first argument is "less" than the second, zero if they are "equal", and positive if the first argument is "greater".

Here is an example of a comparison function which works with an array of numbers of type double:

int
compare_doubles (const double *a, const double *b)
{
  return (int) (*a - *b);
}

The header file 'stdlib.h' defines a name for the data type of comparison functions.  This type is a GNU extension.

int comparison_fn_t (const void *, const void *);


Array Search Function
Node: Array Search Function, Next: Array Sort Function, Prev: Comparison Functions, Up: Searching and Sorting

Array Search Function

To search a sorted array for an element matching the key, use the bsearch function.  The prototype for this function is in the header file 'stdlib.h'.  K

 - Function: void * bsearch (const void *key, const void *array, size_t count, size_t size, comparison_fn_t compare)

The bsearch function searches the sorted array array for an object that is equivalent to key.  The array contains count elements, each of which is of size size bytes.

The compare function is used to perform the comparison.  This function is called with two pointer arguments and should return an integer less than, equal to, or greater than zero corresponding to whether its first argument is considered less than, equal to, or greater than its second argument.  The elements of the array must already be sorted in ascending order according to this comparison function.

The return value is a pointer to the matching array element, or a null pointer if no match is found.  If the array contains more than one element that matches, the one that is returned is unspecified.

This function derives its name from the fact that it is implemented using the binary search algorithm.


Array Sort Function
Node: Array Sort Function, Next: Search/Sort Example, Prev: Array Search Function, Up: Searching and Sorting

Array Sort Function

To sort an array using an arbitrary comparison function, use the qsort function.  The prototype for this function is in 'stdlib.h'.  K

 - Function: void qsort (void *array, size_t count, size_t size, comparison_fn_t compare)

The qsort function sorts the array array.  The array contains count elements, each of which is of size size.

The compare function is used to perform the comparison on the array elements.  This function is called with two pointer arguments and should return an integer less than, equal to, or greater than zero corresponding to whether its first argument is considered less than, equal to, or greater than its second argument.

Warning: If two objects compare as equal, their order after sorting is unpredictable.  That is to say, the sorting is not stable.  This can make a difference when the comparison considers only part of the elements.  Two elements with the same sort key may differ in other respects.

If you want the effect of a stable sort, you can get this result by writing the comparison function so that, lacking other reason distinguish between two elements, it compares them by their addresses.

Here is a simple example of sorting an array of doubles in numerical order, using the comparison function defined above (see Comparison Functions):

{
  double *array;
  int size;
  ...
  qsort (array, size, sizeof (double), compare_doubles);
}

The qsort function derives its name from the fact that it was originally implemented using the "quick sort" algorithm.


Searching and Sorting Example
Node: Search/Sort Example, Next: , Prev: Array Sort Function, Up: Searching and Sorting

Searching and Sorting Example

Here is an example showing the use of qsort and bsearch with an array of structures.  The objects in the array are sorted by comparing their name fields with the strcmp function.  Then, we can look up individual objects based on their names.

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

/* Define an array of critters to sort. */

struct critter
  {
    const char *name;
    const char *species;
  };

struct critter muppets[] =
  {
    {"Kermit", "frog"},
    {"Piggy", "pig"},
    {"Gonzo", "whatever"},
    {"Fozzie", "bear"},
    {"Sam", "eagle"},
    {"Robin", "frog"},
    {"Animal", "animal"},
    {"Camilla", "chicken"},
    {"Sweetums", "monster"},
    {"Dr. Strangepork", "pig"},
    {"Link Hogthrob", "pig"},
    {"Zoot", "human"},
    {"Dr. Bunsen Honeydew", "human"},
    {"Beaker", "human"},
    {"Swedish Chef", "human"}
  };

int count = sizeof (muppets) / sizeof (struct critter);

/* This is the comparison function used for sorting and searching. */

int
critter_cmp (const struct critter *c1, const struct critter *c2)
{
  return strcmp (c1->name, c2->name);
}

/* Print information about a critter. */

void
print_critter (const struct critter *c)
{
  printf ("%s, the %s\n", c->name, c->species);
}

/* Do the lookup into the sorted array. */

void
find_critter (const char *name)
{
  struct critter target, *result;
  target.name = name;
  result = bsearch (&target, muppets, count, sizeof (struct critter),
                    critter_cmp);
  if (result)
    print_critter (result);
  else
    printf ("Couldn't find %s.\n", name);
}

/* Main program. */

int
main (void)
{
  int i;

  for (i = 0; i < count; i++)
    print_critter (&muppets[i]);
  printf ("\n");

  qsort (muppets, count, sizeof (struct critter), critter_cmp);

  for (i = 0; i < count; i++)
    print_critter (&muppets[i]);
  printf ("\n");

  find_critter ("Kermit");
  find_critter ("Gonzo");
  find_critter ("Janice");

  return 0;
}

The output from this program looks like:

Kermit, the frog
Piggy, the pig
Gonzo, the whatever
Fozzie, the bear
Sam, the eagle
Robin, the frog
Animal, the animal
Camilla, the chicken
Sweetums, the monster
Dr. Strangepork, the pig
Link Hogthrob, the pig
Zoot, the human
Dr. Bunsen Honeydew, the human
Beaker, the human
Swedish Chef, the human

Animal, the animal
Beaker, the human
Camilla, the chicken
Dr. Bunsen Honeydew, the human
Dr. Strangepork, the pig
Fozzie, the bear
Gonzo, the whatever
Kermit, the frog
Link Hogthrob, the pig
Piggy, the pig
Robin, the frog
Sam, the eagle
Swedish Chef, the human
Sweetums, the monster
Zoot, the human

Kermit, the frog
Gonzo, the whatever
Couldn't find Janice.


Pattern Matching
Node: Pattern Matching, Next: I/O Overview, Prev: Searching and Sorting, Up: Top

Pattern Matching

The GNU C Library provides pattern matching facilities for two kinds of patterns: regular expressions and file-name wildcards.  The library also provides a facility for expanding variable and command references and parsing text into words in the way the shell does.

* Menu:

Wildcard Matching	Matching a wildcard pattern against a single string.

Globbing	Finding the files that match a wildcard pattern.

Regular Expressions	Matching regular expressions against strings.

Word Expansion	Expanding shell variables, nested commands, arithmetic, and wildcards. This is what the shell does with shell commands.


Wildcard Matching
Node: Wildcard Matching, Next: Globbing, Prev: , Up: Pattern Matching

Wildcard Matching

This section describes how to match a wildcard pattern against a particular string.  The result is a yes or no answer: does the string fit the pattern or not.  The symbols described here are all declared in 'fnmatch.h'.

 - Function: int fnmatch (const char *pattern, const char *string, int flags)

This function tests whether the string string matches the pattern pattern.  It returns 0 if they do match; otherwise, it returns the nonzero value FNM_NOMATCH.  The arguments pattern and string are both strings.

The argument flags is a combination of flag bits that alter the details of matching.  See below for a list of the defined flags.

In the GNU C Library, fnmatch cannot experience an "error"--it always returns an answer for whether the match succeeds.  However, other implementations of fnmatch might sometimes report "errors".  They would do so by returning nonzero values that are not equal to FNM_NOMATCH.

These are the available flags for the flags argument:

FNM_FILE_NAME

Treat the '/' character specially, for matching file names.  If this flag is set, wildcard constructs in pattern cannot match '/' in string.  Thus, the only way to match '/' is with an explicit '/' in pattern.

FNM_PATHNAME

This is an alias for FNM_FILE_NAME; it comes from POSIX.2.  We don't recommend this name because we don't use the term "pathname" for file names.

FNM_PERIOD

Treat the '.' character specially if it appears at the beginning of string.  If this flag is set, wildcard constructs in pattern cannot match '.' as the first character of string.

If you set both FNM_PERIOD and FNM_FILE_NAME, then the special treatment applies to '.' following '/' as well as to '.' at the beginning of string.  (The shell uses the FNM_PERIOD and FNM_FILE_NAME falgs together for matching file names.)

FNM_NOESCAPE

Don't treat the '\' character specially in patterns.  Normally, '\' quotes the following character, turning off its special meaning (if any) so that it matches only itself.  When quoting is enabled, the pattern '\?' matches only the string '?', because the question mark in the pattern acts like an ordinary character.

If you use FNM_NOESCAPE, then '\' is an ordinary character.

FNM_LEADING_DIR

Ignore a trailing sequence of characters starting with a '/' in string; that is to say, test whether string starts with a directory name that pattern matches.

If this flag is set, either 'foo*' or 'foobar' as a pattern would match the string 'foobar/frobozz'.

FNM_CASEFOLD

Ignore case in comparing string to pattern.


Globbing
Node: Globbing, Next: Regular Expressions, Prev: Wildcard Matching, Up: Pattern Matching

Globbing

The archetypal use of wildcards is for matching against the files in a directory, and making a list of all the matches.  This is called "globbing".

You could do this using fnmatch, by reading the directory entries one by one and testing each one with fnmatch.  But that would be slow (and complex, since you would have to handle subdirectories by hand).

The library provides a function glob to make this particular use of wildcards convenient.  glob and the other symbols in this section are declared in 'glob.h'.

* Menu:

Calling Glob	Basic use of glob.

Flags for Globbing	Flags that enable various options in glob.


Calling glob
Node: Calling Glob, Next: Flags for Globbing, Prev: , Up: Globbing

Calling glob

The result of globbing is a vector of file names (strings).  To return this vector, glob uses a special data type, glob_t, which is a structure.  You pass glob the address of the structure, and it fills in the structure's fields to tell you about the results.

 - Data Type: glob_t

This data type holds a pointer to a word vector.  More precisely, it records both the address of the word vector and its size.

gl_pathc

The number of elements in the vector.

gl_pathv

The address of the vector.  This field has type char **.

gl_offs

The offset of the first real element of the vector, from its nominal address in the gl_pathv field.  Unlike the other fields, this is always an input to glob, rather than an output from it.

If you use a nonzero offset, then that many elements at the beginning of the vector are left empty.  (The glob function fills them with null pointers.)

The gl_offs field is meaningful only if you use the GLOB_DOOFFS flag.  Otherwise, the offset is always zero regardless of what is in this field, and the first real element comes at the beginning of the vector.

 - Function: int glob (const char *pattern, int flags, int (*errfunc) (const char *filename, int error-code), glob_t *vector_ptr)

The function glob does globbing using the pattern pattern in the current directory.  It puts the result in a newly allocated vector, and stores the size and address of this vector into *vector_ptr.  The argument flags is a combination of bit flags; see See Flags for Globbing, for details of the flags.

The result of globbing is a sequence of file names.  The function glob allocates a string for each resulting word, then allocates a vector of type char ** to store the addresses of these strings.  The last element of the vector is a null pointer.  This vector is called the "word vector".

To return this vector, glob stores both its address and its length (number of elements, not counting the terminating null pointer) into *vector_ptr.

Normally, glob sorts the file names alphabetically before returning them.  You can turn this off with the flag GLOB_NOSORT if you want to get the information as fast as possible.  Usually it's a good idea to let glob sort them--if you process the files in alphabetical order, the users will have a feel for the rate of progress that your application is making.

If glob succeeds, it returns 0.  Otherwise, it returns one of these error codes:

GLOB_ABORTED

There was an error opening a directory, and you used the flag GLOB_ERR or your specified errfunc returned a nonzero value.  See Flags for Globbing, for an explanation of the GLOB_ERR flag and errfunc.

GLOB_NOMATCH

The pattern didn't match any existing files.  If you use the GLOB_NOCHECK flag, then you never get this error code, because that flag tells glob to pretend that the pattern matched at least one file.

GLOB_NOSPACE

It was impossible to allocate memory to hold the result.

In the event of an error, glob stores information in *vector_ptr about all the matches it has found so far.


Flags for Globbing
Node: Flags for Globbing, Next: , Prev: Calling Glob, Up: Globbing

Flags for Globbing

This section describes the flags that you can specify in the flags argument to glob.  Choose the flags you want, and combine them with the C bitwise OR operator |.

GLOB_APPEND

Append the words from this expansion to the vector of words produced by previous calls to glob.  This way you can effectively expand several words as if they were concatenated with spaces between them.

In order for appending to work, you must not modify the contents of the word vector structure between calls to glob.  And, if you set GLOB_DOOFFS in the first call to glob, you must also set it when you append to the results.

Note that the pointer stored in gl_pathv may no longer be valid after you call glob the second time, because glob might have relocated the vector.  So always fetch gl_pathv from the glob_t structure after each glob call; never save the pointer across calls.

GLOB_DOOFFS

Leave blank slots at the beginning of the vector of words.  The gl_offs field says how many slots to leave.  The blank slots contain null pointers.

GLOB_ERR

Give up right away and report an error if there is any difficulty reading the directories that must be read in order to expand pattern fully.  Such difficulties might include a directory in which you don't have the requisite access.  Normally, glob tries its best to keep on going despite any errors, reading whatever directories it can.

You can exercise even more control than this by specifying an error-handler function errfunc when you call glob.  If errfunc is not a null pointer, then glob doesn't give up right away when it can't read a directory; instead, it calls errfunc with two arguments, like this:

(*errfunc) (filename, error-code)

The argument filename is the name of the directory that glob couldn't open or couldn't read, and error-code is the errno value that was reported to glob.

If the error handler function returns nonzero, then glob gives up right away.  Otherwise, it continues.

GLOB_MARK

If the pattern matches the name of a directory, append '/' to the directory's name when returning it.

GLOB_NOCHECK

If the pattern doesn't match any file names, return the pattern itself as if it were a file name that had been matched.  (Normally, when the pattern doesn't match anything, glob returns that there were no matches.)

GLOB_NOSORT

Don't sort the file names; return them in no particular order.  (In practice, the order will depend on the order of the entries in the directory.)  The only reason not to sort is to save time.

GLOB_NOESCAPE

Don't treat the '\' character specially in patterns.  Normally, '\' quotes the following character, turning off its special meaning (if any) so that it matches only itself.  When quoting is enabled, the pattern '\?' matches only the string '?', because the question mark in the pattern acts like an ordinary character.

If you use GLOB_NOESCAPE, then '\' is an ordinary character.

glob does its work by calling the function fnmatch repeatedly.  It handles the flag GLOB_NOESCAPE by turning on the FNM_NOESCAPE flag in calls to fnmatch.


Regular Expression Matching
Node: Regular Expressions, Next: Word Expansion, Prev: Globbing, Up: Pattern Matching

Regular Expression Matching

The GNU C library supports two interfaces for matching regular expressions.  One is the standard POSIX.2 interface, and the other is what the GNU system has had for many years.

Both interfaces are declared in the header file 'regex.h'.  If you define _POSIX_C_SOURCE, then only the POSIX.2 functions, structures, and constants are declared.

* Menu:

POSIX Regexp Compilation	Using regcomp to prepare to match.

Flags for POSIX Regexps	Syntax variations for regcomp.

Matching POSIX Regexps	Using regexec to match the compiled pattern that you get from regcomp.

Regexp Subexpressions	Finding which parts of the string were matched.

Subexpression Complications	Find points of which parts were matched.

Regexp Cleanup	Freeing storage; reporting errors.


POSIX Regular Expression Compilation
Node: POSIX Regexp Compilation, Next: Flags for POSIX Regexps, Prev: , Up: Regular Expressions

POSIX Regular Expression Compilation

Before you can actually match a regular expression, you must "compile" it.  This is not true compilation--it produces a special data structure, not machine instructions.  But it is like ordinary compilation in that its purpose is to enable you to "execute" the pattern fast.  (See Matching POSIX Regexps, for how to use the compiled regular expression for matching.)

There is a special data type for compiled regular expressions:

 - Data Type: regex_t

This type of object holds a compiled regular expression.  It is actually a structure.  It has just one field that your programs should look at:

re_nsub

This field holds the number of parenthetical subexpressions in the regular expression that was compiled.

There are several other fields, but we don't describe them here, because only the functions in the library should use them.

After you create a regex_t object, you can compile a regular expression into it by calling regcomp.

 - Function: int regcomp (regex_t *compiled, const char *pattern, int cflags)

The function regcomp "compiles" a regular expression into a data structure that you can use with regexec to match against a string.  The compiled regular expression format is designed for efficient matching.  regcomp stores it into *compiled.

It's up to you to allocate an object of type regex_t and pass its address to regcomp.

The argument cflags lets you specify various options that control the syntax and semantics of regular expressions.  See Flags for POSIX Regexps.

If you use the flag REG_NOSUB, then regcomp omits from the compiled regular expression the information necessary to record how subexpressions actually match.  In this case, you might as well pass 0 for the matchptr and nmatch arguments when you call regexec.

If you don't use REG_NOSUB, then the compiled regular expression does have the capacity to record how subexpressions match.  Also, regcomp tells you how many subexpressions pattern has, by storing the number in compiled->re_nsub.  You can use that value to decide how long an array to allocate to hold information about subexpression matches.

regcomp returns 0 if it succeeds in compiling the regular expression; otherwise, it returns a nonzero error code (see the table below).  You can use regerror to produce an error message string describing the reason for a nonzero value; see See Regexp Cleanup.

Here are the possible nonzero values that regcomp can return:

REG_BADBR

There was an invalid '\{...\}' construct in the regular expression.  A valid '\{...\}' construct must contain either a single number, or two numbers in increasing order separated by a comma.

REG_BADPAT

There was a syntax error in the regular expression.

REG_BADRPT

A repetition operator such as '?' or '*' appeared in a bad position (with no preceding subexpression to act on).

REG_ECOLLATE

The regular expression referred to an invalid collating element (one not defined in the current locale for string collation).  See Locale Categories.

REG_ECTYPE

The regular expression referred to an invalid character class name.

REG_EESCAPE

The regular expression ended with '\'.

REG_ESUBREG

There was an invalid number in the '\digit' construct.

REG_EBRACK

There were unbalanced square brackets in the regular expression.

REG_EPAREN

An extended regular expression had unbalanced parentheses, or a basic regular expression had unbalanced '\(' and '\)'.

REG_EBRACE

The regular expression had unbalanced '\{' and '\}'.

REG_ERANGE

One of the endpoints in a range expression was invalid.

REG_ESPACE

regcomp ran out of memory.


Flags for POSIX Regular Expressions
Node: Flags for POSIX Regexps, Next: Matching POSIX Regexps, Prev: POSIX Regexp Compilation, Up: Regular Expressions

Flags for POSIX Regular Expressions

These are the bit flags that you can use in the cflags operand when compiling a regular expression with regcomp.

REG_EXTENDED

Treat the pattern as an extended regular expression, rather than as a basic regular expression.

REG_ICASE

Ignore case when matching letters.

REG_NOSUB

Don't bother storing the contents of the matches_ptr array.

REG_NEWLINE

Treat a newline in string as dividing string into multiple lines, so that '$' can match before the newline and '^' can match after.  Also, don't permit '.' to match a newline, and don't permit '[^...]' to match a newline.

Otherwise, newline acts like any other ordinary character.


Matching a Compiled POSIX Regular Expression
Node: Matching POSIX Regexps, Next: Regexp Subexpressions, Prev: Flags for POSIX Regexps, Up: Regular Expressions

Matching a Compiled POSIX Regular Expression

Once you have compiled a regular expression, as described in See POSIX Regexp Compilation, you can match it against strings using regexec.  A match anywhere inside the string counts as success, unless the regular expression contains anchor characters ('^' or '$').

 - Function: int regexec (regex_t *compiled, char *string, size_t nmatch, regmatch_t matchptr [], int eflags)

This function tries to match the compiled regular expression *compiled against string.

regexec returns 0 if the regular expression matches; otherwise, it returns a nonzero value.  See the table below for what nonzero values mean.  You can use regerror to produce an error message string describing the reason for a nonzero value; see See Regexp Cleanup.

The argument eflags is a word of bit flags that enable various options.

If you want to get information about what part of string actually matched the regular expression or its subexpressions, use the arguments matchptr and nmatch.  Otherwise, pass 0 for nmatch, and NULL for matchptr.  See Regexp Subexpressions.

You must match the regular expression with the same set of current locales that were in effect when you compiled the regular expression.

The function regexec accepts the following flags in the eflags argument:

REG_NOTBOL

Do not regard the beginning of the specified string as the beginning of a line; more generally, don't make any assumptions about what text might precede it.

REG_NOTEOL

Do not regard the end of the specified string as the end of a line; more generally, don't make any assumptions about what text might follow it.

Here are the possible nonzero values that regexec can return:

REG_NOMATCH

The pattern didn't match the string.  This isn't really an error.

REG_ESPACE

regexec ran out of memory.


Match Results with Subexpressions
Node: Regexp Subexpressions, Next: Subexpression Complications, Prev: Matching POSIX Regexps, Up: Regular Expressions

Match Results with Subexpressions

When regexec matches parenthetical subexpressions of pattern, it records which parts of string they match.  It returns that information by storing the offsets into an array whose elements are structures of type regmatch_t.  The first element of the array (index 0) records the part of the string that matched the entire regular expression.  Each other element of the array records the beginning and end of the part that matched a single parenthetical subexpression.

 - Data Type: regmatch_t

This is the data type of the matcharray array that you pass to regexec.  It containes two structure fields, as follows:

rm_so

The offset in string of the beginning of a substring.  Add this value to string to get the address of that part.

rm_eo

The offset in string of the end of the substring.

 - Data Type: regoff_t

regoff_t is an alias for another signed integer type.  The fields of regmatch_t have type regoff_t.

The regmatch_t elements correspond to subexpressions positionally; the first element (index 1) records where the first subexpression matched, the second element records the second subexpression, and so on.  The order of the subexpressions is the order in which they begin.

When you call regexec, you specify how long the matchptr array is, with the nmatch argument.  This tells regexec how many elements to store.  If the actual regular expression has more than nmatch subexpressions, then you won't get offset information about the rest of them.  But this doesn't alter whether the pattern matches a particular string or not.

If you don't want regexec to return any information about where the subexpressions matched, you can either supply 0 for nmatch, or use the flag REG_NOSUB when you compile the pattern with regcomp.


Complications in Subexpression Matching
Node: Subexpression Complications, Next: Regexp Cleanup, Prev: Regexp Subexpressions, Up: Regular Expressions

Complications in Subexpression Matching

Sometimes a subexpression matches a substring of no characters.  This happens when 'f\(o*\)' matches the string 'fum'.  (It really matches just the 'f'.)  In this case, both of the offsets identify the point in the string where the null substring was found.  In this example, the offsets are both 1.

Sometimes the entire regular expression can match without using some of its subexpressions at all--for example, when 'ba\(na\)*' matches the string 'ba', the parenthetical subexpression is not used.  When this happens, regexec stores -1 in both fields of the element for that subexpression.

Sometimes matching the entire regular expression can match a particular subexpression more than once--for example, when 'ba\(na\)*' matches the string 'bananana', the parenthetical subexpression matches three times.  When this happens, regexec usually stores the offsets of the last part of the string that matched the subexpression.  In the case of 'bananana', these offsets are 6 and 8.

But the last match is not always the one that is chosen.  It's more accurate to say that the last opportunity to match is the one that takes precedence.  What this means is that when one subexpression appears within another, then the results reported for the inner subexpression reflect whatever happened on the last match of the outer subexpression.  For an example, consider '\(ba\(na\)*s \)*' matching the string 'bananas bas '.  The last time the inner expression actually matches is near the end of the first word.  But it is considered again in the second word, and fails to match there.  regexec reports nonuse of the "na" subexpression.

Another place where this rule applies is when the regular expression '\(ba\(na\)*s \|nefer\(ti\)* \)*' matches 'bananas nefertiti'.  The "na" subexpression does match in the first word, but it doesn't match in the second word because the other alternative is used there.  Once again, the second repetition of the outer subexpression overrides the first, and within that second repetition, the "na" subexpression is not used.  So regexec reports nonuse of the "na" subexpression.


POSIX Regexp Matching Cleanup
Node: Regexp Cleanup, Next: , Prev: Subexpression Complications, Up: Regular Expressions

POSIX Regexp Matching Cleanup

When you are finished using a compiled regular expression, you can free the storage it uses by calling regfree.

 - Function: void regfree (regex_t *compiled)

Calling regfree frees all the storage that *compiled points to.  This includes various internal fields of the regex_t structure that aren't documented in this manual.

regfree does not free the object *compiled itself.

You should always free the space in a regex_t structure with regfree before using the structure to compile another regular expression.

When regcomp or regexec reports an error, you can use the function regerror to turn it into an error message string.

 - Function: size_t regerror (int errcode, regex_t *compiled, char *buffer, size_t length)

This function produces an error message string for the error code errcode, and stores the string in length bytes of memory starting at buffer.  For the compiled argument, supply the same compiled regular expression structure that regcomp or regexec was working with when it got the error.  Alternatively, you can supply NULL for compiled; you will still get a meaningful error message, but it might not be as detailed.

If the error message can't fit in length bytes (including a terminating null character), then regerror truncates it.  The string that regerror stores is always null-terminated even if it has been truncated.

The return value of regerror is the minimum length needed to store the entire error message.  If this is less than length, then the error message was not truncated, and you can use it.  Otherwise, you should call regerror again with a larger buffer.

Here is a function which uses regerror, but always dynamically allocates a buffer for the error message:

char *get_regerror (int errcode, regex_t *compiled)
{
  size_t length = regerror (errcode, compiled, NULL, 0);
  char *buffer = xmalloc (length);
  (void) regerror (errcode, compiled, buffer, length);
  return buffer;
}


Shell-Style Word Expansion
Node: Word Expansion, Next: , Prev: Regular Expressions, Up: Pattern Matching

Shell-Style Word Expansion

"Word expansion" means the process of splitting a string into "words" and substituting for variables, commands, and wildcards just as the shell does.

For example, when you write 'ls -l foo.c', this string is split into three separate words--'ls', '-l' and 'foo.c'.  This is the most basic function of word expansion.

When you write 'ls *.c', this can become many words, because the word '*.c' can be replaced with any number of file names.  This is called "wildcard expansion", and it is also a part of word expansion.

When you use 'echo $PATH' to print your path, you are taking advantage of "variable substitution", which is also part of word expansion.

Ordinary programs can perform word expansion just like the shell by calling the library function wordexp.

* Menu:

Expansion Stages	What word expansion does to a string.

Calling Wordexp	How to call wordexp.

Flags for Wordexp	Options you can enable in wordexp.

Wordexp Example	A sample program that does word expansion.


The Stages of Word Expansion
Node: Expansion Stages, Next: Calling Wordexp, Prev: , Up: Word Expansion

The Stages of Word Expansion

When word expansion is applied to a sequence of words, it performs the following transformations in the order shown here:

1. 	"Tilde expansion": Replacement of '~foo' with the name of the home directory of 'foo'.

2. 	Next, three different transformations are applied in the same step, from left to right:

•	"Variable substitution": Environment variables are substituted for references such as '$foo'.

•	"Command substitution": Constructs such as '`cat foo`' and the equivalent '$(cat foo)' are replaced with the output from the inner command.

•	"Arithmetic expansion": Constructs such as '$(($x-1))' are replaced with the result of the arithmetic computation.

3. 	"Field splitting": subdivision of the text into "words".

4. 	"Wildcard expansion": The replacement of a construct such as '*.c' with a list of '.c' file names.  Wildcard expansion applies to an entire word at a time, and replaces that word with 0 or more file names that are themselves words.

5. 	"Quote removal": The deletion of string-quotes, now that they have done their job by inhibiting the above transformations when appropriate.

For the details of these transformations, and how to write the constructs that use them, see `The BASH Manual' (to appear).


Calling wordexp
Node: Calling Wordexp, Next: Flags for Wordexp, Prev: Expansion Stages, Up: Word Expansion

Calling wordexp

All the functions, constants and data types for word expansion are declared in the header file 'wordexp.h'.

Word expansion produces a vector of words (strings).  To return this vector, wordexp uses a special data type, wordexp_t, which is a structure.  You pass wordexp the address of the structure, and it fills in the structure's fields to tell you about the results.

 - Data Type: wordexp_t

This data type holds a pointer to a word vector.  More precisely, it records both the address of the word vector and its size.

we_wordc

The number of elements in the vector.

we_wordv

The address of the vector.  This field has type char **.

we_offs

The offset of the first real element of the vector, from its nominal address in the we_wordv field.  Unlike the other fields, this is always an input to wordexp, rather than an output from it.

If you use a nonzero offset, then that many elements at the beginning of the vector are left empty.  (The wordexp function fills them with null pointers.)

The we_offs field is meaningful only if you use the WRDE_DOOFFS flag.  Otherwise, the offset is always zero regardless of what is in this field, and the first real element comes at the beginning of the vector.

 - Function: int wordexp (const char *words, wordexp_t *word-vector-ptr, int flags)

Perform word expansion on the string words, putting the result in a newly allocated vector, and store the size and address of this vector into *word-vector-ptr.  The argument flags is a combination of bit flags; see See Flags for Wordexp, for details of the flags.

You shouldn't use any of the characters '|&;<>' in the string words unless they are quoted; likewise for newline.  If you use these characters unquoted, you will get the WRDE_BADCHAR error code.  Don't use parentheses or braces unless they are quoted or part of a word expansion construct.  If you use quotation characters ''"`', they should come in pairs that balance.

The results of word expansion are a sequence of words.  The function wordexp allocates a string for each resulting word, then allocates a vector of type char ** to store the addresses of these strings.  The last element of the vector is a null pointer.  This vector is called the "word vector".

To return this vector, wordexp stores both its address and its length (number of elements, not counting the terminating null pointer) into *word-vector-ptr.

If wordexp succeeds, it returns 0.  Otherwise, it returns one of these error codes:

WRDE_BADCHAR

The input string words contains an unquoted invalid character such as '|'.

WRDE_BADVAL

The input string refers to an undefined shell variable, and you used the flag WRDE_UNDEF to forbid such references.

WRDE_CMDSUB

The input string uses command substitution, and you used the flag WRDE_NOCMD to forbid command substitution.

WRDE_NOSPACE

It was impossible to allocate memory to hold the result.  In this case, wordexp can store part of the results--as much as it could allocate room for.

WRDE_SYNTAX

There was a syntax error in the input string.  For example, an unmatched quoting character is a syntax error.

 - Function: void wordfree (wordexp_t *word-vector-ptr)

Free the storage used for the word-strings and vector that *word-vector-ptr points to.  This does not free the structure *word-vector-ptr itself--only the other data it points to.


Flags for Word Expansion
Node: Flags for Wordexp, Next: Wordexp Example, Prev: Calling Wordexp, Up: Word Expansion

Flags for Word Expansion

This section describes the flags that you can specify in the flags argument to wordexp.  Choose the flags you want, and combine them with the C operator |.

WRDE_APPEND

Append the words from this expansion to the vector of words produced by previous calls to wordexp.  This way you can effectively expand several words as if they were concatenated with spaces between them.

In order for appending to work, you must not modify the contents of the word vector structure between calls to wordexp.  And, if you set WRDE_DOOFFS in the first call to wordexp, you must also set it when you append to the results.

WRDE_DOOFFS

Leave blank slots at the beginning of the vector of words.  The we_offs field says how many slots to leave.  The blank slots contain null pointers.

WRDE_NOCMD

Don't do command substitution; if the input requests command substitution, report an error.

WRDE_REUSE

Reuse a word vector made by a previous call to wordexp.  Instead of allocating a new vector of words, this call to wordexp will use the vector that already exists (making it larger if necessary).

Note that the vector may move, so it is not safe to save an old pointer and use it again after calling wordexp.  You must fetch we_pathv anew after each call.

WRDE_SHOWERR

Do show any error messages printed by commands run by command substitution.  More precisely, allow these commands to inherit the standard error output stream of the current process.  By default, wordexp gives these commands a standard error stream that discards all output.

WRDE_UNDEF

If the input refers to a shell variable that is not defined, report an error.


wordexp Example
Node: Wordexp Example, Next: , Prev: Flags for Wordexp, Up: Word Expansion

wordexp Example

Here is an example of using wordexp to expand several strings and use the results to run a shell command.  It also shows the use of WRDE_APPEND to concatenate the expansions and of wordfree to free the space allocated by wordexp.

int
expand_and_execute (const char *program, const char *options)
{
  wordexp_t result;
  pid_t pid
  int status, i;

  /* Expand the string for the program to run.  */
  switch (wordexp (program, &result, 0))
    {
    case 0:	/* Successful.  */
      break;
    case WRDE_NOSPACE:
      /* If the error was WRDE_NOSPACE,
         then perhaps part of the result was allocated.  */
      wordfree (&result);
    default:                    /* Some other error.  */
      return -1;
    }

  /* Expand the strings specified for the arguments.  */
  for (i = 0; args[i]; i++)
    {
      if (wordexp (options, &result, WRDE_APPEND))
        {
          wordfree (&result);
          return -1;
        }
    }

  pid = fork ();
  if (pid == 0)
    {
      /* This is the child process.  Execute the command. */
      execv (result.we_wordv[0], result.we_wordv);
      exit (EXIT_FAILURE);
    }
  else if (pid < 0)
    /* The fork failed.  Report failure.  */
    status = -1;
  else
    /* This is the parent process.  Wait for the child to complete.  */
    if (waitpid (pid, &status, 0) != pid)
      status = -1;

  wordfree (&result);
  return status;
}

In practice, since wordexp is executed by running a subshell, it would be faster to do this by concatenating the strings with spaces between them and running that as a shell command using 'sh -c'.


Date and Time
Node: Date and Time, Next: Non-Local Exits, Prev: Arithmetic, Up: Top

Date and Time

This chapter describes functions for manipulating dates and times, including functions for determining what the current time is and conversion between different time representations.

The time functions fall into three main categories:

•	Functions for measuring elapsed CPU time are discussed in See Processor Time.

•	Functions for measuring absolute clock or calendar time are discussed in See Calendar Time.

•	Functions for setting alarms and timers are discussed in See Setting an Alarm.

* Menu:

Processor Time	Measures processor time used by a program.

Calendar Time	Manipulation of "real" dates and times.

Setting an Alarm	Sending a signal after a specified time.

Sleeping	Waiting for a period of time.

Resource Usage	Measuring various resources used.

Limits on Resources	Specifying limits on resource usage.

Priority	Reading or setting process run priority.


Processor Time
Node: Processor Time, Next: Calendar Time, Prev: , Up: Date and Time

Processor Time

If you're trying to optimize your program or measure its efficiency, it's very useful to be able to know how much "processor time" or "CPU time" it has used at any given point.  Processor time is different from actual wall clock time because it doesn't include any time spent waiting for I/O or when some other process is running.  Processor time is represented by the data type clock_t, and is given as a number of "clock ticks" relative to an arbitrary base time marking the beginning of a single program invocation.  K

* Menu:

Basic CPU Time	The clock function.

Detailed CPU Time	The times function.


Basic CPU Time Inquiry
Node: Basic CPU Time, Next: Detailed CPU Time, Prev: , Up: Processor Time

Basic CPU Time Inquiry

To get the elapsed CPU time used by a process, you can use the clock function.  This facility is declared in the header file 'time.h'.  KIn typical usage, you call the clock function at the beginning and end of the interval you want to time, subtract the values, and then divide by CLOCKS_PER_SEC (the number of clock ticks per second), like this:

#include <time.h>

clock_t start, end;
double elapsed;

start = clock();
... /* Do the work. */
end = clock();
elapsed = ((double) (end - start)) / CLOCKS_PER_SEC;

Different computers and operating systems vary wildly in how they keep track of processor time.  It's common for the internal processor clock to have a resolution somewhere between hundredths and millionths of a second.

In the GNU system, clock_t is equivalent to long int and CLOCKS_PER_SEC is an integer value.  But in other systems, both clock_t and the type of the macro CLOCKS_PER_SEC can be either integer or floating-point types.  Casting processor time values to double, as in the example above, makes sure that operations such as arithmetic and printing work properly and consistently no matter what the underlying representation is.

 - Macro: int CLOCKS_PER_SEC

The value of this macro is the number of clock ticks per second measured by the clock function.

 - Macro: int CLK_TCK

This is an obsolete name for CLOCKS_PER_SEC.

 - Data Type: clock_t

This is the type of the value returned by the clock function.  Values of type clock_t are in units of clock ticks.

 - Function: clock_t clock (void)

This function returns the elapsed processor time.  The base time is arbitrary but doesn't change within a single process.  If the processor time is not available or cannot be represented, clock returns the value (clock_t)(-1).


Detailed Elapsed CPU Time Inquiry
Node: Detailed CPU Time, Next: , Prev: Basic CPU Time, Up: Processor Time

Detailed Elapsed CPU Time Inquiry

The times function returns more detailed information about elapsed processor time in a struct tms object.  You should include the header file 'sys/times.h' to use this facility.  K

 - Data Type: struct tms

The tms structure is used to return information about process times.  It contains at least the following members:

clock_t tms_utime

This is the CPU time used in executing the instructions of the calling process.

clock_t tms_stime

This is the CPU time used by the system on behalf of the calling process.

clock_t tms_cutime

This is the sum of the tms_utime values and the tms_cutime values of all terminated child processes of the calling process, whose status has been reported to the parent process by wait or waitpid; see See Process Completion.  In other words, it represents the total CPU time used in executing the instructions of all the terminated child processes of the calling process.

clock_t tms_cstime

This is similar to tms_cutime, but represents the total CPU time used by the system on behalf of all the terminated child processes of the calling process.

All of the times are given in clock ticks.  These are absolute values; in a newly created process, they are all zero.  See Creating a Process.

 - Function: clock_t times (struct tms *buffer)

The times function stores the processor time information for the calling process in buffer.

The return value is the same as the value of clock(): the elapsed real time relative to an arbitrary base.  The base is a constant within a particular process, and typically represents the time since system start-up.  A value of (clock_t)(-1) is returned to indicate failure.

Portability Note: The clock function described in See Basic CPU Time, is specified by the ANSI C standard.  The times function is a feature of POSIX.1.  In the GNU system, the value returned by the clock function is equivalent to the sum of the tms_utime and tms_stime fields returned by times.


Calendar Time
Node: Calendar Time, Next: Setting an Alarm, Prev: Processor Time, Up: Date and Time

Calendar Time

This section describes facilities for keeping track of dates and times according to the Gregorian calendar.  KThere are three representations for date and time information:

•	"Calendar time" (the time_t data type) is a compact representation, typically giving the number of seconds elapsed since some implementation-specific base time.  K

•	There is also a "high-resolution time" representation (the struct timeval data type) that includes fractions of a second.  Use this time representation instead of ordinary calendar time when you need greater precision.  K

•	"Local time" or "broken-down time" (the struct tm data type) represents the date and time as a set of components specifying the year, month, and so on, for a specific time zone.  This time representation is usually used in conjunction with formatting date and time values.  K

* Menu:

Simple Calendar Time	Facilities for manipulating calendar time.

High-Resolution Calendar	A time representation with greater precision.

Broken-down Time	Facilities for manipulating local time.

Formatting Date and Time	Converting times to strings.

TZ Variable	How users specify the time zone.

Time Zone Functions	Functions to examine or specify the time zone.

Time Functions Example	An example program showing use of some of the time functions.


Simple Calendar Time
Node: Simple Calendar Time, Next: High-Resolution Calendar, Prev: , Up: Calendar Time

Simple Calendar Time

This section describes the time_t data type for representing calendar time, and the functions which operate on calendar time objects.  These facilities are declared in the header file 'time.h'.  K

- Data Type: time_t

This is the data type used to represent calendar time.  In the GNU C library and other POSIX-compliant implementations, time_t is equivalent to long int.  When interpreted as an absolute time value, it represents the number of seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time.  (This date is sometimes referred to as the "epoch".)

In other systems, time_t might be either an integer or floating-point type.

 - Function: double difftime (time_t time1, time_t time0)

The difftime function returns the number of seconds elapsed between time time1 and time time0, as a value of type double.

In the GNU system, you can simply subtract time_t values.  But on other systems, the time_t data type might use some other encoding where subtraction doesn't work directly.

 - Function: time_t time (time_t *result)

The time function returns the current time as a value of type time_t.  If the argument result is not a null pointer, the time value is also stored in *result.  If the calendar time is not available, the value (time_t)(-1) is returned.


High-Resolution Calendar
Node: High-Resolution Calendar, Next: Broken-down Time, Prev: Simple Calendar Time, Up: Calendar Time

High-Resolution Calendar

The time_t data type used to represent calendar times has a resolution of only one second.  Some applications need more precision.

So, the GNU C library also contains functions which are capable of representing calendar times to a higher resolution than one second.  The functions and the associated data types described in this section are declared in 'sys/time.h'.  K

 - Data Type: struct timeval

The struct timeval structure represents a calendar time.  It has the following members:

long int tv_sec

This represents the number of seconds since the epoch.  It is equivalent to a normal time_t value.

long int tv_usec

This is the fractional second value, represented as the number of microseconds.

Some times struct timeval values are user for time intervals.  Then the tv_sec member is the number of seconds in the interval, and tv_usec is the number of addictional microseconds.

 - Data Type: struct timezone

The struct timezone structure is used to hold minimal information about the local time zone.  It has the following members:

int tz_minuteswest

This is the number of minutes west of GMT.

int tz_dsttime

If nonzero, daylight savings time applies during some part of the year.

The struct timezone type is obsolete and should never be used.  Instead, use the facilities described in See Time Zone Functions.

It is often necessary to subtract two values of type struct timeval.  Here is the best way to do this.  It works even on some peculiar operating systems where the tv_sec member has an unsigned type.

/* Subtract the `struct timeval' values X and Y,
   storing the result in RESULT.
   Return 1 if the difference is negative, otherwise 0.  */

int
timeval_subtract (result, x, y)
     struct timeval *result, *x, *y;
{
  /* Perform the carry for the later subtraction by updating y. */
  if (x->tv_usec < y->tv_usec) {
    int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1;
    y->tv_usec -= 1000000 * nsec;
    y->tv_sec += nsec;
  }
  if (x->tv_usec - y->tv_usec > 1000000) {
    int nsec = (y->tv_usec - x->tv_usec) / 1000000;
    y->tv_usec += 1000000 * nsec;
    y->tv_sec -= nsec;
  }

  /* Compute the time remaining to wait.
     tv_usec is certainly positive. */
  result->tv_sec = x->tv_sec - y->tv_sec;
  result->tv_usec = x->tv_usec - y->tv_usec;

  /* Return 1 if result is negative. */
  return x->tv_sec < y->tv_sec;
}

 - Function: int gettimeofday (struct timeval *tp, struct timezone *tzp)

The gettimeofday function returns the current date and time in the struct timeval structure indicated by tp.  Information about the time zone is returned in the structure pointed at tzp.  If the tzp argument is a null pointer, time zone information is ignored.

The return value is 0 on success and -1 on failure.  The following errno error condition is defined for this function:

ENOSYS

The operating system does not support getting time zone information, and tzp is not a null pointer.  The GNU operating system does not support using struct timezone to represent time zone information; that is an obsolete feature of 4.3 BSD.  Instead, use the facilities described in See Time Zone Functions.

 - Function: int settimeofday (const struct timeval *tp, const struct timezone *tzp)

The settimeofday function sets the current date and time according to the arguments.  As for gettimeofday, time zone information is ignored if tzp is a null pointer.

You must be a privileged user in order to use settimeofday.

The return value is 0 on success and -1 on failure.  The following errno error conditions are defined for this function:

EPERM

This process cannot set the time because it is not privileged.

ENOSYS

The operating system does not support setting time zone information, and tzp is not a null pointer.

 - Function: int adjtime (const struct timeval *delta, struct timeval *olddelta)

This function speeds up or slows down the system clock in order to make gradual adjustments in the current time.  This ensures that the time reported by the system clock is always monotonically increasing, which might not happen if you simply set the current time.

The delta argument specifies a relative adjustment to be made to the current time.  If negative, the system clock is slowed down for a while until it has lost this much time.  If positive, the system clock is speeded up for a while.

If the olddelta argument is not a null pointer, the adjtime function returns information about any previous time adjustment that has not yet completed.

This function is typically used to synchronize the clocks of computers in a local network.  You must be a privileged user to use it.  The return value is 0 on success and -1 on failure.  The following errno error condition is defined for this function:

EPERM

You do not have privilege to set the time.

Portability Note:  The gettimeofday, settimeofday, and adjtime functions are derived from BSD.


Broken-down Time
Node: Broken-down Time, Next: Formatting Date and Time, Prev: High-Resolution Calendar, Up: Calendar Time

Broken-down Time

Calender time is represented as a number of seconds.  This is convenient for calculation, but has no resemblance to the way people normally represent dates and times.  By contrast, "broken-down time" is a binary representation separated into year, month, day, and so on.  Broken down time values are not useful for calculations, but they are useful for printing human readable time.

A broken-down time value is always relative to a choice of local time zone, and it also indicates which time zone was used.

The symbols in this section are declared in the header file 'time.h'.

 - Data Type: struct tm

This is the data type used to represent a broken-down time.  The structure contains at least the following members, which can appear in any order:

int tm_sec

This is the number of seconds after the minute, normally in the range 0 to 59.  (The actual upper limit is 61, to allow for "leap seconds".) K

int tm_min

This is the number of minutes after the hour, in the range 0 to 59.

int tm_hour

This is the number of hours past midnight, in the range 0 to 23.

int tm_mday

This is the day of the month, in the range 1 to 31.

int tm_mon

This is the number of months since January, in the range 0 to 11.

int tm_year

This is the number of years since 1900.

int tm_wday

This is the number of days since Sunday, in the range 0 to 6.

int tm_yday

This is the number of days since January 1, in the range 0 to 365.

int tm_isdst

This is a flag that indicates whether Daylight Saving Time is (or was, or will be) in effect at the time described.  The value is positive if Daylight Saving Time is in effect, zero if it is not, and negative if the information is not available.

long int tm_gmtoff

This field describes the time zone that was used to compute this broken-down time value; it is the amount you must add to the local time in that zone to get GMT, in units of seconds.  The value is like that of the variable timezone (see Time Zone Functions).  You can also think of this as the "number of seconds west" of GMT.  The tm_gmtoff field is a GNU library extension.

const char *tm_zone

This field is the three-letter name for the time zone that was used to compute this broken-down time value.  It is a GNU library extension.

 - Function: struct tm * localtime (const time_t *time)

The localtime function converts the calendar time pointed to by time to broken-down time representation, expressed relative to the user's specified time zone.

The return value is a pointer to a static broken-down time structure, which might be overwritten by subsequent calls to any of the date and time functions.  (But no other library function overwrites the contents of this object.)

Calling localtime has one other effect: it sets the variable tzname with information about the current time zone.  See Time Zone Functions.

 - Function: struct tm * gmtime (const time_t *time)

This function is similar to localtime, except that the broken-down time is expressed as Coordinated Universal Time (UTC)--that is, as Greenwich Mean Time (GMT) rather than relative to the local time zone.

Recall that calendar times are always expressed in coordinated universal time.

 - Function: time_t mktime (struct tm *brokentime)

The mktime function is used to convert a broken-down time structure to a calendar time representation.  It also "normalizes" the contents of the broken-down time structure, by filling in the day of week and day of year based on the other date and time components.

The mktime function ignores the specified contents of the tm_wday and tm_yday members of the broken-down time structure.  It uses the values of the other components to compute the calendar time; it's permissible for these components to have unnormalized values outside of their normal ranges.  The last thing that mktime does is adjust the components of the brokentime structure (including the tm_wday and tm_yday).

If the specified broken-down time cannot be represented as a calendar time, mktime returns a value of (time_t)(-1) and does not modify the contents of brokentime.

Calling mktime also sets the variable tzname with information about the current time zone.  See Time Zone Functions.


Formatting Date and Time
Node: Formatting Date and Time, Next: TZ Variable, Prev: Broken-down Time, Up: Calendar Time

Formatting Date and Time

The functions described in this section format time values as strings.  These functions are declared in the header file 'time.h'.  K

 - Function: char * asctime (const struct tm *brokentime)

The asctime function converts the broken-down time value that brokentime points to into a string in a standard format:

"Tue May 21 13:46:22 1991\n"

The abbreviations for the days of week are: 'Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', and 'Sat'.

The abbreviations for the months are: 'Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', and 'Dec'.

The return value points to a statically allocated string, which might be overwritten by subsequent calls to any of the date and time functions.  (But no other library function overwrites the contents of this string.)

 - Function: char * ctime (const time_t *time)

The ctime function is similar to asctime, except that the time value is specified in calendar time (rather than local time) format.  It is equivalent to

asctime (localtime (time))

ctime sets the variable tzname, because localtime does so.  See Time Zone Functions.

 - Function: size_t strftime (char *s, size_t size, const char *template, const struct tm *brokentime)

This function is similar to the sprintf function (see Formatted Input), but the conversion specifications that can appear in the format template template are specialized for printing components of the date and time brokentime according to the locale currently specified for time conversion (see Locales).

Ordinary characters appearing in the template are copied to the output string s; this can include multibyte character sequences.  Conversion specifiers are introduced by a '%' character, and are replaced in the output string as follows:

%a

The abbreviated weekday name according to the current locale.

%A

The full weekday name according to the current locale.

%b

The abbreviated month name according to the current locale.

%B

The full month name according to the current locale.

%c

The preferred date and time representation for the current locale.

%d

The day of the month as a decimal number (range 01 to 31).

%H

The hour as a decimal number, using a 24-hour clock (range 00 to 23).

%I

The hour as a decimal number, using a 12-hour clock (range 01 to 12).

%j

The day of the year as a decimal number (range 001 to 366).

%m

The month as a decimal number (range 01 to 12).

%M

The minute as a decimal number.

%p

Either 'am' or 'pm', according to the given time value; or the corresponding strings for the current locale.

%S

The second as a decimal number.

%U

The week number of the current year as a decimal number, starting with the first Sunday as the first day of the first week.

%W

The week number of the current year as a decimal number, starting with the first Monday as the first day of the first week.

%w

The day of the week as a decimal number, Sunday being 0.

%x

The preferred date representation for the current locale, but without the time.

%X

The preferred time representation for the current locale, but with no date.

%y

The year as a decimal number, but without a century (range 00 to 99).

%Y

The year as a decimal number, including the century.

%Z

The time zone or name or abbreviation (empty if the time zone can't be determined).

%%

A literal '%' character.

The size parameter can be used to specify the maximum number of characters to be stored in the array s, including the terminating null character.  If the formatted time requires more than size characters, the excess characters are discarded.  The return value from strftime is the number of characters placed in the array s, not including the terminating null character.  If the value equals size, it means that the array s was too small; you should repeat the call, providing a bigger array.

If s is a null pointer, strftime does not actually write anything, but instead returns the number of characters it would have written.

For an example of strftime, see See Time Functions Example.


Specifying the Time Zone with TZ
Node: TZ Variable, Next: Time Zone Functions, Prev: Formatting Date and Time, Up: Calendar Time

Specifying the Time Zone with TZ

In the GNU system, a user can specify the time zone by means of the TZ environment variable.  For information about how to set environment variables, see See Environment Variables.  The functions for accessing the time zone are declared in 'time.h'.  KThe value of the TZ variable can be of one of three formats.  The first format is used when there is no Daylight Saving Time (or summer time) in the local time zone:

std offset

The std string specifies the name of the time zone.  It must be three or more characters long and must not contain a leading colon or embedded digits, commas, or plus or minus signs.  There is no space character separating the time zone name from the offset, so these restrictions are necessary to parse the specification correctly.

The offset specifies the time value one must add to the local time to get a Coordinated Universal Time value.  It has syntax like [+|-]hh[:mm[:ss]].  This is positive if the local time zone is west of the Prime Meridian and negative if it is east.  The hour must be between 0 and 24, and the minute and seconds between 0 and 59.

For example, here is how we would specify Eastern Standard Time, but without any daylight savings time alternative:

EST+5

The second format is used when there is Daylight Saving Time:

std offset dst [offset],start[/time],end[/time]

The initial std and offset specify the standard time zone, as described above.  The dst string and offset specify the name and offset for the corresponding daylight savings time time zone; if the offset is omitted, it defaults to one hour ahead of standard time.

The remainder of the specification describes when daylight savings time is in effect.  The start field is when daylight savings time goes into effect and the end field is when the change is made back to standard time.  The following formats are recognized for these fields:

Jn

This specifies the Julian day, with n between 1 and 365.  February 29 is never counted, even in leap years.

n

This specifies the Julian day, with n between 0 and 365.  February 29 is counted in leap years.

Mm.w.d

This specifies day d of week w of month m.  The day d must be between 0 (Sunday) and 6.  The week w must be between 1 and 5; week 1 is the first week in which day d occurs, and week 5 specifies the last d day in the month.  The month m should be between 1 and 12.

The time fields specify when, in the local time currently in effect, the change to the other time occurs.  If omitted, the default is 02:00:00.

For example, here is how one would specify the Eastern time zone in the United States, including the appropriate daylight saving time and its dates of applicability.  The normal offset from GMT is 5 hours; since this is west of the prime meridian, the sign is positive.  Summer time begins on the first Sunday in April at 2:00am, and ends on the last Sunday in October at 2:00am.

EST+5EDT,M4.1.0/M10.5.0

The schedule of daylight savings time in any particular jurisdiction has changed over the years.  To be strictly correct, the conversion of dates and times in the past should be based on the schedule that was in effect then.  However, the system has no facilities to let you specify how the schedule has changed from year to year.  The most you can do is specify one particular schedule--usually the present day schedule--and this is used to convert any date, no matter when.

The third format looks like this:

:characters

Each operating system interprets this format differently; in the GNU C library, characters is the name of a file which describes the time zone.

If the TZ environment variable does not have a value, the operation chooses a time zone by default.  Each operating system has its own rules for choosing the default time zone, so there is little we can say about them.


Functions and Variables for Time Zones
Node: Time Zone Functions, Next: Time Functions Example, Prev: TZ Variable, Up: Calendar Time

Functions and Variables for Time Zones

 - Variable: char *

The array tzname contains two strings, which are the standard three-letter names of the pair of time zones (standard and daylight savings) that the user has selected.  tzname[0] is the name of the standard time zone (for example, "EST"), and tzname[1] is the name for the time zone when daylight savings time is in use (for example, "EDT").  These correspond to the std and dst strings (respectively) from the TZ environment variable.

The tzname array is initialized from the TZ environment variable whenever tzset, ctime, strftime, mktime, or localtime is called.

 - Function: void tzset (void)

The tzset function initializes the tzname variable from the value of the TZ environment variable.  It is not usually necessary for your program to call this function, because it is called automatically when you use the other time conversion functions that depend on the time zone.

The following variables are defined for compatibility with System V Unix.  These variables are set by calling localtime.

 - Variable: long int timezone

This contains the difference between GMT and local standard time, in seconds.  For example, in the U.S. Eastern time zone, the value is 5*60*60.

 - Variable: int daylight

This variable has a nonzero value if the standard U.S. daylight savings time rules apply.


Time Functions Example
Node: Time Functions Example, Next: , Prev: Time Zone Functions, Up: Calendar Time

Time Functions Example

Here is an example program showing the use of some of the local time and calendar time functions.

#include <time.h>
#include <stdio.h>

#define SIZE 256

int
main (void)
{
  char buffer[SIZE];
  time_t curtime;
  struct tm *loctime;

  /* Get the current time. */
  curtime = time (NULL);

  /* Convert it to local time representation. */
  loctime = localtime (&curtime);

  /* Print out the date and time in the standard format. */
  fputs (asctime (loctime), stdout);

/* Print it out in a nice format. */
  strftime (buffer, SIZE, "Today is %A, %B %d.\n", loctime);
  fputs (buffer, stdout);
  strftime (buffer, SIZE, "The time is %I:%M %p.\n", loctime);
  fputs (buffer, stdout);

  return 0;
}

It produces output like this:

Wed Jul 31 13:02:36 1991
Today is Wednesday, July 31.
The time is 01:02 PM.


Setting an Alarm
Node: Setting an Alarm, Next: Sleeping, Prev: Calendar Time, Up: Date and Time

Setting an Alarm

The alarm and setitimer functions provide a mechanism for a process to interrupt itself at some future time.  They do this by setting a timer; when the timer expires, the process receives a signal.

Each process has three independent interval timers available:

•	A real-time timer that counts clock time.  This timer sends a SIGALRM signal to the process when it expires.  K

•	A virtual timer that counts CPU time used by the process.  This timer sends a SIGVTALRM signal to the process when it expires.  K

•	A profiling timer that counts both CPU time used by the process, and CPU time spent in system calls on behalf of the process.  This timer sends a SIGPROF signal to the process when it expires.  KYou can only have one timer of each kind set at any given time.  If you set a timer that has not yet expired, that timer is simply reset to the new value.

You should establish a handler for the appropriate alarm signal using signal or sigaction before issuing a call to setitimer or alarm.  Otherwise, an unusual chain of events could cause the timer to expire before your program establishes the handler, and in that case it would be terminated, since that is the default action for the alarm signals.  See Signal Handling.

The setitimer function is the primary means for setting an alarm.  This facility is declared in the header file 'sys/time.h'.  The alarm function, declared in 'unistd.h', provides a somewhat simpler interface for setting the real-time timer.  K

 - Data Type: struct itimerval

This structure is used to specify when a timer should expire.  It contains the following members:

struct timeval it_interval

This is the interval between successive timer interrupts.  If zero, the alarm will only be sent once.

struct timeval it_value

This is the interval to the first timer interrupt.  If zero, the alarm is disabled.

The struct timeval data type is described in See High-Resolution Calendar.

 - Function: int setitimer (int which, struct itimerval *new, struct itimerval *old)

The setitimer function sets the timer specified by which according to new.  The which argument can have a value of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF.

If old is not a null pointer, setitimer returns information about any previous unexpired timer of the same kind in the structure it points to.

The return value is 0 on success and -1 on failure.  The following errno error conditions are defined for this function:

EINVAL

The timer interval was too large.

 - Function: int getitimer (int which, struct itimerval *old)

The getitimer function stores information about the timer specified by which in the structure pointed at by old.

The return value and error conditions are the same as for setitimer.

ITIMER_REAL

This constant can be used as the which argument to the setitimer and getitimer functions to specify the real-time timer.

ITIMER_VIRTUAL

This constant can be used as the which argument to the setitimer and getitimer functions to specify the virtual timer.

ITIMER_PROF

This constant can be used as the which argument to the setitimer and getitimer functions to specify the profiling timer.

 - Function: unsigned int alarm (unsigned int seconds)

The alarm function sets the real-time timer to expire in seconds seconds.  If you want to cancel any existing alarm, you can do this by calling alarm with a seconds argument of zero.

The return value indicates how many seconds remain before the previous alarm would have been sent.  If there is no previous alarm, alarm returns zero.

The alarm function could be defined in terms of setitimer like this:

unsigned int
alarm (unsigned int seconds)
{
  struct itimerval old, new;
  new.it_interval.tv_usec = 0;
  new.it_interval.tv_sec = 0;
  new.it_value.tv_usec = 0;
  new.it_value.tv_sec = (long int) seconds;
  if (setitimer (ITIMER_REAL, &new, &old) < 0)
    return 0;
  else
    return old.it_value.tv_sec;
}

There is an example showing the use of the alarm function in See Handler Returns.

If you simply want your process to wait for a given number of seconds, you should use the sleep function.  See Sleeping.

You shouldn't count on the signal arriving precisely when the timer expires.  In a multiprocessing environment there is typically some amount of delay involved.

Portability Note: The setitimer and getitimer functions are derived from BSD Unix, while the alarm function is specified by the POSIX.1 standard.  setitimer is more powerful than alarm, but alarm is more widely used.


Sleeping
Node: Sleeping, Next: Resource Usage, Prev: Setting an Alarm, Up: Date and Time

Sleeping

The function sleep gives a simple way to make the program wait for short periods of time.  If your program doesn't use signals (except to terminate), then you can expect sleep to wait reliably for the specified amount of time.  Otherwise, sleep can return sooner if a signal arrives; if you want to wait for a given period regardless of signals, use select (see Waiting for I/O) and don't specify any descriptors to wait for.

 - Function: unsigned int sleep (unsigned int seconds)

The sleep function waits for seconds or until a signal is delivered, whichever happens first.

If sleep function returns because the requested time has elapsed, it returns a value of zero.  If it returns because of delivery of a signal, its return value is the remaining time in the sleep period.

The sleep function is declared in 'unistd.h'.

Resist the temptation to implement a sleep for a fixed amount of time by using the return value of sleep, when nonzero, to call sleep again.  This will work with a certain amount of accuracy as long as signals arrive infrequently.  But each signal can cause the eventual wakeup time to be off by an additional second or so.  Suppose a few signals happen to arrive in rapid succession by bad luck--there is no limit on how much this could shorten or lengthen the wait.

Instead, compute the time at which the program should stop waiting, and keep trying to wait until that time.  This won't be off by more than a second.  With just a little more work, you can use select and make the waiting period quite accurate.  (Of course, heavy system load can cause unavoidable additional delays--unless the machine is dedicated to one application, there is no way you can avoid this.)

On some systems, sleep can do strange things if your program uses SIGALRM explicitly.  Even if SIGALRM signals are being ignored or blocked when sleep is called, sleep might return prematurely on delivery of a SIGALRM signal.  If you have established a handler for SIGALRM signals and a SIGALRM signal is delivered while the process is sleeping, the action taken might be just to cause sleep to return instead of invoking your handler.  And, if sleep is interrupted by delivery of a signal whose handler requests an alarm or alters the handling of SIGALRM, this handler and sleep will interfere.

On the GNU system, it is safe to use sleep and SIGALRM in the same program, because sleep does not work by means of SIGALRM.


Resource Usage
Node: Resource Usage, Next: Limits on Resources, Prev: Sleeping, Up: Date and Time

Resource Usage

The function getrusage and the data type struct rusage are used for examining the usage figures of a process.  They are declared in 'sys/resource.h'.

 - Function: int getrusage (int processes, struct rusage *rusage)

This function reports the usage totals for processes specified by processes, storing the information in *rusage.

In most systems, processes has only two valid values:

RUSAGE_SELF

Just the current process.

RUSAGE_CHILDREN

All child processes (direct and indirect) that have terminated already.

In the GNU system, you can also inquire about a particular child process by specifying its process ID.

The return value of getrusage is zero for success, and -1 for failure.

EINVAL

The argument processes is not valid.

One way of getting usage figures for a particular child process is with the function wait4, which returns totals for a child when it terminates.  See BSD Wait Functions.

 - Data Type: struct rusage

This data type records a collection usage amounts for various sorts of resources.  It has the following members, and possibly others:

struct timeval ru_utime

User time used.

struct timeval ru_stime

System time used.

long ru_majflt

Number of page faults.

long ru_inblock

Number of block input operations.

long ru_oublock

Number of block output operations.

long ru_msgsnd

Number of messages sent.

long ru_msgrcv

Number of messages received.

long ru_nsignals

Number of signals received.

An additional historical function for examining usage figures, vtimes, is supported but not documented here.  It is declared in 'sys/vtimes.h'.


Limiting Resource Usage
Node: Limits on Resources, Next: Priority, Prev: Resource Usage, Up: Date and Time

Limiting Resource Usage

You can specify limits for the resource usage of a process.  When the process tries to exceed a limit, it may get a signal, or the system call by which it tried to do so may fail, depending on the limit.  Each process initially inherits its limit values from its parent, but it can subsequently change them.

The symbols in this section are defined in 'sys/resource.h'.

 - Function: int getrlimit (int resource, struct rlimit *rlp)

Read the current value and the maximum value of resource resource and store them in *rlp.

The return value is 0 on success and -1 on failure.  The only possible errno error condition is EFAULT.

 - Function: int setrlimit (int resource, struct rlimit *rlp)

Store the current value and the maximum value of resource resource in *rlp.

The return value is 0 on success and -1 on failure.  The following errno error condition is possible:

EPERM

You tried to change the maximum permissible limit value, but you don't have privileges to do so.

 - Data Type: struct rlimit

This structure is used with getrlimit to receive limit values, and with setrlimit to specify limit values.  It has two fields:

rlim_cur

The current value of the limit in question.

rlim_max

The maximum permissible value of the limit in question.  You cannot set the current value of the limit to a larger number than this maximum.  Only the super user can change the maximum permissible value.

In getrlimit, the structure is an output; it receives the current values.  In setrlimit, it specifies the new values.

Here is a list of resources that you can specify a limit for.  Those that are sizes are measured in bytes.

RLIMIT_CPU

The maximum amount of cpu time the process can use.  If it runs for longer than this, it gets a signal: SIGXCPU.  The value is measured in seconds.  See Nonstandard Signals.

RLIMIT_FSIZE

The maximum size of file the process can create.  Trying to write a larger file causes a signal: SIGXFSZ.  See Nonstandard Signals.

RLIMIT_DATA

The maximum size of data memory for the process.  If the process tries to allocate data memory beyond this amount, the allocation function fails.

RLIMIT_STACK

The maximum stack size for the process.  If the process tries to extend its stack past this size, it gets a SIGSEGV signal.  See Program Error Signals.

RLIMIT_CORE

The maximum size core file that this process can create.  If the process terminates and a core file is made, and this maximum size is not enough, the core file is truncated.

RLIMIT_RSS

The maximum amount of physical memory that this process should get.  This parameter is a guide for the system's scheduler and memory allocator; the system may give the process more memory when there is a surplus.

RLIMIT_OPEN_FILES

The maximum number of files that the process can open.  If it tries to open more files than this, it gets error code EMFILE.  See Error Codes.

RLIM_NLIMITS

The number of different resource limits.  Any valid resource operand must be less than RLIM_NLIMITS.

 - Constant: int

This constant stands for a value of "infinity" when supplied as the limit value in setrlimit.

Two historical functions for setting resource limits, ulimit and vlimit, are not documented here.  The latter is declared in 'sys/vlimit.h' and comes from BSD.


Process Priority
Node: Priority, Next: , Prev: Limits on Resources, Up: Date and Time

Process Priority

When several processes try to run, their respective priorities determine what share of the CPU each process gets.  This section describes how you can read and set the priority of a process.  All these functions and macros are declared in 'sys/resource.h'.

The range of valid priority values depends on the operating system, but typically it runs from -20 to 20.  A lower priority value means the process runs more often.  These constants describe the range of priority values:

PRIO_MIN

The smallest valid priority value.

PRIO_MAX

The smallest valid priority value.

 - Function: int getpriority (int class, int id)

Read the priority of a class of processes; class and id specify which ones (see below).

The return value is the priority value on success, and -1 on failure.  The following errno error condition are possible for this function:

ESRCH

The combination of class and id does not match any existing process.

EINVAL

The value of class is not valid.

When the return value is -1, it could indicate failure, or it could be the priority value.  The only way to make certain is to set errno = 0 before calling getpriority, then use errno != 0 afterward as the criterion for failure.

 - Function: int setpriority (int class, int id, int priority)

Read the priority of a class of processes; class and id specify which ones (see below).

The return value is 0 on success and -1 on failure.  The following errno error condition are defined for this function:

ESRCH

The combination of class and id does not match any existing process.

EINVAL

The value of class is not valid.

EPERM

You tried to set the priority of some other user's process, and you don't have privileges for that.

EACCES

You tried to lower the priority of a process, and you don't have privileges for that.

The arguments class and id together specify a set of processes you are interested in.  These are the possible values for class:

PRIO_PROCESS

Read or set the priority of one process.  The argument id is a process ID.

PRIO_PGRP

Read or set the priority of one process group.  The argument id is a process group ID.

PRIO_USER

Read or set the priority of one user's processes.  The argument id is a user ID.

If the argument id is 0, it stands for the current process, current process group, or the current user, according to class.

 - Function: int nice (int increment)

Increment the priority of the current process by increment.  The return value is not meaningful.

Here is an equivalent definition for nice:

int
nice (int increment)
{
  int old = getpriority (PRIO_PROCESS, 0);
  setpriority (PRIO_PROCESS, 0, old + increment);
}


Extended Characters
Node: Extended Characters, Next: Locales, Prev: String and Array Utilities, Up: Top

Extended Characters

A number of languages use character sets that are larger than the range of values of type char.  Japanese and Chinese are probably the most familiar examples.

The GNU C library includes support for two mechanisms for dealing with extended character sets: multibyte characters and wide characters.  This chapter describes how to use these mechanisms, and the functions for converting between them.  KThe behavior of the functions in this chapter is affected by the current locale for character classification--the LC_CTYPE category; see See Locale Categories.  This choice of locale selects which multibyte code is used, and also controls the meanings and characteristics of wide character codes.

* Menu:

Extended Char Intro	Multibyte codes versus wide characters.

Locales and Extended Chars	The locale selects the character codes.

Multibyte Char Intro	How multibyte codes are represented.

Wide Char Intro	How wide characters are represented.

Wide String Conversion	Converting wide strings to multibyte code and vice versa.

Length of Char	how many bytes make up one multibyte char.

Converting One Char	Converting a string character by character.

Example of Conversion	Example showing why converting one character at a time may be useful.

Shift State	Multibyte codes with "shift characters".


Introduction to Extended Characters
Node: Extended Char Intro, Next: Locales and Extended Chars, Prev: , Up: Extended Characters

Introduction to Extended Characters

You can represent extended characters in either of two ways:

•	As "multibyte characters" which can be embedded in an ordinary string, an array of char objects.  Their advantage is that many programs and operating systems can handle occasional multibyte characters scattered among ordinary ASCII characters, without any change.

•	As "wide characters", which are like ordinary characters except that they occupy more bits.  The wide character data type, wchar_t, has a range large enough to hold extended character codes as well as old-fashioned ASCII codes.

An advantage of wide characters is that each character is a single data object, just like ordinary ASCII characters.  There are a few disadvantages:

•	Each existing program must be modified and recompiled to make it use wide characters.

•	Files of wide characters cannot be read by programs that expect ordinary characters.

Typically, you use the multibyte character representation as part of the external program interface, such as reading or writing text to files.  However, it's usually easier to perform internal manipulations on strings containing extended characters on arrays of wchar_t objects, since the uniform representation makes most editing operations easier.  If you do use multibyte characters for files and wide characters for internal operations, you need to convert between them when you read and write data.

If your system supports extended characters, then it supports them both as multibyte characters and as wide characters.  The library includes functions you can use to convert between the two representations.  These functions are described in this chapter.


Locales and Extended Characters
Node: Locales and Extended Chars, Next: Multibyte Char Intro, Prev: Extended Char Intro, Up: Extended Characters

Locales and Extended Characters

A computer system can support more than one multibyte character code, and more than one wide character code.  The user controls the choice of codes through the current locale for character classification (see Locales).  Each locale specifies a particular multibyte character code and a particular wide character code.  The choice of locale influences the behavior of the conversion functions in the library.

Some locales support neither wide characters nor nontrivial multibyte characters.  In these locales, the library conversion functions still work, even though what they do is basically trivial.

If you select a new locale for character classification, the internal shift state maintained by these functions can become confused, so it's not a good idea to change the locale while you are in the middle of processing a string.


Multibyte Characters
Node: Multibyte Char Intro, Next: Wide Char Intro, Prev: Locales and Extended Chars, Up: Extended Characters

Multibyte Characters

In the ordinary ASCII code, a sequence of characters is a sequence of bytes, and each character is one byte.  This is very simple, but allows for only 256 distinct characters.

In a "multibyte character code", a sequence of characters is a sequence of bytes, but each character may occupy one or more consecutive bytes of the sequence.

There are many different ways of designing a multibyte character code; different systems use different codes.  To specify a particular code means designating the "basic" byte sequences--those which represent a single character--and what characters they stand for.  A code that a computer can actually use must have a finite number of these basic sequences, and typically none of them is more than a few characters long.

These sequences need not all have the same length.  In fact, many of them are just one byte long.  Because the basic ASCII characters in the range from 0 to 0177 are so important, they stand for themselves in all multibyte character codes.  That is to say, a byte whose value is 0 through 0177 is always a character in itself.  The characters which are more than one byte must always start with a byte in the range from 0200 through 0377.

The byte value 0 can be used to terminate a string, just as it is often used in a string of ASCII characters.

Specifying the basic byte sequences that represent single characters automatically gives meanings to many longer byte sequences, as more than one character.  For example, if the two byte sequence 0205 049 stands for the Greek letter alpha, then 0205 049 065 must stand for an alpha followed by an 'A' (ASCII code 065), and 0205 049 0205 049 must stand for two alphas in a row.

If any byte sequence can have more than one meaning as a sequence of characters, then the multibyte code is ambiguous--and no good.  The codes that systems actually use are all unambiguous.

In most codes, there are certain sequences of bytes that have no meaning as a character or characters.  These are called "invalid".

The simplest possible multibyte code is a trivial one:

The basic sequences consist of single bytes.

This particular code is equivalent to not using multibyte characters at all.  It has no invalid sequences.  But it can handle only 256 different characters.

Here is another possible code which can handle 9376 different characters:

The basic sequences consist of

•	single bytes with values in the range 0 through 0237.

•	two-byte sequences, in which both of the bytes have values in the range from 0240 through 0377.

This code or a similar one is used on some systems to represent Japanese characters.  The invalid sequences are those which consist of an odd number of consecutive bytes in the range from 0240 through 0377.

Here is another multibyte code which can handle more distinct extended characters--in fact, almost thirty million:

The basic sequences consist of

•	single bytes with values in the range 0 through 0177.

•	sequences of up to four bytes in which the first byte is in the range from 0200 through 0237, and the remaining bytes are in the range from 0240 through 0377.

In this code, any sequence that starts with a byte in the range from 0240 through 0377 is invalid.

And here is another variant which has the advantage that removing the last byte or bytes from a valid character can never produce another valid character.  (This property is convenient when you want to search strings for particular characters.)

The basic sequences consist of

•	single bytes with values in the range 0 through 0177.

•	two-byte sequences in which the first byte is in the range from 0200 through 0207, and the second byte is in the range from 0240 through 0377.

•	three-byte sequences in which the first byte is in the range from 0210 through 0217, and the other bytes are in the range from 0240 through 0377.

•	four-byte sequences in which the first byte is in the range from 0220 through 0227, and the other bytes are in the range from 0240 through 0377.

The list of invalid sequences for this code is long and not worth stating in full; examples of invalid sequences include 0240 and 0220 0300 065.

The number of possible multibyte codes is astronomical.  But a given computer system will support at most a few different codes.  (One of these codes may allow for thousands of different characters.) Another computer system may support a completely different code.  The library facilities described in this chapter are helpful because they package up the knowledge of the details of a particular computer system's multibyte code, so your programs need not know them.

You can use special standard macros to find out the maximum possible number of bytes in a character in the currently selected multibyte code with MB_CUR_MAX, and the maximum for any multibyte code supported on your computer with MB_LEN_MAX.

 - Macro: int MB_LEN_MAX

This is the maximum length of a multibyte character for any supported locale.  It is defined in 'limits.h'.  K

- Macro: int MB_CUR_MAX

This macro expands into a (possibly non-constant) positive integer expression that is the maximum number of bytes in a multibyte character in the current locale.  The value is never greater than MB_LEN_MAX.

MB_CUR_MAX is defined in 'stdlib.h'.

Normally, each basic sequence in a particular character code stands for one character, the same character regardless of context.  Some multibyte character codes have a concept of "shift state"; certain codes, called "shift sequences", change to a different shift state, and the meaning of some or all basic sequences varies according to the current shift state.  In fact, the set of basic sequences might even be different depending on the current shift state.  See Shift State, for more information on handling this sort of code.

What happens if you try to pass a string containing multibyte characters to a function that doesn't know about them?  Normally, such a function treats a string as a sequence of bytes, and interprets certain byte values specially; all other byte values are "ordinary".  As long as a multibyte character doesn't contain any of the special byte values, the function should pass it through as if it were several ordinary characters.

For example, let's figure out what happens if you use multibyte characters in a file name.  The functions such as open and unlink that operate on file names treat the name as a sequence of byte values, with '/' as the only special value.  Any other byte values are copied, or compared, in sequence, and all byte values are treated alike.  Thus, you may think of the file name as a sequence of bytes or as a string containing multibyte characters; the same behavior makes sense equally either way, provided no multibyte character contains a '/'.


Wide Character Introduction
Node: Wide Char Intro, Next: Wide String Conversion, Prev: Multibyte Char Intro, Up: Extended Characters

Wide Character Introduction

"Wide characters" are much simpler than multibyte characters.  They are simply characters with more than eight bits, so that they have room for more than 256 distinct codes.  The wide character data type, wchar_t, has a range large enough to hold extended character codes as well as old-fashioned ASCII codes.

An advantage of wide characters is that each character is a single data object, just like ordinary ASCII characters.  Wide characters also have some disadvantages:

•	A program must be modified and recompiled in order to use wide characters at all.

•	Files of wide characters cannot be read by programs that expect ordinary characters.

Wide character values 0 through 0177 are always identical in meaning to the ASCII character codes.  The wide character value zero is often used to terminate a string of wide characters, just as a single byte with value zero often terminates a string of ordinary characters.

 - Data Type: wchar_t

This is the "wide character" type, an integer type whose range is large enough to represent all distinct values in any extended character set in the supported locales.  See Locales, for more information about locales.  This type is defined in the header file 'stddef.h'.  K

If your system supports extended characters, then each extended character has both a wide character code and a corresponding multibyte basic sequence.

In this chapter, the term "code" is used to refer to a single extended character object to emphasize the distinction from the char data type.


Conversion of Extended Strings
Node: Wide String Conversion, Next: Length of Char, Prev: Wide Char Intro, Up: Extended Characters

Conversion of Extended Strings

The mbstowcs function converts a string of multibyte characters to a wide character array.  The wcstombs function does the reverse.  These functions are declared in the header file 'stdlib.h'.

In most programs, these functions are the only ones you need for conversion between wide strings and multibyte character strings.  But they have limitations.  If your data is not null-terminated or is not all in core at once, you probably need to use the low-level conversion functions to convert one character at a time.  See Converting One Char.

 - Function: size_t mbstowcs (wchar_t *wstring, const char *string, size_t size)

The mbstowcs ("multibyte string to wide character string") function converts the null-terminated string of multibyte characters string to an array of wide character codes, storing not more than size wide characters into the array beginning at wstring.  The terminating null character counts towards the size, so if size is less than the actual number of wide characters resulting from string, no terminating null character is stored.

The conversion of characters from string begins in the initial shift state.

If an invalid multibyte character sequence is found, this function returns a value of -1.  Otherwise, it returns the number of wide characters stored in the array wstring.  This number does not include the terminating null character, which is present if the number is less than size.

Here is an example showing how to convert a string of multibyte characters, allocating enough space for the result.

wchar_t *
mbstowcs_alloc (const char *string)
{
  size_t size = strlen (string) + 1;
  wchar_t *buf = xmalloc (size * sizeof (wchar_t));

  size = mbstowcs (buf, string, size);
  if (size == (size_t) -1)
    return NULL;
  buf = xrealloc (buf, (size + 1) * sizeof (wchar_t));
  return buf;
}

 - Function: size_t wcstombs (char *string, const wchar_t wstring, size_t size)

The wcstombs ("wide character string to multibyte string") function converts the null-terminated wide character array wstring into a string containing multibyte characters, storing not more than size bytes starting at string, followed by a terminating null character if there is room.  The conversion of characters begins in the initial shift state.

The terminating null character counts towards the size, so if size is less than or equal to the number of bytes needed in wstring, no terminating null character is stored.

If a code that does not correspond to a valid multibyte character is found, this function returns a value of -1.  Otherwise, the return value is the number of bytes stored in the array string.  This number does not include the terminating null character, which is present if the number is less than size.


Multibyte Character Length
Node: Length of Char, Next: Converting One Char, Prev: Wide String Conversion, Up: Extended Characters

Multibyte Character Length

This section describes how to scan a string containing multibyte characters, one character at a time.  The difficulty in doing this is to know how many bytes each character contains.  Your program can use mblen to find this out.

 - Function: int mblen (const char *string, size_t size)

The mblen function with a non-null string argument returns the number of bytes that make up the multibyte character beginning at string, never examining more than size bytes.  (The idea is to supply for size the number of bytes of data you have in hand.)

The return value of mblen distinguishes three possibilities: the first size bytes at string start with valid multibyte character, they start with an invalid byte sequence or just part of a character, or string points to an empty string (a null character).

For a valid multibyte character, mblen returns the number of bytes in that character (always at least 1, and never more than size).  For an invalid byte sequence, mblen returns -1.  For an empty string, it returns 0.

If the multibyte character code uses shift characters, then mblen maintains and updates a shift state as it scans.  If you call mblen with a null pointer for string, that initializes the shift state to its standard initial value.  It also returns nonzero if the multibyte character code in use actually has a shift state.  See Shift State.

The function mblen is declared in 'stdlib.h'.


Conversion of Extended Characters One by One
Node: Converting One Char, Next: Example of Conversion, Prev: Length of Char, Up: Extended Characters

Conversion of Extended Characters One by One

You can convert multibyte characters one at a time to wide characters with the mbtowc function.  The wctomb function does the reverse.  These functions are declared in 'stdlib.h'.

 - Function: int mbtowc (wchar_t *result, const char *string, size_t size)

The mbtowc ("multibyte to wide character") function when called with non-null string converts the first multibyte character beginning at string to its corresponding wide character code.  It stores the result in *result.

mbtowc never examines more than size bytes.  (The idea is to supply for size the number of bytes of data you have in hand.)

mbtowc with non-null string distinguishes three possibilities: the first size bytes at string start with valid multibyte character, they start with an invalid byte sequence or just part of a character, or string points to an empty string (a null character).

For a valid multibyte character, mbtowc converts it to a wide character and stores that in *result, and returns the number of bytes in that character (always at least 1, and never more than size).

For an invalid byte sequence, mbtowc returns -1.  For an empty string, it returns 0, also storing 0 in *result.

If the multibyte character code uses shift characters, then mbtowc maintains and updates a shift state as it scans.  If you call mbtowc with a null pointer for string, that initializes the shift state to its standard initial value.  It also returns nonzero if the multibyte character code in use actually has a shift state.  See Shift State.

 - Function: int wctomb (char *string, wchar_t wchar)

The wctomb ("wide character to multibyte") function converts the wide character code wchar to its corresponding multibyte character sequence, and stores the result in bytes starting at string.  At most MB_CUR_MAX characters are stored.

wctomb with non-null string distinguishes three possibilities for wchar: a valid wide character code (one that can be translated to a multibyte character), an invalid code, and 0.

Given a valid code, wctomb converts it to a multibyte character, storing the bytes starting at string.  Then it returns the number of bytes in that character (always at least 1, and never more than MB_CUR_MAX).

If wchar is an invalid wide character code, wctomb returns -1.  If wchar is 0, it returns 0, also storing 0 in *string.

If the multibyte character code uses shift characters, then wctomb maintains and updates a shift state as it scans.  If you call wctomb with a null pointer for string, that initializes the shift state to its standard initial value.  It also returns nonzero if the multibyte character code in use actually has a shift state.  See Shift State.

Calling this function with a wchar argument of zero when string is not null has the side-effect of reinitializing the stored shift state as well as storing the multibyte character 0 and returning 0.


Character-by-Character Conversion Example
Node: Example of Conversion, Next: Shift State, Prev: Converting One Char, Up: Extended Characters

Character-by-Character Conversion Example

Here is an example that reads multibyte character text from descriptor input and writes the corresponding wide characters to descriptor output.  We need to convert characters one by one for this example because mbstowcs is unable to continue past a null character, and cannot cope with an apparently invalid partial character by reading more input.

int
file_mbstowcs (int input, int output)
{
  char buffer[BUFSIZ + MB_LEN_MAX];
  int filled = 0;
  int eof = 0;

  while (!eof)
    {
      int nread;
      int nwrite;
      char *inp = buffer;
      wchar_t outbuf[BUFSIZ];
      wchar_t *outp = outbuf;

      /* Fill up the buffer from the input file.  */
      nread = read (input, buffer + filled, BUFSIZ);
      if (nread < 0)
        {
          perror ("read");
          return 0;
        }
      /* If we reach end of file, make a note to read no more. */
      if (nread == 0)
        eof = 1;

      /* filled is now the number of bytes in buffer. */
      filled += nread;

      /* Convert those bytes to wide characters--as many as we can. */
      while (1)
        {
          int thislen = mbtowc (outp, inp, filled);
          /* Stop converting at invalid character;
             this can mean we have read just the first part
             of a valid character.  */
          if (thislen == -1)
            break;
          /* Treat null character like any other,
             but also reset shift state. */
          if (thislen == 0) {
            thislen = 1;
            mbtowc (NULL, NULL, 0);
          }
          /* Advance past this character. */
          inp += thislen;
          filled -= thislen;
          outp++;
        }

      /* Write the wide characters we just made.  */
      nwrite = write (output, outbuf,
                      (outp - outbuf) * sizeof (wchar_t));
      if (nwrite < 0)
        {
          perror ("write");
          return 0;
        }

      /* See if we have a real invalid character. */
      if ((eof && filled > 0) || filled >= MB_CUR_MAX)
        {
          error ("invalid multibyte character");
          return 0;
        }

      /* If any characters must be carried forward,
         put them at the beginning of buffer. */
      if (filled > 0)
        memcpy (inp, buffer, filled);
      }
    }

  return 1;
}


Multibyte Codes Using Shift Sequences
Node: Shift State, Next: , Prev: Example of Conversion, Up: Extended Characters

Multibyte Codes Using Shift Sequences

In some multibyte character codes, the meaning of any particular byte sequence is not fixed; it depends on what other sequences have come earlier in the same string.  Typically there are just a few sequences that can change the meaning of other sequences; these few are called "shift sequences" and we say that they set the "shift state" for other sequences that follow.

To illustrate shift state and shift sequences, suppose we decide that the sequence 0200 (just one byte) enters Japanese mode, in which pairs of bytes in the range from 0240 to 0377 are single characters, while 0201 enters Latin-1 mode, in which single bytes in the range from 0240 to 0377 are characters, and interpreted according to the ISO Latin-1 character set.  This is a multibyte code which has two alternative shift states ("Japanese mode" and "Latin-1 mode"), and two shift sequences that specify particular shift states.

When the multibyte character code in use has shift states, then mblen, mbtowc and wctomb must maintain and update the current shift state as they scan the string.  To make this work properly, you must follow these rules:

•	Before starting to scan a string, call the function with a null pointer for the multibyte character address--for example, mblen (NULL, 0).  This initializes the shift state to its standard initial value.

•	Scan the string one character at a time, in order.  Do not "back up" and rescan characters already scanned, and do not intersperse the processing of different strings.

Here is an example of using mblen following these rules:

void
scan_string (char *s)
{
  int length = strlen (s);

  /* Initialize shift state. */
  mblen (NULL, 0);

  while (1)
    {
      int thischar = mblen (s, length);
      /* Deal with end of string and invalid characters. */
      if (thischar == 0)
        break;
      if (thischar == -1)
        {
          error ("invalid multibyte character");
          break;
        }
      /* Advance past this character. */
      s += thischar;
      length -= thischar;
    }
}

The functions mblen, mbtowc and wctomb are not reentrant when using a multibyte code that uses a shift state.  However, no other library functions call these functions, so you don't have to worry that the shift state will be changed mysteriously.


Locales and Internationalization
Node: Locales, Next: Searching and Sorting, Prev: Extended Characters, Up: Top

Locales and Internationalization

Different countries and cultures have varying conventions for how to communicate.  These conventions range from very simple ones, such as the format for representing dates and times, to very complex ones, such as the language spoken.

"Internationalization" of software means programming it to be able to adapt to the user's favorite conventions.  In ANSI C, internationalization works by means of "locales".  Each locale specifies a collection of conventions, one convention for each purpose.  The user chooses a set of conventions by specifying a locale (via environment variables).

All programs inherit the chosen locale as part of their environment.  Provided the programs are written to obey the choice of locale, they will follow the conventions preferred by the user.

* Menu:

Effects of Locale	Actions affected by the choice of locale.

Choosing Locale	How the user specifies a locale.

Locale Categories	Different purposes for which you can select a locale.

Setting the Locale	How a program specifies the locale with library functions.

Standard Locales	Locale names available on all systems.

Numeric Formatting	How to format numbers according to the chosen locale.


What Effects a Locale Has
Node: Effects of Locale, Next: Choosing Locale, Prev: , Up: Locales

What Effects a Locale Has

Each locale specifies conventions for several purposes, including the following:

•	What multibyte character sequences are valid, and how they are interpreted (see Extended Characters).

•	Classification of which characters in the local character set are considered alphabetic, and upper- and lower-case conversion conventions (see Character Handling).

•	The collating sequence for the local language and character set (see Collation Functions).

•	Formatting of numbers and currency amounts (see Numeric Formatting).

•	Formatting of dates and times (see Formatting Date and Time).

•	What language to use for output, including error messages.  (The C library doesn't yet help you implement this.)

•	What language to use for user answers to yes-or-no questions.

•	What language to use for more complex user input.  (The C library doesn't yet help you implement this.)

Some aspects of adapting to the specified locale are handled automatically by the library subroutines.  For example, all your program needs to do in order to use the collating sequence of the chosen locale is to use strcoll or strxfrm to compare strings.

Other aspects of locales are beyond the comprehension of the library.  For example, the library can't automatically translate your program's output messages into other languages.  The only way you can support output in the user's favorite language is to program this more or less by hand.  (Eventually, we hope to provide facilities to make this easier.)

This chapter discusses the mechanism by which you can modify the current locale.  The effects of the current locale on specific library functions are discussed in more detail in the descriptions of those functions.


Choosing a Locale
Node: Choosing Locale, Next: Locale Categories, Prev: Effects of Locale, Up: Locales

Choosing a Locale

The simplest way for the user to choose a locale is to set the environment variable LANG.  This specifies a single locale to use for all purposes.  For example, a user could specify a hypothetical locale named 'espana-castellano' to use the standard conventions of most of Spain.

The set of locales supported depends on the operating system you are using, and so do their names.  We can't make any promises about what locales will exist, except for one standard locale called 'C' or 'POSIX'.

A user also has the option of specifying different locales for different purposes--in effect, choosing a mixture of multiple locales.

For example, the user might specify the locale 'espana-castellano' for most purposes, but specify the locale 'usa-english' for currency formatting.  This might make sense if the user is a Spanish-speaking American, working in Spanish, but representing monetary amounts in US dollars.

Note that both locales 'espana-castellano' and 'usa-english', like all locales, would include conventions for all of the purposes to which locales apply.  However, the user can choose to use each locale for a particular subset of those purposes.


Categories of Activities that Locales Affect
Node: Locale Categories, Next: Setting the Locale, Prev: Choosing Locale, Up: Locales

Categories of Activities that Locales Affect

The purposes that locales serve are grouped into "categories", so that a user or a program can choose the locale for each category independently.  Here is a table of categories; each name is both an environment variable that a user can set, and a macro name that you can use as an argument to setlocale.

LC_COLLATE

This category applies to collation of strings (functions strcoll and strxfrm); see See Collation Functions.

LC_CTYPE

This category applies to classification and conversion of characters, and to multibyte and wide characters; see See Character Handling and See Extended Characters.

LC_MONETARY

This category applies to formatting monetary values; see See Numeric Formatting.

LC_NUMERIC

This category applies to formatting numeric values that are not monetary; see See Numeric Formatting.

LC_TIME

This category applies to formatting date and time values; see See Formatting Date and Time.

LC_ALL

This is not an environment variable; it is only a macro that you can use with setlocale to set a single locale for all purposes.

LANG

If this environment variable is defined, its value specifies the locale to use for all purposes except as overridden by the variables above.


How Programs Set the Locale
Node: Setting the Locale, Next: Standard Locales, Prev: Locale Categories, Up: Locales

How Programs Set the Locale

A C program inherits its locale environment variables when it starts up.  This happens automatically.  However, these variables do not automatically control the locale used by the library functions, because ANSI C says that all programs start by default in the standard 'C' locale.  To use the locales specified by the environment, you must call setlocale.  Call it as follows:

setlocale (LC_ALL, "");

to select a locale based on the appropriate environment variables.

You can also use setlocale to specify a particular locale, for general use or for a specific category.

The symbols in this section are defined in the header file 'locale.h'.

 - Function: char * setlocale (int category, const char *locale)

The function setlocale sets the current locale for category category to locale.

If category is LC_ALL, this specifies the locale for all purposes.  The other possible values of category specify an individual purpose (see Locale Categories).

You can also use this function to find out the current locale by passing a null pointer as the locale argument.  In this case, setlocale returns a string that is the name of the locale currently selected for category category.

The string returned by setlocale can be overwritten by subsequent calls, so you should make a copy of the string (see Copying and Concatenation) if you want to save it past any further calls to setlocale.  (The standard library is guaranteed never to call setlocale itself.)

You should not modify the string returned by setlocale.  It might be the same string that was passed as an argument in a previous call to setlocale.

When you read the current locale for category LC_ALL, the value encodes the entire combination of selected locales for all categories.  In this case, the value is not just a single locale name.  In fact, we don't make any promises about what it looks like.  But if you specify the same "locale name" with LC_ALL in a subsequent call to setlocale, it restores the same combination of locale selections.

When the locale argument is not a null pointer, the string returned by setlocale reflects the newly modified locale.

If you specify an empty string for locale, this means to read the appropriate environment variable and use its value to select the locale for category.

If you specify an invalid locale name, setlocale returns a null pointer and leaves the current locale unchanged.

Here is an example showing how you might use setlocale to temporarily switch to a new locale.

#include <stddef.h>
#include <locale.h>
#include <stdlib.h>
#include <string.h>

void
with_other_locale (char *new_locale,
                   void (*subroutine) (int),
                   int argument)
{
  char *old_locale, *saved_locale;

  /* Get the name of the current locale.  */
  old_locale = setlocale (LC_ALL, NULL);

  /* Copy the name so it won't be clobbered by setlocale. */
  saved_locale = strdup (old_locale);
  if (old_locale == NULL)
    fatal ("Out of memory");

  /* Now change the locale and do some stuff with it. */
  setlocale (LC_ALL, new_locale);
  (*subroutine) (argument);

  /* Restore the original locale. */
  setlocale (LC_ALL, saved_locale);
  free (saved_locale);
}

Portability Note: Some ANSI C systems may define additional locale categories.  For portability, assume that any symbol beginning with 'LC_' might be defined in 'locale.h'.


Standard Locales
Node: Standard Locales, Next: Numeric Formatting, Prev: Setting the Locale, Up: Locales

Standard Locales

The only locale names you can count on finding on all operating systems are these three standard ones:

"C"

This is the standard C locale.  The attributes and behavior it provides are specified in the ANSI C standard.  When your program starts up, it initially uses this locale by default.

"POSIX"

This is the standard POSIX locale.  Currently, it is an alias for the standard C locale.

""

The empty name says to select a locale based on environment variables.  See Locale Categories.

Defining and installing named locales is normally a responsibility of the system administrator at your site (or the person who installed the GNU C library).  Some systems may allow users to create locales, but we don't discuss that here.

If your program needs to use something other than the 'C' locale, it will be more portable if you use whatever locale the user specifies with the environment, rather than trying to specify some non-standard locale explicitly by name.  Remember, different machines might have different sets of locales installed.


Numeric Formatting
Node: Numeric Formatting, Next: , Prev: Standard Locales, Up: Locales

Numeric Formatting

When you want to format a number or a currency amount using the conventions of the current locale, you can use the function localeconv to get the data on how to do it.  The function localeconv is declared in the header file 'locale.h'.  K

 - Function: struct lconv * localeconv (void)

The localeconv function returns a pointer to a structure whose components contain information about how numeric and monetary values should be formatted in the current locale.

You shouldn't modify the structure or its contents.  The structure might be overwritten by subsequent calls to localeconv, or by calls to setlocale, but no other function in the library overwrites this value.

 - Data Type: struct lconv

This is the data type of the value returned by localeconv.

If a member of the structure struct lconv has type char, and the value is CHAR_MAX, it means that the current locale has no value for that parameter.

* Menu:

General Numeric	Parameters for formatting numbers and currency amounts.

Currency Symbol	How to print the symbol that identifies an amount of money (e.g. '$').

Sign of Money Amount	How to print the (positive or negative) sign for a monetary amount, if one exists.


Generic Numeric Formatting Parameters
Node: General Numeric, Next: Currency Symbol, Prev: , Up: Numeric Formatting

Generic Numeric Formatting Parameters

These are the standard members of struct lconv; there may be others.

char *decimal_point

char *mon_decimal_point

These are the decimal-point separators used in formatting non-monetary and monetary quantities, respectively.  In the 'C' locale, the value of decimal_point is ".", and the value of mon_decimal_point is "".  K

char *thousands_sep

char *mon_thousands_sep

These are the separators used to delimit groups of digits to the left of the decimal point in formatting non-monetary and monetary quantities, respectively.  In the 'C' locale, both members have a value of "" (the empty string).

char *grouping

char *mon_grouping

These are strings that specify how to group the digits to the left of the decimal point.  grouping applies to non-monetary quantities and mon_grouping applies to monetary quantities.  Use either thousands_sep or mon_thousands_sep to separate the digit groups.  KEach string is made up of decimal numbers separated by semicolons.  Successive numbers (from left to right) give the sizes of successive groups (from right to left, starting at the decimal point).  The last number in the string is used over and over for all the remaining groups.

If the last integer is -1, it means that there is no more grouping--or, put another way, any remaining digits form one large group without separators.

For example, if grouping is "4;3;2", the correct grouping for the number 123456787654321 is '12', '34', '56', '78', '765', '4321'.  This uses a group of 4 digits at the end, preceded by a group of 3 digits, preceded by groups of 2 digits (as many as needed).  With a separator of ',', the number would be printed as '12,34,56,78,765,4321'.

A value of "3" indicates repeated groups of three digits, as normally used in the U.S.

In the standard 'C' locale, both grouping and mon_grouping have a value of "".  This value specifies no grouping at all.

char int_frac_digits

char frac_digits

These are small integers indicating how many fractional digits (to the right of the decimal point) should be displayed in a monetary value in international and local formats, respectively.  (Most often, both members have the same value.)

In the standard 'C' locale, both of these members have the value CHAR_MAX, meaning "unspecified".  The ANSI standard doesn't say what to do when you find this the value; we recommend printing no fractional digits.  (This locale also specifies the empty string for mon_decimal_point, so printing any fractional digits would be confusing!)


Printing the Currency Symbol
Node: Currency Symbol, Next: Sign of Money Amount, Prev: General Numeric, Up: Numeric Formatting

Printing the Currency Symbol

These members of the struct lconv structure specify how to print the symbol to identify a monetary value--the international analog of '$' for US dollars.

Each country has two standard currency symbols.  The "local currency symbol" is used commonly within the country, while the "international currency symbol" is used internationally to refer to that country's currency when it is necessary to indicate the country unambiguously.

For example, many countries use the dollar as their monetary unit, and when dealing with international currencies it's important to specify that one is dealing with (say) Canadian dollars instead of U.S. dollars or Australian dollars.  But when the context is known to be Canada, there is no need to make this explicit--dollar amounts are implicitly assumed to be in Canadian dollars.

char *currency_symbol

The local currency symbol for the selected locale.

In the standard 'C' locale, this member has a value of "" (the empty string), meaning "unspecified".  The ANSI standard doesn't say what to do when you find this value; we recommend you simply print the empty string as you would print any other string found in the appropriate member.

char *int_curr_symbol

The international currency symbol for the selected locale.

The value of int_curr_symbol should normally consist of a three-letter abbreviation determined by the international standard `ISO 4217 Codes for the Representation of Currency and Funds', followed by a one-character separator (often a space).

In the standard 'C' locale, this member has a value of "" (the empty string), meaning "unspecified".  We recommend you simply print the empty string as you would print any other string found in the appropriate member.

char p_cs_precedes

char n_cs_precedes

These members are 1 if the currency_symbol string should precede the value of a monetary amount, or 0 if the string should follow the value.  The p_cs_precedes member applies to positive amounts (or zero), and the n_cs_precedes member applies to negative amounts.

In the standard 'C' locale, both of these members have a value of CHAR_MAX, meaning "unspecified".  The ANSI standard doesn't say what to do when you find this value, but we recommend printing the currency symbol before the amount.  That's right for most countries.  In other words, treat all nonzero values alike in these members.

The POSIX standard says that these two members apply to the int_curr_symbol as well as the currency_symbol.  The ANSI C standard seems to imply that they should apply only to the currency_symbol--so the int_curr_symbol should always precede the amount.

We can only guess which of these (if either) matches the usual conventions for printing international currency symbols.  Our guess is that they should always preceed the amount.  If we find out a reliable answer, we will put it here.

char p_sep_by_space

char n_sep_by_space

These members are 1 if a space should appear between the currency_symbol string and the amount, or 0 if no space should appear.  The p_sep_by_space member applies to positive amounts (or zero), and the n_sep_by_space member applies to negative amounts.

In the standard 'C' locale, both of these members have a value of CHAR_MAX, meaning "unspecified".  The ANSI standard doesn't say what you should do when you find this value; we suggest you treat it as one (print a space).  In other words, treat all nonzero values alike in these members.

These members apply only to currency_symbol.  When you use int_curr_symbol, you never print an additional space, because int_curr_symbol itself contains the appropriate separator.

The POSIX standard says that these two members apply to the int_curr_symbol as well as the currency_symbol.  But an example in the ANSI C standard clearly implies that they should apply only to the currency_symbol--that the int_curr_symbol contains any appropriate separator, so you should never print an additional space.

Based on what we know now, we recommend you ignore these members when printing international currency symbols, and print no extra space.


Printing the Sign of an Amount of Money
Node: Sign of Money Amount, Next: , Prev: Currency Symbol, Up: Numeric Formatting

Printing the Sign of an Amount of Money

These members of the struct lconv structure specify how to print the sign (if any) in a monetary value.

char *positive_sign

char *negative_sign

These are strings used to indicate positive (or zero) and negative (respectively) monetary quantities.

In the standard 'C' locale, both of these members have a value of "" (the empty string), meaning "unspecified".

The ANSI standard doesn't say what to do when you find this value; we recommend printing positive_sign as you find it, even if it is empty.  For a negative value, print negative_sign as you find it unless both it and positive_sign are empty, in which case print '-' instead.  (Failing to indicate the sign at all seems rather unreasonable.)

char p_sign_posn

char n_sign_posn

These members have values that are small integers indicating how to position the sign for nonnegative and negative monetary quantities, respectively.  (The string used by the sign is what was specified with positive_sign or negative_sign.)  The possible values are as follows:

0

The currency symbol and quantity should be surrounded by parentheses.

1

Print the sign string before the quantity and currency symbol.

2

Print the sign string after the quantity and currency symbol.

3

Print the sign string right before the currency symbol.

4

Print the sign string right after the currency symbol.

CHAR_MAX

"Unspecified".  Both members have this value in the standard 'C' locale.

The ANSI standard doesn't say what you should do when the value is CHAR_MAX.  We recommend you print the sign after the currency symbol.

It is not clear whether you should let these members apply to the international currency format or not.  POSIX says you should, but intuition plus the examples in the ANSI C standard suggest you should not.  We hope that someone who knows well the conventions for formatting monetary quantities will tell us what we should recommend.


Non-Local Exits
Node: Non-Local Exits, Next: Signal Handling, Prev: Date and Time, Up: Top

Non-Local Exits

Sometimes when your program detects an unusual situation inside a deeply nested set of function calls, you would like to be able to immediately return to an outer level of control.  This section describes how you can do such "non-local exits" using the setjmp and longjmp functions.

* Menu:

Intro	When and how to use these facilities.

Details	Functions for nonlocal exits.

Non-Local Exits and Signals	Portability issues.


Introduction to Non-Local Exits
Node: Non-Local Intro, Next: Non-Local Details, Prev: , Up: Non-Local Exits

Introduction to Non-Local Exits

As an example of a situation where a non-local exit can be useful, suppose you have an interactive program that has a "main loop" that prompts for and executes commands.  Suppose the "read" command reads input from a file, doing some lexical analysis and parsing of the input while processing it.  If a low-level input error is detected, it would be useful to be able to return immediately to the "main loop" instead of having to make each of the lexical analysis, parsing, and processing phases all have to explicitly deal with error situations initially detected by nested calls.

(On the other hand, if each of these phases has to do a substantial amount of cleanup when it exits--such as closing files, deallocating buffers or other data structures, and the like--then it can be more appropriate to do a normal return and have each phase do its own cleanup, because a non-local exit would bypass the intervening phases and their associated cleanup code entirely.  Alternatively, you could use a non-local exit but do the cleanup explicitly either before or after returning to the "main loop".)

In some ways, a non-local exit is similar to using the 'return' statement to return from a function.  But while 'return' abandons only a single function call, transferring control back to the point at which it was called, a non-local exit can potentially abandon many levels of nested function calls.

You identify return points for non-local exits calling the function setjmp.  This function saves information about the execution environment in which the call to setjmp appears in an object of type jmp_buf.  Execution of the program continues normally after the call to setjmp, but if a exit is later made to this return point by calling longjmp with the corresponding jmp_buf object, control is transferred back to the point where setjmp was called.  The return value from setjmp is used to distinguish between an ordinary return and a return made by a call to longjmp, so calls to setjmp usually appear in an 'if' statement.

Here is how the example program described above might be set up:

#include <setjmp.h>
#include <stdlib.h>
#include <stdio.h>

jmp_buf main_loop;

void
abort_to_main_loop (int status)
{
  longjmp (main_loop, status);
}

int
main (void)
{
  while (1)
    if (setjmp (main_loop))
      puts ("Back at main loop....");
    else
      do_command ();
}

void
do_command (void)
{
  char buffer[128];
  if (fgets (buffer, 128, stdin) == NULL)
    abort_to_main_loop (-1);
  else
    exit (EXIT_SUCCESS);
}

The function abort_to_main_loop causes an immediate transfer of control back to the main loop of the program, no matter where it is called from.

The flow of control inside the main function may appear a little mysterious at first, but it is actually a common idiom with setjmp.  A normal call to setjmp returns zero, so the "else" clause of the conditional is executed.  If abort_to_main_loop is called somewhere within the execution of do_command, then it actually appears as if the same call to setjmp in main were returning a second time with a value of -1.

So, the general pattern for using setjmp looks something like:

if (setjmp (buffer))
  /* Code to clean up after premature return. */
  ...
else
  /* Code to be executed normally after setting up the return point. */
  ...


Details of Non-Local Exits
Node: Non-Local Details, Next: Non-Local Exits and Signals, Prev: Non-Local Intro, Up: Non-Local Exits

Details of Non-Local Exits

Here are the details on the functions and data structures used for performing non-local exits.  These facilities are declared in 'setjmp.h'.  K

 - Data Type: jmp_buf

Objects of type jmp_buf hold the state information to be restored by a non-local exit.  The contents of a jmp_buf identify a specific place to return to.

 - Macro: int setjmp (jmp_buf state)

When called normally, setjmp stores information about the execution state of the program in state and returns zero.  If longjmp is later used to perform a non-local exit to this state, setjmp returns a nonzero value.

 - Function: void longjmp (jmp_buf state, int value)

This function restores current execution to the state saved in state, and continues execution from the call to setjmp that established that return point.  Returning from setjmp by means of longjmp returns the value argument that was passed to longjmp, rather than 0.  (But if value is given as 0, setjmp returns 1).

There are a lot of obscure but important restrictions on the use of setjmp and longjmp.  Most of these restrictions are present because non-local exits require a fair amount of magic on the part of the C compiler and can interact with other parts of the language in strange ways.

The setjmp function is actually a macro without an actual function definition, so you shouldn't try to '#undef' it or take its address.  In addition, calls to setjmp are safe in only the following contexts:

•	As the test expression of a selection or iteration statement (such as 'if' or 'while').

•	As one operand of a equality or comparison operator that appears as the test expression of a selection or iteration statement.  The other operand must be an integer constant expression.

•	As the operand of a unary '!' operator, that appears as the test expression of a selection or iteration statement.

•	By itself as an expression statement.

Return points are valid only during the dynamic extent of the function that called setjmp to establish them.  If you longjmp to a return point that was established in a function that has already returned, unpredictable and disastrous things are likely to happen.

You should use a nonzero value argument to longjmp.  While longjmp refuses to pass back a zero argument as the return value from setjmp, this is intended as a safety net against accidental misuse and is not really good programming style.

When you perform a non-local exit, accessible objects generally retain whatever values they had at the time longjmp was called.  The exception is that the values of automatic variables local to the function containing the setjmp call that have been changed since the call to setjmp are indeterminate, unless you have declared them volatile.


Non-Local Exits and Signals
Node: Non-Local Exits and Signals, Next: , Prev: Non-Local Details, Up: Non-Local Exits

Non-Local Exits and Signals

In BSD Unix systems, setjmp and longjmp also save and restore the set of blocked signals; see See Blocking Signals.  However, the POSIX.1 standard requires setjmp and longjmp not to change the set of blocked signals, and provides an additional pair of functions (sigsetjmp and sigsetjmp) to get the BSD behavior.

The behavior of setjmp and longjmp in the GNU library is controlled by feature test macros; see See Feature Test Macros.  The default in the GNU system is the POSIX.1 behavior rather than the BSD behavior.

The facilities in this section are declared in the header file 'setjmp.h'.  K

 - Data Type: sigjmp_buf

This is similar to jmp_buf, except that it can also store state information about the set of blocked signals.

 - Function: int sigsetjmp (sigjmp_buf state, int savesigs)

This is similar to setjmp.  If savesigs is nonzero, the set of blocked signals is saved in state and will be restored if a siglongjmp is later performed with this state.

 - Function: void siglongjmp (sigjmp_buf state, int value)

This is similar to longjmp except for the type of its state argument.  If the sigsetjmp call that set this state used a nonzero savesigs flag, siglongjmp also restores the set of blocked signals.


Signal Handling
Node: Signal Handling, Next: Process Startup, Prev: Non-Local Exits, Up: Top

Signal Handling

A "signal" is a software interrupt delivered to a process.  The operating system uses signals to report exceptional situations to an executing program.  Some signals report errors such as references to invalid memory addresses; others report asynchronous events, such as disconnection of a phone line.

The GNU C library defines a variety of signal types, each for a particular kind of event.  Some kinds of events make it inadvisable or impossible for the program to proceed as usual, and the corresponding signals normally abort the program.  Other kinds of signals that report harmless events are ignored by default.

If you anticipate an event that causes signals, you can define a handler function and tell the operating system to run it when that particular type of signal arrives.

Finally, one process can send a signal to another process; this allows a parent process to abort a child, or two related processes to communicate and synchronize.

* Menu:

Concepts of Signals	Introduction to the signal facilities.

Standard Signals	Particular kinds of signals with standard names and meanings.

Signal Actions	Specifying what happens when a particular signal is delivered.

Defining Handlers	How to write a signal handler function.

Interrupted Primitives	Signal handlers affect use of open, read, write and other functions.

Generating Signals	How to send a signal to a process.

Blocking Signals	Making the system hold signals temporarily.

Waiting for a Signal	Suspending your program until a signal arrives.

BSD Signal Handling	Additional functions for backward compatibility with BSD.

BSD Handler	BSD Function to Establish a Handler.


Basic Concepts of Signals
Node: Concepts of Signals, Next: Standard Signals, Prev: , Up: Signal Handling

Basic Concepts of Signals

This section explains basic concepts of how signals are generated, what happens after a signal is delivered, and how programs can handle signals.

* Menu:

Kinds of Signals	Some examples of what can cause a signal.

Signal Generation	Concepts of why and how signals occur.

Delivery of Signal	Concepts of what a signal does to the process.


Some Kinds of Signals
Node: Kinds of Signals, Next: Signal Generation, Prev: , Up: Concepts of Signals

Some Kinds of Signals

A signal reports the occurrence of an exceptional event.  These are some of the events that can cause (or "generate", or "raise") a signal:

•	A program error such as dividing by zero or issuing an address outside the valid range.

•	A user request to interrupt or terminate the program.  Most environments are set up to let a user suspend the program by typing ^C-z, or terminate it with ^C-c.  Whatever key sequence is used, the operating system sends the proper signal to interrupt the process.

•	The termination of a child process.

•	Expiration of a timer or alarm.

•	A call to kill or raise by the same process.

•	A call to kill from another process.  Signals are a limited but useful form of interprocess communication.

Each of these kinds of events (excepting explicit calls to kill and raise) generates its own particular kind of signal.  The various kinds of signals are listed and described in detail in See Standard Signals.


Concepts of Signal Generation
Node: Signal Generation, Next: Delivery of Signal, Prev: Kinds of Signals, Up: Concepts of Signals

Concepts of Signal Generation

In general, the events that generate signals fall into three major categories: errors, external events, and explicit requests.

An error means that a program has done something invalid and cannot continue execution.  But not all kinds of errors generate signals--in fact, most do not.  For example, opening a nonexistent file is an error, but it does not raise a signal; instead, open returns -1.  In general, errors that are necessarily associated with certain library functions are reported by returning a value that indicates an error.  The errors which raise signals are those which can happen anywhere in the program, not just in library calls.  These include division by zero and invalid memory addresses.

An external event generally has to do with I/O or other processes.  These include the arrival of input, the expiration of a timer, and the termination of a child process.

An explicit request means the use of a library function such as kill whose purpose is specifically to generate a signal.

Signals may be generated "synchronously" or "asynchronously".  A synchronous signal pertains to a specific action in the program, and is delivered (unless blocked) during that action.  Errors generate signals synchronously, and so do explicit requests by a process to generate a signal for that same process.

Asynchronous signals are generated by events outside the control of the process that receives them.  These signals arrive at unpredictable times during execution.  External events generate signals asynchronously, and so do explicit requests that apply to some other process.

A given type of signal is either typically synchrous or typically asynchronous.  For example, signals for errors are typically synchronous because errors generate signals synchronously.  But any type of signal can be generated synchronously or asynchronously with an explicit request.


How Signals Are Delivered
Node: Delivery of Signal, Next: , Prev: Signal Generation, Up: Concepts of Signals

How Signals Are Delivered

When a signal is generated, it becomes "pending".  Normally it remains pending for just a short period of time and then is "delivered" to the process that was signaled.  However, if that kind of signal is currently "blocked", it may remain pending indefinitely--until signals of that kind are "unblocked".  Once unblocked, it will be delivered immediately.  See Blocking Signals.

When the signal is delivered, whether right away or after a long delay, the "specified action" for that signal is taken.  For certain signals, such as SIGKILL and SIGSTOP, the action is fixed, but for most signals, the program has a choice: ignore the signal, specify a "handler function", or accept the "default action" for that kind of signal.  The program specifies its choice using functions such as signal or sigaction (see Signal Actions).  We sometimes say that a handler "catches" the signal.  While the handler is running, that particular signal is normally blocked.

If the specified action for a kind of signal is to ignore it, then any such signal which is generated is discarded immediately.  This happens even if the signal is also blocked at the time.  A signal discarded in this way will never be delivered, not even if the program subsequently specifies a different action for that kind of signal and then unblocks it.

If a signal arrives which the program has neither handled nor ignored, its "default action" takes place.  Each kind of signal has its own default action, documented below (see Standard Signals).  For most kinds of signals, the default action is to terminate the process.  For certain kinds of signals that represent "harmless" events, the default action is to do nothing.

When a signal terminates a process, its parent process can determine the cause of termination by examining the termination status code reported by the wait or waitpid functions.  (This is discussed in more detail in See Process Completion.)  The information it can get includes the fact that termination was due to a signal, and the kind of signal involved.  If a program you run from a shell is terminated by a signal, the shell typically prints some kind of error message.

The signals that normally represent program errors have a special property: when one of these signals terminates the process, it also writes a "core dump file" which records the state of the process at the time of termination.  You can examine the core dump with a debugger to investigate what caused the error.

If you raise a "program error" signal by explicit request, and this terminates the process, it makes a core dump file just as if the signal had been due directly to an error.


Standard Signals
Node: Standard Signals, Next: Signal Actions, Prev: Concepts of Signals, Up: Signal Handling

Standard Signals

This section lists the names for various standard kinds of signals and describes what kind of event they mean.  Each signal name is a macro which stands for a positive integer--the "signal number" for that kind of signal.  Your programs should never make assumptions about the numeric code for a particular kind of signal, but rather refer to them always by the names defined here.  This is because the number for a given kind of signal can vary from system to system, but the meanings of the names are standardized and fairly uniform.

The signal names are defined in the header file 'signal.h'.

 - Macro: int NSIG

The value of this symbolic constant is the total number of signals defined.  Since the signal numbers are allocated consecutively, NSIG is also one greater than the largest defined signal number.

* Menu:

Program Error Signals	Used to report serious program errors.

Termination Signals	Used to interrupt and/or terminate the program.

Alarm Signals	Used to indicate expiration of timers.

Asynchronous I/O Signals	Used to indicate input is available.

Job Control Signals	Signals used to support job control.

Miscellaneous Signals	Miscellaneous Signals.

Nonstandard Signals	Implementations can support other signals.

Signal Messages	Printing a message describing a signal.


Program Error Signals
Node: Program Error Signals, Next: Termination Signals, Prev: , Up: Standard Signals

Program Error Signals

The following signals are generated when a serious program error is detected by the operating system or the computer itself.  In general, all of these signals are indications that your program is seriously broken in some way, and there's usually no way to continue the computation which encountered the error.

Some programs handle program error signals in order to tidy up before terminating; for example, programs that turn off echoing of terminal input should handle program error signals in order to turn echoing back on.  The handler should end by specifying the default action for the signal that happened and then reraising it; this will cause the program to terminate with that signal, as if it had not had a handler.  (See Termination in Handler.)

Termination is the sensible ultimate outcome from a program error in most programs.  However, programming systems such as Lisp that can load compiled user programs might need to keep executing even if a user program incurs an error.  These programs have handlers which use longjmp to return control to the command level.

The default action for all of these signals is to cause the process to terminate.  If you block or ignore these signals or establish handlers for them that return normally, your program will probably break horribly when such signals happen, unless they are generated by raise or kill instead of a real error.

When one of these program error signals terminates a process, it also writes a "core dump file" which records the state of the process at the time of termination.  The core dump file is named 'core' and is written in whichever directory is current in the process at the time.  (On the GNU system, you can specify the file name for core dumps with the environment variable COREFILE.)  The purpose of core dump files is so that you can examine them with a debugger to investigate what caused the error.

 - Macro: int SIGFPE

The SIGFPE signal reports a fatal arithmetic error.  Although the name is derived from "floating-point exception", this signal actually covers all arithmetic errors, including division by zero and overflow.  If a program stores integer data in a location which is then used in a floating-point operation, this often causes an "invalid operation" exception, because the processor cannot recognize the data as a floating-point number.  KActual floating-point exceptions are a complicated subject because there are many types of exceptions with subtly different meanings, and the SIGFPE signal doesn't distinguish between them.  The `IEEE Standard for Binary Floating-Point Arithmetic (ANSI/IEEE Std 754-1985)' defines various floating-point exceptions and requires conforming computer systems to report their occurrences.  However, this standard does not specify how the exceptions are reported, or what kinds of handling and control the operating system can offer to the programmer.

BSD systems provide the SIGFPE handler with an extra argument that distinguishes various causes of the exception.  In order to access this argument, you must define the handler to accept two arguments, which means you must cast it to a one-argument function type in order to establish the handler.  The GNU library does provide this extra argument, but the value is meaningful only on operating systems that provide the information (BSD systems and GNU systems).

FPE_INTOVF_TRAP

Integer overflow (impossible in a C program unless you enable overflow trapping in a hardware-specific fashion).

FPE_INTDIV_TRAP

Integer division by zero.

FPE_SUBRNG_TRAP

Subscript-range (something that C programs never check for).

FPE_FLTOVF_TRAP

Floating overflow trap.

FPE_FLTDIV_TRAP

Floating/decimal division by zero.

FPE_FLTUND_TRAP

Floating underflow trap.  (Trapping on floating underflow is not normally enabled.)

FPE_DECOVF_TRAP

Decimal overflow trap.  (Only a few machines have decimal arithmetic and C never uses it.)

 - Macro: int SIGILL

The name of this signal is derived from "illegal instruction"; it means your program is trying to execute garbage or a privileged instruction.  Since the C compiler generates only valid instructions, SIGILL typically indicates that the executable file is corrupted, or that you are trying to execute data.  Some common ways of getting into the latter situation are by passing an invalid object where a pointer to a function was expected, or by writing past the end of an automatic array (or similar problems with pointers to automatic variables) and corrupting other data on the stack such as the return address of a stack frame.

 - Macro: int SIGSEGV

This signal is generated when a program tries to read or write outside the memory that is allocated for it.  (Actually, the signals only occur when the program goes far enough outside to be detected by the system's memory protection mechanism.)  The name is an abbreviation for "segmentation violation".

The most common way of getting a SIGSEGV condition is by dereferencing a null or uninitialized pointer.  A null pointer refers to the address 0, and most operating systems make sure this address is always invalid precisely so that dereferencing a null pointer will cause SIGSEGV.  (Some operating systems place valid memory at address 0, and dereferencing a null pointer does not cause a signal on these systems.)  As for uninitialized pointer variables, they contain random addresses which may or may not be valid.

Another common way of getting into a SIGSEGV situation is when you use a pointer to step through an array, but fail to check for the end of the array.

 - Macro: int SIGBUS

This signal is generated when an invalid pointer is dereferenced.  Like SIGSEGV, this signal is typically the result of dereferencing an uninitialized pointer.  The difference between the two is that SIGSEGV indicates an invalid access to valid memory, while SIGBUS indicates an access to an invalid address.  In particular, SIGBUS signals often result from dereferencing a misaligned pointer, such as referring to a four-word integer at an address not divisible by four.  (Each kind of computer has its own requirements for address alignment.)

The name of this signal is an abbreviation for "bus error".

 - Macro: int SIGABRT

This signal indicates an error detected by the program itself and reported by calling abort.  See Aborting a Program.


Termination Signals
Node: Termination Signals, Next: Alarm Signals, Prev: Program Error Signals, Up: Standard Signals

Termination Signals

These signals are all used to tell a process to terminate, in one way or another.  They have different names because they're used for slightly different purposes, and programs might want to handle them differently.

The reason for handling these signals is usually so your program can tidy up as appropriate before actually terminating.  For example, you might want to save state information, delete temporary files, or restore the previous terminal modes.  Such a handler should end by specifying the default action for the signal that happened and then reraising it; this will cause the program to terminate with that signal, as if it had not had a handler.  (See Termination in Handler.)

The (obvious) default action for all of these signals is to cause the process to terminate.

 - Macro: int SIGHUP

The SIGHUP ("hang-up") signal is used to report that the user's terminal is disconnected, perhaps because a network or telephone connection was broken.  For more information about this, see See Control Modes.

This signal is also used to report the termination of the controlling process on a terminal to jobs associated with that session; this termination effectively disconnects all processes in the session from the controlling terminal.  For more information, see See Termination Internals.

 - Macro: int SIGINT

The SIGINT ("program interrupt") signal is sent when the user types the INTR character (normally ^C-c).  See Special Characters, for information about terminal driver support for ^C-c.

 - Macro: int SIGQUIT

The SIGQUIT signal is similar to SIGINT, except that it's controlled by a different key--the QUIT character, usually ^C-\--and produces a core dump when it terminates the process, just like a program error signal.  You can think of this as a program error condition "detected" by the user.

See Program Error Signals, for information about core dumps.  See Special Characters, for information about terminal driver support.

Certain kinds of cleanups are best omitted in handling SIGQUIT.  For example, if the program creates temporary files, it should handle the other termination requests by deleting the temporary files.  But it is better for SIGQUIT not to delete them, so that the user can examine them in conjunction with the core dump.

 - Macro: int SIGTERM

The SIGTERM signal is a generic signal used to cause program termination.  Unlike SIGKILL, this signal can be blocked, handled, and ignored.

The shell command kill generates SIGTERM by default.  K

- Macro: int SIGKILL

The SIGKILL signal is used to cause immediate program termination.  It cannot be handled or ignored, and is therefore always fatal.  It is also not possible to block this signal.

This signal is generated only by explicit request.  Since it cannot be handled, you should generate it only as a last resort, after first trying a less drastic method such as ^C-c or SIGTERM.  If a process does not respond to any other termination signals, sending it a SIGKILL signal will almost always cause it to go away.

In fact, if SIGKILL fails to terminate a process, that by itself constitutes an operating system bug which you should report.


Alarm Signals
Node: Alarm Signals, Next: Asynchronous I/O Signals, Prev: Termination Signals, Up: Standard Signals

Alarm Signals

These signals are used to indicate the expiration of timers.  See Setting an Alarm, for information about functions that cause these signals to be sent.

The default behavior for these signals is to cause program termination.  This default is rarely useful, but no other default would be useful; most of the ways of using these signals would require handler functions in any case.

 - Macro: int SIGALRM

This signal typically indicates expiration of a timer that measures real or clock time.  It is used by the alarm function, for example.

 - Macro: int SIGVTALRM

This signal typically indicates expiration of a timer that measures CPU time used by the current process.  The name is an abbreviation for "virtual time alarm".

 - Macro: int SIGPROF

This signal is typically indicates expiration of a timer that measures both CPU time used by the current process, and CPU time expended on behalf of the process by the system.  Such a timer is used to implement code profiling facilities, hence the name of this signal.


Asynchronous I/O Signals
Node: Asynchronous I/O Signals, Next: Job Control Signals, Prev: Alarm Signals, Up: Standard Signals

Asynchronous I/O Signals

The signals listed in this section are used in conjunction with asynchronous I/O facilities.  You have to take explicit action by calling fcntl to enable a particular file descriptior to generate these signals (see Interrupt Input).  The default action for these signals is to ignore them.

 - Macro: int SIGIO

This signal is sent when a file descriptor is ready to perform input or output.

On most operating systems, terminals and sockets are the only kinds of files that can generate SIGIO; other kinds, including ordinary files, never generate SIGIO even if you ask them to.

 - Macro: int SIGURG

This signal is sent when "urgent" or out-of-band data arrives on a socket.  See Out-of-Band Data.


Job Control Signals
Node: Job Control Signals, Next: Miscellaneous Signals, Prev: Asynchronous I/O Signals, Up: Standard Signals

Job Control Signals

These signals are used to support job control.  If your system doesn't support job control, then these macros are defined but the signals themselves can't be raised or handled.

You should generally leave these signals alone unless you really understand how job control works.  See Job Control.

 - Macro: int SIGCHLD

This signal is sent to a parent process whenever one of its child processes terminates or stops.

The default action for this signal is to ignore it.  If you establish a handler for this signal while there are child processes that have terminated but not reported their status via wait or waitpid (see Process Completion), whether your new handler applies to those processes or not depends on the particular operating system.

 - Macro: int SIGCONT

You can send a SIGCONT signal to a process to make it continue.  The default behavior for this signal is to make the process continue if it is stopped, and to ignore it otherwise.

Most programs have no reason to handle SIGCONT; they simply resume execution without realizing they were ever stopped.  You can use a handler for SIGCONT to make a program do something special when it is stopped and continued--for example, to reprint a prompt when it is suspended while waiting for input.

 - Macro: int SIGSTOP

The SIGSTOP signal stops the process.  It cannot be handled, ignored, or blocked.

 - Macro: int SIGTSTP

The SIGTSTP signal is an interactive stop signal.  Unlike SIGSTOP, this signal can be handled and ignored.

Your program should handle this signal if you have a special need to leave files or system tables in a secure state when a process is stopped.  For example, programs that turn off echoing should handle SIGTSTP so they can turn echoing back on before stopping.

This signal is generated when the user types the SUSP character (normally ^C-z).  For more information about terminal driver support, see See Special Characters.

 - Macro: int SIGTTIN

A process cannot read from the the user's terminal while it is running as a background job.  When any process in a background job tries to read from the terminal, all of the processes in the job are sent a SIGTTIN signal.  The default action for this signal is to stop the process.  For more information about how this interacts with the terminal driver, see See Access to the Terminal.

 - Macro: int SIGTTOU

This is similar to SIGTTIN, but is generated when a process in a background job attempts to write to the terminal or set its modes.  Again, the default action is to stop the process.

While a process is stopped, no more signals can be delivered to it until it is continued, except SIGKILL signals and (obviously) SIGCONT signals.  The SIGKILL signal always causes termination of the process and can't be blocked or ignored.  You can block or ignore SIGCONT, but it always causes the process to be continued anyway if it is stopped.  Sending a SIGCONT signal to a process causes any pending stop signals for that process to be discarded.  Likewise, any pending SIGCONT signals for a process are discarded when it receives a stop signal.

When a process in an orphaned process group (see Orphaned Process Groups) receives a SIGTSTP, SIGTTIN, or SIGTTOU signal and does not handle it, the process does not stop.  Stopping the process would be unreasonable since there would be no way to continue it.  What happens instead depends on the operating system you are using.  Some systems may do nothing; others may deliver another signal instead, such as SIGKILL or SIGHUP.


Miscellaneous Signals
Node: Miscellaneous Signals, Next: Nonstandard Signals, Prev: Job Control Signals, Up: Standard Signals

Miscellaneous Signals

These signals are used to report various other conditions.  The default action for all of them is to cause the process to terminate.

 - Macro: int SIGPIPE

If you use pipes or FIFOs, you have to design your application so that one process opens the pipe for reading before another starts writing.  If the reading process never starts, or terminates unexpectedly, writing to the pipe or FIFO raises a SIGPIPE signal.  If SIGPIPE is blocked, handled or ignored, the offending call fails with EPIPE instead.

Pipes and FIFO special files are discussed in more detail in See Pipes and FIFOs.

Another cause of SIGPIPE is when you try to output to a socket that isn't connected.  See Sending Data.

 - Macro: int SIGUSR1

- Macro: int SIGUSR2

The SIGUSR1 and SIGUSR2 signals are set aside for you to use any way you want.  They're useful for interprocess communication.  Since these signals are normally fatal, you should write a signal handler for them in the program that receives the signal.

There is an example showing the use of SIGUSR1 and SIGUSR2 in See Signaling Another Process.


Nonstandard Signals
Node: Nonstandard Signals, Next: Signal Messages, Prev: Miscellaneous Signals, Up: Standard Signals

Nonstandard Signals

Particular operating systems support additional signals not listed above.  The ANSI C standard reserves all identifiers beginning with 'SIG' followed by an uppercase letter for the names of signals.  You should consult the documentation or header files for your particular operating system and processor type to find out about the specific signals it supports.

For example, some systems support extra signals which correspond to hardware traps.  Some other kinds of signals commonly supported are used to implement limits on CPU time or file system usage, asynchronous changes to terminal configuration, and the like.  Systems may also define signal names that are aliases for standard signal names.

You can generally assume that the default action (or the action set up by the shell) for implementation-defined signals is reasonable, and not worry about them yourself.  In fact, it's usually a bad idea to ignore or block signals you don't know anything about, or try to establish a handler for signals whose meanings you don't know.

Here are some of the other signals found on commonly used operating systems:

SIGCLD

Obsolete name for SIGCHLD.

SIGTRAP

Generated by the machine's breakpoint instruction.  Used by debuggers.  Default action is to dump core.

SIGIOT

Generated by the PDP-11 "iot" instruction; equivalent to SIGABRT.  Default action is to dump core.

SIGEMT

Emulator trap; this results from certain unimplemented instructions.  It is a program error signal.

SIGSYS

Bad system call; that is to say, the instruction to trap to the operating system was executed, but the code number for the system call to perform was invalid.  This is a program error signal.

SIGPOLL

This is a System V signal name, more or less similar to SIGIO.

SIGXCPU

CPU time limit exceeded.  This is used for batch processing.  Default action is program termination.

SIGXFSZ

File size limit exceeded.  This is used for batch processing.  Default action is program termination.

SIGWINCH

Window size change.  This is generated on certain systems when the size of the current window on the screen is changed.  Default action is to ignore it.


Signal Messages
Node: Signal Messages, Next: , Prev: Nonstandard Signals, Up: Standard Signals

Signal Messages

We mentioned above that the shell prints a message describing the signal that terminated a child process.  The clean way to print a message describing a signal is to use the functions strsignal and psignal.  These functions use a signal number to specify which kind of signal to describe.  The signal number may come from the termination status of a child process (see Process Completion) or it may come from a signal handler in the same process.

 - Function: char * strsignal (int signum)

This function returns a pointer to a statically-allocated string containing a message describing the signal signum.  You should not modify the contents of this string; and, since it can be rewritten on subsequent calls, you should save a copy of it if you need to reference it later.

This function is a GNU extension, declared in the header file 'string.h'.

 - Function: void psignal (int signum, const char *message)

This function prints a message describing the signal signum to the standard error output stream stderr; see See Standard Streams.

If you call psignal with a message that is either a null pointer or an empty string, psignal just prints the message corresponding to signum, adding a trailing newline.

If you supply a non-null message argument, then psignal prefixes its output with this string.  It adds a colon and a space character to separate the message from the string corresponding to signum.

This function is a BSD feature, declared in the header file 'signal.h'.

There is also an array sys_siglist which contains the messages for the various signal codes.  This array exists on BSD systems, unlike strsignal.


Specifying Signal Actions
Node: Signal Actions, Next: Defining Handlers, Prev: Standard Signals, Up: Signal Handling

Specifying Signal Actions

The simplest way to change the action for a signal is to use the signal function.  You can specify a built-in action (such as to ignore the signal), or you can "establish a handler".

The GNU library also implements the more versatile sigaction facility.  This section describes both facilities and gives suggestions on which to use when.

* Menu:

Basic Signal Handling	The simple signal function.

Advanced Signal Handling	The more powerful sigaction function.

Signal and Sigaction	How those two functions interact.

Sigaction Function Example	An example of using the sigaction function.

Flags for Sigaction	Specifying options for signal handling.

Initial Signal Actions	How programs inherit signal actions.


Basic Signal Handling
Node: Basic Signal Handling, Next: Advanced Signal Handling, Prev: , Up: Signal Actions

Basic Signal Handling

The signal function provides a simple interface for establishing an action for a particular signal.  The function and associated macros are declared in the header file 'signal.h'.  K

 - Data Type: sighandler_t

This is the type of signal handler functions.  Signal handlers take one integer argument specifying the signal number, and have return type void.  So, you should define handler functions like this:

void handler (int signum) { ... }

The name sighandler_t for this data type is a GNU extension.

 - Function: sighandler_t signal (int signum, sighandler_t action)

The signal function establishes action as the action for the signal signum.

The first argument, signum, identifies the signal whose behavior you want to control, and should be a signal number.  The proper way to specify a signal number is with one of the symbolic signal names described in See Standard Signals--don't use an explicit number, because the numerical code for a given kind of signal may vary from operating system to operating system.

The second argument, action, specifies the action to use for the signal signum.  This can be one of the following:

SIG_DFL

SIG_DFL specifies the default action for the particular signal.  The default actions for various kinds of signals are stated in See Standard Signals.

SIG_IGN

SIG_IGN specifies that the signal should be ignored.

Your program generally should not ignore signals that represent serious events or that are normally used to request termination.  You cannot ignore the SIGKILL or SIGSTOP signals at all.  You can ignore program error signals like SIGSEGV, but ignoring the error won't enable the program to continue executing meaningfully.  Ignoring user requests such as SIGINT, SIGQUIT, and SIGTSTP is unfriendly.

When you do not wish signals to be delivered during a certain part of the program, the thing to do is to block them, not ignore them.  See Blocking Signals.

handler

Supply the address of a handler function in your program, to specify running this handler as the way to deliver the signal.

For more information about defining signal handler functions, see See Defining Handlers.

If you set the action for a signal to SIG_IGN, or if you set it to SIG_DFL and the default action is to ignore that signal, then any pending signals of that type are discarded (even if they are blocked).  Discarding the pending signals means that they will never be delivered, not even if you subsequently specify another action and unblock this kind of signal.

The signal function returns the action that was previously in effect for the specified signum.  You can save this value and restore it later by calling signal again.

If signal can't honor the request, it returns SIG_ERR instead.  The following errno error conditions are defined for this function:

EINVAL

You specified an invalid signum; or you tried to ignore or provide a handler for SIGKILL or SIGSTOP.

Here is a simple example of setting up a handler to delete temporary files when certain fatal signals happen:

#include <signal.h>

void
termination_handler (int signum)
{
  struct temp_file *p;

  for (p = temp_file_list; p; p = p->next)
    unlink (p->name);
}

int
main (void)
{
  ...
  if (signal (SIGINT, termination_handler) == SIG_IGN)
    signal (SIGINT, SIG_IGN);
  if (signal (SIGHUP, termination_handler) == SIG_IGN)
    signal (SIGHUP, SIG_IGN);
  if (signal (SIGTERM, termination_handler) == SIG_IGN)
    signal (SIGTERM, SIG_IGN);
  ...
}

Note how if a given signal was previously set to be ignored, this code avoids altering that setting.  This is because non-job-control shells often ignore certain signals when starting children, and it is important for the children to respect this.

We do not handle SIGQUIT or the program error signals in this example because these are designed to provide information for debugging (a core dump), and the temporary files may give useful information.

 - Function: sighandler_t ssignal (int signum, sighandler_t action)

The ssignal function does the same thing as signal; it is provided only for compatibility with SVID.

 - Macro: sighandler_t SIG_ERR

The value of this macro is used as the return value from signal to indicate an error.


Advanced Signal Handling
Node: Advanced Signal Handling, Next: Signal and Sigaction, Prev: Basic Signal Handling, Up: Signal Actions

Advanced Signal Handling

The sigaction function has the same basic effect as signal: to specify how a signal should be handled by the process.  However, sigaction offers more control, at the expense of more complexity.  In particular, sigaction allows you to specify additional flags to control when the signal is generated and how the handler is invoked.

The sigaction function is declared in 'signal.h'.  K

 - Data Type: struct sigaction

Structures of type struct sigaction are used in the sigaction function to specify all the information about how to handle a particular signal.  This structure contains at least the following members:

sighandler_t sa_handler

This is used in the same way as the action argument to the signal function.  The value can be SIG_DFL, SIG_IGN, or a function pointer.  See Basic Signal Handling.

sigset_t sa_mask

This specifies a set of signals to be blocked while the handler runs.  Blocking is explained in See Blocking for Handler.  Note that the signal that was delivered is automatically blocked by default before its handler is started; this is true regardless of the value in sa_mask.  If you want that signal not to be blocked within its handler, you must write code in the handler to unblock it.

int sa_flags

This specifies various flags which can affect the behavior of the signal.  These are described in more detail in See Flags for Sigaction.

 - Function: int sigaction (int signum, const struct sigaction *action, struct sigaction *old_action)

The action argument is used to set up a new action for the signal signum, while the old_action argument is used to return information about the action previously associated with this symbol.  (In other words, old_action has the same purpose as the signal function's return value--you can check to see what the old action in effect for the signal was, and restore it later if you want.)

Either action or old_action can be a null pointer.  If old_action is a null pointer, this simply suppresses the return of information about the old action.  If action is a null pointer, the action associated with the signal signum is unchanged; this allows you to inquire about how a signal is being handled without changing that handling.

The return value from sigaction is zero if it succeeds, and -1 on failure.  The following errno error conditions are defined for this function:

EINVAL

The signum argument is not valid, or you are trying to trap or ignore SIGKILL or SIGSTOP.


Interaction of signal and sigaction
Node: Signal and Sigaction, Next: Sigaction Function Example, Prev: Advanced Signal Handling, Up: Signal Actions

Interaction of signal and sigaction

It's possible to use both the signal and sigaction functions within a single program, but you have to be careful because they can interact in slightly strange ways.

The sigaction function specifies more information than the signal function, so the return value from signal cannot express the full range of sigaction possibilities.  Therefore, if you use signal to save and later reestablish an action, it may not be able to reestablish properly a handler that was established with sigaction.

To avoid having problems as a result, always use sigaction to save and restore a handler if your program uses sigaction at all.  Since sigaction is more general, it can properly save and reestablish any action, regardless of whether it was established originally with signal or sigaction.

If you establish an action with signal and then examine it with sigaction, the handler address that you get may not be the same as what you specified with signal.  It may not even be suitable for use as an action argument with signal.  But you can rely on using it as an argument to sigaction.

So, you're better off using one or the other of the mechanisms consistently within a single program.

Portability Note: The basic signal function is a feature of ANSI C, while sigaction is part of the POSIX.1 standard.  If you are concerned about portability to non-POSIX systems, then you should use the signal function instead.


sigaction Function Example
Node: Sigaction Function Example, Next: Flags for Sigaction, Prev: Signal and Sigaction, Up: Signal Actions

sigaction Function Example

In See Basic Signal Handling, we gave an example of establishing a simple handler for termination signals using signal.  Here is an equivalent example using sigaction:

#include <signal.h>

void
termination_handler (int signum)
{
  struct temp_file *p;

  for (p = temp_file_list; p; p = p->next)
    unlink (p->name);
}

int
main (void)
{
  ...
  struct sigaction new_action, old_action;

  /* Set up the structure to specify the new action. */
  new_action.sa_handler = termination_handler;
  sigemptyset (&new_action.sa_mask);
  new_action.sa_flags = 0;

  sigaction (SIGINT, NULL, &old_action);
  if (old_action.sa_handler != SIG_IGN)
    sigaction (SIGINT, &new_action, NULL);
  sigaction (SIGHUP, NULL, &old_action);
  if (old_action.sa_handler != SIG_IGN)
    sigaction (SIGHUP, &new_action, NULL);
  sigaction (SIGTERM, NULL, &old_action);
  if (old_action.sa_handler != SIG_IGN)
    sigaction (SIGTERM, &new_action, NULL);
  ...
}

The program just loads the new_action structure with the desired parameters and passes it in the sigaction call.  The usage of sigemptyset is described later; see See Blocking Signals.

As in the example using signal, we avoid handling signals previously set to be ignored.  Here we can avoid altering the signal handler even momentarily, by using the feature of sigaction that lets us examine the current action without specifying a new one.

Here is another example.  It retrieves information about the current action for SIGINT without changing that action.

struct sigaction query_action;

if (sigaction (SIGINT, NULL, &query_action) < 0)
  /* sigaction returns -1 in case of error. */
else if (query_action.sa_handler == SIG_DFL)
  /* SIGINT is handled in the default, fatal manner. */
else if (query_action.sa_handler == SIG_IGN)
  /* SIGINT is ignored. */
else
  /* A programmer-defined signal handler is in effect. */


Flags for sigaction
Node: Flags for Sigaction, Next: Initial Signal Actions, Prev: Sigaction Function Example, Up: Signal Actions

Flags for sigaction

The sa_flags member of the sigaction structure is a catch-all for special features.  Most of the time, SA_RESTART is a good value to use for this field.

The value of sa_flags is interpreted as a bit mask.  Thus, you should choose the flags you want to set, OR those flags together, and store the result in the sa_flags member of your sigaction structure.

Each signal number has its own set of flags.  Each call to sigaction affects one particular signal number, and the flags that you specify apply only to that particular signal.

In the GNU C library, establishing a handler with signal sets all the flags to zero except for SA_RESTART, whose value depends on the settings you have made with siginterrupt.  See Interrupted Primitives, to see what this is about.

These macros are defined in the header file 'signal.h'.

 - Macro: int SA_NOCLDSTOP

This flag is meaningful only for the SIGCHLD signal.  When the flag is set, the system delivers the signal for a terminated child process but not for one that is stopped.  By default, SIGCHLD is delivered for both terminated children and stopped children.

Setting this flag for a signal other than SIGCHLD has no effect.

 - Macro: int SA_ONSTACK

If this flag is set for a particular signal number, the system uses the signal stack when delivering that kind of signal.  See BSD Signal Handling.

 - Macro: int SA_RESTART

This flag controls what happens when a signal is delivered during certain primitives (such as open, read or write), and the signal handler returns normally.  There are two alternatives: the library function can resume, or it can return failure with error code EINTR.

The choice is controlled by the SA_RESTART flag for the particular kind of signal that was delivered.  If the flag is set, returning from a handler resumes the library function.  If the flag is clear, returning from a handler makes the function fail.  See Interrupted Primitives.


Initial Signal Actions
Node: Initial Signal Actions, Next: , Prev: Flags for Sigaction, Up: Signal Actions

Initial Signal Actions

When a new process is created (see Creating a Process), it inherits handling of signals from its parent process.  However, when you load a new process image using the exec function (see Executing a File), any signals that you've defined your own handlers for revert to their SIG_DFL handling.  (If you think about it a little, this makes sense; the handler functions from the old program are specific to that program, and aren't even present in the address space of the new program image.)  Of course, the new program can establish its own handlers.

When a program is run by a shell, the shell normally sets the initial actions for the child process to SIG_DFL or SIG_IGN, as appropriate.  It's a good idea to check to make sure that the shell has not set up an initial action of SIG_IGN before you establish your own signal handlers.

Here is an example of how to establish a handler for SIGHUP, but not if SIGHUP is currently ignored:

...
struct sigaction temp;

sigaction (SIGHUP, NULL, &temp);

if (temp.sa_handler != SIG_IGN)
  {
    temp.sa_handler = handle_sighup;
    sigemptyset (&temp.sa_mask);
    sigaction (SIGHUP, &temp, NULL);
  }


Defining Signal Handlers
Node: Defining Handlers, Next: Interrupted Primitives, Prev: Signal Actions, Up: Signal Handling

Defining Signal Handlers

This section describes how to write a signal handler function that can be established with the signal or sigaction functions.

A signal handler is just a function that you compile together with the rest of the program.  Instead of directly invoking the function, you use signal or sigaction to tell the operating system to call it when a signal arrives.  This is known as "establishing" the handler.  See Signal Actions.

There are two basic strategies you can use in signal handler functions:

•	You can have the handler function note that the signal arrived by tweaking some global data structures, and then return normally.

•	You can have the handler function terminate the program or transfer control to a point where it can recover from the situation that caused the signal.

You need to take special care in writing handler functions because they can be called asynchronously.  That is, a handler might be called at any point in the program, unpredictably.  If two signals arrive during a very short interval, one handler can run within another.  This section describes what your handler should do, and what you should avoid.

* Menu:

Handler Returns	Handlers that return normally, and what this means.

Termination in Handler	How handler functions terminate a program.

Longjmp in Handler	Nonlocal transfer of control out of a signal handler.

Signals in Handler	What happens when signals arrive while the handler is already occupied.

Merged Signals	When a second signal arrives before the first is handled.

Nonreentrancy	Do not call any functions unless you know they are reentrant with respect to signals.

Atomic Data Access	A single handler can run in the middle of reading or writing a single object.


Signal Handlers That Return
Node: Handler Returns, Next: Termination in Handler, Prev: , Up: Defining Handlers

Signal Handlers That Return

Handlers which return normally are usually used for signals such as SIGALRM and the I/O and interprocess communication signals.  But a handler for SIGINT might also return normally after setting a flag that tells the program to exit at a convenient time.

It is not safe to return normally from the handler for a program error signal, because the behavior of the program when the handler function returns is not defined after a program error.  See Program Error Signals.

Handlers that return normally must modify some global variable in order to have any effect.  Typically, the variable is one that is examined periodically by the program during normal operation.  Its data type should be sig_atomic_t for reasons described in See Atomic Data Access.

Here is a simple example of such a program.  It executes the body of the loop until it has noticed that a SIGALRM signal has arrived.  This technique is useful because it allows the iteration in progress when the signal arrives to complete before the loop exits.

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>

/* This flag controls termination of the main loop. */
volatile sig_atomic_t keep_going = 1;

/* The signal handler just clears the flag and re-enables itself. */
void
catch_alarm (int sig)
{
  keep_going = 0;
  signal (sig, catch_alarm);
}

void
do_stuff (void)
{
  puts ("Doing stuff while waiting for alarm....");
}

int
main (void)
{
  /* Establish a handler for SIGALRM signals. */
  signal (SIGALRM, catch_alarm);

  /* Set an alarm to go off in a little while. */
  alarm (2);

  /* Check the flag once in a while to see when to quit. */
  while (keep_going)
    do_stuff ();

  return EXIT_SUCCESS;
}


Handlers That Terminate the Process
Node: Termination in Handler, Next: Longjmp in Handler, Prev: Handler Returns, Up: Defining Handlers

Handlers That Terminate the Process

Handler functions that terminate the program are typically used to cause orderly cleanup or recovery from program error signals and interactive interrupts.

The cleanest way for a handler to terminate the process is to raise the same signal that ran the handler in the first place.  Here is how to do this:

volatile sig_atomic_t fatal_error_in_progress = 0;

void
fatal_error_signal (int sig)
{

/* Since this handler is established for more than one kind of signal,
     it might still get invoked recursively by delivery of some other kind
     of signal.  Use a static variable to keep track of that. */
  if (fatal_error_in_progress)
    raise (sig);
  fatal_error_in_progress = 1;

/* Now do the clean up actions:
     - reset terminal modes
     - kill child processes
     - remove lock files */
  ...

/* Now reraise the signal.  Since the signal is blocked,
     it will receive its default handling, which is
     to terminate the process.  We could just call
     exit or abort, but reraising the signal
     sets the return status from the process correctly. */
  raise (sig);
}


Nonlocal Control Transfer in Handlers
Node: Longjmp in Handler, Next: Signals in Handler, Prev: Termination in Handler, Up: Defining Handlers

Nonlocal Control Transfer in Handlers

You can do a nonlocal transfer of control out of a signal handler using the setjmp and longjmp facilities (see Non-Local Exits).

When the handler does a nonlocal control transfer, the part of the program that was running will not continue.  If this part of the program was in the middle of updating an important data structure, the data structure will remain inconsistent.  Since the program does not terminate, the inconsistency is likely to be noticed later on.

There are two ways to avoid this problem.  One is to block the signal for the parts of the program that update important data structures.  Blocking the signal delays its delivery until it is unblocked, once the critical updating is finished.  See Blocking Signals.

The other way to re-initialize the crucial data structures in the signal handler, or make their values consistent.

Here is a rather schematic example showing the reinitialization of one global variable.

#include <signal.h>
#include <setjmp.h>

jmp_buf return_to_top_level;

volatile sig_atomic_t waiting_for_input;

void
handle_sigint (int signum)
{
  /* We may have been waiting for input when the signal arrived,
     but we are no longer waiting once we transfer control. */
  waiting_for_input = 0;
  longjmp (return_to_top_level, 1);
}

int
main (void)
{
  ...
  signal (SIGINT, sigint_handler);
  ...
  while (1) {
    prepare_for_command ();
    if (setjmp (return_to_top_level) == 0)
      read_and_execute_command ();
  }
}

/* Imagine this is a subroutine used by various commands. */
char *
read_data ()
{
  if (input_from_terminal) {
    waiting_for_input = 1;
    ...
    waiting_for_input = 0;
  } else {
    ...
  }
}


Signals Arriving While a Handler Runs
Node: Signals in Handler, Next: Merged Signals, Prev: Longjmp in Handler, Up: Defining Handlers

Signals Arriving While a Handler Runs

What happens if another signal arrives when your signal handler function is running?

When the handler for a particular signal is invoked, that signal is normally blocked until the handler returns.  That means that if two signals of the same kind arrive close together, the second one will be held until the first has been handled.  (The handler can explicitly unblock the signal using sigprocmask, if you want to allow more signals of this type to arrive; see See Process Signal Mask.)

However, your handler can still be interrupted by delivery of another kind of signal.  To avoid this, you can use the sa_mask member of the action structure passed to sigaction to explicitly specify which signals should be blocked while the signal handler runs.  These signals are in addition to the signal for which the handler was invoked, and any other signals that are normally blocked by the process.  See Blocking for Handler.

Portability Note: Always use sigaction to establish a handler for a signal that you expect to receive asynchronously, if you want your program to work properly on System V Unix.  On this system, the handling of a signal whose handler was established with signal automatically sets the signal's action back to SIG_DFL, and the handler must re-establish itself each time it runs.  This practice, while inconvenient, does work when signals cannot arrive in succession.  However, if another signal can arrive right away, it may arrive before the handler can re-establish itself.  Then the second signal would receive the default handling, which could terminate the process.


Signals Close Together Merge into One
Node: Merged Signals, Next: Nonreentrancy, Prev: Signals in Handler, Up: Defining Handlers

Signals Close Together Merge into One

If multiple signals of the same type are delivered to your process before your signal handler has a chance to be invoked at all, the handler may only be invoked once, as if only a single signal had arrived.  In effect, the signals merge into one.  This situation can arise when the signal is blocked, or in a multiprocessing environment where the system is busy running some other processes while the signals are delivered.  This means, for example, that you cannot reliably use a signal handler to count signals.  The only distinction you can reliably make is whether at least one signal has arrived since a given time in the past.

Here is an example of a handler for SIGCHLD that compensates for the fact that the number of signals recieved may not equal the number of child processes generate them.  It assumes that the program keeps track of all the child processes with a chain of structures as follows:

struct process
{
  struct process *next;
  /* The process ID of this child.  */
  int pid;
  /* The descriptor of the pipe or pseudo terminal
     on which output comes from this child.  */
  int input_descriptor;
  /* Nonzero if this process has stopped or terminated.  */
  sig_atomic_t have_status;
  /* The status of this child; 0 if running,
     otherwise a status value from waitpid.  */
  int status;
};

struct process *process_list;

This example also uses a flag to indicate whether signals have arrived since some time in the past--whenever the program last cleared it to zero.

/* Nonzero means some child's status has changed
   so look at process_list for the details.  */
int process_status_change;

Here is the handler itself:

void
sigchld_handler (int signo)
{
  int old_errno = errno;

  while (1) {
    register int pid;
    int w;
    struct process *p;

    /* Keep asking for a status until we get a definitive result.  */
    do
      {
        errno = 0;
        pid = waitpid (WAIT_ANY, &w, WNOHANG | WUNTRACED);
      }
    while (pid <= 0 && errno == EINTR);

    if (pid <= 0) {
      /* A real failure means there are no more
         stopped or terminated child processes, so return.  */
      errno = old_errno;
      return;
    }

    /* Find the process that signaled us, and record its status.  */

    for (p = process_list; p; p = p->next)
      if (p->pid == pid) {
        p->status = w;
        /* Indicate that the status field
           has data to look at.  We do this only after storing it.  */
        p->have_status = 1;

        /* If process has terminated, stop waiting for its output.  */
        if (WIFSIGNALED (w) || WIFEXITED (w))
          if (p->input_descriptor)
            FD_CLR (p->input_descriptor, &input_wait_mask);

        /* The program should check this flag from time to time
           to see if there is any news in process_list.  */
        ++process_status_change;
      }

    /* Loop around to handle all the processes
       that have something to tell us.  */
  }
}

Here is the proper way to check the flag process_status_change:

if (process_status_change) {
  struct process *p;
  process_status_change = 0;
  for (p = process_list; p; p = p->next)
    if (p->have_status) {
      ... Examine p->status ...
    }
}

It is vital to clear the flag before examining the list; otherwise, if a signal were delivered just before the clearing of the flag, and after the appropriate element of the process list had been checked, the status change would go unnoticed until the next signal arrived to set the flag again.  You could, of course, avoid this problem by blocking the signal while scanning the list, but it is much more elegant to guarantee correctness by doing things in the right order.

The loop which checks process status avoids examining p->status until it sees that status has been validly stored.  This is to make sure that the status cannot change in the middle of accessing it.  Once p->have_status is set, it means that the child process is stopped or terminated, and in either case, it cannot stop or terminate again until the program has taken notice.  See Atomic Usage, for more information about coping with interruptions during accessings of a variable.

Here is another way you can test whether the handler has run since the last time you checked.  This technique uses a counter which is never changed outside the handler.  Instead of clearing the count, the program remembers the previous value and sees whether it has changed since the previous check.  The advantage of this method is that different parts of the program can check independently, each part checking whether there has been a signal since that part last checked.

sig_atomic_t process_status_change;

sig_atomic_t last_process_status_change;

...
{
  sig_atomic_t prev = last_process_status_change;
  last_process_status_change = process_status_change;
  if (last_process_status_change != prev) {
    struct process *p;
    for (p = process_list; p; p = p->next)
      if (p->have_status) {
        ... Examine p->status ...
      }
  }
}


Signal Handling and Nonreentrant Functions
Node: Nonreentrancy, Next: Atomic Data Access, Prev: Merged Signals, Up: Defining Handlers

Signal Handling and Nonreentrant Functions

Handler functions usually don't do very much.  The best practice is to write a handler that does nothing but set an external variable that the program checks regularly, and leave all serious work to the program.  This is best because the handler can be called at asynchronously, at unpredictable times--perhaps in the middle of a system call, or even between the beginning and the end of a C operator that requires multiple instructions.  The data structures being manipulated might therefore be in an inconsistent state when the handler function is invoked.  Even copying one int variable into another can take two instructions on most machines.

This means you have to be very careful about what you do in a signal handler.

•	If your handler needs to access any global variables from your program, declare those variables volatile.  This tells the compiler that the value of the variable might change asynchronously, and inhibits certain optimizations that would be invalidated by such modifications.

•	If you call a function in the handler, make sure it is "reentrant" with respect to signals, or else make sure that the signal cannot interrupt a call to a related function.

A function can be non-reentrant if it uses memory that is not on the stack.

•	If a function uses a static variable or a global variable, or a dynamically-allocated object that it finds for itself, then it is non-reentrant and any two calls to the function can interfere.

For example, suppose that the signal handler uses gethostbyname.  This function returns its value in a static object, reusing the same object each time.  If the signal happens to arrive during a call to gethostbyname, or even after one (while the program is still using the value), it will clobber the value that the program asked for.

However, if the program does not use gethostbyname or any other function that returns information in the same object, or if it always blocks signals around each use, then you are safe.

There are a large number of library functions that return values in a fixed object, always reusing the same object in this fashion, and all of them cause the same problem.  The description of a function in this manual always mentions this behavior.

•	If a function uses and modifies an object that you supply, then it is potentially non-reentrant; two calls can interfere if they use the same object.

This case arises when you do I/O using streams.  Suppose that the signal handler prints a message with fprintf.  Suppose that the program was in the middle of an fprintf call using the same stream when the signal was delivered.  Both the signal handler's message and the program's data could be corrupted, because both calls operate on the same data structure--the stream itself.

However, if you know that the stream that the handler uses cannot possibly be used by the program at a time when signals can arrive, then you are safe.  It is no problem if the program uses some other stream.

•	On most systems, malloc and free are not reentrant, because they use a static data structure which records what memory blocks are free.  As a result, no library functions that allocate or free memory are reentrant.  This includes functions that allocate space to store a result.

The best way to avoid the need to allocate memory in a handler is to allocate in advance space for signal handlers to use.

The best way to avoid freeing memory in a handler is to flag or record the objects to be freed, and have the program check from time to time whether anything is waiting to be freed.  But this must be done with care, because placing an object on a chain is not atomic, and if it is interrupted by another signal handler that does the same thing, you could "lose" one of the objects.

On the GNU system, malloc and free are safe to use in signal handlers because it blocks signals.  As a result, the library functions that allocate space for a result are also safe in signal handlers.  The obstack allocation functions are safe as long as you don't use the same obstack both inside and outside of a signal handler.

The relocating allocation functions (see Relocating Allocator) are certainly not safe to use in a signal handler.

•	Any function that modifies errno is non-reentrant, but you can correct for this: in the handler, save the original value of errno and restore it before returning normally.  This prevents errors that occur within the signal handler from being confused with errors from system calls at the point the program is interrupted to run the handler.

This technique is generally applicable; if you want to call in a handler a function that modifies a particular object in memory, you can make this safe by saving and restoring that object.

•	Merely reading from a memory object is safe provided that you can deal with any of the values that might appear in the object at a time when the signal can be delivered.  Keep in mind that assignment to some data types requires more than one instruction, which means that the handler could run "in the middle of" an assignment to the variable if its type is not atomic.  See Atomic Data Access.

•	Merely writing into a memory object is safe as long as a sudden change in the value, at any time when the handler might run, will not disturb anything.


Atomic Data Access and Signal Handling
Node: Atomic Data Access, Next: , Prev: Nonreentrancy, Up: Defining Handlers

Atomic Data Access and Signal Handling

Whether the data in your application concerns atoms, or mere text, you have to be careful about the fact that access to a single datum is not necessarily "atomic".  This means that it can take more than one instruction to read or write a single object.  In such cases, a signal handler can run in the middle of reading or writing the object.

There are three ways you can cope with this problem.  You can use data types that are always accessed atomically; you can carefully arrange that nothing untoward happens if an access is interrupted, or you can block all signals around any access that had better not be interrupted (see Blocking Signals).

* Menu:

Non-atomic Example	A program illustrating interrupted access.

Types	Data types that guarantee no interruption.

Usage	Proving that interruption is harmless.


Problems with Non-Atomic Access
Node: Non-atomic Example, Next: Atomic Types, Prev: , Up: Atomic Data Access

Problems with Non-Atomic Access

Here is an example which shows what can happen if a signal handler runs in the middle of modifying a variable.  (Interrupting the reading of a variable can also lead to paradoxical results, but here we only show writing.)

#include <signal.h>
#include <stdio.h>

struct two_words { int a, b; } memory;

void
handler(int signum)
{
   printf ("%d,%d\n", memory.a, memory.b);
   alarm (1);
}

int
main (void)
{
   static struct two_words zeros = { 0, 0 }, ones = { 1, 1 };
   signal (SIGALRM, handler);
   memory = zeros;
   alarm (1);
   while (1)
     {
       memory = zeros;
       memory = ones;
     }
}

This program fills memory with zeros, ones, zeros, ones, alternating forever; meanwhile, once per second, the alarm signal handler prints the current contents.  (Calling printf in the handler is safe in this program because it is certainly not being called outside the handler when the signal happens.)

Clearly, this program can print a pair of zeros or a pair of ones.  But that's not all it can do!  On most machines, it takes several instructions to store a new value in memory, and the value is stored one word at a time.  If the signal is delivered in between these instructions, the handler might find that memory.a is zero and memory.b is one (or vice versa).

On some machines it may be possible to store a new value in memory with just one instruction that cannot be interrupted.  On these machines, the handler will always print two zeros or two ones.


Atomic Types
Node: Atomic Types, Next: Atomic Usage, Prev: Non-atomic Example, Up: Atomic Data Access

Atomic Types

To avoid uncertainty about interrupting access to a variable, you can use a particular data type for which access is always atomic: sig_atomic_t.  Reading and writing this data type is guaranteed to happen in a single instruction, so there's no way for a handler to run "in the middle" of an access.

The type sig_atomic_t is always an integer data type, but which one it is, and how many bits it contains, may vary from machine to machine.

 - Data Type: sig_atomic_t

This is an integer data type.  Objects of this type are always accessed atomically.

In practice, you can assume that int and other integer types no longer than int are atomic.  You can also assume that pointer types are atomic; that is very convenient.  Both of these are true on all of the machines that the GNU C library supports, and on all POSIX systems we know of.


Atomic Usage Patterns
Node: Atomic Usage, Next: , Prev: Atomic Types, Up: Atomic Data Access

Atomic Usage Patterns

Certain patterns of access avoid any problem even if an access is interrupted.  For example, a flag which is set by the handler, and tested and cleared by the main program from time to time, is always safe even if access actually requires two instructions.  To show that this is so, we must consider each access that could be interrupted, and show that there is no problem if it is interrupted.

An interrupt in the middle of testing the flag is safe because either it's recognized to be nonzero, in which case the precise value doesn't matter, or it will be seen to be nonzero the next time it's tested.

An interrupt in the middle of clearing the flag is no problem because either the value ends up zero, which is what happens if a signal comes in just before the flag is cleared, or the value ends up nonzero, and subsequent events occur as if the signal had come in just after the flag was cleared.  As long as the code handles both of these cases properly, it can also handle a signal in the middle of clearing the flag.  (This is an example of the sort of reasoning you need to do to figure out whether non-atomic usage is safe.)

Sometimes you can insure uninterrupted access to one object by protecting its use with another object, perhaps one whose type guarantees atomicity.  See Merged Signals, for an example.


Primitives Interrupted by Signals
Node: Interrupted Primitives, Next: Generating Signals, Prev: Defining Handlers, Up: Signal Handling

Primitives Interrupted by Signals

A signal can arrive and be handled while an I/O primitive such as open or read is waiting for an I/O device.  If the signal handler returns, the system faces the question: what should happen next?

POSIX specifies one approach: make the primitive fail right away.  The error code for this kind of failure is EINTR.  This is flexible, but usually inconvenient.  Typically, POSIX applications that use signal handlers must check for EINTR after each library function that can return it, in order to try the call again.  Often programmers forget to check, which is a common source of error.

The GNU library provides a convenient way to retry a call after a temporary failure, with the macro TEMP_FAILURE_RETRY:

 - Macro: TEMP_FAILURE_RETRY (expression)

This macro evaluates expression once.  If it fails and reports error code EINTR, TEMP_FAILURE_RETRY evaluates it again, and over and over until the result is not a temporary failure.

The value returned by TEMP_FAILURE_RETRY is whatever value expression produced.

BSD avoids EINTR entirely and provides a more convenient approach: to restart the interrupted primitive, instead of making it fail.  If you choose this approach, you need not be concerned with EINTR.

You can choose either approach with the GNU library.  If you use sigaction to establish a signal handler, you can specify how that handler should behave.  If you specify the SA_RESTART flag, return from that handler will resume a primitive; otherwise, return from that handler will cause EINTR.  See Flags for Sigaction.

Another way to specify the choice is with the siginterrupt function.  See POSIX vs BSD.

When you don't specify with sigaction or siginterrupt what a particular handler should do, it uses a default choice.  The default choice in the GNU library depends on the feature test macros you have defined.  If you define _BSD_SOURCE or _GNU_SOURCE before calling signal, the default is to resume primitives; otherwise, the default is to make them fail with EINTR.  (The library contains alternate versions of the signal function, and the feature test macros determine which one you really call.)  See Feature Test Macros.

The primitives affected by this issue are close, fcntl (operation F_SETLK), open, read, recv, recvfrom, select, send, sendto, tcdrain, waitpid, wait, and write.

There is one situation where resumption never happens no matter which choice you make: when a data-transfer function such as read or write is interrupted by a signal after transferring part of the data.  In this case, the function returns the number of bytes already transferred, indicating partial success.

This might at first appear to cause unreliable behavior on record-oriented devices (including datagram sockets; see Datagrams), where splitting one read or write into two would read or write two records.  Actually, there is no problem, because interruption after a partial transfer cannot happen on such devices; they always transfer an entire record in one burst, with no waiting once data transfer has started.


Generating Signals
Node: Generating Signals, Next: Blocking Signals, Prev: Interrupted Primitives, Up: Signal Handling

Generating Signals

Besides signals that are generated as a result of a hardware trap or interrupt, your program can explicitly send signals to itself or to another process.

* Menu:

Signaling Yourself	A process can send a signal to itself.

Signaling Another Process	Send a signal to another process.

Permission for kill	Permission for using kill.

Kill Example	Using kill for Communication.


Signaling Yourself
Node: Signaling Yourself, Next: Signaling Another Process, Prev: , Up: Generating Signals

Signaling Yourself

A process can send itself a signal with the raise function.  This function is declared in 'signal.h'.  K

 - Function: int raise (int signum)

The raise function sends the signal signum to the calling process.  It returns zero if successful and a nonzero value if it fails.  About the only reason for failure would be if the value of signum is invalid.

 - Function: int gsignal (int signum)

The gsignal function does the same thing as raise; it is provided only for compatibility with SVID.

One convenient use for raise is to reproduce the default behavior of a signal that you have trapped.  For instance, suppose a user of your program types the SUSP character (usually ^C-z; see Special Characters) to send it an interactive stop stop signal (SIGTSTP), and you want to clean up some internal data buffers before stopping.  You might set this up like this:

#include <signal.h>

/* When a stop signal arrives, set the action back to the default
   and then resend the signal after doing cleanup actions. */

void
tstp_handler (int sig)
{
  signal (SIGTSTP, SIG_DFL);
  /* Do cleanup actions here. */
  ...
  raise (SIGTSTP);
}

/* When the process is continued again, restore the signal handler. */

void
cont_handler (int sig)
{
  signal (SIGCONT, cont_handler);
  signal (SIGTSTP, tstp_handler);
}

/* Enable both handlers during program initialization. */

int
main (void)
{
  signal (SIGCONT, cont_handler);
  signal (SIGTSTP, tstp_handler);
  ...
}

Portability note: raise was invented by the ANSI C committee.  Older systems may not support it, so using kill may be more portable.  See Signaling Another Process.


Signaling Another Process
Node: Signaling Another Process, Next: Permission for kill, Prev: Signaling Yourself, Up: Generating Signals

Signaling Another Process

The kill function can be used to send a signal to another process.  In spite of its name, it can be used for a lot of things other than causing a process to terminate.  Some examples of situations where you might want to send signals between processes are:

•	A parent process starts a child to perform a task--perhaps having the child running an infinite loop--and then terminates the child when the task is no longer needed.

•	A process executes as part of a group, and needs to terminate or notify the other processes in the group when an error or other event occurs.

•	Two processes need to synchronize while working together.

This section assumes that you know a little bit about how processes work.  For more information on this subject, see See Child Processes.

The kill function is declared in 'signal.h'.  K

 - Function: int kill (pid_t pid, int signum)

The kill function sends the signal signum to the process or process group specified by pid.  Besides the signals listed in See Standard Signals, signum can also have a value of zero to check the validity of the pid.

The pid specifies the process or process group to receive the signal:

pid > 0

The process whose identifier is pid.

pid == 0

All processes in the same process group as the sender.  The sender itself does not receive the signal.

pid < -1

The process group whose identifier is -pid.

pid == -1

If the process is privileged, send the signal to all processes except for some special system processes.  Otherwise, send the signal to all processes with the same effective user ID.

A process can send a signal signum to itself with a call like kill (getpid(), signum).  If kill is used by a process to send a signal to itself, and the signal is not blocked, then kill delivers at least one signal (which might be some other pending unblocked signal instead of the signal signum) to that process before it returns.

The return value from kill is zero if the signal can be sent successfully.  Otherwise, no signal is sent, and a value of -1 is returned.  If pid specifies sending a signal to several processes, kill succeeds if it can send the signal to at least one of them.  There's no way you can tell which of the processes got the signal or whether all of them did.

The following errno error conditions are defined for this function:

EINVAL

The signum argument is an invalid or unsupported number.

EPERM

You do not have the privilege to send a signal to the process or any of the processes in the process group named by pid.

ESCRH

The pid argument does not refer to an existing process or group.

 - Function: int killpg (int pgid, int signum)

This is similar to kill, but sends signal signum to the process group pgid.  This function is provided for compatibility with BSD; using kill to do this is more portable.

As a simple example of kill, the call kill (getpid (), sig) has the same effect as raise (sig).


Permission for using kill
Node: Permission for kill, Next: Kill Example, Prev: Signaling Another Process, Up: Generating Signals

Permission for using kill

There are restrictions that prevent you from using kill to send signals to any random process.  These are intended to prevent antisocial behavior such as arbitrarily killing off processes belonging to another user.  In typical use, kill is used to pass signals between parent, child, and sibling processes, and in these situations you normally do have permission to send signals.  The only common execption is when you run a setuid program in a child process; if the program changes its real UID as well as its effective UID, you may not have permission to send a signal.  The su program does this.

Whether a process has permission to send a signal to another process is determined by the user IDs of the two processes.  This concept is discussed in detail in See Process Persona.

Generally, for a process to be able to send a signal to another process, either the sending process must belong to a privileged user (like 'root'), or the real or effective user ID of the sending process must match the real or effective user ID of the receiving process.  If the receiving process has changed its effective user ID from the set-user-ID mode bit on its process image file, then the owner of the process image file is used in place of its current effective user ID.  In some implementations, a parent process might be able to send signals to a child process even if the user ID's don't match, and other implementations might enforce other restrictions.

The SIGCONT signal is a special case.  It can be sent if the sender is part of the same session as the receiver, regardless of user IDs.


Using kill for Communication
Node: Kill Example, Next: , Prev: Permission for kill, Up: Generating Signals

Using kill for Communication

Here is a longer example showing how signals can be used for interprocess communication.  This is what the SIGUSR1 and SIGUSR2 signals are provided for.  Since these signals are fatal by default, the process that is supposed to receive them must trap them through signal or sigaction.

In this example, a parent process forks a child process and then waits for the child to complete its initialization.  The child process tells the parent when it is ready by sending it a SIGUSR1 signal, using the kill function.

#include <signal.h>
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>

/* When a SIGUSR1 signal arrives, set this variable. */
volatile sig_atomic_t usr_interrupt = 0;

void
synch_signal (int sig)
{
  usr_interrupt = 1;
}

/* The child process executes this function. */
void
child_function (void)
{
  /* Perform initialization. */
  printf ("I'm here!!!  My pid is %d.\n", (int) getpid ());

  /* Let parent know you're done. */
  kill (getppid (), SIGUSR1);

  /* Continue with execution. */
  puts ("Bye, now....");
  exit (0);
}

int
main (void)
{
  struct sigaction usr_action;
  sigset_t block_mask;
  pid_t child_id;

  /* Establish the signal handler. */
  sigfillset (&block_mask);
  usr_action.sa_handler = synch_signal;
  usr_action.sa_mask = block_mask;
  usr_action.sa_flags = 0;
  sigaction (SIGUSR1, &usr_action, NULL);

  /* Create the child process. */
  child_id = fork ();
  if (child_id == 0)
    child_function ();          /* Does not return. */

/* Busy wait for the child to send a signal. */
  while (!usr_interrupt)
    ;

  /* Now continue execution. */
  puts ("That's all, folks!");

  return 0;
}

This example uses a busy wait, which is bad, because it wastes CPU cycles that other programs could otherwise use.  It is better to ask the system to wait until the signal arrives.  See the example in See Waiting for a Signal.


Blocking Signals
Node: Blocking Signals, Next: Waiting for a Signal, Prev: Generating Signals, Up: Signal Handling

Blocking Signals

Blocking a signal means telling the operating system to hold it and deliver it later.  Generally, a program does not block signals indefinitely--it might as well ignore them by setting their actions to SIG_IGN.  But it is useful to block signals briefly, to prevent them from interrupting sensitive operations.  For instance:

•	You can use the sigprocmask function to block signals while you modify global variables that are also modified by the handlers for these signals.

•	You can set sa_mask in your sigaction call to block certain signals while a particular signal handler runs.  This way, the signal handler can run without being interrupted itself by signals.

* Menu:

Why Block	The purpose of blocking signals.

Signal Sets	How to specify which signals to block.

Process Signal Mask	Blocking delivery of signals to your process during normal execution.

Testing for Delivery	Blocking to Test for Delivery of a Signal.

Blocking for Handler	Blocking additional signals while a handler is being run.

Checking for Pending Signals	Checking for Pending Signals

Remembering a Signal	How you can get almost the same effect as blocking a signal, by handling it and setting a flag to be tested later.


Why Blocking Signals is Useful
Node: Why Block, Next: Signal Sets, Prev: , Up: Blocking Signals

Why Blocking Signals is Useful

Temporary blocking of signals with sigprocmask gives you a way to prevent interrupts during critical parts of your code.  If signals arrive in that part of the program, they are delivered later, after you unblock them.

One example where this is useful is for sharing data between a signal handler and the rest of the program.  If the type of the data is not sig_atomic_t (see Atomic Data Access), then the signal handler could run when the rest of the program has only half finished reading or writing the data.  This would lead to confusing consequences.

To make the program reliable, you can prevent the signal handler from running while the rest of the program is examining or modifying that data--by blocking the appropriate signal around the parts of the program that touch the data.

Blocking signals is also necessary when you want to perform a certain action only if a signal has not arrived.  Suppose that the handler for the signal sets a flag of type sig_atomic_t; you would like to test the flag and perform the action if the flag is not set.  This is unreliable.  Suppose the signal is delivered immediately after you test the flag, but before the consequent action: then the program will perform the action even though the signal has arrived.

The only way to test reliably for whether a signal has yet arrived is to test while the signal is blocked.


Signal Sets
Node: Signal Sets, Next: Process Signal Mask, Prev: Why Block, Up: Blocking Signals

Signal Sets

All of the signal blocking functions use a data structure called a "signal set" to specify what signals are affected.  Thus, every activity involves two stages: creating the signal set, and then passing it as an argument to a library function.  KThese facilities are declared in the header file 'signal.h'.  K

 - Data Type: sigset_t

The sigset_t data type is used to represent a signal set.  Internally, it may be implemented as either an integer or structure type.

For portability, use only the functions described in this section to initialize, change, and retrieve information from sigset_t objects--don't try to manipulate them directly.

There are two ways to initialize a signal set.  You can initially specify it to be empty with sigemptyset and then add specified signals individually.  Or you can specify it to be full with sigfillset and then delete specified signals individually.

You must always initialize the signal set with one of these two functions before using it in any other way.  Don't try to set all the signals explicitly because the sigset_t object might include some other information (like a version field) that needs to be initialized as well.  (In addition, it's not wise to put into your program an assumption that the system has no signals aside from the ones you know about.)

 - Function: int sigemptyset (sigset_t *set)

This function initializes the signal set set to exclude all of the defined signals.  It always returns 0.

 - Function: int sigfillset (sigset_t *set)

This function initializes the signal set set to include all of the defined signals.  Again, the return value is 0.

 - Function: int sigaddset (sigset_t *set, int signum)

This function adds the signal signum to the signal set set.  All sigaddset does is modify set; it does not block or unblock any signals.

The return value is 0 on success and -1 on failure.  The following errno error condition is defined for this function:

EINVAL

The signum argument doesn't specify a valid signal.

 - Function: int sigdelset (sigset_t *set, int signum)

This function removes the signal signum from the signal set set.  All sigdelset does is modify set; it does not block or unblock any signals.  The return value and error conditions are the same as for sigaddset.

Finally, there is a function to test what signals are in a signal set:

 - Function: int sigismember (const sigset_t *set, int signum)

The sigismember function tests whether the signal signum is a member of the signal set set.  It returns 1 if the signal is in the set, 0 if not, and -1 if there is an error.

The following errno error condition is defined for this function:

EINVAL

The signum argument doesn't specify a valid signal.


Process Signal Mask
Node: Process Signal Mask, Next: Testing for Delivery, Prev: Signal Sets, Up: Blocking Signals

Process Signal Mask

The collection of signals that are currently blocked is called the "signal mask".  Each process has its own signal mask.  When you create a new process (see Creating a Process), it inherits its parent's mask.  You can block or unblock signals with total flexibility by modifying the signal mask.

The prototype for the sigprocmask function is in 'signal.h'.  K

 - Function: int sigprocmask (int how, const sigset_t *set, sigset_t *oldset)

The sigprocmask function is used to examine or change the calling process's signal mask.  The how argument determines how the signal mask is changed, and must be one of the following values:

SIG_BLOCK

Block the signals in set--add them to the existing mask.  In other words, the new mask is the union of the existing mask and set.

SIG_UNBLOCK

Unblock the signals in set--remove them from the existing mask.

SIG_SETMASK

Use set for the mask; ignore the previous value of the mask.

The last argument, oldset, is used to return information about the old process signal mask.  If you just want to change the mask without looking at it, pass a null pointer as the oldset argument.  Similarly, if you want to know what's in the mask without changing it, pass a null pointer for set (in this case the how argument is not significant).  The oldset argument is often used to remember the previous signal mask in order to restore it later.  (Since the signal mask is inherited over fork and exec calls, you can't predict what its contents are when your program starts running.)

If invoking sigprocmask causes any pending signals to be unblocked, at least one of those signals is delivered to the process before sigprocmask returns.  The order in which pending signals are delivered is not specified, but you can control the order explicitly by making multiple sigprocmask calls to unblock various signals one at a time.

The sigprocmask function returns 0 if successful, and -1 to indicate an error.  The following errno error conditions are defined for this function:

EINVAL

The how argument is invalid.

You can't block the SIGKILL and SIGSTOP signals, but if the signal set includes these, sigprocmask just ignores them instead of returning an error status.

Remember, too, that blocking program error signals such as SIGFPE leads to undesirable results for signals generated by an actual program error (as opposed to signals sent with raise or kill).  This is because your program may be too broken to be able to continue executing to a point where the signal is unblocked again.  See Program Error Signals.


Blocking to Test for Delivery of a Signal
Node: Testing for Delivery, Next: Blocking for Handler, Prev: Process Signal Mask, Up: Blocking Signals

Blocking to Test for Delivery of a Signal

Now for a simple example.  Suppose you establish a handler for SIGALRM signals that sets a flag whenever a signal arrives, and your main program checks this flag from time to time and then resets it.  You can prevent additional SIGALRM signals from arriving in the meantime by wrapping the critical part of the code with calls to sigprocmask, like this:

/* This variable is set by the SIGALRM signal handler. */
volatile sig_atomic_t flag = 0;

int
main (void)
{
  sigset_t block_alarm;

  ...

  /* Initialize the signal mask. */
  sigemptyset (&block_alarm);
  sigaddset (&block_alarm, SIGALRM);

while (1)
    {
      /* Check if a signal has arrived; if so, reset the flag. */
      sigprocmask (SIG_BLOCK, &block_alarm, NULL);
      if (flag)
        {
          actions-if-not-arrived
          flag = 0;
        }
      sigprocmask (SIG_UNBLOCK, &block_alarm, NULL);

      ...
    }
}


Blocking Signals for a Handler
Node: Blocking for Handler, Next: Checking for Pending Signals, Prev: Testing for Delivery, Up: Blocking Signals

Blocking Signals for a Handler

When a signal handler is invoked, you usually want it to be able to finish without being interrupted by another signal.  From the moment the handler starts until the moment it finishes, you must block signals that might confuse it or corrupt its data.

When a handler function is invoked on a signal, that signal is automatically blocked (in addition to any other signals that are already in the process's signal mask) during the time the handler is running.  If you set up a handler for SIGTSTP, for instance, then the arrival of that signal forces further SIGTSTP signals to wait during the execution of the handler.

However, by default, other kinds of signals are not blocked; they can arrive during handler execution.

The reliable way to block other kinds of signals during the execution of the handler is to use the sa_mask member of the sigaction structure.

Here is an example:

#include <signal.h>
#include <stddef.h>

void catch_stop ();

void
install_handler (void)
{
  struct sigaction setup_action;
  sigset_t block_mask;

  sigemptyset (&block_mask);
  /* Block other terminal-generated signals while handler runs. */
  sigaddset (&block_mask, SIGINT);
  sigaddset (&block_mask, SIGQUIT);
  setup_action.sa_handler = catch_stop;
  setup_action.sa_mask = block_mask;
  setup_action.sa_flags = 0;
  sigaction (SIGTSTP, &setup_action, NULL);
}

This is more reliable than blocking the other signals explicitly in the code for the handler.  If you block signals explicity in the handler, you can't avoid at least a short interval at the beginning of the handler where they are not yet blocked.

You cannot remove signals from the process's current mask using this mechanism.  However, you can make calls to sigprocmask within your handler to block or unblock signals as you wish.

In any case, when the handler returns, the system restores the mask that was in place before the handler was entered.


Checking for Pending Signals
Node: Checking for Pending Signals, Next: Remembering a Signal, Prev: Blocking for Handler, Up: Blocking Signals

Checking for Pending Signals

You can find out which signals are pending at any time by calling sigpending.  This function is declared in 'signal.h'.  K

 - Function: int sigpending (sigset_t *set)

The sigpending function stores information about pending signals in set.  If there is a pending signal that is blocked from delivery, then that signal is a member of the returned set.  (You can test whether a particular signal is a member of this set using sigismember; see See Signal Sets.)

The return value is 0 if successful, and -1 on failure.

Testing whether a signal is pending is not often useful.  Testing when that signal is not blocked is almost certainly bad design.

Here is an example.

#include <signal.h>
#include <stddef.h>

sigset_t base_mask, waiting_mask;

sigemptyset (&base_mask);
sigaddset (&base_mask, SIGINT);
sigaddset (&base_mask, SIGTSTP);

/* Block user interrupts while doing other processing. */
sigprocmask (SIG_SETMASK, &base_mask, NULL);
...

/* After a while, check to see whether any signals are pending. */
sigpending (&waiting_mask);
if (sigismember (&waiting_mask, SIGINT)) {
  /* User has tried to kill the process. */
}
else if (sigismember (&waiting_mask, SIGTSTP)) {
  /* User has tried to stop the process. */
}

Remember that if there is a particular signal pending for your process, additional signals of that same type that arrive in the meantime might be discarded.  For example, if a SIGINT signal is pending when another SIGINT signal arrives, your program will probably only see one of them when you unblock this signal.

Portability Note: The sigpending function is new in POSIX.1.  Older systems have no equivalent facility.


Remembering a Signal to Act On Later
Node: Remembering a Signal, Next: , Prev: Checking for Pending Signals, Up: Blocking Signals

Remembering a Signal to Act On Later

Instead of blocking a signal using the library facilities, you can get almost the same results by making the handler set a flag to be tested later, when you "unblock".  Here is an example:

/* If this flag is nonzero, don't handle the signal right away. */
volatile sig_atomic_t signal_pending;

/* This is nonzero if a signal arrived and was not handled. */
volatile sig_atomic_t defer_signal;

void
handler (int signum)
{
  if (defer_signal)
    signal_pending = signum;
  else
    ... /* ``Really'' handle the signal. */
}

...

void
update_mumble (int frob)
{
  /* Prevent signals from having immediate effect. */
  defer_signal++;
  /* Now update mumble, without worrying about interruption. */
  mumble.a = 1;
  mumble.b = hack ();
  mumble.c = frob;
  /* We have updated mumble.  Handle any signal that came in. */
  defer_signal--;
  if (defer_signal == 0 && signal_pending != 0)
    raise (signal_pending);
}

Note how the particular signal that arrives is stored in signal_pending.  That way, we can handle several types of inconvenient signals with the same mechanism.

We increment and decrement defer_signal so that nested critical sections will work properly; thus, if update_mumble were called with signal_pending already nonzero, signals would be deferred not only within update_mumble, but also within the caller.  This is also why we do not check signal_pending if defer_signal is still nonzero.

The incrementing and decrementing of defer_signal require more than one instruction; it is possible for a signal to happen in the middle.  But that does not cause any problem.  If the signal happens early enough to see the value from before the increment or decrement, that is equivalent to a signal which came before the beginning of the increment or decrement, which is a case that works properly.

It is absolutely vital to decrement defer_signal before testing signal_pending, because this avoids a subtle bug.  If we did these things in the other order, like this,

  if (defer_signal == 1 && signal_pending != 0)
    raise (signal_pending);
  defer_signal--;

then a signal arriving in between the if statement and the decrement would be effetively "lost" for an indefinite amount of time.  The handler would merely set defer_signal, but the program having already tested this variable, it would not test the variable again.

Bugs like these are called "timing errors".  They are especially bad because they happen only rarely and are nearly impossible to reproduce.  You can't expect to find them with a debugger as you would find a reproducible bug.  So it is worth being especially careful to avoid them.

(You would not be tempted to write the code in this order, given the use of defer_signal as a counter which must be tested along with signal_pending.  After all, testing for zero is cleaner than testing for one.  But if you did not use defer_signal as a counter, and gave it values of zero and one only, then either order might seem equally simple.  This is a further advantage of using a counter for defer_signal: it will reduce the chance you will write the code in the wrong order and create a subtle bug.)


Waiting for a Signal
Node: Waiting for a Signal, Next: BSD Signal Handling, Prev: Blocking Signals, Up: Signal Handling

Waiting for a Signal

If your program is driven by external events, or uses signals for synchronization, then when it has nothing to do it should probably wait until a signal arrives.

* Menu:

Using Pause	The simple way, using pause.

Pause Problems	Why the simple way is often not very good.

Sigsuspend	Reliably waiting for a specific signal.


Using pause
Node: Using Pause, Next: Pause Problems, Prev: , Up: Waiting for a Signal

Using pause

The simple way to wait until a signal arrives is to call pause.  Please read about its disadvantages, in the following section, before you use it.

 - Function: int pause ()

The pause function suspends program execution until a signal arrives whose action is either to execute a handler function, or to terminate the process.

If the signal causes a handler function to be executed, then pause returns.  This is considered an unsuccessful return (since "successful" behavior would be to suspend the program forever), so the return value is -1.  Even if you specify that other primitives should resume when a system handler returns (see Interrupted Primitives), this has no effect on pause; it always fails when a signal is handled.

The following errno error conditions are defined for this function:

EINTR

The function was interrupted by delivery of a signal.

If the signal causes program termination, pause doesn't return (obviously).

The pause function is declared in  'unistd.h'.


Problems with pause
Node: Pause Problems, Next: Sigsuspend, Prev: Using Pause, Up: Waiting for a Signal

Problems with pause

The simplicity of pause can conceal serious timing errors that can make a program hang mysteriously.

It is safe to use pause if the real work of your program is done by the signal handlers themselves, and the "main program" does nothing but call pause.  Each time a signal is delivered, the handler will do the next batch of work that is to be done, and then return, so that the main loop of the program can call pause again.

You can't safely use pause to wait until one more signal arrives, and then resume real work.  Even if you arrange for the signal handler to cooperate by setting a flag, you still can't use pause reliably.  Here is an example of this problem:

/* usr_interrupt is set by the signal handler.  */
if (!usr_interrupt)
  pause ();

/* Do work once the signal arrives.  */
...

This has a bug: the signal could arrive after the variable usr_interrupt is checked, but before the call to pause.  If no further signals arrive, the process would never wake up again.

You can put an upper limit on the excess waiting by using sleep in a loop, instead of using pause.  (See Sleeping, for more about sleep.)  Here is what this looks like:

/* usr_interrupt is set by the signal handler.
while (!usr_interrupt)
  sleep (1);

/* Do work once the signal arrives.  */
...

For some purposes, that is good enough.  But with a little more complexity, you can wait reliably until a particular signal handler is run, using sigsuspend.  See Sigsuspend.


Using sigsuspend
Node: Sigsuspend, Next: , Prev: Pause Problems, Up: Waiting for a Signal

Using sigsuspend

The clean and reliable way to wait for a signal to arrive is to block it and then use sigsuspend.  By using sigsuspend in a loop, you can wait for certain kinds of signals, while letting other kinds of signals be handled by their handlers.

 - Function: int sigsuspend (const sigset_t *set)

This function replaces the process's signal mask with set and then suspends the process until a signal is delivered whose action is either to terminate the process or invoke a signal handling function.  In other words, the program is effectively suspended until one of the signals that is not a member of set arrives.

If the process is woken up by deliver of a signal that invokes a handler function, and the handler function returns, then sigsuspend also returns.

The mask remains set only as long as sigsuspend is waiting.  The function sigsuspend always restores the previous signal mask when it returns.

The return value and error conditions are the same as for pause.

With sigsuspend, you can replace the pause or sleep loop in the previous section with something completely reliable:

sigset_t mask, oldmask;

...

/* Set up the mask of signals to temporarily block. */
sigemptyset (&mask);
sigaddset (&mask, SIGUSR1);

...

/* Wait for a signal to arrive. */
sigprocmask (SIG_BLOCK, &mask, &oldmask);
while (!usr_interrupt)
  sigsuspend (&oldmask);
sigprocmask (SIG_UNBLOCK, &mask, NULL);

This last piece of code is a little tricky.  The key point to remember here is that when sigsuspend returns, it resets the process's signal mask to the original value, the value from before the call to sigsuspend--in this case, the SIGUSR1 signal is once again blocked.  The second call to sigprocmask is necessary to explicitly unblock this signal.

One other point: you may be wondering why the while loop is necessary at all, since the program is apparently only waiting for one SIGUSR1 signal.  The answer is that the mask passed to sigsuspend permits the process to be woken up by the delivery of other kinds of signals, as well--for example, job control signals.  If the process is woken up by a signal that doesn't set usr_interrupt, it just suspends itself again until the "right" kind of signal eventually arrives.

This technique takes a few more lines of preparation, but that is needed just once for each kind of wait criterion you want to use.  The code that actually waits is just four lines.


BSD Signal Handling
Node: BSD Signal Handling, Next: BSD Handler, Prev: Waiting for a Signal, Up: Signal Handling

BSD Signal Handling

This section describes alternative signal handling functions derived from BSD Unix.  These facilities were an advance, in their time; today, they are mostly obsolete, and supported mainly for compatibility with BSD Unix.

They do provide one feature that is not available through the POSIX functions: You can specify a separate stack for use in certain signal handlers.  Using a signal stack is the only way you can handle a signal caused by stack overflow.

* Menu:

POSIX vs BSD	Overview comparing BSD and POSIX signal functions.


POSIX and BSD Signal Facilities
Node: POSIX vs BSD, Next: , Prev: , Up: BSD Signal Handling

POSIX and BSD Signal Facilities

There are many similarities between the BSD and POSIX signal handling facilities, because the POSIX facilities were inspired by the BSD facilities.  Besides having different names for all the functions to avoid conflicts, the main differences between the two are:

•	BSD Unix represents signal masks as an int bit mask, rather than as a sigset_t object.

•	The BSD facilities use a different default for whether an interrupted primitive should fail or resume.  The POSIX facilities make system calls fail unless you specify that they should resume.  With the BSD facility, the default is to make system calls resume unless you say they should fail.  See Interrupted Primitives.

•	BSD Unix has a concept of a "signal stack".  This is an alternate stack that is used during the execution of signal handler functions, instead of its normal execution stack.  KThe BSD facilities are declared in 'signal.h'.  K


BSD Function to Establish a Handler
Node: BSD Handler, Next: , Prev: BSD Signal Handling, Up: Signal Handling

BSD Function to Establish a Handler

 - Data Type: struct sigvec

This data type is the BSD equivalent of struct sigaction (see Advanced Signal Handling); it is used to specify signal actions to the sigvec function.  It contains the following members:

sighandler_t sv_handler

This is the handler function.

int sv_mask

This is the mask of additional signals to be blocked while the handler function is being called.

int sv_flags

This is a bit mask used to specify various flags which affect the behavior of the signal.  You can also refer to this field as sv_onstack.

These symbolic constants can be used to provide values for the sv_flags field of a sigvec structure.  This field is a bit mask value, so you bitwise-OR the flags of interest to you together.

 - Macro: int SV_ONSTACK

If this bit is set in the sv_flags field of a sigvec structure, it means to use the signal stack when delivering the signal.

 - Macro: int SV_INTERRUPT

If this bit is set in the sv_flags field of a sigvec structure, it means that system calls interrupted by this kind of signal should not be restarted if the handler returns; instead, the system calls should return with a EINTR error status.  See Interrupted Primitives.

 - Macro: int SV_RESETHAND

If this bit is set in the sv_flags field of a sigvec structure, it means to reset the action for the signal back to SIG_DFL when the signal is received.

 - Function: int sigvec (int signum, const struct sigvec *action,struct sigvec *old_action)

This function is the equivalent of sigaction (see Advanced Signal Handling); it installs the action action for the signal signum, returning information about the previous action in effect for that signal in old_action.

 - Function: int siginterrupt (int signum, int failflag)

This function specifies which approach to use when certain primitives are interrupted by handling signal signum.  If failflag is false, signal signum restarts primitives.  If failflag is true, handling signum causes these primitives to fail with error code EINTR.  See Interrupted Primitives.

* Menu:

Blocking in BSD	BSD Functions for Blocking Signals.

Signal Stack	Using a Separate Signal Stack.


BSD Functions for Blocking Signals
Node: Blocking in BSD, Next: Signal Stack, Prev: , Up: BSD Handler

BSD Functions for Blocking Signals

 - Macro: int sigmask (int signum)

This macro returns a signal mask that has the bit for signal signum set.  You can bitwise-OR the results of several calls to sigmask together to specify more than one signal.  For example,

(sigmask (SIGTSTP) | sigmask (SIGSTOP)
 | sigmask (SIGTTIN) | sigmask (SIGTTOU))

specifies a mask that includes all the job-control stop signals.

 - Function: int sigblock (int mask)

This function is equivalent to sigprocmask (see Process Signal Mask) with a how argument of SIG_BLOCK: it adds the signals specified by mask to the calling process's set of blocked signals.  The return value is the previous set of blocked signals.

 - Function: int sigsetmask (int mask)

This function equivalent to sigprocmask (see Process Signal Mask) with a how argument of SIG_SETMASK: it sets the calling process's signal mask to mask.  The return value is the previous set of blocked signals.

 - Function: int sigpause (int mask)

This function is the equivalent of sigsuspend (see Waiting for a Signal):  it sets the calling process's signal mask to mask, and waits for a signal to arrive.  On return the previous set of blocked signals is restored.


Using a Separate Signal Stack
Node: Signal Stack, Next: , Prev: Blocking in BSD, Up: BSD Handler

Using a Separate Signal Stack

A signal stack is a special area of memory to be used as the execution stack during signal handlers.  It should be fairly large, to avoid any danger that it will overflow in turn; the macro SIGSTKSZ is defined to a canonical size for signal stacks.  You can use malloc to allocate the space for the stack.  Then call sigaltstack or sigstack to tell the system to use that space for the signal stack.

You don't need to write signal handlers differently in order to use a signal stack.  Switching from one stack to the other happens automatically.  However, some debuggers on some machines may get confused if you examine a stack trace while a handler that uses the signal stack is running.

There are two interfaces for telling the system to use a separate signal stack.  sigstack is the older interface, which comes from 4.2 BSD.  sigaltstack is the newer interface, and comes from 4.4 BSD.  The sigaltstack interface has the advantage that it does not require your program to know which direction the stack grows, which depends on the specific machine and operating system.

 - Data Type: struct sigaltstack

This structure describes a signal stack.  It contains the following members:

void *ss_sp

This points to the base of the signal stack.

size_t ss_size

This is the size (in bytes) of the signal stack which 'ss_sp' points to.  You should set this to however much space you allocated for the stack.

There are two macros defined in 'signal.h' that you should use in calculating this size:

SIGSTKSZ

This is the canonical size for a signal stack.  It is judged to be sufficient for normal uses.

MINSIGSTKSZ

This is the amount of signal stack space the operating system needs just to implement signal delivery.  The size of a signal stack must be greater than this.

For most cases, just using SIGSTKSZ for ss_size is sufficient.  But if you know how much stack space your program's signal handlers will need, you may want to use a different size.  In this case, you should allocate MINSIGSTKSZ additional bytes for the signal stack and increase ss_size accordinly.

int ss_flags

This field contains the bitwise OR of these flags:

SA_DISABLE

This tells the system that it should not use the signal stack.

SA_ONSTACK

This is set by the system, and indicates that the signal stack is currently in use.  If this bit is not set, then signals will be delivered on the normal user stack.

 - Function: int sigaltstack (const struct sigaltstack *stack, struct sigaltstack *oldstack)

The sigaltstack function specifies an alternate stack for use during signal handling.  When a signal is received by the process and its action indicates that the signal stack is used, the system arranges a switch to the currently installed signal stack while the handler for that signal is executed.

If oldstack is not a null pointer, information about the currently installed signal stack is returned in the location it points to.  If stack is not a null pointer, then this is installed as the new stack for use by signal handlers.

The return value is 0 on success and -1 on failure.  If sigaltstack fails, it sets errno to one of these values:

EINVAL

You tried to disable a stack that was in fact currently in use.

ENOMEM

The size of the alternate stack was too small.  It must be greater than MINSIGSTKSZ.

Here is the older sigstack interface.  You should use sigaltstack instead on systems that have it.

 - Data Type: struct sigstack

This structure describes a signal stack.  It contains the following members:

void *ss_sp

This is the stack pointer.  If the stack grows downwards on your machine, this should point to the top of the area you allocated.  If the stack grows upwards, it should point to the bottom.

int ss_onstack

This field is true if the process is currently using this stack.

 - Function: int sigstack (const struct sigstack *stack, struct sigstack *oldstack)

The sigstack function specifies an alternate stack for use during signal handling.  When a signal is received by the process and its action indicates that the signal stack is used, the system arranges a switch to the currently installed signal stack while the handler for that signal is executed.

If oldstack is not a null pointer, information about the currently installed signal stack is returned in the location it points to.  If stack is not a null pointer, then this is installed as the new stack for use by signal handlers.

The return value is 0 on success and -1 on failure.


Process Startup and Termination
Node: Process Startup, Next: Child Processes, Prev: Signal Handling, Up: Top

Process Startup and Termination

"Processes" are the primitive units for allocation of system resources.  Each process has its own address space and (usually) one thread of control.  A process executes a program; you can have multiple processes executing the same program, but each process has its own copy of the program within its own address space and executes it independently of the other copies.

This chapter explains what your program should do to handle the startup of a process, to terminate its process, and to receive information (arguments and the environment) from the parent process.

* Menu:

Program Arguments	Parsing your program's command-line arguments.

Environment Variables	How to access parameters inherited from a parent process.

Program Termination	How to cause a process to terminate and return status information to its parent.


Program Arguments
Node: Program Arguments, Next: Environment Variables, Prev: , Up: Process Startup

Program Arguments

The system starts a C program by calling the function main.  It is up to you to write a function named main--otherwise, you won't even be able to link your program without errors.

You can define main either to take no arguments, or to take two arguments that represent the command line arguments to the program, like this:

int main (int argc, char *argv[])

The command line arguments are the whitespace-separated tokens given in the shell command used to invoke the program; thus, in 'cat foo bar', the arguments are 'foo' and 'bar'.  The only way a program can look at its command line arguments is via the arguments of main.  If main doesn't take arguments, then you cannot get at the command line.

The value of the argc argument is the number of command line arguments.  The argv argument is a vector of C strings; its elements are the individual command line argument strings.  The file name of the program being run is also included in the vector as the first element; the value of argc counts this element.  A null pointer always follows the last element: argv[argc] is this null pointer.

For the command 'cat foo bar', argc is 3 and argv has three elements, "cat", "foo" and "bar".

If the syntax for the command line arguments to your program is simple enough, you can simply pick the arguments off from argv by hand.  But unless your program takes a fixed number of arguments, or all of the arguments are interpreted in the same way (as file names, for example), you are usually better off using getopt to do the parsing.

* Menu:

Argument Syntax	By convention, options start with a hyphen.

Parsing Options	The getopt function.

Example of Getopt	An example of parsing options with getopt.

Long Options	GNU suggests utilities accept long-named options. Here is how to do that.

Long Option Example	An example of using getopt_long.


Program Argument Syntax Conventions
Node: Argument Syntax, Next: Parsing Options, Prev: , Up: Program Arguments

Program Argument Syntax Conventions

POSIX recommends these conventions for command line arguments.  getopt (see Parsing Options) makes it easy to implement them.

•	Arguments are options if they begin with a hyphen delimiter ('-').

•	Multiple options may follow a hyphen delimiter in a single token if the options do not take arguments.  Thus, '-abc' is equivalent to '-a -b -c'.

•	Option names are single alphanumeric characters (as for isalnum; see See Classification of Characters).

•	Certain options require an argument.  For example, the '-o' command of the ld command requires an argument--an output file name.

•	An option and its argument may or may not appear as separate tokens.  (In other words, the whitespace separating them is optional.)  Thus, '-o foo' and '-ofoo' are equivalent.

•	Options typically precede other non-option arguments.

The implementation of getopt in the GNU C library normally makes it appear as if all the option arguments were specified before all the non-option arguments for the purposes of parsing, even if the user of your program intermixed option and non-option arguments.  It does this by reordering the elements of the argv array.  This behavior is nonstandard; if you want to suppress it, define the _POSIX_OPTION_ORDER environment variable.  See Standard Environment.

•	The argument '--' terminates all options; any following arguments are treated as non-option arguments, even if they begin with a hyphen.

•	A token consisting of a single hyphen character is interpreted as an ordinary non-option argument.  By convention, it is used to specify input from or output to the standard input and output streams.

•	Options may be supplied in any order, or appear multiple times.  The interpretation is left up to the particular application program.

GNU adds "long options" to these conventions.  Long options consist of '--' followed by a name made of alphanumeric characters and dashes.  Option names are typically one to three words long, with hyphens to separate words.  Users can abbreviate the option names as long as the abbreviations are unique.

To specify an argument for a long option, write '--name=value'.  This syntax enables a long option to accept an argument that is itself optional.

Eventually, the GNU system will provide completion for long option names in the shell.


Parsing Program Options
Node: Parsing Options, Next: Example of Getopt, Prev: Argument Syntax, Up: Program Arguments

Parsing Program Options

Here are the details about how to call the getopt function.  To use this facility, your program must include the header file 'unistd.h'.  K

 - Variable: int opterr

If the value of this variable is nonzero, then getopt prints an error message to the standard error stream if it encounters an unknown option character or an option with a missing required argument.  This is the default behavior.  If you set this variable to zero, getopt does not print any messages, but it still returns the character ?  to indicate an error.

 - Variable: int optopt

When getopt encounters an unknown option character or an option with a missing required argument, it stores that option character in this variable.  You can use this for providing your own diagnostic messages.

 - Variable: int optind

This variable is set by getopt to the index of the next element of the argv array to be processed.  Once getopt has found all of the option arguments, you can use this variable to determine where the remaining non-option arguments begin.  The initial value of this variable is 1.

 - Variable: char * optarg

This variable is set by getopt to point at the value of the option argument, for those options that accept arguments.

 - Function: int getopt (int argc, char **argv, const char *options)

The getopt function gets the next option argument from the argument list specified by the argv and argc arguments.  Normally these values come directly from the arguments received by main.

The options argument is a string that specifies the option characters that are valid for this program.  An option character in this string can be followed by a colon (':') to indicate that it takes a required argument.

If the options argument string begins with a hyphen ('-'), this is treated specially.  It permits arguments that are not options to be returned as if they were associated with option character '\0'.

The getopt function returns the option character for the next command line option.  When no more option arguments are available, it returns -1.  There may still be more non-option arguments; you must compare the external variable optind against the argc parameter to check this.

If the option has an argument, getopt returns the argument by storing it in the varables optarg.  You don't ordinarily need to copy the optarg string, since it is a pointer into the original argv array, not into a static area that might be overwritten.

If getopt finds an option character in argv that was not included in options, or a missing option argument, it returns '?' and sets the external variable optopt to the actual option character.  If the first character of options is a colon (':'), then getopt returns ':' instead of '?' to indicate a missing option argument.  In addition, if the external variable opterr is nonzero (which is the default), getopt prints an error message.


Example of Parsing Arguments with getopt
Node: Example of Getopt, Next: Long Options, Prev: Parsing Options, Up: Program Arguments

Example of Parsing Arguments with getopt

Here is an example showing how getopt is typically used.  The key points to notice are:

•	Normally, getopt is called in a loop.  When getopt returns -1, indicating no more options are present, the loop terminates.

•	A switch statement is used to dispatch on the return value from getopt.  In typical use, each case just sets a variable that is used later in the program.

•	A second loop is used to process the remaining non-option arguments.

#include <unistd.h>
#include <stdio.h>

int
main (int argc, char **argv)
{
  int aflag = 0;
  int bflag = 0;
  char *cvalue = NULL;
  int index;
  int c;

  opterr = 0;

while ((c = getopt (argc, argv, "abc:")) != -1)
    switch (c)
      {
      case 'a':
        aflag = 1;
        break;
      case 'b':
        bflag = 1;
        break;
      case 'c':
        cvalue = optarg;
        break;
      case '?':
        if (isprint (optopt))
          fprintf (stderr, "Unknown option `-%c'.\n", optopt);
        else
          fprintf (stderr,
                   "Unknown option character `\\x%x'.\n",
                   optopt);
        return 1;
      default:
        abort ();
      }

printf ("aflag = %d, bflag = %d, cvalue = %s\n", aflag, bflag, cvalue);

  for (index = optind; index < argc; index++)
    printf ("Non-option argument %s\n", argv[index]);
  return 0;
}

Here are some examples showing what this program prints with different combinations of arguments:

% testopt
aflag = 0, bflag = 0, cvalue = (null)

% testopt -a -b
aflag = 1, bflag = 1, cvalue = (null)

% testopt -ab
aflag = 1, bflag = 1, cvalue = (null)

% testopt -c foo
aflag = 0, bflag = 0, cvalue = foo

% testopt -cfoo
aflag = 0, bflag = 0, cvalue = foo

% testopt arg1
aflag = 0, bflag = 0, cvalue = (null)
Non-option argument arg1

% testopt -a arg1
aflag = 1, bflag = 0, cvalue = (null)
Non-option argument arg1

% testopt -c foo arg1
aflag = 0, bflag = 0, cvalue = foo
Non-option argument arg1

% testopt -a -- -b
aflag = 1, bflag = 0, cvalue = (null)
Non-option argument -b

% testopt -a -
aflag = 1, bflag = 0, cvalue = (null)
Non-option argument -


Parsing Long Options
Node: Long Options, Next: Long Option Example, Prev: Example of Getopt, Up: Program Arguments

Parsing Long Options

To accept GNU-style long options as well as single-character options, use getopt_long instead of getopt.  You should make every program accept long options if it uses any options, for this takes little extra work and helps beginners remember how to use the program.

 - Data Type: struct option

This structure describes a single long option name for the sake of getopt_long.  The argument longopts must be an array of these structures, one for each long option.  Terminate the array with an element containing all zeros.

The struct option structure has these fields:

const char *name

This field is the name of the option.  It is a string.

int has_arg

This field says whether the option takes an argument.  It is an integer, and there are three legitimate values: no_argument, required_argument and optional_argument.

int *flag

int val

These fields control how to report or act on the option when it occurs.

If flag is a null pointer, then the val is a value which identifies this option.  Often these values are chosen to uniquely identify particular long options.

If flag is not a null pointer, it should be the address of an int variable which is the flag for this option.  The value in val is the value to store in the flag to indicate that the option was seen.

 - Function: int getopt_long (int argc, char **argv, const char *shortopts, struct option *longopts, int *indexptr)

Decode options from the vector argv (whose length is argc).  The argument shortopts describes the short options to accept, just as it does in getopt.  The argument longopts describes the long options to accept (see above).

When getopt_long encounters a short option, it does the same thing that getopt would do: it returns the character code for the option, and stores the options argument (if it has one) in optarg.

When getopt_long encounters a long option, it takes actions based on the flag and val fields of the definition of that option.

If flag is a null pointer, then getopt_long returns the contents of val to indicate which option it found.  You should arrange distinct values in the val field for options with different meanings, so you can decode these values after getopt_long returns.  If the long option is equivalent to a short option, you can use the short option's character code in val.

If flag is not a null pointer, that means this option should just set a flag in the program.  The flag is a variable of type int that you define.  Put the address of the flag in the flag field.  Put in the val field the value you would like this option to store in the flag.  In this case, getopt_long returns 0.

For any long option, getopt_long tells you the index in the array longopts of the options definition, by storing it into *indexptr.  You can get the name of the option with longopts[*indexptr].name.  So you can distinguish among long options either by the values in their val fields or by their indices.  You can also distinguish in this way among long options that set flags.

When a long option has an argument, getopt_long puts the argument value in the variable optarg before returning.  When the option has no argument, the value in optarg is a null pointer.  This is how you can tell whether an optional argument was supplied.

When getopt_long has no more options to handle, it returns -1, and leaves in the variable optind the index in argv of the next remaining argument.


Example of Parsing Long Options
Node: Long Option Example, Next: , Prev: Long Options, Up: Program Arguments

Example of Parsing Long Options

#include <stdio.h>

/* Flag set by '--verbose'. */
static int verbose_flag;

int
main (argc, argv)
     int argc;
     char **argv;
{
  int c;

  while (1)
    {
      static struct option long_options[] =
        {
          /* These options set a flag. */
          {"verbose", 0, &verbose_flag, 1},
          {"brief", 0, &verbose_flag, 0},
          /* These options don't set a flag.
             We distinguish them by their indices. */
          {"add", 1, 0, 0},
          {"append", 0, 0, 0},
          {"delete", 1, 0, 0},
          {"create", 0, 0, 0},
          {"file", 1, 0, 0},
          {0, 0, 0, 0}
        };
      /* getopt_long stores the option index here. */
      int option_index = 0;

      c = getopt_long (argc, argv, "abc:d:",
                       long_options, &option_index);

      /* Detect the end of the options. */
      if (c == -1)
        break;

      switch (c)
        {
        case 0:
          /* If this option set a flag, do nothing else now. */
          if (long_options[option_index].flag != 0)
            break;
          printf ("option %s", long_options[option_index].name);
          if (optarg)
            printf (" with arg %s", optarg);
          printf ("\n");
          break;

        case 'a':
          puts ("option -a\n");
          break;

        case 'b':
          puts ("option -b\n");
          break;

        case 'c':
          printf ("option -c with value `%s'\n", optarg);
          break;

        case 'd':
          printf ("option -d with value `%s'\n", optarg);
          break;

        case '?':
          /* getopt_long already printed an error message. */
          break;

        default:
          abort ();
        }
    }

  /* Instead of reporting '--verbose'
     and '--brief' as they are encountered,
     we report the final status resulting from them. */
  if (verbose_flag)
    puts ("verbose flag is set");

  /* Print any remaining command line arguments (not options). */
  if (optind < argc)
    {
      printf ("non-option ARGV-elements: ");
      while (optind < argc)
        printf ("%s ", argv[optind++]);
      putchar ('\n');
    }

  exit (0);
}


Environment Variables
Node: Environment Variables, Next: Program Termination, Prev: Program Arguments, Up: Process Startup

Environment Variables

When a program is executed, it receives information about the context in which it was invoked in two ways.  The first mechanism uses the argv and argc arguments to its main function, and is discussed in See Program Arguments.  The second mechanism uses "environment variables" and is discussed in this section.

The argv mechanism is typically used to pass command-line arguments specific to the particular program being invoked.  The environment, on the other hand, keeps track of information that is shared by many programs, changes infrequently, and that is less frequently accessed.

The environment variables discussed in this section are the same environment variables that you set using assignments and the export command in the shell.  Programs executed from the shell inherit all of the environment variables from the shell.

Standard environment variables are used for information about the user's home directory, terminal type, current locale, and so on; you can define additional variables for other purposes.  The set of all environment variables that have values is collectively known as the "environment".

Names of environment variables are case-sensitive and must not contain the character '='.  System-defined environment variables are invariably uppercase.

The values of environment variables can be anything that can be represented as a string.  A value must not contain an embedded null character, since this is assumed to terminate the string.

* Menu:

Environment Access	How to get and set the values of environment variables.

Standard Environment	These environment variables have standard interpretations.


Environment Access
Node: Environment Access, Next: Standard Environment, Prev: , Up: Environment Variables

Environment Access

The value of an environment variable can be accessed with the getenv function.  This is declared in the header file 'stdlib.h'.  K

 - Function: char * getenv (const char *name)

This function returns a string that is the value of the environment variable name.  You must not modify this string.  In some non-Unix systems not using the GNU library, it might be overwritten by subsequent calls to getenv (but not by any other library function).  If the environment variable name is not defined, the value is a null pointer.

 - Function: int putenv (const char *string)

The putenv function adds or removes definitions from the environment.  If the string is of the form 'name=value', the definition is added to the environment.  Otherwise, the string is interpreted as the name of an environment variable, and any definition for this variable in the environment is removed.

The GNU library provides this function for compatibility with SVID; it may not be available in other systems.

You can deal directly with the underlying representation of environment objects to add more variables to the environment (for example, to communicate with another program you are about to execute; see See Executing a File).

 - Variable: char ** environ

The environment is represented as an array of strings.  Each string is of the format 'name=value'.  The order in which strings appear in the environment is not significant, but the same name must not appear more than once.  The last element of the array is a null pointer.

This variable is declared in the header file 'unistd.h'.

If you just want to get the value of an environment variable, use getenv.


Standard Environment Variables
Node: Standard Environment, Next: , Prev: Environment Access, Up: Environment Variables

Standard Environment Variables

These environment variables have standard meanings.  This doesn't mean that they are always present in the environment; but if these variables are present, they have these meanings, and that you shouldn't try to use these environment variable names for some other purpose.

HOME

This is a string representing the user's "home directory", or initial default working directory.

The user can set HOME to any value.  If you need to make sure to obtain the proper home directory for a particular user, you should not use HOME; instead, look up the user's name in the user database (see User Database).

For most purposes, it is better to use HOME, precisely because this lets the user specify the value.

LOGNAME

This is the name that the user used to log in.  Since the value in the environment can be tweaked arbitrarily, this is not a reliable way to identify the user who is running a process; a function like getlogin (see Who Logged In) is better for that purpose.

For most purposes, it is better to use LOGNAME, precisely because this lets the user specify the value.

PATH

A "path" is a sequence of directory names which is used for searching for a file.  The variable PATH holds a path used for searching for programs to be run.

The execlp and execvp functions (see Executing a File) use this environment variable, as do many shells and other utilities which are implemented in terms of those functions.

The syntax of a path is a sequence of directory names separated by colons.  An empty string instead of a directory name stands for the current directory (see Working Directory).

A typical value for this environment variable might be a string like:

:/bin:/etc:/usr/bin:/usr/new/X11:/usr/new:/usr/local/bin

This means that if the user tries to execute a program named foo, the system will look for files named 'foo', '/bin/foo', '/etc/foo', and so on.  The first of these files that exists is the one that is executed.

TERM

This specifies the kind of terminal that is receiving program output.  Some programs can make use of this information to take advantage of special escape sequences or terminal modes supported by particular kinds of terminals.  Many programs which use the termcap library (see Find) use the TERM environment variable, for example.

TZ

This specifies the time zone.  See TZ Variable, for information about the format of this string and how it is used.

LANG

This specifies the default locale to use for attribute categories where neither LC_ALL nor the specific environment variable for that category is set.  See Locales, for more information about locales.

LC_COLLATE

This specifies what locale to use for string sorting.

LC_CTYPE

This specifies what locale to use for character sets and character classification.

LC_MONETARY

This specifies what locale to use for formatting monetary values.

LC_NUMERIC

This specifies what locale to use for formatting numbers.

LC_TIME

This specifies what locale to use for formatting date/time values.

_POSIX_OPTION_ORDER

 If this environment variable is defined, it suppresses the usual reordering of command line arguments by getopt.  See Argument Syntax.


Program Termination
Node: Program Termination, Next: , Prev: Environment Variables, Up: Process Startup

Program Termination

The usual way for a program to terminate is simply for its main function to return.  The "exit status value" returned from the main function is used to report information back to the process's parent process or shell.

A program can also terminate normally by calling the exit function.

In addition, programs can be terminated by signals; this is discussed in more detail in See Signal Handling.  The abort function causes a signal that kills the program.

* Menu:

Normal Termination	If a program calls exit, a process terminates normally.

Exit Status	The exit status provides information about why the process terminated.

Cleanups on Exit	A process can run its own cleanup functions upon normal termination.

Aborting a Program	The abort function causes abnormal program termination.

Termination Internals	What happens when a process terminates.


Normal Termination
Node: Normal Termination, Next: Exit Status, Prev: , Up: Program Termination

Normal Termination

A process terminates normally when the program calls exit.  Returning from main is equivalent to calling exit, and the value that main returns is used as the argument to exit.

 - Function: void exit (int status)

The exit function terminates the process with status status.  This function does not return.

Normal termination causes the following actions:

1. 	Functions that were registered with the atexit or on_exit functions are called in the reverse order of their registration.  This mechanism allows your application to specify its own "cleanup" actions to be performed at program termination.  Typically, this is used to do things like saving program state information in a file, or unlocking locks in shared data bases.

2. 	All open streams are closed, writing out any buffered output data.  See See Closing Streams.  In addition, temporary files opened with the tmpfile function are removed; see See Temporary Files.

3. 	_exit is called, terminating the program.  See Termination Internals.


Exit Status
Node: Exit Status, Next: Cleanups on Exit, Prev: Normal Termination, Up: Program Termination

Exit Status

When a program exits, it can return to the parent process a small amount of information about the cause of termination, using the "exit status".  This is a value between 0 and 255 that the exiting process passes as an argument to exit.

Normally you should use the exit status to report very broad information about success or failure.  You can't provide a lot of detail about the reasons for the failure, and most parent processes would not want much detail anyway.

There are conventions for what sorts of status values certain programs should return.  The most common convention is simply 0 for success and 1 for failure.  Programs that perform comparison use a different convention: they use status 1 to indicate a mismatch, and status 2 to indicate an inability to compare.  Your program should follow an existing convention if an existing convention makes sense for it.

A general convention reserves status values 128 and up for special purposes.  In particular, the value 128 is used to indicate failure to execute another program in a subprocess.  This convention is not universally obeyed, but it is a good idea to follow it in your programs.

Warning: Don't try to use the number of errors as the exit status.  This is actually not very useful; a parent process would generally not care how many errors occurred.  Worse than that, it does not work, because the status value is truncated to eight bits.  Thus, if the program tried to report 256 errors, the parent would receive a report of 0 errors--that is, success.

For the same reason, it does not work to use the value of errno as the exit status--these can exceed 255.

Portability note: Some non-POSIX systems use different conventions for exit status values.  For greater portability, you can use the macros EXIT_SUCCESS and EXIT_FAILURE for the conventional status value for success and failure, respectively.  They are declared in the file 'stdlib.h'.  K

 - Macro: int EXIT_SUCCESS

This macro can be used with the exit function to indicate successful program completion.

On POSIX systems, the value of this macro is 0.  On other systems, the value might be some other (possibly non-constant) integer expression.

 - Macro: int EXIT_FAILURE

This macro can be used with the exit function to indicate unsuccessful program completion in a general sense.

On POSIX systems, the value of this macro is 1.  On other systems, the value might be some other (possibly non-constant) integer expression.  Other nonzero status values also indicate future.  Certain programs use different nonzero status values to indicate particular kinds of "non-success".  For example, diff uses status value 1 to mean that the files are different, and 2 or more to mean that there was difficulty in opening the files.


Cleanups on Exit
Node: Cleanups on Exit, Next: Aborting a Program, Prev: Exit Status, Up: Program Termination

Cleanups on Exit

Your program can arrange to run its own cleanup functions if normal termination happens.  If you are writing a library for use in various application programs, then it is unreliable to insist that all applications call the library's cleanup functions explicitly before exiting.  It is much more robust to make the cleanup invisible to the application, by setting up a cleanup function in the library itself using atexit or on_exit.

 - Function: int atexit (void (*function) (void))

The atexit function registers the function function to be called at normal program termination.  The function is called with no arguments.

The return value from atexit is zero on success and nonzero if the function cannot be registered.

 - Function: int on_exit (void (*function)(int status, void *arg), void *arg)

This function is a somewhat more powerful variant of atexit.  It accepts two arguments, a function function and an arbitrary pointer arg.  At normal program termination, the function is called with two arguments:  the status value passed to exit, and the arg.

This function is included in the GNU C library only for compatibility for SunOS, and may not be supported by other implementations.

Here's a trivial program that illustrates the use of exit and atexit:

#include <stdio.h>
#include <stdlib.h>

void
bye (void)
{
  puts ("Goodbye, cruel world....");
}

int
main (void)
{
  atexit (bye);
  exit (EXIT_SUCCESS);
}

When this program is executed, it just prints the message and exits.


Aborting a Program
Node: Aborting a Program, Next: Termination Internals, Prev: Cleanups on Exit, Up: Program Termination

Aborting a Program

You can abort your program using the abort function.  The prototype for this function is in 'stdlib.h'.  K

 - Function: void abort (void)

The abort function causes abnormal program termination.  This does not execute cleanup functions registered with atexit or on_exit.

This function actually terminates the process by raising a SIGABRT signal, and your program can include a handler to intercept this signal; see See Signal Handling.

Future Change Warning: Proposed Federal censorship regulations may prohibit us from giving you information about the possibility of calling this function.  We would be required to say that this is not an acceptable way of terminating a program.


Termination Internals
Node: Termination Internals, Next: , Prev: Aborting a Program, Up: Program Termination

Termination Internals

The _exit function is the primitive used for process termination by exit.  It is declared in the header file 'unistd.h'.  K

 - Function: void _exit (int status)

The _exit function is the primitive for causing a process to terminate with status status.  Calling this function does not execute cleanup functions registered with atexit or on_exit.

When a process terminates for any reason--either by an explicit termination call, or termination as a result of a signal--the following things happen:

•	All open file descriptors in the process are closed.  See Low-Level I/O.

•	The low-order 8 bits of the return status code are saved to be reported back to the parent process via wait or waitpid; see See Process Completion.

•	Any child processes of the process being terminated are assigned a new parent process.  (This is the init process, with process ID 1.)

•	A SIGCHLD signal is sent to the parent process.

•	If the process is a session leader that has a controlling terminal, then a SIGHUP signal is sent to each process in the foreground job, and the controlling terminal is disassociated from that session.  See Job Control.

•	If termination of a process causes a process group to become orphaned, and any member of that process group is stopped, then a SIGHUP signal and a SIGCONT signal are sent to each process in the group.  See Job Control.


Child Processes
Node: Child Processes, Next: Job Control, Prev: Process Startup, Up: Top

Child Processes

"Processes" are the primitive units for allocation of system resources.  Each process has its own address space and (usually) one thread of control.  A process executes a program; you can have multiple processes executing the same program, but each process has its own copy of the program within its own address space and executes it independently of the other copies.

Processes are organized hierarchically.  Each process has a "parent process" which explicitly arranged to create it.  The processes created by a given parent are called its "child processes".  A child inherits many of its attributes from the parent process.

This chapter describes how a program can create, terminate, and control child processes.  Actually, there are three distinct operations involved: creating a new child process, causing the new process to execute a program, and coordinating the completion of the child process with the original program.

The system function provides a simple, portable mechanism for running another program; it does all three steps automatically.  If you need more control over the details of how this is done, you can use the primitive functions to do each step individually instead.

* Menu:

Running a Command	The easy way to run another program.

Process Creation Concepts	An overview of the hard way to do it.

Process Identification	How to get the process ID of a process.

Creating a Process	How to fork a child process.

Executing a File	How to make a process execute another program.

Process Completion	How to tell when a child process has completed.

Process Completion Status	How to interpret the status value returned from a child process.

BSD Wait Functions	More functions, for backward compatibility.

Process Creation Example	A complete example program.


Running a Command
Node: Running a Command, Next: Process Creation Concepts, Prev: , Up: Child Processes

Running a Command

The easy way to run another program is to use the system function.  This function does all the work of running a subprogram, but it doesn't give you much control over the details: you have to wait until the subprogram terminates before you can do anything else.

 - Function: int system (const char *command)

This function executes command as a shell command.  In the GNU C library, it always uses the default shell sh to run the command.  In particular, it searches the directories in PATH to find programs to execute.  The return value is -1 if it wasn't possible to create the shell process, and otherwise is the status of the shell process.  See Process Completion, for details on how this status code can be interpreted.

The system function is declared in the header file 'stdlib.h'.

Portability Note: Some C implementations may not have any notion of a command processor that can execute other programs.  You can determine whether a command processor exists by executing system (NULL); if the return value is nonzero, a command processor is available.

The popen and pclose functions (see Pipe to a Subprocess) are closely related to the system function.  They allow the parent process to communicate with the standard input and output channels of the command being executed.


Process Creation Concepts
Node: Process Creation Concepts, Next: Process Identification, Prev: Running a Command, Up: Child Processes

Process Creation Concepts

This section gives an overview of processes and of the steps involved in creating a process and making it run another program.

Each process is named by a "process ID" number.  A unique process ID is allocated to each process when it is created.  The "lifetime" of a process ends when its termination is reported to its parent process; at that time, all of the process resources, including its process ID, are freed.

Processes are created with the fork system call (so the operation of creating a new process is sometimes called "forking" a process).  The "child process" created by fork is an exact clone of the original "parent process", except that it has its own process ID.

After forking a child process, both the parent and child processes continue to execute normally.  If you want your program to wait for a child process to finish executing before continuing, you must do this explicitly after the fork operation, by calling wait or waitpid (see Process Completion).  These functions give you limited information about why the child terminated--for example, its exit status code.

A newly forked child process continues to execute the same program as its parent process, at the point where the fork call returns.  You can use the return value from fork to tell whether the program is running in the parent process or the child.

Having several processes run the same program is only occasionally useful.  But the child can execute another program using one of the exec functions; see See Executing a File.  The program that the process is executing is called its "process image".  Starting execution of a new program causes the process to forget all about its previous process image; when the new program exits, the process exits too, instead of returning to the previous process image.


Process Identification
Node: Process Identification, Next: Creating a Process, Prev: Process Creation Concepts, Up: Child Processes

Process Identification

The pid_t data type represents process IDs.  You can get the process ID of a process by calling getpid.  The function getppid returns the process ID of the parent of the current process (this is also known as the "parent process ID").  Your program should include the header files 'unistd.h' and 'sys/types.h' to use these functions.  K

 - Data Type: pid_t

The pid_t data type is a signed integer type which is capable of representing a process ID.  In the GNU library, this is an int.

 - Function: pid_t getpid (void)

The getpid function returns the process ID of the current process.

 - Function: pid_t getppid (void)

The getppid function returns the process ID of the parent of the current process.


Creating a Process
Node: Creating a Process, Next: Executing a File, Prev: Process Identification, Up: Child Processes

Creating a Process

The fork function is the primitive for creating a process.  It is declared in the header file 'unistd.h'.  K

 - Function: pid_t fork (void)

The fork function creates a new process.

If the operation is successful, there are then both parent and child processes and both see fork return, but with different values: it returns a value of 0 in the child process and returns the child's process ID in the parent process.

If process creation failed, fork returns a value of -1 in the parent process.  The following errno error conditions are defined for fork:

EAGAIN

There aren't enough system resources to create another process, or the user already has too many processes running.

ENOMEM

The process requires more space than the system can supply.

The specific attributes of the child process that differ from the parent process are:

•	The child process has its own unique process ID.

•	The parent process ID of the child process is the process ID of its parent process.

•	The child process gets its own copies of the parent process's open file descriptors.  Subsequently changing attributes of the file descriptors in the parent process won't affect the file descriptors in the child, and vice versa.  See Control Operations.

•	The elapsed processor times for the child process are set to zero; see See Processor Time.

•	The child doesn't inherit file locks set by the parent process.  See Control Operations.

•	The child doesn't inherit alarms set by the parent process.  See Setting an Alarm.

•	The set of pending signals (see Delivery of Signal) for the child process is cleared.  (The child process inherits its mask of blocked signals and signal actions from the parent process.)

 - Function: pid_t vfork (void)

The vfork function is similar to fork but more efficient; however, there are restrictions you must follow to use it safely.

While fork makes a complete copy of the calling process's address space and allows both the parent and child to execute independently, vfork does not make this copy.  Instead, the child process created with vfork shares its parent's address space until it calls one of the exec functions.  In the meantime, the parent process suspends execution.

You must be very careful not to allow the child process created with vfork to modify any global data or even local variables shared with the parent.  Furthermore, the child process cannot return from (or do a long jump out of) the function that called vfork!  This would leave the parent process's control information very confused.  If in doubt, use fork instead.

Some operating systems don't really implement vfork.  The GNU C library permits you to use vfork on all systems, but actually executes fork if vfork isn't available.  If you follow the proper precautions for using vfork, your program will still work even if the system uses fork instead.


Executing a File
Node: Executing a File, Next: Process Completion, Prev: Creating a Process, Up: Child Processes

Executing a File

This section describes the exec family of functions, for executing a file as a process image.  You can use these functions to make a child process execute a new program after it has been forked.

The functions in this family differ in how you specify the arguments, but otherwise they all do the same thing.  They are declared in the header file 'unistd.h'.

 - Function: int execv (const char *filename, char *const argv[])

The execv function executes the file named by filename as a new process image.

The argv argument is an array of null-terminated strings that is used to provide a value for the argv argument to the main function of the program to be executed.  The last element of this array must be a null pointer.  By convention, the first element of this array is the file name of the program sans directory names.  See Program Arguments, for full details on how programs can access these arguments.

The environment for the new process image is taken from the environ variable of the current process image; see See Environment Variables, for information about environments.

 - Function: int execl (const char *filename, const char *arg0, ...)

This is similar to execv, but the argv strings are specified individually instead of as an array.  A null pointer must be passed as the last such argument.

 - Function: int execve (const char *filename, char *const argv[], char *const env[])

This is similar to execv, but permits you to specify the environment for the new program explicitly as the env argument.  This should be an array of strings in the same format as for the environ variable; see See Environment Access.

 - Function: int execle (const char *filename, const char *arg0, char *const env[], ...)

This is similar to execl, but permits you to specify the environment for the new program explicitly.  The environment argument is passed following the null pointer that marks the last argv argument, and should be an array of strings in the same format as for the environ variable.

 - Function: int execvp (const char *filename, char *const argv[])

The execvp function is similar to execv, except that it searches the directories listed in the PATH environment variable (see Standard Environment) to find the full file name of a file from filename if filename does not contain a slash.

This function is useful for executing system utility programs, because it looks for them in the places that the user has chosen.  Shells use it to run the commands that users type.

 - Function: int execlp (const char *filename, const char *arg0, ...)

This function is like execl, except that it performs the same file name searching as the execvp function.

The size of the argument list and environment list taken together must not be greater than ARG_MAX bytes.  See General Limits.  In the GNU system, the size (which compares against ARG_MAX) includes, for each string, the number of characters in the string, plus the size of a char *, plus one, rounded up to a multiple of the size of a char *.  Other systems may have somewhat different rules for counting.

These functions normally don't return, since execution of a new program causes the currently executing program to go away completely.  A value of -1 is returned in the event of a failure.  In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for these functions:

E2BIG

The combined size of the new program's argument list and environment list is larger than ARG_MAX bytes.  The GNU system has no specific limit on the argument list size, so this error code cannot result, but you may get ENOMEM instead if the arguments are too big for available memory.

ENOEXEC

The specified file can't be executed because it isn't in the right format.

ENOMEM

Executing the specified file requires more storage than is available.

If execution of the new file succeeds, it updates the access time field of the file as if the file had been read.  See File Times, for more details about access times of files.

The point at which the file is closed again is not specified, but is at some point before the process exits or before another process image is executed.

Executing a new process image completely changes the contents of memory, copying only the argument and environment strings to new locations.  But many other attributes of the process are unchanged:

•	The process ID and the parent process ID.  See Process Creation Concepts.

•	Session and process group membership.  See Concepts of Job Control.

•	Real user ID and group ID, and supplementary group IDs.  See Process Persona.

•	Pending alarms.  See Setting an Alarm.

•	Current working directory and root directory.  See Working Directory.

•	File mode creation mask.  See Setting Permissions.

•	Process signal mask; see See Process Signal Mask.

•	Pending signals; see See Blocking Signals.

•	Elapsed processor time associated with the process; see See Processor Time.

If the set-user-ID and set-group-ID mode bits of the process image file are set, this affects the effective user ID and effective group ID (respectively) of the process.  These concepts are discussed in detail in See Process Persona.

Signals that are set to be ignored in the existing process image are also set to be ignored in the new process image.  All other signals are set to the default action in the new process image.  For more information about signals, see See Signal Handling.

File descriptors open in the existing process image remain open in the new process image, unless they have the FD_CLOEXEC (close-on-exec) flag set.  The files that remain open inherit all attributes of the open file description from the existing process image, including file locks.  File descriptors are discussed in See Low-Level I/O.

Streams, by contrast, cannot survive through exec functions, because they are located in the memory of the process itself.  The new process image has no streams except those it creates afresh.  Each of the streams in the pre-exec process image has a descriptor inside it, and these descriptors do survive through exec (provided that they do not have FD_CLOEXEC set.  The new process image can reconnect these to new streams using fdopen (see Descriptors and Streams).


Process Completion
Node: Process Completion, Next: Process Completion Status, Prev: Executing a File, Up: Child Processes

Process Completion

The functions described in this section are used to wait for a child process to terminate or stop, and determine its status.  These functions are declared in the header file 'sys/wait.h'.  K

 - Function: pid_t waitpid (pid_t pid, int *status_ptr, int options)

The waitpid function is used to request status information from a child process whose process ID is pid.  Normally, the calling process is suspended until the child process makes status information available by terminating.

Other values for the pid argument have special interpretations.  A value of -1 or WAIT_ANY requests status information for any child process; a value of 0 or WAIT_MYPGRP requests information for any child process in the same process group as the calling process; and any other negative value - pgid requests information for any child process whose process group ID is pgid.

If status information for a child process is available immediately, this function returns immediately without waiting.  If more than one eligible child process has status information available, one of them is chosen randomly, and its status is returned immediately.  To get the status from the other eligible child processes, you need to call waitpid again.

The options argument is a bit mask.  Its value should be the bitwise OR (that is, the '|' operator) of zero or more of the WNOHANG and WUNTRACED flags.  You can use the WNOHANG flag to indicate that the parent process shouldn't wait; and the WUNTRACED flag to request status information from stopped processes as well as processes that have terminated.

The status information from the child process is stored in the object that status_ptr points to, unless status_ptr is a null pointer.

The return value is normally the process ID of the child process whose status is reported.  If the WNOHANG option was specified and no child process is waiting to be noticed, the value is zero.  A value of -1 is returned in case of error.  The following errno error conditions are defined for this function:

EINTR

The function was interrupted by delivery of a signal to the calling process.  See Interrupted Primitives.

ECHILD

There are no child processes to wait for, or the specified pid is not a child of the calling process.

EINVAL

An invalid value was provided for the options argument.

These symbolic constants are defined as values for the pid argument to the waitpid function.

WAIT_ANY

This constant macro (whose value is -1) specifies that waitpid should return status information about any child process.

WAIT_MYPGRP

This constant (with value 0) specifies that waitpid should return status information about any child process in the same process group as the calling process.

These symbolic constants are defined as flags for the options argument to the waitpid function.  You can bitwise-OR the flags together to obtain a value to use as the argument.

WNOHANG

This flag specifies that waitpid should return immediately instead of waiting, if there is no child process ready to be noticed.

WUNTRACED

This flag specifies that waitpid should report the status of any child processes that have been stopped as well as those that have terminated.

 - Function: pid_t wait (int *status_ptr)

This is a simplified version of waitpid, and is used to wait until any one child process terminates.  The call:

wait (&status)

is exactly equivalent to:

waitpid (-1, &status, 0)

Here's an example of how to use waitpid to get the status from all child processes that have terminated, without ever waiting.  This function is designed to be a handler for SIGCHLD, the signal that indicates that at least one child process has terminated.

void
sigchld_handler (int signum)
{
  int pid;
  int status;
  while (1)
    {
      pid = waitpid (WAIT_ANY, &status, WNOHANG);
      if (pid < 0)
        {
          perror ("waitpid");
          break;
        }
      if (pid == 0)
        break;
      notice_termination (pid, status);
    }
}


Process Completion Status
Node: Process Completion Status, Next: BSD Wait Functions, Prev: Process Completion, Up: Child Processes

Process Completion Status

If the exit status value (see Program Termination) of the child process is zero, then the status value reported by waitpid or wait is also zero.  You can test for other kinds of information encoded in the returned status value using the following macros.  These macros are defined in the header file 'sys/wait.h'.  K

 - Macro: int WIFEXITED (int status)

This macro returns a nonzero value if the child process terminated normally with exit or _exit.

 - Macro: int WEXITSTATUS (int status)

If WIFEXITED is true of status, this macro returns the low-order 8 bits of the exit status value from the child process.  See Exit Status.

 - Macro: int WIFSIGNALED (int status)

This macro returns a nonzero value if the child process terminated because it received a signal that was not handled.  See Signal Handling.

 - Macro: int WTERMSIG (int status)

If WIFSIGNALED is true of status, this macro returns the signal number of the signal that terminated the child process.

 - Macro: int WCOREDUMP (int status)

This macro returns a nonzero value if the child process terminated and produced a core dump.

 - Macro: int WIFSTOPPED (int status)

This macro returns a nonzero value if the child process is stopped.

 - Macro: int WSTOPSIG (int status)

If WIFSTOPPED is true of status, this macro returns the signal number of the signal that caused the child process to stop.


BSD Process Wait Functions
Node: BSD Wait Functions, Next: Process Creation Example, Prev: Process Completion Status, Up: Child Processes

BSD Process Wait Functions

The GNU library also provides these related facilities for compatibility with BSD Unix.  BSD uses the union wait data type to represent status values rather than an int.  The two representations are actually interchangeable; they describe the same bit patterns.  The GNU C Library defines macros such as WEXITSTATUS so that they will work on either kind of object, and the wait function is defined to accept either type of pointer as its status_ptr argument.

These functions are declared in 'sys/wait.h'.  K

 - Data Type: union wait

This data type represents program termination status values.  It has the following members:

int w_termsig

The value of this member is the same as the result of the WTERMSIG macro.

int w_coredump

The value of this member is the same as the result of the WCOREDUMP macro.

int w_retcode

The value of this member is the same as the result of the WEXITSTATUS macro.

int w_stopsig

The value of this member is the same as the result of the WSTOPSIG macro.

Instead of accessing these members directly, you should use the equivalent macros.

 - Function: pid_t wait3 (union wait *status_ptr, int options, struct rusage *usage)

If usage is a null pointer, wait3 is equivalent to waitpid (-1, status_ptr, options).

If usage is not null, wait3 stores usage figures for the child process in *rusage (but only if the child has terminated, not if it has stopped).  See Resource Usage.

 - Function: pid_t wait4 (pid_t pid, union wait *status_ptr, int options, struct rusage *usage)

If usage is a null pointer, wait4 is equivalent to waitpid (pid, status_ptr, options).

If usage is not null, wait4 stores usage figures for the child process in *rusage (but only if the child has terminated, not if it has stopped).  See Resource Usage.


Process Creation Example
Node: Process Creation Example, Next: , Prev: BSD Wait Functions, Up: Child Processes

Process Creation Example

Here is an example program showing how you might write a function similar to the built-in system.  It executes its command argument using the equivalent of 'sh -c command'.

#include <stddef.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

/* Execute the command using this shell program.  */
#define SHELL "/bin/sh"

int
my_system (const char *command)
{
  int status;
  pid_t pid;

  pid = fork ();
  if (pid == 0)
    {
      /* This is the child process.  Execute the shell command. */
      execl (SHELL, SHELL, "-c", command, NULL);
      _exit (EXIT_FAILURE);
    }
  else if (pid < 0)
    /* The fork failed.  Report failure.  */
    status = -1;
  else
    /* This is the parent process.  Wait for the child to complete.  */
    if (waitpid (pid, &status, 0) != pid)
      status = -1;
  return status;
}

There are a couple of things you should pay attention to in this example.

Remember that the first argv argument supplied to the program represents the name of the program being executed.  That is why, in the call to execl, SHELL is supplied once to name the program to execute and a second time to supply a value for argv[0].

The execl call in the child process doesn't return if it is successful.  If it fails, you must do something to make the child process terminate.  Just returning a bad status code with return would leave two processes running the original program.  Instead, the right behavior is for the child process to report failure to its parent process.

Call _exit to accomplish this.  The reason for using _exit instead of exit is to avoid flushing fully buffered streams such as stdout.  The buffers of these streams probably contain data that was copied from the parent process by the fork, data that will be output eventually by the parent process.  Calling exit in the child would output the data twice.  See Termination Internals.


Job Control
Node: Job Control, Next: Users and Groups, Prev: Child Processes, Up: Top

Job Control

"Job control" refers to the protocol for allowing a user to move between multiple "process groups" (or "jobs") within a single "login session".  The job control facilities are set up so that appropriate behavior for most programs happens automatically and they need not do anything special about job control.  So you can probably ignore the material in this chapter unless you are writing a shell or login program.

You need to be familiar with concepts relating to process creation (see Process Creation Concepts) and signal handling (see Signal Handling) in order to understand this material presented in this chapter.

* Menu:

Concepts of Job Control	Jobs can be controlled by a shell.

Job Control is Optional	Not all POSIX systems support job control.

Controlling Terminal	How a process gets its controlling terminal.

Access to the Terminal	How processes share the controlling terminal.

Orphaned Process Groups	Jobs left after the user logs out.

Implementing a Shell	What a shell must do to implement job control.

Functions for Job Control	Functions to control process groups.


Concepts of Job Control
Node: Concepts of Job Control, Next: Job Control is Optional, Prev: , Up: Job Control

Concepts of Job Control

The fundamental purpose of an interactive shell is to read commands from the user's terminal and create processes to execute the programs specified by those commands.  It can do this using the fork (see Creating a Process) and exec (see Executing a File) functions.

A single command may run just one process--but often one command uses several processes.  If you use the '|' operator in a shell command, you explicitly request several programs in their own processes.  But even if you run just one program, it can use multiple processes internally.  For example, a single compilation command such as 'cc -c foo.c' typically uses four processes (though normally only two at any given time).  If you run make, its job is to run other programs in separate processes.

The processes belonging to a single command are called a "process group" or "job".  This is so that you can operate on all of them at once.  For example, typing ^C-c sends the signal SIGINT to terminate all the processes in the foreground process group.

A "session" is a larger group of processes.  Normally all the proccesses that stem from a single login belong to the same session.

Every process belongs to a process group.  When a process is created, it becomes a member of the same process group and session as its parent process.  You can put it in another process group using the setpgid function, provided the process group belongs to the same session.

The only way to put a process in a different session is to make it the initial process of a new session, or a "session leader", using the setsid function.  This also puts the session leader into a new process group, and you can't move it out of that process group again.

Usually, new sessions are created by the system login program, and the session leader is the process running the user's login shell.

A shell that supports job control must arrange to control which job can use the terminal at any time.  Otherwise there might be multiple jobs trying to read from the terminal at once, and confusion about which process should receive the input typed by the user.  To prevent this, the shell must cooperate with the terminal driver using the protocol described in this chapter.

The shell can give unlimited access to the controlling terminal to only one process group at a time.  This is called the "foreground job" on that controlling terminal.  Other process groups managed by the shell that are executing without such access to the terminal are called "background jobs".

If a background job needs to read from or write to its controlling terminal, it is "stopped" by the terminal driver.  The user can stop a foreground job by typing the SUSP character (see Special Characters) and a program can stop any job by sending it a SIGSTOP signal.  It's the responsibility of the shell to notice when jobs stop, to notify the user about them, and to provide mechanisms for allowing the user to interactively continue stopped jobs and switch jobs between foreground and background.

See Access to the Terminal, for more information about I/O to the controlling terminal,


Job Control is Optional
Node: Job Control is Optional, Next: Controlling Terminal, Prev: Concepts of Job Control, Up: Job Control

Job Control is Optional

Not all operating systems support job control.  The GNU system does support job control, but if you are using the GNU library on some other system, that system may not support job control itself.

You can use the _POSIX_JOB_CONTROL macro to test at compile-time whether the system supports job control.  See System Options.

If job control is not supported, then there can be only one process group per session, which behaves as if it were always in the foreground.  The functions for creating additional process groups simply fail with the error code ENOSYS.

The macros naming the various job control signals (see Job Control Signals) are defined even if job control is not supported.  However, the system never generates these signals, and attempts to send a job control signal or examine or specify their actions report errors or do nothing.


Controlling Terminal of a Process
Node: Controlling Terminal, Next: Access to the Terminal, Prev: Job Control is Optional, Up: Job Control

Controlling Terminal of a Process

One of the attributes of a process is its controlling terminal.  Child processes created with fork inherit the controlling terminal from their parent process.  In this way, all the processes in a session inherit the controlling terminal from the session leader.  A session leader that has control of a terminal is called the "controlling process" of that terminal.

You generally do not need to worry about the exact mechanism used to allocate a controlling terminal to a session, since it is done for you by the system when you log in.

An individual process disconnects from its controlling terminal when it calls setsid to become the leader of a new session.  See Process Group Functions.


Access to the Controlling Terminal
Node: Access to the Terminal, Next: Orphaned Process Groups, Prev: Controlling Terminal, Up: Job Control

Access to the Controlling Terminal

Processes in the foreground job of a controlling terminal have unrestricted access to that terminal; background proesses do not.  This section describes in more detail what happens when a process in a background job tries to access its controlling terminal.

When a process in a background job tries to read from its controlling terminal, the process group is usually sent a SIGTTIN signal.  This normally causes all of the processes in that group to stop (unless they handle the signal and don't stop themselves).  However, if the reading process is ignoring or blocking this signal, then read fails with an EIO error instead.

Similarly, when a process in a background job tries to write to its controlling terminal, the default behavior is to send a SIGTTOU signal to the process group.  However, the behavior is modified by the TOSTOP bit of the local modes flags (see Local Modes).  If this bit is not set (which is the default), then writing to the controlling terminal is always permitted without sending a signal.  Writing is also permitted if the SIGTTOU signal is being ignored or blocked by the writing process.

Most other terminal operations that a program can do are treated as reading or as writing.  (The description of each operation should say which.)

For more information about the primitive read and write functions, see See I/O Primitives.


Orphaned Process Groups
Node: Orphaned Process Groups, Next: Implementing a Shell, Prev: Access to the Terminal, Up: Job Control

Orphaned Process Groups

When a controlling process terminates, its terminal becomes free and a new session can be established on it.  (In fact, another user could log in on the terminal.)  This could cause a problem if any processes from the old session are still trying to use that terminal.

To prevent problems, process groups that continue running even after the session leader has terminated are marked as "orphaned process groups".  Processes in an orphaned process group cannot read from or write to the controlling terminal.  Attempts to do so will fail with an EIO error.

When a process group becomes an orphan, its processes are sent a SIGHUP signal.  Ordinarily, this causes the processes to terminate.  However, if a program ignores this signal or establishes a handler for it (see Signal Handling), it can continue running as in the orphan process group even after its controlling process terminates; but it still cannot access the terminal any more.


Implementing a Job Control Shell
Node: Implementing a Shell, Next: Functions for Job Control, Prev: Orphaned Process Groups, Up: Job Control

Implementing a Job Control Shell

This section describes what a shell must do to implement job control, by presenting an extensive sample program to illustrate the concepts involved.

* Menu:

Data Structures	Introduction to the sample shell.

Initializing the Shell	What the shell must do to take responsibility for job control.

Launching Jobs	Creating jobs to execute commands.

Foreground and Background	Putting a job in foreground of background.

Stopped and Terminated Jobs	Reporting job status.

Continuing Stopped Jobs	How to continue a stopped job in the foreground or background.

Missing Pieces	Other parts of the shell.


Data Structures for the Shell
Node: Data Structures, Next: Initializing the Shell, Prev: , Up: Implementing a Shell

Data Structures for the Shell

All of the program examples included in this chapter are part of a simple shell program.  This section presents data structures and utility functions which are used throughout the example.

The sample shell deals mainly with two data structures.  The job type contains information about a job, which is a set of subprocesses linked together with pipes.  The process type holds information about a single subprocess.  Here are the relevant data structure declarations:

/* A process is a single process.  */
typedef struct process
{
  struct process *next;       /* next process in pipeline */
  char **argv;                /* for exec */
  pid_t pid;                  /* process ID */
  char completed;             /* true if process has completed */
  char stopped;               /* true if process has stopped */
  int status;                 /* reported status value */
} process;

/* A job is a pipeline of processes.  */
typedef struct job
{
  struct job *next;           /* next active job */
  char *command;              /* command line, used for messages */
  process *first_process;     /* list of processes in this job */
  pid_t pgid;                 /* process group ID */
  char notified;              /* true if user told about stopped job */
  struct termios tmodes;      /* saved terminal modes */
  int stdin, stdout, stderr;  /* standard i/o channels */
} job;

/* The active jobs are linked into a list.  This is its head.   */
job *first_job = NULL;

Here are some utility functions that are used for operating on job objects.

/* Find the active job with the indicated pgid.  */
job *
find_job (pid_t pgid)
{
  job *j;

  for (j = first_job; j; j = j->next)
    if (j->pgid == pgid)
      return j;
  return NULL;
}

/* Return true if all processes in the job have stopped or completed.  */
int
job_is_stopped (job *j)
{
  process *p;

  for (p = j->first_process; p; p = p->next)
    if (!p->completed && !p->stopped)
      return 0;
  return 1;
}

/* Return true if all processes in the job have completed.  */
int
job_is_completed (job *j)
{
  process *p;

  for (p = j->first_process; p; p = p->next)
    if (!p->completed)
      return 0;
  return 1;
}


Initializing the Shell
Node: Initializing the Shell, Next: Launching Jobs, Prev: Data Structures, Up: Implementing a Shell

Initializing the Shell

When a shell program that normally performs job control is started, it has to be careful in case it has been invoked from another shell that is already doing its own job control.

A subshell that runs interactively has to ensure that it has been placed in the foreground by its parent shell before it can enable job control itself.  It does this by getting its initial process group ID with the getpgrp function, and comparing it to the process group ID of the current foreground job associated with its controlling terminal (which can be retrieved using the tcgetpgrp function).

If the subshell is not running as a foreground job, it must stop itself by sending a SIGTTIN signal to its own process group.  It may not arbitrarily put itself into the foreground; it must wait for the user to tell the parent shell to do this.  If the subshell is continued again, it should repeat the check and stop itself again if it is still not in the foreground.

Once the subshell has been placed into the foreground by its parent shell, it can enable its own job control.  It does this by calling setpgid to put itself into its own process group, and then calling tcsetpgrp to place this process group into the foreground.

When a shell enables job control, it should set itself to ignore all the job control stop signals so that it doesn't accidentally stop itself.  You can do this by setting the action for all the stop signals to SIG_IGN.

A subshell that runs non-interactively cannot and should not support job control.  It must leave all processes it creates in the same process group as the shell itself; this allows the non-interactive shell and its child processes to be treated as a single job by the parent shell.  This is easy to do--just don't use any of the job control primitives--but you must remember to make the shell do it.

Here is the initialization code for the sample shell that shows how to do all of this.

/* Keep track of attributes of the shell.  */

#include <sys/types.h>
#include <termios.h>
#include <unistd.h>

pid_t shell_pgid;
struct termios shell_tmodes;
int shell_terminal;
int shell_is_interactive;

/* Make sure the shell is running interactively as the foreground job
   before proceeding. */

void
init_shell ()
{

  /* See if we are running interactively.  */
  shell_terminal = STDIN_FILENO;
  shell_is_interactive = isatty (shell_terminal);

  if (shell_is_interactive)
    {
      /* Loop until we are in the foreground.  */
      while (tcgetpgrp (shell_terminal) != (shell_pgid = getpgrp ()))
        kill (- shell_pgid, SIGTTIN);

      /* Ignore interactive and job-control signals.  */
      signal (SIGINT, SIG_IGN);
      signal (SIGQUIT, SIG_IGN);
      signal (SIGTSTP, SIG_IGN);
      signal (SIGTTIN, SIG_IGN);
      signal (SIGTTOU, SIG_IGN);
      signal (SIGCHLD, SIG_IGN);

      /* Put ourselves in our own process group.  */
      shell_pgid = getpid ();
      if (setpgid (shell_pgid, shell_pgid) < 0)
        {
          perror ("Couldn't put the shell in its own process group");
          exit (1);
        }

      /* Grab control of the terminal.  */
      tcsetpgrp (shell_terminal, shell_pgid);

      /* Save default terminal attributes for shell.  */
      tcgetattr (shell_terminal, &shell_tmodes);
    }
}


Launching Jobs
Node: Launching Jobs, Next: Foreground and Background, Prev: Initializing the Shell, Up: Implementing a Shell

Launching Jobs

Once the shell has taken responsibility for performing job control on its controlling terminal, it can launch jobs in response to commands typed by the user.

To create the processes in a process group, you use the same fork and exec functions described in See Process Creation Concepts.  Since there are multiple child processes involved, though, things are a little more complicated and you must be careful to do things in the right order.  Otherwise, nasty race conditions can result.

You have two choices for how to structure the tree of parent-child relationships among the processes.  You can either make all the processes in the process group be children of the shell process, or you can make one process in group be the ancestor of all the other processes in that group.  The sample shell program presented in this chapter uses the first approach because it makes bookkeeping somewhat simpler.

As each process is forked, it should put itself in the new process group by calling setpgid; see See Process Group Functions.  The first process in the new group becomes its "process group leader", and its process ID becomes the "process group ID" for the group.

The shell should also call setpgid to put each of its child processes into the new process group.  This is because there is a potential timing problem: each child process must be put in the process group before it begins executing a new program, and the shell depends on having all the child processes in the group before it continues executing.  If both the child processes and the shell call setpgid, this ensures that the right things happen no matter which process gets to it first.

If the job is being launched as a foreground job, the new process group also needs to be put into the foreground on the controlling terminal using tcsetpgrp.  Again, this should be done by the shell as well as by each of its child processes, to avoid race conditions.

The next thing each child process should do is to reset its signal actions.

During initialization, the shell process set itself to ignore job control signals; see See Initializing the Shell.  As a result, any child processes it creates also ignore these signals by inheritance.  This is definitely undesirable, so each child process should explicitly set the actions for these signals back to SIG_DFL just after it is forked.

Since shells follow this convention, applications can assume that they inherit the correct handling of these signals from the parent process.  But every application has a responsibility not to mess up the handling of stop signals.  Applications that disable the normal interpretation of the SUSP character should provide some other mechanism for the user to stop the job.  When the user invokes this mechanism, the program should send a SIGTSTP signal to the process group of the process, not just to the process itself.  See Signaling Another Process.

Finally, each child process should call exec in the normal way.  This is also the point at which redirection of the standard input and output channels should be handled.  See Duplicating Descriptors, for an explanation of how to do this.

Here is the function from the sample shell program that is responsible for launching a program.  The function is executed by each child process immediately after it has been forked by the shell, and never returns.

void
launch_process (process *p, pid_t pgid,
                int infile, int outfile, int errfile,
                int foreground)
{
  pid_t pid;

  if (shell_is_interactive)
    {
      /* Put the process into the process group and give the process group
         the terminal, if appropriate.
         This has to be done both by the shell and in the individual
         child processes because of potential race conditions.  */
      pid = getpid ();
      if (pgid == 0) pgid = pid;
      setpgid (pid, pgid);
      if (foreground)
        tcsetpgrp (shell_terminal, pgid);

      /* Set the handling for job control signals back to the default.  */
      signal (SIGINT, SIG_DFL);
      signal (SIGQUIT, SIG_DFL);
      signal (SIGTSTP, SIG_DFL);
      signal (SIGTTIN, SIG_DFL);
      signal (SIGTTOU, SIG_DFL);
      signal (SIGCHLD, SIG_DFL);
    }

  /* Set the standard input/output channels of the new process.  */
  if (infile != STDIN_FILENO)
    {
      dup2 (infile, STDIN_FILENO);
      close (infile);
    }
  if (outfile != STDOUT_FILENO)
    {
      dup2 (outfile, STDOUT_FILENO);
      close (outfile);
    }
  if (errfile != STDERR_FILENO)
    {
      dup2 (errfile, STDERR_FILENO);
      close (errfile);
    }

  /* Exec the new process.  Make sure we exit.  */
  execvp (p->argv[0], p->argv);
  perror ("execvp");
  exit (1);
}

If the shell is not running interactively, this function does not do anything with process groups or signals.  Remember that a shell not performing job control must keep all of its subprocesses in the same process group as the shell itself.

Next, here is the function that actually launches a complete job.  After creating the child processes, this function calls some other functions to put the newly created job into the foreground or background; these are discussed in See Foreground and Background.

void
launch_job (job *j, int foreground)
{
  process *p;
  pid_t pid;
  int mypipe[2], infile, outfile;

  infile = j->stdin;
  for (p = j->first_process; p; p = p->next)
    {
      /* Set up pipes, if necessary.  */
      if (p->next)
        {
          if (pipe (mypipe) < 0)
            {
              perror ("pipe");
              exit (1);
            }
          outfile = mypipe[1];
        }
      else
        outfile = j->stdout;

      /* Fork the child processes.  */
      pid = fork ();
      if (pid == 0)
        /* This is the child process.  */
        launch_process (p, j->pgid, infile,
                        outfile, j->stderr, foreground);
      else if (pid < 0)
        {
          /* The fork failed.  */
          perror ("fork");
          exit (1);
        }
      else
        {
          /* This is the parent process.  */
          p->pid = pid;
          if (shell_is_interactive)
            {
              if (!j->pgid)
                j->pgid = pid;
              setpgid (pid, j->pgid);
            }
        }

      /* Clean up after pipes.  */
      if (infile != j->stdin)
        close (infile);
      if (outfile != j->stdout)
        close (outfile);
      infile = mypipe[0];
    }

  format_job_info (j, "launched");

  if (!shell_is_interactive)
    wait_for_job (j);
  else if (foreground)
    put_job_in_foreground (j, 0);
  else
    put_job_in_background (j, 0);
}


Foreground and Background
Node: Foreground and Background, Next: Stopped and Terminated Jobs, Prev: Launching Jobs, Up: Implementing a Shell

Foreground and Background

Now let's consider what actions must be taken by the shell when it launches a job into the foreground, and how this differs from what must be done when a background job is launched.

When a foreground job is launched, the shell must first give it access to the controlling terminal by calling tcsetpgrp.  Then, the shell should wait for processes in that process group to terminate or stop.  This is discussed in more detail in See Stopped and Terminated Jobs.

When all of the processes in the group have either completed or stopped, the shell should regain control of the terminal for its own process group by calling tcsetpgrp again.  Since stop signals caused by I/O from a background process or a SUSP character typed by the user are sent to the process group, normally all the processes in the job stop together.

The foreground job may have left the terminal in a strange state, so the shell should restore its own saved terminal modes before continuing.  In case the job is merely been stopped, the shell should first save the current terminal modes so that it can restore them later if the job is continued.  The functions for dealing with terminal modes are tcgetattr and tcsetattr; these are described in See Terminal Modes.

Here is the sample shell's function for doing all of this.

/* Put job j in the foreground.  If cont is nonzero,
   restore the saved terminal modes and send the process group a
   SIGCONT signal to wake it up before we block.  */

void
put_job_in_foreground (job *j, int cont)
{
  /* Put the job into the foreground.  */
  tcsetpgrp (shell_terminal, j->pgid);

/* Send the job a continue signal, if necessary.  */
  if (cont)
    {
      tcsetattr (shell_terminal, TCSADRAIN, &j->tmodes);
      if (kill (- j->pgid, SIGCONT) < 0)
        perror ("kill (SIGCONT)");
    }

  /* Wait for it to report.  */
  wait_for_job (j);

  /* Put the shell back in the foreground.  */
  tcsetpgrp (shell_terminal, shell_pgid);

/* Restore the shell's terminal modes.  */
  tcgetattr (shell_terminal, &j->tmodes);
  tcsetattr (shell_terminal, TCSADRAIN, &shell_tmodes);
}

If the process group is launched as a background job, the shell should remain in the foreground itself and continue to read commands from the terminal.

In the sample shell, there is not much that needs to be done to put a job into the background.  Here is the function it uses:

/* Put a job in the background.  If the cont argument is true, send
   the process group a SIGCONT signal to wake it up.  */

void
put_job_in_background (job *j, int cont)
{
  /* Send the job a continue signal, if necessary.  */
  if (cont)
    if (kill (-j->pgid, SIGCONT) < 0)
      perror ("kill (SIGCONT)");
}


Stopped and Terminated Jobs
Node: Stopped and Terminated Jobs, Next: Continuing Stopped Jobs, Prev: Foreground and Background, Up: Implementing a Shell

Stopped and Terminated Jobs

When a foreground process is launched, the shell must block until all of the processes in that job have either terminated or stopped.  It can do this by calling the waitpid function; see See Process Completion.  Use the WUNTRACED option so that status is reported for processes that stop as well as processes that terminate.

The shell must also check on the status of background jobs so that it can report terminated and stopped jobs to the user; this can be done by calling waitpid with the WNOHANG option.  A good place to put a such a check for terminated and stopped jobs is just before prompting for a new command.

The shell can also receive asynchronous notification that there is status information available for a child process by establishing a handler for SIGCHLD signals.  See Signal Handling.

In the sample shell program, the SIGCHLD signal is normally ignored.  This is to avoid reentrancy problems involving the global data structures the shell manipulates.  But at specific times when the shell is not using these data structures--such as when it is waiting for input on the terminal--it makes sense to enable a handler for SIGCHLD.  The same function that is used to do the synchronous status checks (do_job_notification, in this case) can also be called from within this handler.

Here are the parts of the sample shell program that deal with checking the status of jobs and reporting the information to the user.

/* Store the status of the process pid that was returned by waitpid.
   Return 0 if all went well, nonzero otherwise.  */

int
mark_process_status (pid_t pid, int status)
{
  job *j;
  process *p;

if (pid > 0)
    {
      /* Update the record for the process.  */
      for (j = first_job; j; j = j->next)
        for (p = j->first_process; p; p = p->next)
          if (p->pid == pid)
            {
              p->status = status;
              if (WIFSTOPPED (status))
                p->stopped = 1;
              else
                {
                  p->completed = 1;
                  if (WIFSIGNALED (status))
                    fprintf (stderr, "%d: Terminated by signal %d.\n",
                             (int) pid, WTERMSIG (p->status));
                }
              return 0;
             }
      fprintf (stderr, "No child process %d.\n", pid);
      return -1;
    }

else if (pid == 0 || errno == ECHILD)
    /* No processes ready to report.  */
    return -1;
  else {
    /* Other weird errors.  */
    perror ("waitpid");
    return -1;
  }
}

/* Check for processes that have status information available,
   without blocking.  */

void
update_status (void)
{
  int status;
  pid_t pid;

  do
    pid = waitpid (WAIT_ANY, &status, WUNTRACED|WNOHANG);
  while (!mark_process_status (pid, status));
}

/* Check for processes that have status information available,
   blocking until all processes in the given job have reported.  */

void
wait_for_job (job *j)
{
  int status;
  pid_t pid;

  do
    pid = waitpid (WAIT_ANY, &status, WUNTRACED);
  while (!mark_process_status (pid, status)
         && !job_is_stopped (j)
         && !job_is_completed (j));
}

/* Format information about job status for the user to look at.  */

void
format_job_info (job *j, const char *status)
{
  fprintf (stderr, "%ld (%s): %s\n", (long)j->pgid, status, j->command);
}

/* Notify the user about stopped or terminated jobs.
   Delete terminated jobs from the active job list.  */

void
do_job_notification (void)
{
  job *j, *jlast, *jnext;
  process *p;

  /* Update status information for child processes.  */
  update_status ();

  jlast = NULL;
  for (j = first_job; j; j = jnext)
    {
      jnext = j->next;

      /* If all processes have completed, tell the user the job has
         completed and delete it from the list of active jobs.  */
      if (job_is_completed (j)) {
        format_job_info (j, "completed");
        if (jlast)
          jlast->next = jnext;
        else
          first_job = jnext;
        free_job (j);
      }

      /* Notify the user about stopped jobs,
         marking them so that we won't do this more than once.  */
      else if (job_is_stopped (j) && !j->notified) {
        format_job_info (j, "stopped");
        j->notified = 1;
        jlast = j;
      }

      /* Don't say anything about jobs that are still running.  */
      else
        jlast = j;
    }
}


Continuing Stopped Jobs
Node: Continuing Stopped Jobs, Next: Missing Pieces, Prev: Stopped and Terminated Jobs, Up: Implementing a Shell

Continuing Stopped Jobs

The shell can continue a stopped job by sending a SIGCONT signal to its process group.  If the job is being continued in the foreground, the shell should first invoke tcsetpgrp to give the job access to the terminal, and restore the saved terminal settings.  After continuing a job in the foreground, the shell should wait for the job to stop or complete, as if the job had just been launched in the foreground.

The sample shell program handles both newly created and continued jobs with the same pair of functions, put_job_in_foreground and put_job_in_background.  The definitions of these functions were given in See Foreground and Background.  When continuing a stopped job, a nonzero value is passed as the cont argument to ensure that the SIGCONT signal is sent and the terminal modes reset, as appropriate.

This leaves only a function for updating the shell's internal bookkeeping about the job being continued:

/* Mark a stopped job J as being running again.  */

void
mark_job_as_running (job *j)
{
  Process *p;

  for (p = j->first_process; p; p = p->next)
    p->stopped = 0;
  j->notified = 0;
}

/* Continue the job J.  */

void
continue_job (job *j, int foreground)
{
  mark_job_as_running (j);
  if (foreground)
    put_job_in_foreground (j, 1);
  else
    put_job_in_background (j, 1);
}


The Missing Pieces
Node: Missing Pieces, Next: , Prev: Continuing Stopped Jobs, Up: Implementing a Shell

The Missing Pieces

The code extracts for the sample shell included in this chapter are only a part of the entire shell program.  In particular, nothing at all has been said about how job and program data structures are allocated and initialized.

Most real shells provide a complex user interface that has support for a command language; variables; abbreviations, substitutions, and pattern matching on file names; and the like.  All of this is far too complicated to explain here!  Instead, we have concentrated on showing how to implement the core process creation and job control functions that can be called from such a shell.

Here is a table summarizing the major entry points we have presented:

void init_shell (void)

Initialize the shell's internal state.  See Initializing the Shell.

void launch_job (job *j, int foreground)

Launch the job j as either a foreground or background job.  See Launching Jobs.

void do_job_notification (void)

Check for and report any jobs that have terminated or stopped.  Can be called synchronously or within a handler for SIGCHLD signals.  See Stopped and Terminated Jobs.

void continue_job (job *j, int foreground)

Continue the job j.  See Continuing Stopped Jobs.

Of course, a real shell would also want to provide other functions for managing jobs.  For example, it would be useful to have commands to list all active jobs or to send a signal (such as SIGKILL) to a job.


Functions for Job Control
Node: Functions for Job Control, Next: , Prev: Implementing a Shell, Up: Job Control

Functions for Job Control

This section contains detailed descriptions of the functions relating to job control.

* Menu:

Identifying the Terminal	Determining the controlling terminal's name.

Process Group Functions	Functions for manipulating process groups.

Terminal Access Functions	Functions for controlling terminal access.


Identifying the Controlling Terminal
Node: Identifying the Terminal, Next: Process Group Functions, Prev: , Up: Functions for Job Control

Identifying the Controlling Terminal

You can use the ctermid function to get a file name that you can use to open the controlling terminal.  In the GNU library, it returns the same string all the time: "/dev/tty".  That is a special "magic" file name that refers to the controlling terminal of the current process (if it has one).  The function ctermid is declared in the header file 'stdio.h'.  K

 - Function: char * ctermid (char *string)

The ctermid function returns a string containing the file name of the controlling terminal for the current process.  If string is not a null pointer, it should be an array that can hold at least L_ctermid characters; the string is returned in this array.  Otherwise, a pointer to a string in a static area is returned, which might get overwritten on subsequent calls to this function.

An empty string is returned if the file name cannot be determined for any reason.  Even if a file name is returned, access to the file it represents is not guaranteed.

 - Macro: int L_ctermid

The value of this macro is an integer constant expression that represents the size of a string large enough to hold the file name returned by ctermid.

See also the isatty and ttyname functions, in See Is It a Terminal.


Process Group Functions
Node: Process Group Functions, Next: Terminal Access Functions, Prev: Identifying the Terminal, Up: Functions for Job Control

Process Group Functions

Here are descriptions of the functions for manipulating process groups.  Your program should include the header files 'sys/types.h' and 'unistd.h' to use these functions.  K

 - Function: pid_t setsid (void)

The setsid function creates a new session.  The calling process becomes the session leader, and is put in a new process group whose process group ID is the same as the process ID of that process.  There are initially no other processes in the new process group, and no other process groups in the new session.

This function also makes the calling process have no controlling terminal.

The setsid function returns the new process group ID of the calling process if successful.  A return value of -1 indicates an error.  The following errno error conditions are defined for this function:

EPERM

The calling process is already a process group leader, or there is already another process group around that has the same process group ID.

The getpgrp function has two definitions: one derived from BSD Unix, and one from the POSIX.1 standard.  The feature test macros you have selected (see Feature Test Macros) determine which definition you get.  Specifically, you get the BSD version if you define _BSD_SOURCE; otherwise, you get the POSIX version if you define _POSIX_SOURCE or _GNU_SOURCE.  Programs written for old BSD systems will not include 'unistd.h', which defines getpgrp specially under _BSD_SOURCE.  You must link such programs with the -lbsd-compat option to get the BSD definition.  K

 - POSIX.1 Function: pid_t getpgrp (void)

The POSIX.1 definition of getpgrp returns the process group ID of the calling process.

 - BSD Function: pid_t getpgrp (pid_t pid)

The BSD definition of getpgrp returns the process group ID of the process pid.  You can supply a value of 0 for the pid argument to get information about the calling process.

 - Function: int setpgid (pid_t pid, pid_t pgid)

The setpgid function puts the process pid into the process group pgid.  As a special case, either pid or pgid can be zero to indicate the process ID of the calling process.

This function fails on a system that does not support job control.  See Job Control is Optional, for more information.

If the operation is successful, setpgid returns zero.  Otherwise it returns -1.  The following errno error conditions are defined for this function:

EACCES

The child process named by pid has executed an exec function since it was forked.

EINVAL

The value of the pgid is not valid.

ENOSYS

The system doesn't support job control.

EPERM

The process indicated by the pid argument is a session leader, or is not in the same session as the calling process, or the value of the pgid argument doesn't match a process group ID in the same session as the calling process.

ESRCH

The process indicated by the pid argument is not the calling process or a child of the calling process.

 - Function: int setpgrp (pid_t pid, pid_t pgid)

This is the BSD Unix name for setpgid.  Both functions do exactly the same thing.


Functions for Controlling Terminal Access
Node: Terminal Access Functions, Next: , Prev: Process Group Functions, Up: Functions for Job Control

Functions for Controlling Terminal Access

These are the functions for reading or setting the foreground process group of a terminal.  You should include the header files 'sys/types.h' and 'unistd.h' in your application to use these functions.  KAlthough these functions take a file descriptor argument to specify the terminal device, the foreground job is associated with the terminal file itself and not a particular open file descriptor.

 - Function: pid_t tcgetpgrp (int filedes)

This function returns the process group ID of the foreground process group associated with the terminal open on descriptor filedes.

If there is no foreground process group, the return value is a number greater than 1 that does not match the process group ID of any existing process group.  This can happen if all of the processes in the job that was formerly the foreground job have terminated, and no other job has yet been moved into the foreground.

In case of an error, a value of -1 is returned.  The following errno error conditions are defined for this function:

EBADF

The filedes argument is not a valid file descriptor.

ENOSYS

The system doesn't support job control.

ENOTTY

The terminal file associated with the filedes argument isn't the controlling terminal of the calling process.

 - Function: int tcsetpgrp (int filedes, pid_t pgid)

This function is used to set a terminal's foreground process group ID.  The argument filedes is a descriptor which specifies the terminal; pgid specifies the process group.  The calling process must be a member of the same session as pgid and must have the same controlling terminal.

For terminal access purposes, this function is treated as output.  If it is called from a background process on its controlling terminal, normally all processes in the process group are sent a SIGTTOU signal.  The exception is if the calling process itself is ignoring or blocking SIGTTOU signals, in which case the operation is performed and no signal is sent.

If successful, tcsetpgrp returns 0.  A return value of -1 indicates an error.  The following errno error conditions are defined for this function:

EBADF

The filedes argument is not a valid file descriptor.

EINVAL

The pgid argument is not valid.

ENOSYS

The system doesn't support job control.

ENOTTY

The filedes isn't the controlling terminal of the calling process.

EPERM

The pgid isn't a process group in the same session as the calling process.


Users and Groups
Node: Users and Groups, Next: System Information, Prev: Job Control, Up: Top

Users and Groups

Every user who can log in on the system is identified by a unique number called the "user ID".  Each process has an effective user ID which says which user's access permissions it has.

Users are classified into "groups" for access control purposes.  Each process has one or more "group ID values" which say which groups the process can use for access to files.

The effective user and group IDs of a process collectively form its "persona".  This determines which files the process can access.  Normally, a process inherits its persona from the parent process, but under special circumstances a process can change its persona and thus change its access permissions.

Each file in the system also has a user ID and a group ID.  Access control works by comparing the user and group IDs of the file with those of the running process.

The system keeps a database of all the registered users, and another database of all the defined groups.  There are library functions you can use to examine these databases.

* Menu:

User and Group IDs	Each user has a unique numeric ID; likewise for groups.

Process Persona	The user IDs and group IDs of a process.

Why Change Persona	Why a program might need to change its user and/or group IDs.

How Change Persona	Changing the user and group IDs.

Reading Persona	How to examine the user and group IDs.

Setting User ID	Functions for setting the user ID.

Setting Groups	Functions for setting the group IDs.

Enable/Disable Setuid	Turning setuid access on and off.

Setuid Program Example	The pertinent parts of one sample program.

Tips for Setuid	How to avoid granting unlimited access.

Who Logged In	Getting the name of the user who logged in, or of the real user ID of the current process.

User Database	Functions and data structures for accessing the user database.

Group Database	Functions and data structures for accessing the group database.

Database Example	Example program showing use of database inquiry functions.


User and Group IDs
Node: User and Group IDs, Next: Process Persona, Prev: , Up: Users and Groups

User and Group IDs

Each user account on a computer system is identified by a "user name" (or "login name") and "user ID".  Normally, each user name has a unique user ID, but it is possible for several login names to have the same user ID.  The user names and corresponding user IDs are stored in a data base which you can access as described in See User Database.

Users are classified in "groups".  Each user name also belongs to one or more groups, and has one "default group".  Users who are members of the same group can share resources (such as files) that are not accessible to users who are not a member of that group.  Each group has a "group name" and "group ID".  See Group Database, for how to find information about a group ID or group name.


The Persona of a Process
Node: Process Persona, Next: Why Change Persona, Prev: User and Group IDs, Up: Users and Groups

The Persona of a Process

At any time, each process has a single user ID and a group ID which determine the privileges of the process.  These are collectively called the "persona" of the process, because they determine "who it is" for purposes of access control.  These IDs are also called the "effective user ID" and "effective group ID" of the process.

Your login shell starts out with a persona which consists of your user ID and your default group ID.  In normal circumstances, all your other processes inherit these values.

A process also has a "real user ID" which identifies the user who created the process, and a "real group ID" which identifies that user's default group.  These values do not play a role in access control, so we do not consider them part of the persona.  But they are also important.

Both the real and effective user ID can be changed during the lifetime of a process.  See Why Change Persona.

In addition, a user can belong to multiple groups, so the persona includes "supplementary group IDs" that also contribute to access permission.

For details on how a process's effective user IDs and group IDs affect its permission to access files, see See Access Permission.

The user ID of a process also controls permissions for sending signals using the kill function.  See Signaling Another Process.


Why Change the Persona of a Process?
Node: Why Change Persona, Next: How Change Persona, Prev: Process Persona, Up: Users and Groups

Why Change the Persona of a Process?

The most obvious situation where it is necessary for a process to change its user and/or group IDs is the login program.  When login starts running, its user ID is root.  Its job is to start a shell whose user and group IDs are those of the user who is logging in.  (To accomplish this fully, login must set the real user and group IDs as well as its persona.  But this is a special case.)

The more common case of changing persona is when an ordinary user program needs access to a resource that wouldn't ordinarily be accessible to the user actually running it.

For example, you may have a file that is controlled by your program but that shouldn't be read or modified directly by other users, either because it implements some kind of locking protocol, or because you want to preserve the integrity or privacy of the information it contains.  This kind of restricted access can be implemented by having the program change its effective user or group ID to match that of the resource.

Thus, imagine a game program that saves scores in a file.  The game program itself needs to be able to update this file no matter who is running it, but if users can write the file without going through the game, they can give themselves any scores they like.  Some people consider this undesirable, or even reprehensible.  It can be prevented by creating a new user ID and login name (say, games) to own the scores file, and make the file writable only by this user.  Then, when the game program wants to update this file, it can change its effective user ID to be that for games.  In effect, the program must adopt the persona of games so it can write the scores file.


How an Application Can Change Persona
Node: How Change Persona, Next: Reading Persona, Prev: Why Change Persona, Up: Users and Groups

How an Application Can Change Persona

The ability to change the persona of a process can be a source of unintentional privacy violations, or even intentional abuse.  Because of the potential for problems, changing persona is restricted to special circumstances.

You can't arbitrarily set your user ID or group ID to anything you want; only privileged processes can do that.  Instead, the normal way for a program to change its persona is that it has been set up in advance to change to a particular user or group.  This is the function of the setuid and setgid bits of a file's access mode.  See Permission Bits.

When the setuid bit of an executable file is set, executing that file automatically changes the effective user ID to the user that owns the file.  Likewise, executing a file whose setgid bit is set changes the effective group ID to the group of the file.  See Executing a File.  Creating a file that changes to a particular user or group ID thus requires full access to that user or group ID.

See File Attributes, for a more general discussion of file modes and accessibility.

A process can always change its effective user (or group) ID back to its real ID.  Programs do this so as to turn off their special privileges when they are not needed, which makes for more robustness.


Reading the Persona of a Process
Node: Reading Persona, Next: Setting User ID, Prev: How Change Persona, Up: Users and Groups

Reading the Persona of a Process

Here are detailed descriptions of the functions for reading the user and group IDs of a process, both real and effective.  To use these facilities, you must include the header files 'sys/types.h' and 'unistd.h'.  K

 - Data Type: uid_t

This is an integer data type used to represent user IDs.  In the GNU library, this is an alias for unsigned int.

 - Data Type: gid_t

This is an integer data type used to represent group IDs.  In the GNU library, this is an alias for unsigned int.

 - Function: uid_t getuid (void)

The getuid function returns the real user ID of the process.

 - Function: gid_t getgid (void)

The getgid function returns the real group ID of the process.

 - Function: uid_t geteuid (void)

The geteuid function returns the effective user ID of the process.

 - Function: gid_t getegid (void)

The getegid function returns the effective group ID of the process.

 - Function: int getgroups (int count, gid_t *groups)

The getgroups function is used to inquire about the supplementary group IDs of the process.  Up to count of these group IDs are stored in the array groups; the return value from the function is the number of group IDs actually stored.  If count is smaller than the total number of supplementary group IDs, then getgroups returns a value of -1 and errno is set to EINVAL.

If count is zero, then getgroups just returns the total number of supplementary group IDs.  On systems that do not support supplementary groups, this will always be zero.

Here's how to use getgroups to read all the supplementary group IDs:

gid_t *
read_all_groups (void)
{
  int ngroups = getgroups (NULL, 0);
  gid_t *groups
    = (gid_t *) xmalloc (ngroups * sizeof (gid_t));
  int val = getgroups (ngroups, groups);
  if (val < 0)
    {
      free (groups);
      return NULL;
    }
  return groups;
}


Setting the User ID
Node: Setting User ID, Next: Setting Groups, Prev: Reading Persona, Up: Users and Groups

Setting the User ID

This section describes the functions for altering the user ID (real and/or effective) of a process.  To use these facilities, you must include the header files 'sys/types.h' and 'unistd.h'.  K

 - Function: int setuid (uid_t newuid)

This function sets both the real and effective user ID of the process to newuid, provided that the process has appropriate privileges.

If the process is not privileged, then newuid must either be equal to the real user ID or the saved user ID (if the system supports the _POSIX_SAVED_IDS feature).  In this case, setuid sets only the effective user ID and not the real user ID.

The setuid function returns a value of 0 to indicate successful completion, and a value of -1 to indicate an error.  The following errno error conditions are defined for this function:

EINVAL

The value of the newuid argument is invalid.

EPERM

The process does not have the appropriate privileges; you do not have permission to change to the specified ID.

 - Function: int setreuid (uid_t ruid, uid_t euid)

This function sets the real user ID of the process to ruid and the effective user ID to euid.  If ruid is -1, it means not to change the real user ID; likewise if euid is -1, it means not to change the effective user ID.

The setreuid function exists for compatibility with 4.3 BSD Unix, which does not support saved IDs.  You can use this function to swap the effective and real user IDs of the process.  (Privileged processes are not limited to this particular usage.)  If saved IDs are supported, you should use that feature instead of this function.  See Enable/Disable Setuid.

The return value is 0 on success and -1 on failure.  The following errno error conditions are defined for this function:

EPERM

The process does not have the appropriate privileges; you do not have permission to change to the specified ID.


Setting the Group IDs
Node: Setting Groups, Next: Enable/Disable Setuid, Prev: Setting User ID, Up: Users and Groups

Setting the Group IDs

This section describes the functions for altering the group IDs (real and effective) of a process.  To use these facilities, you must include the header files 'sys/types.h' and 'unistd.h'.  K

 - Function: int setgid (gid_t newgid)

This function sets both the real and effective group ID of the process to newgid, provided that the process has appropriate privileges.

If the process is not privileged, then newgid must either be equal to the real group ID or the saved group ID.  In this case, setgid sets only the effective group ID and not the real group ID.

The return values and error conditions for setgid are the same as those for setuid.

 - Function: int setregid (gid_t rgid, fid_t egid)

This function sets the real group ID of the process to rgid and the effective group ID to egid.  If rgid is -1, it means not to change the real group ID; likewise if egid is -1, it means not to change the effective group ID.

The setregid function is provided for compatibility with 4.3 BSD Unix, which does not support saved IDs.  You can use this function to swap the effective and real group IDs of the process.  (Privileged processes are not limited to this usage.)  If saved IDs are supported, you should use that feature instead of using this function.  See Enable/Disable Setuid.

The return values and error conditions for setregid are the same as those for setreuid.

The GNU system also lets privileged processes change their supplementary group IDs.  To use setgroups or initgroups, your programs should include the header file 'grp.h'.  K

 - Function: int setgroups (size_t count, gid_t *groups)

This function sets the process's supplementary group IDs.  It can only be called from privileged processes.  The count argument specifies the number of group IDs in the array groups.

This function returns 0 if successful and -1 on error.  The following errno error conditions are defined for this function:

EPERM

The calling process is not privileged.

 - Function: int initgroups (const char *user, gid_t gid)

The initgroups function effectively calls setgroups to set the process's supplementary group IDs to be the normal default for the user name user.  The group ID gid is also included.


Enabling and Disabling Setuid Access
Node: Enable/Disable Setuid, Next: Setuid Program Example, Prev: Setting Groups, Up: Users and Groups

Enabling and Disabling Setuid Access

A typical setuid program does not need its special access all of the time.  It's a good idea to turn off this access when it isn't needed, so it can't possibly give unintended access.

If the system supports the saved user ID feature, you can accomplish this with setuid.  When the game program starts, its real user ID is jdoe, its effective user ID is games, and its saved user ID is also games.  The program should record both user ID values once at the beginning, like this:

user_user_id = getuid ();
game_user_id = geteuid ();

Then it can turn off game file access with

setuid (user_user_id);

and turn it on with

setuid (game_user_id);

Throughout this process, the real user ID remains jdoe and the saved user ID remains games, so the program can always set its effective user ID to either one.

On other systems that don't support the saved user ID feature, you can turn setuid access on and off by using setreuid to swap the real and effective user IDs of the process, as follows:

setreuid (geteuid (), getuid ());

This special case is always allowed--it cannot fail.

Why does this have the effect of toggling the setuid access?  Suppose a game program has just started, and its real user ID is jdoe while its effective user ID is games.  In this state, the game can write the scores file.  If it swaps the two uids, the real becomes games and the effective becomes jdoe; now the program has only jdoe access.  Another swap brings games back to the effective user ID and restores access to the scores file.

In order to handle both kinds of systems, test for the saved user ID feature with a preprocessor conditional, like this:

#ifdef _POSIX_SAVED_IDS
  setuid (user_user_id);
#else
  setreuid (geteuid (), getuid ());
#endif


Setuid Program Example
Node: Setuid Program Example, Next: Tips for Setuid, Prev: Enable/Disable Setuid, Up: Users and Groups

Setuid Program Example

Here's an example showing how to set up a program that changes its effective user ID.

This is part of a game program called caber-toss that manipulates a file 'scores' that should be writable only by the game program itself.  The program assumes that its executable file will be installed with the set-user-ID bit set and owned by the same user as the 'scores' file.  Typically, a system administrator will set up an account like games for this purpose.

The executable file is given mode 4755, so that doing an 'ls -l' on it produces output like:

-rwsr-xr-x   1 games    184422 Jul 30 15:17 caber-toss

The set-user-ID bit shows up in the file modes as the 's'.

The scores file is given mode 644, and doing an 'ls -l' on it shows:

-rw-r--r--  1 games           0 Jul 31 15:33 scores

Here are the parts of the program that show how to set up the changed user ID.  This program is conditionalized so that it makes use of the saved IDs feature if it is supported, and otherwise uses setreuid to swap the effective and real user IDs.

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>

/* Save the effective and real UIDs. */

static uid_t euid, ruid;

/* Restore the effective UID to its original value. */

void
do_setuid (void)
{
  int status;

#ifdef _POSIX_SAVED_IDS
  status = setuid (euid);
#else
  status = setreuid (ruid, euid);
#endif
  if (status < 0) {
    fprintf (stderr, "Couldn't set uid.\n");
    exit (status);
    }
}

/* Set the effective UID to the real UID. */

void
undo_setuid (void)
{
  int status;

#ifdef _POSIX_SAVED_IDS
  status = setuid (ruid);
#else
  status = setreuid (euid, ruid);
#endif
  if (status < 0) {
    fprintf (stderr, "Couldn't set uid.\n");
    exit (status);
    }
}

/* Main program. */

int
main (void)
{
  /* Save the real and effective user IDs.  */
  ruid = getuid ();
  euid = geteuid ();
  undo_setuid ();

  /* Do the game and record the score.  */
  ...
}

Notice how the first thing the main function does is to set the effective user ID back to the real user ID.  This is so that any other file accesses that are performed while the user is playing the game use the real user ID for determining permissions.  Only when the program needs to open the scores file does it switch back to the original effective user ID, like this:

/* Record the score. */

int
record_score (int score)
{
  FILE *stream;
  char *myname;

  /* Open the scores file. */
  do_setuid ();
  stream = fopen (SCORES_FILE, "a");
  undo_setuid ();

/* Write the score to the file. */
  if (stream)
    {
      myname = cuserid (NULL);
      if (score < 0)
        fprintf (stream, "%10s: Couldn't lift the caber.\n", myname);
      else
        fprintf (stream, "%10s: %d feet.\n", myname, score);
      fclose (stream);
      return 0;
    }
  else
    return -1;
}


Tips for Writing Setuid Programs
Node: Tips for Setuid, Next: Who Logged In, Prev: Setuid Program Example, Up: Users and Groups

Tips for Writing Setuid Programs

It is easy for setuid programs to give the user access that isn't intended--in fact, if you want to avoid this, you need to be careful.  Here are some guidelines for preventing unintended access and minimizing its consequences when it does occur:

•	Don't have setuid programs with privileged user IDs such as root unless it is absolutely necessary.  If the resource is specific to your particular program, it's better to define a new, nonprivileged user ID or group ID just to manage that resource.

•	Be cautious about using the system and exec functions in combination with changing the effective user ID.  Don't let users of your program execute arbitrary programs under a changed user ID.  Executing a shell is especially bad news.  Less obviously, the execlp and execvp functions are a potential risk (since the program they execute depends on the user's PATH environment variable).

If you must exec another program under a changed ID, specify an absolute file name (see File Name Resolution) for the executable, and make sure that the protections on that executable and all containing directories are such that ordinary users cannot replace it with some other program.

•	Only use the user ID controlling the resource in the part of the program that actually uses that resource.  When you're finished with it, restore the effective user ID back to the actual user's user ID.  See Enable/Disable Setuid.

•	If the setuid part of your program needs to access other files besides the controlled resource, it should verify that the real user would ordinarily have permission to access those files.  You can use the access function (see Access Permission) to check this; it uses the real user and group IDs, rather than the effective IDs.


Identifying Who Logged In
Node: Who Logged In, Next: User Database, Prev: Tips for Setuid, Up: Users and Groups

Identifying Who Logged In

You can use the functions listed in this section to determine the login name of the user who is running a process, and the name of the user who logged in the current session.  See also the function getuid and friends (see Reading Persona).

The getlogin function is declared in 'unistd.h', while cuserid and L_cuserid are declared in 'stdio.h'.  K

 - Function: char * getlogin (void)

The getlogin function returns a pointer to a string containing the name of the user logged in on the controlling terminal of the process, or a null pointer if this information cannot be determined.  The string is statically allocated and might be overwritten on subsequent calls to this function or to cuserid.

 - Function: char * cuserid (char *string)

The cuserid function returns a pointer to a string containing a user name associated with the effective ID of the process.  If string is not a null pointer, it should be an array that can hold at least L_cuserid characters; the string is returned in this array.  Otherwise, a pointer to a string in a static area is returned.  This string is statically allocated and might be overwritten on subsequent calls to this function or to getlogin.

 - Macro: int L_cuserid

An integer constant that indicates how long an array you might need to store a user name.

These functions let your program identify positively the user who is running or the user who logged in this session.  (These can differ when setuid programs are involved; See Process Persona.)  The user cannot do anything to fool these functions.

For most purposes, it is more useful to use the environment variable LOGNAME to find out who the user is.  This is more flexible precisely because the user can set LOGNAME arbitrarily.  See Standard Environment.


User Database
Node: User Database, Next: Group Database, Prev: Who Logged In, Up: Users and Groups

User Database

This section describes all about now to search and scan the database of registered users.  The database itself is kept in the file '/etc/passwd' on most systems, but on some systems a special network server gives access to it.

* Menu:

User Data Structure	What each user record contains.

Lookup User	How to look for a particular user.

Scanning All Users	Scanning the list of all users, one by one.

Writing a User Entry	How a program can rewrite a user's record.


The Data Structure that Describes a User
Node: User Data Structure, Next: Lookup User, Prev: , Up: User Database

The Data Structure that Describes a User

The functions and data structures for accessing the system user database are declared in the header file 'pwd.h'.  K

 - Data Type: struct passwd

The passwd data structure is used to hold information about entries in the system user data base.  It has at least the following members:

char *pw_name

The user's login name.

char *pw_passwd.

The encrypted password string.

uid_t pw_uid

The user ID number.

gid_t pw_gid

The user's default group ID number.

char *pw_gecos

A string typically containing the user's real name, and possibly other information such as a phone number.

char *pw_dir

The user's home directory, or initial working directory.  This might be a null pointer, in which case the interpretation is system-dependent.

char *pw_shell

The user's default shell, or the initial program run when the user logs in.  This might be a null pointer, indicating that the system default should be used.


Looking Up One User
Node: Lookup User, Next: Scanning All Users, Prev: User Data Structure, Up: User Database

Looking Up One User

You can search the system user database for information about a specific user using getpwuid or getpwnam.  These functions are declared in 'pwd.h'.

 - Function: struct passwd * getpwuid (uid_t uid)

This function returns a pointer to a statically-allocated structure containing information about the user whose user ID is uid.  This structure may be overwritten on subsequent calls to getpwuid.

A null pointer value indicates there is no user in the data base with user ID uid.

 - Function: struct passwd * getpwnam (const char *name)

This function returns a pointer to a statically-allocated structure containing information about the user whose user name is name.  This structure may be overwritten on subsequent calls to getpwnam.

A null pointer value indicates there is no user named name.


Scanning the List of All Users
Node: Scanning All Users, Next: Writing a User Entry, Prev: Lookup User, Up: User Database

Scanning the List of All Users

This section explains how a program can read the list of all users in the system, one user at a time.  The functions described here are declared in 'pwd.h'.

You can use the fgetpwent function to read user entries from a particular file.

 - Function: struct passwd * fgetpwent (FILE *stream)

This function reads the next user entry from stream and returns a pointer to the entry.  The structure is statically allocated and is rewritten on subsequent calls to fgetpwent.  You must copy the contents of the structure if you wish to save the information.

This stream must correspond to a file in the same format as the standard password database file.  This function comes from System V.

The way to scan all the entries in the user database is with setpwent, getpwent, and endpwent.

 - Function: void setpwent (void)

This function initializes a stream which getpwent uses to read the user database.

 - Function: struct passwd * getpwent (void)

The getpwent function reads the next entry from the stream initialized by setpwent.  It returns a pointer to the entry.  The structure is statically allocated and is rewritten on subsequent calls to getpwent.  You must copy the contents of the structure if you wish to save the information.

 - Function: void endpwent (void)

This function closes the internal stream used by getpwent.


Writing a User Entry
Node: Writing a User Entry, Next: , Prev: Scanning All Users, Up: User Database

Writing a User Entry

 - Function: int putpwent (const struct passwd *p, FILE *stream)

This function writes the user entry *p to the stream stream, in the format used for the standard user database file.  The return value is zero on success and nonzero on failure.

This function exists for compatibility with SVID.  We recommend that you avoid using it, because it makes sense only on the assumption that the struct passwd structure has no members except the standard ones; on a system which merges the traditional Unix data base with other extended information about users, adding an entry using this function would inevitably leave out much of the important information.

The function putpwent is declared in 'pwd.h'.


Group Database
Node: Group Database, Next: Database Example, Prev: User Database, Up: Users and Groups

Group Database

This section describes all about how to search and scan the database of registered groups.  The database itself is kept in the file '/etc/group' on most systems, but on some systems a special network service provides access to it.

* Menu:

Group Data Structure	What each group record contains.

Lookup Group	How to look for a particular group.

Scanning All Groups	Scanning the list of all groups.


The Data Structure for a Group
Node: Group Data Structure, Next: Lookup Group, Prev: , Up: Group Database

The Data Structure for a Group

The functions and data structures for accessing the system group database are declared in the header file 'grp.h'.  K

 - Data Type: struct group

The group structure is used to hold information about an entry in the system group database.  It has at least the following members:

char *gr_name

The name of the group.

gid_t gr_gid

The group ID of the group.

char **gr_mem

A vector of pointers to the names of users in the group.  Each user name is a null-terminated string, and the vector itself is terminated by a null pointer.


Looking Up One Group
Node: Lookup Group, Next: Scanning All Groups, Prev: Group Data Structure, Up: Group Database

Looking Up One Group

You can search the group database for information about a specific group using getgrgid or getgrnam.  These functions are declared in 'grp.h'.

 - Function: struct group * getgrgid (gid_t gid)

This function returns a pointer to a statically-allocated structure containing information about the group whose group ID is gid.  This structure may be overwritten by subsequent calls to getgrgid.

A null pointer indicates there is no group with ID gid.

 - Function: struct group * getgrnam (const char *name)

This function returns a pointer to a statically-allocated structure containing information about the group whose group name is name.  This structure may be overwritten by subsequent calls to getgrnam.

A null pointer indicates there is no group named name.


Scanning the List of All Groups
Node: Scanning All Groups, Next: , Prev: Lookup Group, Up: Group Database

Scanning the List of All Groups

This section explains how a program can read the list of all groups in the system, one group at a time.  The functions described here are declared in 'grp.h'.

You can use the fgetgrent function to read group entries from a particular file.

 - Function: struct group * fgetgrent (FILE *stream)

The fgetgrent function reads the next entry from stream.  It returns a pointer to the entry.  The structure is statically allocated and is rewritten on subsequent calls to fgetgrent.  You must copy the contents of the structure if you wish to save the information.

The stream must correspond to a file in the same format as the standard group database file.

The way to scan all the entries in the group database is with setgrent, getgrent, and endgrent.

 - Function: void setgrent (void)

This function initializes a stream for reading from the group data base.  You use this stream by calling getgrent.

 - Function: struct group * getgrent (void)

The getgrent function reads the next entry from the stream initialized by setgrent.  It returns a pointer to the entry.  The structure is statically allocated and is rewritten on subsequent calls to getgrent.  You must copy the contents of the structure if you wish to save the information.

 - Function: void endgrent (void)

This function closes the internal stream used by getgrent.


User and Group Database Example
Node: Database Example, Next: , Prev: Group Database, Up: Users and Groups

User and Group Database Example

Here is an example program showing the use of the system database inquiry functions.  The program prints some information about the user running the program.

#include <grp.h>
#include <pwd.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>

int
main (void)
{
  uid_t me;
  struct passwd *my_passwd;
  struct group *my_group;
  char **members;

  /* Get information about the user ID. */
  me = getuid ();
  my_passwd = getpwuid (me);
  if (!my_passwd)
    {
      printf ("Couldn't find out about user %d.\n", (int) me);
      exit (EXIT_FAILURE);
    }

  /* Print the information. */
  printf ("I am %s.\n", my_passwd->pw_gecos);
  printf ("My login name is %s.\n", my_passwd->pw_name);
  printf ("My uid is %d.\n", (int) (my_passwd->pw_uid));
  printf ("My home directory is %s.\n", my_passwd->pw_dir);
  printf ("My default shell is %s.\n", my_passwd->pw_shell);

  /* Get information about the default group ID. */
  my_group = getgrgid (my_passwd->pw_gid);
  if (!my_group)
    {
      printf ("Couldn't find out about group %d.\n",
              (int) my_passwd->pw_gid);
      exit (EXIT_FAILURE);
    }

  /* Print the information. */
  printf ("My default group is %s (%d).\n",
          my_group->gr_name, (int) (my_passwd->pw_gid));
  printf ("The members of this group are:\n");
  members = my_group->gr_mem;
  while (*members)
    {
      printf ("  %s\n", *(members));
      members++;
    }

  return EXIT_SUCCESS;
}

Here is some output from this program:

I am Throckmorton Snurd.
My login name is snurd.
My uid is 31093.
My home directory is /home/fsg/snurd.
My default shell is /bin/sh.
My default group is guest (12).
The members of this group are:
  friedman
  tami


System Information
Node: System Information, Next: System Configuration, Prev: Users and Groups, Up: Top

System Information

This chapter describes functions that return information about the particular machine that is in use--the type of hardware, the type of software, and the individual machine's name.

* Menu:

Host Identification	Determining the name of the machine.

Hardware/Software Type ID	Determining the hardware type of the machine and what operating system it is running.


Host Identification
Node: Host Identification, Next: Hardware/Software Type ID, Prev: , Up: System Information

Host Identification

This section explains how to identify the particular machine that your program is running on.  The identification of a machine consists of its Internet host name and Internet address; see See Internet Namespace.

Prototypes for these functions appear in 'unistd.h'.  The shell commands hostname and hostid work by calling them.

 - Function: int gethostname (char *name, size_t size)

This function returns the name of the host machine in the array name.  The size argument specifies the size of this array, in bytes.

The return value is 0 on success and -1 on failure.  In the GNU C library, gethostname fails if size is not large enough; then you can try again with a larger array.  The following errno error condition is defined for this function:

ENAMETOOLONG

The size argument is less than the size of the host name plus one.

On some systems, there is a symbol for the maximum possible host name length: MAXHOSTNAMELEN.  It is defined in 'sys/param.h'.  But you can't count on this to exist, so it is cleaner to handle failure and try again.

gethostname stores the beginning of the host name in name even if the host name won't entirely fit.  For some purposes, a truncated host name is good enough.  If it is, you can ignore the error code.

 - Function: int sethostname (const char *name, size_t length)

The sethostname function sets the name of the host machine to name, a string with length length.  Only privileged processes are allowed to do this.  Usually it happens just once, at system boot time.

The return value is 0 on success and -1 on failure.  The following errno error condition is defined for this function:

EPERM

This process cannot set the host name because it is not privileged.

 - Function: long int gethostid (void)

This function returns the "host ID" of the machine the program is running on.  By convention, this is usually the primary Internet address of that machine, converted to a long int.  But on some systems it is a meaningless but unique number which is hard-coded for each machine.

 - Function: int sethostid (long int id)

The sethostid function sets the "host ID" of the host machine to id.  Only privileged processes are allowed to do this.  Usually it happens just once, at system boot time.

The return value is 0 on success and -1 on failure.  The following errno error condition is defined for this function:

EPERM

This process cannot set the host name because it is not privileged.

ENOSYS

The operating system does not support setting the host ID.  On some systems, the host ID is a meaningless but unique number hard-coded for each machine.


Hardware/Software Type Identification
Node: Hardware/Software Type ID, Next: , Prev: Host Identification, Up: System Information

Hardware/Software Type Identification

You can use the uname function to find out some information about the type of computer your program is running on.  This function and the associated data type are declared in the header file 'sys/utsname.h'.  K

 - Data Type: struct utsname

The utsname structure is used to hold information returned by the uname function.  It has the following members:

char sysname[]

This is the name of the operating system in use.

char nodename[]

This is the network name of this particular computer.  In the GNU library, the value is the same as that returned by gethostname; see See Host Identification.

char release[]

This is the current release level of the operating system implementation.

char version[]

This is the current version level within the release of the operating system.

char machine[]

This is a description of the type of hardware that is in use.

Some systems provide a mechanism to interrogate the kernel directly for this information.  On systems without such a mechanism, the GNU C library fills in this field based on the configuration name that was specified when building and installing the library.

GNU uses a three-part name to describe a system configuration; the three parts are cpu, manufacturer and system-type, and they are separated with dashes.  Any possible combination of three names is potentially meaningful, but most such combinations are meaningless in practice and even the meaningful ones are not necessarily supported by any particular GNU program.

Since the value in machine is supposed to describe just the hardware, it consists of the first two parts of the configuration name: 'cpu-manufacturer'.  For example, it might be one of these:

"sparc-sun", "i386-anything", "m68k-hp", "m68k-sony", "m68k-sun", "mips-dec"

 - Function: int uname (struct utsname *info)

The uname function fills in the structure pointed to by info with information about the operating system and host machine.  A non-negative value indicates that the data was successfully stored.

-1 as the value indicates an error.  The only error possible is EFAULT, which we normally don't mention as it is always a possibility.


System Configuration Parameters
Node: System Configuration, Next: Language Features, Prev: System Information, Up: Top

System Configuration Parameters

The functions and macros listed in this chapter give information about configuration parameters of the operating system--for example, capacity limits, presence of optional POSIX features, and the default path for executable files (see String Parameters).

* Menu:

General Limits	Constants and functions that describe various process-related limits that have one uniform value for any given machine.

System Options	Optional POSIX features.

Version Supported	Version numbers of POSIX.1 and POSIX.2.

Sysconf	Getting specific configuration values of general limits and system options.

Minimums	Minimum values for general limits.

Limits for Files	Size limitations that pertain to individual files. These can vary between file systems or even from file to file.

Options for Files	Optional features that some files may support.

File Minimums	Minimum values for file limits.

Pathconf	Getting the limit values for a particular file.

Utility Limits	Capacity limits of some POSIX.2 utility programs.

Utility Minimums	Minimum allowable values of those limits.

String Parameters	Getting the default search path.


General Capacity Limits
Node: General Limits, Next: System Options, Prev: , Up: System Configuration

General Capacity Limits

The POSIX.1 and POSIX.2 standards specify a number of parameters that describe capacity limitations of the system.  These limits can be fixed constants for a given operating system, or they can vary from machine to machine.  For example, some limit values may be configurable by the system administrator, either at run time or by rebuilding the kernel, and this should not require recompiling application programs.

Each of the following limit parameters has a macro that is defined in 'limits.h' only if the system has a fixed, uniform limit for the parameter in question.  If the system allows different file systems or files to have different limits, then the macro is undefined; use sysconf to find out the limit that applies at a particular time on a particular machine.  See Sysconf.

Each of these parameters also has another macro, with a name starting with '_POSIX', which gives the lowest value that the limit is allowed to have on any POSIX system.  See Minimums.

- Macro: int ARG_MAX

If defined, the unvarying maximum combined length of the argv and environ arguments that can be passed to the exec functions.

- Macro: int CHILD_MAX

If defined, the unvarying maximum number of processes that can exist with the same real user ID at any one time.

- Macro: int OPEN_MAX

If defined, the unvarying maximum number of files that a single process can have open simultaneously.

 - Macro: int STREAM_MAX

If defined, the unvarying maximum number of streams that a single process can have open simultaneously.  See Opening Streams.

- Macro: int TZNAME_MAX

If defined, the unvarying maximum length of a time zone name.  See Time Zone Functions.

These limit macros are always defined in 'limits.h'.

- Macro: int NGROUPS_MAX

The maximum number of supplementary group IDs that one process can have.

The value of this macro is actually a lower bound for the maximum.  That is, you can count on being able to have that many supplementary group IDs, but a particular machine might let you have even more.  You can use sysconf to see whether a particular machine will let you have more (see Sysconf).

 - Macro: int SSIZE_MAX

The largest value that can fit in an object of type ssize_t.  Effectively, this is the limit on the number of bytes that can be read or written in a single operation.

This macro is defined in all POSIX systems because this limit is never configurable.

 - Macro: int RE_DUP_MAX

The largest number of repetitions you are guaranteed is allowed in the construct '\{min,max\}' in a regular expression.

The value of this macro is actually a lower bound for the maximum.  That is, you can count on being able to have that many repetitions, but a particular machine might let you have even more.  You can use sysconf to see whether a particular machine will let you have more (see Sysconf).  And even the value that sysconf tells you is just a lower bound--larger values might work.

This macro is defined in all POSIX.2 systems, because POSIX.2 says it should always be defined even if there is no specific imposed limit.


Overall System Options
Node: System Options, Next: Version Supported, Prev: General Limits, Up: System Configuration

Overall System Options

POSIX defines certain system-specific options that not all POSIX systems support.  Since these options are provided in the kernel, not in the library, simply using the GNU C library does not guarantee any of these features is supported; it depends on the system you are using.

You can test for the availability of a given option using the macros in this section, together with the function sysconf.  The macros are defined only if you include 'unistd.h'.

For the following macros, if the macro is defined in 'unistd.h', then the option is supported.  Otherwise, the option may or may not be supported; use sysconf to find out.  See Sysconf.

 - Macro: int _POSIX_JOB_CONTROL

If this symbol is defined, it indicates that the system supports job control.  Otherwise, the implementation behaves as if all processes within a session belong to a single process group.  See Job Control.

 - Macro: int _POSIX_SAVED_IDS

If this symbol is defined, it indicates that the system remembers the effective user and group IDs of a process before it executes an executable file with the set-user-ID or set-group-ID bits set, and that explicitly changing the effective user or group IDs back to these values is permitted.  If this option is not defined, then if a nonprivileged process changes its effective user or group ID to the real user or group ID of the process, it can't change it back again.  See Enable/Disable Setuid.

For the following macros, if the macro is defined in 'unistd.h', then its value indicates whether the option is supported.  A value of -1 means no, and any other value means yes.  If the macro is not defined, then the option may or may not be supported; use sysconf to find out.  See Sysconf.

 - Macro: int _POSIX2_C_DEV

If this symbol is defined, it indicates that the system has the POSIX.2 C compiler command, c89.  The GNU C library always defines this as 1, on the assumption that you would not have installed it if you didn't have a C compiler.

 - Macro: int _POSIX2_FORT_DEV

If this symbol is defined, it indicates that the system has the POSIX.2 Fortran compiler command, fort77.  The GNU C library never defines this, because we don't know what the system has.

 - Macro: int _POSIX2_FORT_RUN

If this symbol is defined, it indicates that the system has the POSIX.2 asa command to interpret Fortran carriage control.  The GNU C library never defines this, because we don't know what the system has.

 - Macro: int _POSIX2_LOCALEDEF

If this symbol is defined, it indicates that the system has the POSIX.2 localedef command.  The GNU C library never defines this, because we don't know what the system has.

 - Macro: int _POSIX2_SW_DEV

If this symbol is defined, it indicates that the system has the POSIX.2 commands ar, make, and strip.  The GNU C library always defines this as 1, on the assumption that you had to have ar and make to install the library, and it's unlikely that strip would be absent when those are present.


Which Version of POSIX is Supported
Node: Version Supported, Next: Sysconf, Prev: System Options, Up: System Configuration

Which Version of POSIX is Supported

 - Macro: long int _POSIX_VERSION

This constant represents the version of the POSIX.1 standard to which the implementation conforms.  For an implementation conforming to the 1990 POSIX.1 standard, the value is the integer 199009L.

_POSIX_VERSION is always defined (in 'unistd.h') in any POSIX system.

Usage Note: Don't try to test whether the system supports POSIX by including 'unistd.h' and then checking whether _POSIX_VERSION is defined.  On a non-POSIX system, this will probably fail because there is no 'unistd.h'.  We do not know of any way you can reliably test at compilation time whether your target system supports POSIX or whether 'unistd.h' exists.

The GNU C compiler predefines the symbol __POSIX__ if the target system is a POSIX system.  Provided you do not use any other compilers on POSIX systems, testing defined (__POSIX__) will reliably detect such systems.

 - Macro: long int _POSIX2_C_VERSION

This constant represents the version of the POSIX.2 standard which the library and system kernel support.  We don't know what value this will be for the first version of the POSIX.2 standard, because the value is based on the year and month in which the standard is officially adopted.

The value of this symbol says nothing about the utilities installed on the system.

Usage Note: You can use this macro to tell whether a POSIX.1 system library supports POSIX.2 as well.  Any POSIX.1 system contains 'unistd.h', so include that file and then test defined (_POSIX2_C_VERSION).


Using sysconf
Node: Sysconf, Next: Minimums, Prev: Version Supported, Up: System Configuration

Using sysconf

When your system has configurable system limits, you can use the sysconf function to find out the value that applies to any particular machine.  The function and the associated parameter constants are declared in the header file 'unistd.h'.

* Menu:

Sysconf Definition	Detailed specifications of sysconf.

Constants for Sysconf	The list of parameters sysconf can read.

Examples of Sysconf	How to use sysconf and the parameter macros properly together.


Definition of sysconf
Node: Sysconf Definition, Next: Constants for Sysconf, Prev: , Up: Sysconf

Definition of sysconf

 - Function: long int sysconf (int parameter)

This function is used to inquire about runtime system parameters.  The parameter argument should be one of the '_SC_' symbols listed below.

The normal return value from sysconf is the value you requested.  A value of -1 is returned both if the implementation does not impose a limit, and in case of an error.

The following errno error conditions are defined for this function:

EINVAL

The value of the parameter is invalid.


Constants for sysconf Parameters
Node: Constants for Sysconf, Next: Examples of Sysconf, Prev: Sysconf Definition, Up: Sysconf

Constants for sysconf Parameters

Here are the symbolic constants for use as the parameter argument to sysconf.  The values are all integer constants (more specifically, enumeration type values).

_SC_ARG_MAX

Inquire about the parameter corresponding to ARG_MAX.

_SC_CHILD_MAX

Inquire about the parameter corresponding to CHILD_MAX.

_SC_OPEN_MAX

Inquire about the parameter corresponding to OPEN_MAX.

_SC_STREAM_MAX

Inquire about the parameter corresponding to STREAM_MAX.

_SC_TZNAME_MAX

Inquire about the parameter corresponding to TZNAME_MAX.

_SC_NGROUPS_MAX

Inquire about the parameter corresponding to NGROUPS_MAX.

_SC_JOB_CONTROL

Inquire about the parameter corresponding to _POSIX_JOB_CONTROL.

_SC_SAVED_IDS

Inquire about the parameter corresponding to _POSIX_SAVED_IDS.

_SC_VERSION

Inquire about the parameter corresponding to _POSIX_VERSION.

_SC_CLK_TCK

Inquire about the parameter corresponding to CLOCKS_PER_SEC; see Basic CPU Time.

_SC_2_C_DEV

Inquire about whether the system has the POSIX.2 C compiler command, c89.

_SC_2_FORT_DEV

Inquire about whether the system has the POSIX.2 Fortran compiler command, fort77.

_SC_2_FORT_RUN

Inquire about whether the system has the POSIX.2 asa command to interpret Fortran carriage control.

_SC_2_LOCALEDEF

Inquire about whether the system has the POSIX.2 localedef command.

_SC_2_SW_DEV

Inquire about whether the system has the POSIX.2 commands ar, make, and strip.

_SC_BC_BASE_MAX

Inquire about the maximum value of obase in the bc utility.

_SC_BC_DIM_MAX

Inquire about the maximum size of an array in the bc utility.

_SC_BC_SCALE_MAX

Inquire about the maximum value of scale in the bc utility.

_SC_BC_STRING_MAX

Inquire about the maximum size of a string constant in the bc utility.

_SC_COLL_WEIGHTS_MAX

Inquire about the maximum number of weights that can necessarily be used in defining the collating sequence for a locale.

_SC_EXPR_NEST_MAX

Inquire about the maximum number of expressions nested within parentheses when using the expr utility.

_SC_LINE_MAX

Inquire about the maximum size of a text line that the POSIX.2 text utilities can handle.

_SC_EQUIV_CLASS_MAX

Inquire about the maximum number of weights that can be assigned to an entry of the LC_COLLATE category 'order' keyword in a locale definition.  The GNU C library does not presently support locale definitions.

_SC_VERSION

Inquire about the version number of POSIX.1 that the library and kernel support.

_SC_2_VERSION

Inquire about the version number of POSIX.2 that the system utilities support.

_SC_PAGESIZE

Inquire about the virtual memory page size of the machine.  getpagesize returns the same value.


Examples of sysconf
Node: Examples of Sysconf, Next: , Prev: Constants for Sysconf, Up: Sysconf

Examples of sysconf

We recommend that you first test for a macro definition for the parameter you are interested in, and call sysconf only if the macro is not defined.  For example, here is how to test whether job control is supported:

int
have_job_control (void)
{
#ifdef _POSIX_JOB_CONTROL
  return 1;
#else
  int value = sysconf (_SC_JOB_CONTROL);
  if (value < 0)
    /* If the system is that badly wedged,
       there's no use trying to go on.  */
    fatal (strerror (errno));
  return value;
#endif
}

Here is how to get the value of a numeric limit:

int
get_child_max ()
{
#ifdef CHILD_MAX
  return CHILD_MAX;
#else
  int value = sysconf (_SC_CHILD_MAX);
  if (value < 0)
    fatal (strerror (errno));
  return value;
#endif
}


Minimum Values for General Capacity Limits
Node: Minimums, Next: Limits for Files, Prev: Sysconf, Up: System Configuration

Minimum Values for General Capacity Limits

Here are the names for the POSIX minimum upper bounds for the system limit parameters.  The significance of these values is that you can safely push to these limits without checking whether the particular system you are using can go that far.

_POSIX_ARG_MAX

The value of this macro is the most restrictive limit permitted by POSIX for the maximum combined length of the argv and environ arguments that can be passed to the exec functions.  Its value is 4096.

_POSIX_CHILD_MAX

The value of this macro is the most restrictive limit permitted by POSIX for the maximum number of simultaneous processes per real user ID.  Its value is 6.

_POSIX_NGROUPS_MAX

The value of this macro is the most restrictive limit permitted by POSIX for the maximum number of supplementary group IDs per process.  Its value is 0.

_POSIX_OPEN_MAX

The value of this macro is the most restrictive limit permitted by POSIX for the maximum number of files that a single process can have open simultaneously.  Its value is 16.

_POSIX_SSIZE_MAX

The value of this macro is the most restrictive limit permitted by POSIX for the maximum value that can be stored in an object of type ssize_t.  Its value is 32767.

_POSIX_STREAM_MAX

The value of this macro is the most restrictive limit permitted by POSIX for the maximum number of streams that a single process can have open simultaneously.  Its value is 8.

_POSIX_TZNAME_MAX

The value of this macro is the most restrictive limit permitted by POSIX for the maximum length of a time zone name.  Its value is 3.

_POSIX2_RE_DUP_MAX

The value of this macro is the most restrictive limit permitted by POSIX for the numbers used in the '\{min,max\}' construct in a regular expression.  Its value is 255.


Limits on File System Capacity
Node: Limits for Files, Next: Options for Files, Prev: Minimums, Up: System Configuration

Limits on File System Capacity

The POSIX.1 standard specifies a number of parameters that describe the limitations of the file system.  It's possible for the system to have a fixed, uniform limit for a parameter, but this isn't the usual case.  On most systems, it's possible for different file systems (and, for some parameters, even different files) to have different maximum limits.  For example, this is very likely if you use NFS to mount some of the file systems from other machines.

Each of the following macros is defined in 'limits.h' only if the system has a fixed, uniform limit for the parameter in question.  If the system allows different file systems or files to have different limits, then the macro is undefined; use pathconf or fpathconf to find out the limit that applies to a particular file.  See Pathconf.

Each parameter also has another macro, with a name starting with '_POSIX', which gives the lowest value that the limit is allowed to have on any POSIX system.  See File Minimums.

- Macro: int LINK_MAX

The uniform system limit (if any) for the number of names for a given file.  See Hard Links.

- Macro: int MAX_CANON

The uniform system limit (if any) for the amount of text in a line of input when input editing is enabled.  See Canonical or Not.

 - Macro: int MAX_INPUT

The uniform system limit (if any) for the total number of characters typed ahead as input.  See I/O Queues.

- Macro: int NAME_MAX

The uniform system limit (if any) for the length of a file name component.

 - Macro: int PATH_MAX

The uniform system limit (if any) for the length of an entire file name (that is, the argument given to system calls such as open).

- Macro: int PIPE_BUF

The uniform system limit (if any) for the number of bytes that can be written atomically to a pipe.  If multiple processes are writing to the same pipe simultaneously, output from different processes might be interleaved in chunks of this size.  See Pipes and FIFOs.

These are alternative macro names for some of the same information.

 - Macro: int MAXNAMLEN

This is the BSD name for NAME_MAX.  It is defined in 'dirent.h'.

 - Macro: int FILENAME_MAX

The value of this macro is an integer constant expression that represents the maximum length of a file name string.  It is defined in 'stdio.h'.

Unlike PATH_MAX, this macro is defined even if there is no actual limit imposed.  In such a case, its value is typically a very large number.  This is always the case on the GNU system.

Usage Note: Don't use FILENAME_MAX as the size of an array in which to store a file name!  You can't possibly make an array that big!  Use dynamic allocation (see Memory Allocation) instead.


Optional Features in File Support
Node: Options for Files, Next: File Minimums, Prev: Limits for Files, Up: System Configuration

Optional Features in File Support

POSIX defines certain system-specific options in the system calls for operating on files.  Some systems support these options and others do not.  Since these options are provided in the kernel, not in the library, simply using the GNU C library does not guarantee any of these features is supported; it depends on the system you are using.  They can also vary between file systems on a single machine.

This section describes the macros you can test to determine whether a particular option is supported on your machine.  If a given macro is defined in 'unistd.h', then its value says whether the corresponding feature is supported.  (A value of -1 indicates no; any other value indicates yes.)  If the macro is undefined, it means particular files may or may not support the feature.

Since all the machines that support the GNU C library also support NFS, one can never make a general statement about whether all file systems support the _POSIX_CHOWN_RESTRICTED and _POSIX_NO_TRUNC features.  So these names are never defined as macros in the GNU C library.

 - Macro: int _POSIX_CHOWN_RESTRICTED

If this option is in effect, the chown function is restricted so that the only changes permitted to nonprivileged processes is to change the group owner of a file to either be the effective group ID of the process, or one of its supplementary group IDs.  See File Owner.

 - Macro: int _POSIX_NO_TRUNC

If this option is in effect, file name components longer than NAME_MAX generate an ENAMETOOLONG error.  Otherwise, file name components that are too long are silently truncated.

 - Macro: unsigned char _POSIX_VDISABLE

This option is only meaningful for files that are terminal devices.  If it is enabled, then handling for special control characters can be disabled individually.  See Special Characters.

If one of these macros is undefined, that means that the option might be in effect for some files and not for others.  To inquire about a particular file, call pathconf or fpathconf.  See Pathconf.


Minimum Values for File System Limits
Node: File Minimums, Next: Pathconf, Prev: Options for Files, Up: System Configuration

Minimum Values for File System Limits

Here are the names for the POSIX minimum upper bounds for some of the above parameters.  The significance of these values is that you can safely push to these limits without checking whether the particular system you are using can go that far.

_POSIX_LINK_MAX

The most restrictive limit permitted by POSIX for the maximum value of a file's link count.  The value of this constant is 8; thus, you can always make up to eight names for a file without running into a system limit.

_POSIX_MAX_CANON

The most restrictive limit permitted by POSIX for the maximum number of bytes in a canonical input line from a terminal device.  The value of this constant is 255.

_POSIX_MAX_INPUT

The most restrictive limit permitted by POSIX for the maximum number of bytes in a terminal device input queue (or typeahead buffer).  See Input Modes.  The value of this constant is 255.

_POSIX_NAME_MAX

The most restrictive limit permitted by POSIX for the maximum number of bytes in a file name component.  The value of this constant is 14.

_POSIX_PATH_MAX

The most restrictive limit permitted by POSIX for the maximum number of bytes in a file name.  The value of this constant is 255.

_POSIX_PIPE_BUF

The most restrictive limit permitted by POSIX for the maximum number of bytes that can be written atomically to a pipe.  The value of this constant is 512.


Using pathconf
Node: Pathconf, Next: Utility Limits, Prev: File Minimums, Up: System Configuration

Using pathconf

When your machine allows different files to have different values for a file system parameter, you can use the functions in this section to find out the value that applies to any particular file.

These functions and the associated constants for the parameter argument are declared in the header file 'unistd.h'.

 - Function: long int pathconf (const char *filename, int parameter)

This function is used to inquire about the limits that apply to the file named filename.

The parameter argument should be one of the '_PC_' constants listed below.

The normal return value from pathconf is the value you requested.  A value of -1 is returned both if the implementation does not impose a limit, and in case of an error.  In the former case, errno is not set, while in the latter case, errno is set to indicate the cause of the problem.  So the only way to use this function robustly is to store 0 into errno just before calling it.

Besides the usual file name syntax errors (see File Name Errors), the following error condition is defined for this function:

EINVAL

The value of parameter is invalid, or the implementation doesn't support the parameter for the specific file.

 - Function: long int fpathconf (int filedes, int parameter)

This is just like pathconf except that an open file descriptor is used to specify the file for which information is requested, instead of a file name.

The following errno error conditions are defined for this function:

EBADF

The filedes argument is not a valid file descriptor.

EINVAL

The value of parameter is invalid, or the implementation doesn't support the parameter for the specific file.

Here are the symbolic constants that you can use as the parameter argument to pathconf and fpathconf.  The values are all integer constants.

_PC_LINK_MAX

Inquire about the value of LINK_MAX.

_PC_MAX_CANON

Inquire about the value of MAX_CANON.

_PC_MAX_INPUT

Inquire about the value of MAX_INPUT.

_PC_NAME_MAX

Inquire about the value of NAME_MAX.

_PC_PATH_MAX

Inquire about the value of PATH_MAX.

_PC_PIPE_BUF

Inquire about the value of PIPE_BUF.

_PC_CHOWN_RESTRICTED

Inquire about the value of _POSIX_CHOWN_RESTRICTED.

_PC_NO_TRUNC

Inquire about the value of _POSIX_NO_TRUNC.

_PC_VDISABLE

Inquire about the value of _POSIX_VDISABLE.


Utility Program Capacity Limits
Node: Utility Limits, Next: Utility Minimums, Prev: Pathconf, Up: System Configuration

Utility Program Capacity Limits

The POSIX.2 standard specifies certain system limits that you can access through sysconf that apply to utility behavior rather than the behavior of the library or the operating system.

The GNU C library defines macros for these limits, and sysconf returns values for them if you ask; but these values convey no meaningful information.  They are simply the smallest values that POSIX.2 permits.

 - Macro: int BC_BASE_MAX

The largest value of obase that the bc utility is guaranteed to support.

 - Macro: int BC_SCALE_MAX

The largest value of scale that the bc utility is guaranteed to support.

 - Macro: int BC_DIM_MAX

The largest number of elements in one array that the bc utility is guaranteed to support.

 - Macro: int BC_STRING_MAX

The largest number of characters in one string constant that the bc utility is guaranteed to support.

 - Macro: int BC_DIM_MAX

The largest number of elements in one array that the bc utility is guaranteed to support.

 - Macro: int COLL_WEIGHTS_MAX

The largest number of weights that can necessarily be used in defining the collating sequence for a locale.

 - Macro: int EXPR_NEST_MAX

The maximum number of expressions that can be nested within parenthesis by the expr utility.

 - Macro: int LINE_MAX

The largest text line that the text-oriented POSIX.2 utilities can support.  (If you are using the GNU versions of these utilities, then there is no actual limit except that imposed by the available virtual memory, but there is no way that the library can tell you this.)

 - Macro: int EQUIV_CLASS_MAX

The maximum number of weights that can be assigned to an entry of the LC_COLLATE category 'order' keyword in a locale definition.  The GNU C library does not presently support locale definitions.


Minimum Values for Utility Limits
Node: Utility Minimums, Next: String Parameters, Prev: Utility Limits, Up: System Configuration

Minimum Values for Utility Limits

_POSIX2_BC_BASE_MAX

The most restrictive limit permitted by POSIX.2 for the maximum value of obase in the bc utility.  Its value is 99.

_POSIX2_BC_DIM_MAX

The most restrictive limit permitted by POSIX.2 for the maximum size of an array in the bc utility.  Its value is 2048.

_POSIX2_BC_SCALE_MAX

The most restrictive limit permitted by POSIX.2 for the maximum value of scale in the bc utility.  Its value is 99.

_POSIX2_BC_STRING_MAX

The most restrictive limit permitted by POSIX.2 for the maximum size of a string constant in the bc utility.  Its value is 1000.

_POSIX2_COLL_WEIGHTS_MAX

The most restrictive limit permitted by POSIX.2 for the maximum number of weights that can necessarily be used in defining the collating sequence for a locale.  Its value is 2.

_POSIX2_EXPR_NEST_MAX

The most restrictive limit permitted by POSIX.2 for the maximum number of expressions nested within parenthesis when using the expr utility.  Its value is 32.

_POSIX2_LINE_MAX

The most restrictive limit permitted by POSIX.2 for the maximum size of a text line that the text utilities can handle.  Its value is 2048.

_POSIX2_EQUIV_CLASS_MAX

The most restrictive limit permitted by POSIX.2 for the maximum number of weights that can be assigned to an entry of the LC_COLLATE category 'order' keyword in a locale definition.  Its value is 2.  The GNU C library does not presently support locale definitions.


String-Valued Parameters
Node: String Parameters, Next: , Prev: Utility Minimums, Up: System Configuration

String-Valued Parameters

POSIX.2 defines a way to get string-valued parameters from the operating system with the function confstr:

 - Function: size_t confstr (int parameter, char *buf, size_t len)

This function reads the value of a string-valued system parameter, storing the string into len bytes of memory space starting at buf.  The parameter argument should be one of the '_CS_' symbols listed below.

The normal return value from confstr is the length of the string value that you asked for.  If you supply a null pointer for buf, then confstr does not try to store the string; it just returns its length.  A value of 0 indicates an error.

If the string you asked for is too long for the buffer (that is, longer than len - 1), then confstr stores just that much (leaving room for the terminating null character).  You can tell that this has happened because confstr returns a value greater than or equal to len.

The following errno error conditions are defined for this function:

EINVAL

The value of the parameter is invalid.

Currently there is just one parameter you can read with confstr:

_CS_PATH

This parameter's value is the recommended default path for searching for executable files.  This is the path that a user has by default just after logging in.

The way to use confstr without any arbitrary limit on string size is to call it twice: first call it to get the length, allocate the buffer accordingly, and then call confstr again to fill the buffer, like this:

char *
get_default_path (void)
{
  size_t len = confstr (_CS_PATH, NULL, 0);
  char *buffer = (char *) xmalloc (len);

  if (confstr (_CS_PATH, buf, len + 1) == 0)
    {
      free (buffer);
      return NULL;
    }

  return buffer;
}


C Language Facilities in the Library
Node: Language Features, Next: Library Summary, Prev: System Configuration, Up: Top

C Language Facilities in the Library

Some of the facilities implemented by the C library really should be thought of as parts of the C language itself.  These facilities ought to be documented in the C Language Manual, not in the library manual; but since we don't have the language manual yet, and documentation for these features has been written, we are publishing it here.

* Menu:

Consistency Checking	Using assert to abort if something "impossible" happens.

Variadic Functions	Defining functions with varying numbers of args.

Null Pointer Constant	The macro NULL.

Important Data Types	Data types for object sizes.

Data Type Measurements	Parameters of data type representations.


Explicitly Checking Internal Consistency
Node: Consistency Checking, Next: Variadic Functions, Prev: , Up: Language Features

Explicitly Checking Internal Consistency

When you're writing a program, it's often a good idea to put in checks at strategic places for "impossible" errors or violations of basic assumptions.  These checks are helpful in debugging problems due to misunderstandings between different parts of the program.

The assert macro, defined in the header file 'assert.h', provides a convenient way to abort the program while printing a message about where in the program the error was detected.

Once you think your program is debugged, you can disable the error checks performed by the assert macro by recompiling with the macro NDEBUG defined.  This means you don't actually have to change the program source code to disable these checks.

But disabling these consistency checks is undesirable unless they make the program significantly slower.  All else being equal, more error checking is good no matter who is running the program.  A wise user would rather have a program crash, visibly, than have it return nonsense without indicating anything might be wrong.

 - Macro: void assert (int expression)

Verify the programmer's belief that expression should be nonzero at this point in the program.

If NDEBUG is not defined, assert tests the value of expression.  If it is false (zero), assert aborts the program (see Aborting a Program) after printing a message of the form:

'file':linenum: Assertion `expression' failed.

on the standard error stream stderr (see Standard Streams).  The filename and line number are taken from the C preprocessor macros __FILE__ and __LINE__ and specify where the call to assert was written.

If the preprocessor macro NDEBUG is defined at the point where 'assert.h' is included, the assert macro is defined to do absolutely nothing.

Warning: Even the argument expression expression is not evaluated if NDEBUG is in effect.  So never use assert with arguments that involve side effects.  For example, assert (++i > 0); is a bad idea, because i will not be incremented if NDEBUG is defined.

Usage note: The assert facility is designed for detecting internal inconsistency; it is not suitable for reporting invalid input or improper usage by the user of the program.

The information in the diagnostic messages printed by the assert macro is intended to help you, the programmer, track down the cause of a bug, but is not really useful for telling a user of your program why his or her input was invalid or why a command could not be carried out.  So you can't use assert to print the error messages for these eventualities.

What's more, your program should not abort when given invalid input, as assert would do--it should exit with nonzero status (see Exit Status) after printing its error messages, or perhaps read another command or move on to the next input file.

See Error Messages, for information on printing error messages for problems that do not represent bugs in the program.


Variadic Functions
Node: Variadic Functions, Next: Null Pointer Constant, Prev: Consistency Checking, Up: Language Features

Variadic Functions

ANSI C defines a syntax for declaring a function to take a variable number or type of arguments.  (Such functions are referred to as "varargs functions" or "variadic functions".)  However, the language itself provides no mechanism for such functions to access their non-required arguments; instead, you use the variable arguments macros defined in 'stdarg.h'.

This section describes how to declare variadic functions, how to write them, and how to call them properly.

Compatibility Note: Many older C dialects provide a similar, but incompatible, mechanism for defining functions with variable numbers of arguments, using 'varargs.h'.

* Menu:

Why Variadic	Reasons for making functions take variable arguments.

How Variadic	How to define and call variadic functions.

Variadic Example	A complete example.


Why Variadic Functions are Used
Node: Why Variadic, Next: How Variadic, Prev: , Up: Variadic Functions

Why Variadic Functions are Used

Ordinary C functions take a fixed number of arguments.  When you define a function, you specify the data type for each argument.  Every call to the function should supply the expected number of arguments, with types that can be converted to the specified ones.  Thus, if the function 'foo' is declared with int foo (int, char *); then you must call it with two arguments, a number (any kind will do) and a string pointer.

But some functions perform operations that can meaningfully accept an unlimited number of arguments.

In some cases a function can handle any number of values by operating on all of them as a block.  For example, consider a function that allocates a one-dimensional array with malloc to hold a specified set of values.  This operation makes sense for any number of values, as long as the length of the array corresponds to that number.  Without facilities for variable arguments, you would have to define a separate function for each possible array size.

The library function printf (see Formatted Output) is an example of another class of function where variable arguments are useful.  This function prints its arguments (which can vary in type as well as number) under the control of a format template string.

These are good reasons to define a "variadic" function which can handle as many arguments as the caller chooses to pass.

Some functions such as open take a fixed set of arguments, but occasionally ignore the last few.  Strict adherence to ANSI C requires these functions to be defined as variadic; in practice, however, the GNU C compiler and most other C compilers let you define such a function to take a fixed set of arguments--the most it can ever use--and then only declare the function as variadic (or not declare its arguments at all!).


How Variadic Functions are Defined and Used
Node: How Variadic, Next: Variadic Example, Prev: Why Variadic, Up: Variadic Functions

How Variadic Functions are Defined and Used

Defining and using a variadic function involves three steps:

•	Define the function as variadic, using an ellipsis ('...') in the argument list, and using special macros to access the variable arguments.  See Receiving Arguments.

•	Declare the function as variadic, using a prototype with an ellipsis ('...'), in all the files which call it.  See Variadic Prototypes.

•	Call the function by writing the fixed arguments followed by the additional variable arguments.  See Calling Variadics.

* Menu:

Variadic Prototypes	How to make a prototype for a function with variable arguments.

Receiving Arguments	Steps you must follow to access the optional argument values.

How Many Arguments	How to decide whether there are more arguments.

Calling Variadics	Things you need to know about calling variable arguments functions.

Argument Macros	Detailed specification of the macros for accessing variable arguments.

Old Varargs	The pre-ANSI way of defining variadic functions.


Syntax for Variable Arguments
Node: Variadic Prototypes, Next: Receiving Arguments, Prev: , Up: How Variadic

Syntax for Variable Arguments

A function that accepts a variable number of arguments must be declared with a prototype that says so.   You write the fixed arguments as usual, and then tack on '...' to indicate the possibility of additional arguments.  The syntax of ANSI C requires at least one fixed argument before the '...'.  For example,

int
func (const char *a, int b, ...)
{
  ...
}

outlines a definition of a function func which returns an int and takes two required arguments, a const char * and an int.  These are followed by any number of anonymous arguments.

Portability note: For some C compilers, the last required argument must not be declared register in the function definition.  Furthermore, this argument's type must be "self-promoting": that is, the default promotions must not change its type.  This rules out array and function types, as well as float, char (whether signed or not) and short int (whether signed or not).  This is actually an ANSI C requirement.


Receiving the Argument Values
Node: Receiving Arguments, Next: How Many Arguments, Prev: Variadic Prototypes, Up: How Variadic

Receiving the Argument Values

Ordinary fixed arguments have individual names, and you can use these names to access their values.  But optional arguments have no names--nothing but '...'.  How can you access them?

The only way to access them is sequentially, in the order they were written, and you must use special macros from 'stdarg.h' in the following three step process:

1. 	You initialize an argument pointer variable of type va_list using va_start.  The argument pointer when initialized points to the first optional argument.

2. 	You access the optional arguments by successive calls to va_arg.  The first call to va_arg gives you the first optional argument, the next call gives you the second, and so on.

You can stop at any time if you wish to ignore any remaining optional arguments.  It is perfectly all right for a function to access fewer arguments than were supplied in the call, but you will get garbage values if you try to access too many arguments.

3. 	You indicate that you are finished with the argument pointer variable by calling va_end.

(In practice, with most C compilers, calling va_end does nothing and you do not really need to call it.  This is always true in the GNU C compiler.  But you might as well call va_end just in case your program is someday compiled with a peculiar compiler.)

See Argument Macros, for the full definitions of va_start, va_arg and va_end.

Steps 1 and 3 must be performed in the function that accepts the optional arguments.  However, you can pass the va_list variable as an argument to another function and perform all or part of step 2 there.

You can perform the entire sequence of the three steps multiple times within a single function invocation.  If you want to ignore the optional arguments, you can do these steps zero times.

You can have more than one argument pointer variable if you like.  You can initialize each variable with va_start when you wish, and then you can fetch arguments with each argument pointer as you wish.  Each argument pointer variable will sequence through the same set of argument values, but at its own pace.

Portability note: With some compilers, once you pass an argument pointer value to a subroutine, you must not keep using the same argument pointer value after that subroutine returns.  For full portability, you should just pass it to va_end.  This is actually an ANSI C requirement, but most ANSI C compilers work happily regardless.


How Many Arguments Were Supplied
Node: How Many Arguments, Next: Calling Variadics, Prev: Receiving Arguments, Up: How Variadic

How Many Arguments Were Supplied

There is no general way for a function to determine the number and type of the optional arguments it was called with.  So whoever designs the function typically designs a convention for the caller to tell it how many arguments it has, and what kind.  It is up to you to define an appropriate calling convention for each variadic function, and write all calls accordingly.

One kind of calling convention is to pass the number of optional arguments as one of the fixed arguments.  This convention works provided all of the optional arguments are of the same type.

A similar alternative is to have one of the required arguments be a bit mask, with a bit for each possible purpose for which an optional argument might be supplied.  You would test the bits in a predefined sequence; if the bit is set, fetch the value of the next argument, otherwise use a default value.

A required argument can be used as a pattern to specify both the number and types of the optional arguments.  The format string argument to printf is one example of this (see Formatted Output Functions).

Another possibility is to pass an "end marker" value as the last optional argument.  For example, for a function that manipulates an arbitrary number of pointer arguments, a null pointer might indicate the end of the argument list.  (This assumes that a null pointer isn't otherwise meaningful to the function.)  The execl function works in just this way; see See Executing a File.


Calling Variadic Functions
Node: Calling Variadics, Next: Argument Macros, Prev: How Many Arguments, Up: How Variadic

Calling Variadic Functions

You don't have to write anything special when you call a variadic function.  Just write the arguments (required arguments, followed by optional ones) inside parentheses, separated by commas, as usual.  But you should prepare by declaring the function with a prototype, and you must know how the argument values are converted.

In principle, functions that are defined to be variadic must also be declared to be variadic using a function prototype whenever you call them.  (See Variadic Prototypes, for how.)  This is because some C compilers use a different calling convention to pass the same set of argument values to a function depending on whether that function takes variable arguments or fixed arguments.

In practice, the GNU C compiler always passes a given set of argument types in the same way regardless of whether they are optional or required.  So, as long as the argument types are self-promoting, you can safely omit declaring them.  Usually it is a good idea to declare the argument types for variadic functions, and indeed for all functions.  But there are a few functions which it is extremely convenient not to have to declare as variadic--for example, open and printf.

Since the prototype doesn't specify types for optional arguments, in a call to a variadic function the "default argument promotions" are performed on the optional argument values.  This means the objects of type char or short int (whether signed or not) are promoted to either int or unsigned int, as appropriate; and that objects of type float are promoted to type double.  So, if the caller passes a char as an optional argument, it is promoted to an int, and the function should get it with va_arg (ap, int).

Conversion of the required arguments is controlled by the function prototype in the usual way: the argument expression is converted to the declared argument type as if it were being assigned to a variable of that type.


Argument Access Macros
Node: Argument Macros, Next: Old Varargs, Prev: Calling Variadics, Up: How Variadic

Argument Access Macros

Here are descriptions of the macros used to retrieve variable arguments.  These macros are defined in the header file 'stdarg.h'.  K

 - Data Type: va_list

The type va_list is used for argument pointer variables.

 - Macro: void va_start (va_list ap, last_required)

This macro initializes the argument pointer variable ap to point to the first of the optional arguments of the current function; last_required must be the last required argument to the function.

See Old Varargs, for an alternate definition of va_start found in the header file 'varargs.h'.

 - Macro: type va_arg (va_list ap, type)

The va_arg macro returns the value of the next optional argument, and modifies the value of ap to point to the subsequent argument.  Thus, successive uses of va_arg return successive optional arguments.

The type of the value returned by va_arg is type as specified in the call.  type must be a self-promoting type (not char or short int or float) that matches the type of the actual argument.

 - Macro: void va_end (va_list ap)

This ends the use of ap.  After a va_end call, further va_arg calls with the same ap may not work.  You should invoke va_end before returning from the function in which va_start was invoked with the same ap argument.

In the GNU C library, va_end does nothing, and you need not ever use it except for reasons of portability.


Example of a Variadic Function
Node: Variadic Example, Next: , Prev: How Variadic, Up: Variadic Functions

Example of a Variadic Function

Here is a complete sample function that accepts a variable number of arguments.  The first argument to the function is the count of remaining arguments, which are added up and the result returned.  While trivial, this function is sufficient to illustrate how to use the variable arguments facility.

#include <stdarg.h>
#include <stdio.h>

int
add_em_up (int count,...)
{
  va_list ap;
  int i, sum;

  va_start (ap, count);         /* Initialize the argument list. */

  sum = 0;
  for (i = 0; i < count; i++)
    sum += va_arg (ap, int);    /* Get the next argument value. */

  va_end (ap);                  /* Clean up. */
  return sum;
}

int
main (void)
{
  /* This call prints 16. */
  printf ("%d\n", add_em_up (3, 5, 5, 6));

  /* This call prints 55. */
  printf ("%d\n", add_em_up (10, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10));

  return 0;
}


Old-Style Variadic Functions
Node: Old Varargs, Next: , Prev: Argument Macros, Up: How Variadic

Old-Style Variadic Functions

Before ANSI C, programmers used a slightly different facility for writing variadic functions.  The GNU C compiler still supports it; currently, it is more portable than the ANSI C facility, since support for ANSI C is still not universal.  The header file which defines the old-fashioned variadic facility is called 'varargs.h'.

Using 'varargs.h' is almost the same as using 'stdarg.h'.  There is no difference in how you call a variadic function; See Calling Variadics.  The only difference is in how you define them.  First of all, you must use old-style non-prototype syntax, like this:

tree
build (va_alist)
     va_dcl
{

Secondly, you must give va_start just one argument, like this:

  va_list p;
  va_start (p);

These are the special macros used for defining old-style variadic functions:

 - Macro: va_alist

This macro stands for the argument name list required in a variadic function.

 - Macro: va_dcl

This macro declares the implicit argument or arguments for a variadic function.

 - Macro: void va_start (va_list ap)

This macro, as defined in 'varargs.h', initializes the argument pointer variable ap to point to the first argument of the current function.

The other argument macros, va_arg and va_end, are the same in 'varargs.h' as in 'stdarg.h'; see See Argument Macros for details.

It does not work to include both 'varargs.h' and 'stdarg.h' in the same compilation; they define va_start in conflicting ways.


Null Pointer Constant
Node: Null Pointer Constant, Next: Important Data Types, Prev: Variadic Functions, Up: Language Features

Null Pointer Constant

The null pointer constant is guaranteed not to point to any real object.  You can assign it to any pointer variable since it has type void *.  The preferred way to write a null pointer constant is with NULL.

 - Macro: void * NULL

This is a null pointer constant.

You can also use 0 or (void *)0 as a null pointer constant, but using NULL is cleaner because it makes the purpose of the constant more evident.

If you use the null pointer constant as a function argument, then for complete portability you should make sure that the function has a prototype declaration.  Otherwise, if the target machine has two different pointer representations, the compiler won't know which representation to use for that argument.  You can avoid the problem by explicitly casting the constant to the proper pointer type, but we recommend instead adding a prototype for the function you are calling.


Important Data Types
Node: Important Data Types, Next: Data Type Measurements, Prev: Null Pointer Constant, Up: Language Features

Important Data Types

The result of subtracting two pointers in C is always an integer, but the precise data type varies from C compiler to C compiler.  Likewise, the data type of the result of sizeof also varies between compilers.  ANSI defines standard aliases for these two types, so you can refer to them in a portable fashion.  They are defined in the header file 'stddef.h'.  K

 - Data Type: ptrdiff_t

This is the signed integer type of the result of subtracting two pointers.  For example, with the declaration char *p1, *p2;, the expression p2 - p1 is of type ptrdiff_t.  This will probably be one of the standard signed integer types (short int, int or long int), but might be a nonstandard type that exists only for this purpose.

 - Data Type: size_t

This is an unsigned integer type used to represent the sizes of objects.  The result of the sizeof operator is of this type, and functions such as malloc (see Unconstrained Allocation) and memcpy (see Copying and Concatenation) accept arguments of this type to specify object sizes.

Usage Note: size_t is the preferred way to declare any arguments or variables that hold the size of an object.

In the GNU system size_t is equivalent to either unsigned int or unsigned long int.  These types have identical properties on the GNU system, and for most purposes, you can use them interchangeably.  However, they are distinct as data types, which makes a difference in certain contexts.

For example, when you specify the type of a function argument in a function prototype, it makes a difference which one you use.  If the system header files declare malloc with an argument of type size_t and you declare malloc with an argument of type unsigned int, you will get a compilation error if size_t happens to be unsigned long int on your system.  To avoid any possibility of error, when a function argument or value is supposed to have type size_t, never declare its type in any other way.

Compatibility Note: Implementations of C before the advent of ANSI C generally used unsigned int for representing object sizes and int for pointer subtraction results.  They did not necessarily define either size_t or ptrdiff_t.  Unix systems did define size_t, in 'sys/types.h', but the definition was usually a signed type.


Data Type Measurements
Node: Data Type Measurements, Next: , Prev: Important Data Types, Up: Language Features

Data Type Measurements

Most of the time, if you choose the proper C data type for each object in your program, you need not be concerned with just how it is represented or how many bits it uses.  When you do need such information, the C language itself does not provide a way to get it.  The header files 'limits.h' and 'float.h' contain macros which give you this information in full detail.

* Menu:

Width of Type	How many bits does an integer type hold?

Range of Type	What are the largest and smallest values that an integer type can hold?

Floating Type Macros	Parameters that measure the floating point types.

Structure Measurement	Getting measurements on structure types.


Computing the Width of an Integer Data Type
Node: Width of Type, Next: Range of Type, Prev: , Up: Data Type Measurements

Computing the Width of an Integer Data Type

The most common reason that a program needs to know how many bits are in an integer type is for using an array of long int as a bit vector.  You can access the bit at index n with

vector[n / LONGBITS] & (1 << (n % LONGBITS))

provided you define LONGBITS as the number of bits in a long int.

There is no operator in the C language that can give you the number of bits in an integer data type.  But you can compute it from the macro CHAR_BIT, defined in the header file 'limits.h'.

CHAR_BIT

This is the number of bits in a char--eight, on most systems.  The value has type int.

You can compute the number of bits in any data type type like this:

sizeof (type) * CHAR_BIT


Range of an Integer Type
Node: Range of Type, Next: Floating Type Macros, Prev: Width of Type, Up: Data Type Measurements

Range of an Integer Type

Suppose you need to store an integer value which can range from zero to one million.  Which is the smallest type you can use?  There is no general rule; it depends on the C compiler and target machine.  You can use the 'MIN' and 'MAX' macros in 'limits.h' to determine which type will work.

Each signed integer type has a pair of macros which give the smallest and largest values that it can hold.  Each unsigned integer type has one such macro, for the maximum value; the minimum value is, of course, zero.

The values of these macros are all integer constant expressions.  The 'MAX' and 'MIN' macros for char and short int types have values of type int.  The 'MAX' and 'MIN' macros for the other types have values of the same type described by the macro--thus, ULONG_MAX has type unsigned long int.

SCHAR_MIN

This is the minimum value that can be represented by a signed char.

SCHAR_MAX

UCHAR_MAX

These are the maximum values that can be represented by a signed char and unsigned char, respectively.

CHAR_MIN

This is the minimum value that can be represented by a char.  It's equal to SCHAR_MIN if char is signed, or zero otherwise.

CHAR_MAX

This is the maximum value that can be represented by a char.  It's equal to SCHAR_MAX if char is signed, or UCHAR_MAX otherwise.

SHRT_MIN

This is the minimum value that can be represented by a signed short int.  On most machines that the GNU C library runs on, short integers are 16-bit quantities.

SHRT_MAX

USHRT_MAX

These are the maximum values that can be represented by a signed short int and unsigned short int, respectively.

INT_MIN

This is the minimum value that can be represented by a signed int.  On most machines that the GNU C system runs on, an int is a 32-bit quantity.

INT_MAX

UINT_MAX

These are the maximum values that can be represented by, respectively, the type signed int and the type unsigned int.

LONG_MIN

This is the minimum value that can be represented by a signed long int.  On most machines that the GNU C system runs on, long integers are 32-bit quantities, the same size as int.

LONG_MAX

ULONG_MAX

These are the maximum values that can be represented by a signed long int and unsigned long int, respectively.

LONG_LONG_MIN

This is the minimum value that can be represented by a signed long long int.  On most machines that the GNU C system runs on, long long integers are 64-bit quantities.

LONG_LONG_MAX

ULONG_LONG_MAX

These are the maximum values that can be represented by a signed long long int and unsigned long long int, respectively.

WCHAR_MAX

This is the maximum value that can be represented by a wchar_t.  See Wide Char Intro.

The header file 'limits.h' also defines some additional constants that parameterize various operating system and file system limits.  These constants are described in See System Configuration.


Floating Type Macros
Node: Floating Type Macros, Next: Structure Measurement, Prev: Range of Type, Up: Data Type Measurements

Floating Type Macros

The specific representation of floating point numbers varies from machine to machine.  Because floating point numbers are represented internally as approximate quantities, algorithms for manipulating floating point data often need to take account of the precise details of the machine's floating point representation.

Some of the functions in the C library itself need this information; for example, the algorithms for printing and reading floating point numbers (see I/O on Streams) and for calculating trigonometric and irrational functions (see Mathematics) use it to avoid round-off error and loss of accuracy.  User programs that implement numerical analysis techniques also often need this information in order to minimize or compute error bounds.

The header file 'float.h' describes the format used by your machine.

* Menu:

Floating Point Concepts	Definitions of terminology.

Floating Point Parameters	Details of specific macros.

IEEE Floating Point	The measurements for one common representation.


Floating Point Representation Concepts
Node: Floating Point Concepts, Next: Floating Point Parameters, Prev: , Up: Floating Type Macros

Floating Point Representation Concepts

This section introduces the terminology for describing floating point representations.

You are probably already familiar with most of these concepts in terms of scientific or exponential notation for floating point numbers.  For example, the number 123456.0 could be expressed in exponential notation as 1.23456e+05, a shorthand notation indicating that the mantissa 1.23456 is multiplied by the base 10 raised to power 5.

More formally, the internal representation of a floating point number can be characterized in terms of the following parameters:

•	The "sign" is either -1 or 1.

•	The "base" or "radix" for exponentiation, an integer greater than 1.  This is a constant for a particular representation.

•	The "exponent" to which the base is raised.  The upper and lower bounds of the exponent value are constants for a particular representation.

Sometimes, in the actual bits representing the floating point number, the exponent is "biased" by adding a constant to it, to make it always be represented as an unsigned quantity.  This is only important if you have some reason to pick apart the bit fields making up the floating point number by hand, which is something for which the GNU library provides no support.  So this is ignored in the discussion that follows.

•	The "mantissa" or "significand", an unsigned integer which is a part of each floating point number.

•	The "precision" of the mantissa.  If the base of the representation is b, then the precision is the number of base-b digits in the mantissa.  This is a constant for a particular representation.

Many floating point representations have an implicit "hidden bit" in the mantissa.  This is a bit which is present virtually in the mantissa, but not stored in memory because its value is always 1 in a normalized number.  The precision figure (see above) includes any hidden bits.

Again, the GNU library provides no facilities for dealing with such low-level aspects of the representation.

The mantissa of a floating point number actually represents an implicit fraction whose denominator is the base raised to the power of the precision.  Since the largest representable mantissa is one less than this denominator, the value of the fraction is always strictly less than 1.  The mathematical value of a floating point number is then the product of this fraction, the sign, and the base raised to the exponent.

We say that the floating point number is "normalized" if the fraction is at least 1/b, where b is the base.  In other words, the mantissa would be too large to fit if it were multiplied by the base.  Non-normalized numbers are sometimes called "denormal"; they contain less precision than the representation normally can hold.

If the number is not normalized, then you can subtract 1 from the exponent while multiplying the mantissa by the base, and get another floating point number with the same value.  "Normalization" consists of doing this repeatedly until the number is normalized.  Two distinct normalized floating point numbers cannot be equal in value.

(There is an exception to this rule: if the mantissa is zero, it is considered normalized.  Another exception happens on certain machines where the exponent is as small as the representation can hold.  Then it is impossible to subtract 1 from the exponent, so a number may be normalized even if its fraction is less than 1/b.)


Floating Point Parameters
Node: Floating Point Parameters, Next: IEEE Floating Point, Prev: Floating Point Concepts, Up: Floating Type Macros

Floating Point Parameters

These macro definitions can be accessed by including the header file 'float.h' in your program.

Macro names starting with 'FLT_' refer to the float type, while names beginning with 'DBL_' refer to the double type and names beginning with 'LDBL_' refer to the long double type.  (Currently GCC does not support long double as a distinct data type, so the values for the 'LDBL_' constants are equal to the corresponding constants for the double type.)

Of these macros, only FLT_RADIX is guaranteed to be a constant expression.  The other macros listed here cannot be reliably used in places that require constant expressions, such as '#if' preprocessing directives or in the dimensions of static arrays.

Although the ANSI C standard specifies minimum and maximum values for most of these parameters, the GNU C implementation uses whatever values describe the floating point representation of the target machine.  So in principle GNU C actually satisfies the ANSI C requirements only if the target machine is suitable.  In practice, all the machines currently supported are suitable.

FLT_ROUNDS

This value characterizes the rounding mode for floating point addition.  The following values indicate standard rounding modes:

-1

The mode is indeterminable.

0

Rounding is towards zero.

1

Rounding is to the nearest number.

2

Rounding is towards positive infinity.

3

Rounding is towards negative infinity.

Any other value represents a machine-dependent nonstandard rounding mode.

On most machines, the value is 1, in accordance with the IEEE standard for floating point.

Here is a table showing how certain values round for each possible value of FLT_ROUNDS, if the other aspects of the representation match the IEEE single-precision standard.

                0      1             2             3
 1.00000003    1.0    1.0           1.00000012    1.0
 1.00000007    1.0    1.00000012    1.00000012    1.0
-1.00000003   -1.0   -1.0          -1.0          -1.00000012
-1.00000007   -1.0   -1.00000012   -1.0          -1.00000012

FLT_RADIX

This is the value of the base, or radix, of exponent representation.  This is guaranteed to be a constant expression, unlike the other macros described in this section.  The value is 2 on all machines we know of except the IBM 360 and derivatives.

FLT_MANT_DIG

This is the number of base-FLT_RADIX digits in the floating point mantissa for the float data type.  The following expression yields 1.0 (even though mathematically it should not) due to the limited number of mantissa digits:

float radix = FLT_RADIX;

1.0f + 1.0f / radix / radix / ... / radix

where radix appears FLT_MANT_DIG times.

DBL_MANT_DIG

LDBL_MANT_DIG

This is the number of base-FLT_RADIX digits in the floating point mantissa for the data types double and long double, respectively.

FLT_DIG

This is the number of decimal digits of precision for the float data type.  Technically, if p and b are the precision and base (respectively) for the representation, then the decimal precision q is the maximum number of decimal digits such that any floating point number with q base 10 digits can be rounded to a floating point number with p base b digits and back again, without change to the q decimal digits.

The value of this macro is supposed to be at least 6, to satisfy ANSI C.

DBL_DIG

LDBL_DIG

These are similar to FLT_DIG, but for the data types double and long double, respectively.  The values of these macros are supposed to be at least 10.

FLT_MIN_EXP

This is the smallest possible exponent value for type float.  More precisely, is the minimum negative integer such that the value FLT_RADIX raised to this power minus 1 can be represented as a normalized floating point number of type float.

DBL_MIN_EXP

LDBL_MIN_EXP

These are similar to FLT_MIN_EXP, but for the data types double and long double, respectively.

FLT_MIN_10_EXP

This is the minimum negative integer such that 10 raised to this power minus 1 can be represented as a normalized floating point number of type float.  This is supposed to be -37 or even less.

DBL_MIN_10_EXP

LDBL_MIN_10_EXP

These are similar to FLT_MIN_10_EXP, but for the data types double and long double, respectively.

FLT_MAX_EXP

This is the largest possible exponent value for type float.  More precisely, this is the maximum positive integer such that value FLT_RADIX raised to this power minus 1 can be represented as a floating point number of type float.

DBL_MAX_EXP

LDBL_MAX_EXP

These are similar to FLT_MAX_EXP, but for the data types double and long double, respectively.

FLT_MAX_10_EXP

This is the maximum positive integer such that 10 raised to this power minus 1 can be represented as a normalized floating point number of type float.  This is supposed to be at least 37.

DBL_MAX_10_EXP

LDBL_MAX_10_EXP

These are similar to FLT_MAX_10_EXP, but for the data types double and long double, respectively.

FLT_MAX

The value of this macro is the maximum number representable in type float.  It is supposed to be at least 1E+37.  The value has type float.

The smallest representable number is - FLT_MAX.

DBL_MAX

LDBL_MAX

These are similar to FLT_MAX, but for the data types double and long double, respectively.  The type of the macro's value is the same as the type it describes.

FLT_MIN

The value of this macro is the minimum normalized positive floating point number that is representable in type float.  It is supposed to be no more than 1E-37.

DBL_MIN

LDBL_MIN

These are similar to FLT_MIN, but for the data types double and long double, respectively.  The type of the macro's value is the same as the type it describes.

FLT_EPSILON

This is the minimum positive floating point number of type float such that 1.0 + FLT_EPSILON != 1.0 is true.  It's supposed to be no greater than 1E-5.

DBL_EPSILON

LDBL_EPSILON

These are similar to FLT_EPSILON, but for the data types double and long double, respectively.  The type of the macro's value is the same as the type it describes.  The values are not supposed to be greater than 1E-9.


IEEE Floating Point
Node: IEEE Floating Point, Next: , Prev: Floating Point Parameters, Up: Floating Type Macros

IEEE Floating Point

Here is an example showing how the floating type measurements come out for the most common floating point representation, specified by the `IEEE Standard for Binary Floating Point Arithmetic (ANSI/IEEE Std 754-1985)'.  Nearly all computers designed since the 1980s use this format.

The IEEE single-precision float representation uses a base of 2.  There is a sign bit, a mantissa with 23 bits plus one hidden bit (so the total precision is 24 base-2 digits), and an 8-bit exponent that can represent values in the range -125 to 128, inclusive.

So, for an implementation that uses this representation for the float data type, appropriate values for the corresponding parameters are:

FLT_RADIX                             2
FLT_MANT_DIG                         24
FLT_DIG                               6
FLT_MIN_EXP                        -125
FLT_MIN_10_EXP                      -37
FLT_MAX_EXP                         128
FLT_MAX_10_EXP                      +38
FLT_MIN                 1.17549435E-38F
FLT_MAX                 3.40282347E+38F
FLT_EPSILON             1.19209290E-07F

Here are the values for the double data type:

DBL_MANT_DIG                         53
DBL_DIG                              15
DBL_MIN_EXP                       -1021
DBL_MIN_10_EXP                     -307
DBL_MAX_EXP                        1024
DBL_MAX_10_EXP                      308
DBL_MAX         1.7976931348623157E+308
DBL_MIN         2.2250738585072014E-308
DBL_EPSILON     2.2204460492503131E-016


Structure Field Offset Measurement
Node: Structure Measurement, Next: , Prev: Floating Type Macros, Up: Data Type Measurements

Structure Field Offset Measurement

You can use offsetof to measure the location within a structure type of a particular structure member.

 - Macro: size_t offsetof (type, member)

This expands to a integer constant expression that is the offset of the structure member named member in a the structure type type.  For example, offsetof (struct s, elem) is the offset, in bytes, of the member elem in a struct s.

This macro won't work if member is a bit field; you get an error from the C compiler in that case.


Summary of Library Facilities
Node: Library Summary, Next: Maintenance, Prev: Language Features, Up: Top

Summary of Library Facilities

This appendix is a complete list of the facilities declared within the header files supplied with the GNU C library.  Each entry also lists the standard or other source from which each facility is derived, and tells you where in the manual you can find more information about how to use it.

void abort (void)

'stdlib.h' (ANSI):  See Aborting a Program.

int abs (int number)

'stdlib.h' (ANSI):  See Absolute Value.

int accept (int socket, struct sockaddr *addr, size_t *length_ptr)

'sys/socket.h' (BSD):  See Accepting Connections.

int access (const char *filename, int how)

'unistd.h' (POSIX.1):  See Testing File Access.

double acosh (double x)

'math.h' (BSD):  See Hyperbolic Functions.

double acos (double x)

'math.h' (ANSI):  See Inverse Trig Functions.

int adjtime (const struct timeval *delta, struct timeval *olddelta)

'sys/time.h' (BSD):  See High-Resolution Calendar.

AF_FILE

'sys/socket.h' (GNU):  See Address Formats.

AF_INET

'sys/socket.h' (BSD):  See Address Formats.

AF_UNIX

'sys/socket.h' (BSD):  See Address Formats.

AF_UNSPEC

'sys/socket.h' (BSD):  See Address Formats.

unsigned int alarm (unsigned int seconds)

'unistd.h' (POSIX.1):  See Setting an Alarm.

void * alloca (size_t size);

'stdlib.h' (GNU, BSD):  See Variable Size Automatic.

ALTWERASE

'termios.h' (BSD):  See Local Modes.

int ARG_MAX

'limits.h' (POSIX.1):  See General Limits.

char * asctime (const struct tm *brokentime)

'time.h' (ANSI):  See Formatting Date and Time.

double asinh (double x)

'math.h' (BSD):  See Hyperbolic Functions.

double asin (double x)

'math.h' (ANSI):  See Inverse Trig Functions.

int asprintf (char **ptr, const char *template, ...)

'stdio.h' (GNU):  See Dynamic Output.

void assert (int expression)

'assert.h' (ANSI):  See Consistency Checking.

double atan2 (double y, double x)

'math.h' (ANSI):  See Inverse Trig Functions.

double atanh (double x)

'math.h' (BSD):  See Hyperbolic Functions.

double atan (double x)

'math.h' (ANSI):  See Inverse Trig Functions.

int atexit (void (*function) (void))

'stdlib.h' (ANSI):  See Cleanups on Exit.

double atof (const char *string)

'stdlib.h' (ANSI):  See Parsing of Floats.

int atoi (const char *string)

'stdlib.h' (ANSI):  See Parsing of Integers.

long int atol (const char *string)

'stdlib.h' (ANSI):  See Parsing of Integers.

B0

'termios.h' (POSIX.1):  See Line Speed.

B110

'termios.h' (POSIX.1):  See Line Speed.

B1200

'termios.h' (POSIX.1):  See Line Speed.

B134

'termios.h' (POSIX.1):  See Line Speed.

B150

'termios.h' (POSIX.1):  See Line Speed.

B1800

'termios.h' (POSIX.1):  See Line Speed.

B19200

'termios.h' (POSIX.1):  See Line Speed.

B200

'termios.h' (POSIX.1):  See Line Speed.

B2400

'termios.h' (POSIX.1):  See Line Speed.

B300

'termios.h' (POSIX.1):  See Line Speed.

B38400

'termios.h' (POSIX.1):  See Line Speed.

B4800

'termios.h' (POSIX.1):  See Line Speed.

B50

'termios.h' (POSIX.1):  See Line Speed.

B600

'termios.h' (POSIX.1):  See Line Speed.

B75

'termios.h' (POSIX.1):  See Line Speed.

B9600

'termios.h' (POSIX.1):  See Line Speed.

int BC_BASE_MAX

'limits.h' (POSIX.2):  See Utility Limits.

int BC_DIM_MAX

'limits.h' (POSIX.2):  See Utility Limits.

int BC_DIM_MAX

'limits.h' (POSIX.2):  See Utility Limits.

int bcmp (const void *a1, const void *a2, size_t size)

'string.h' (BSD):  See String/Array Comparison.

void * bcopy (void *from, const void *to, size_t size)

'string.h' (BSD):  See Copying and Concatenation.

int BC_SCALE_MAX

'limits.h' (POSIX.2):  See Utility Limits.

int BC_STRING_MAX

'limits.h' (POSIX.2):  See Utility Limits.

int bind (int socket, struct sockaddr *addr, size_t length)

'sys/socket.h' (BSD):  See Setting Address.

BRKINT

'termios.h' (POSIX.1):  See Input Modes.

_BSD_SOURCE

(GNU):  See Feature Test Macros.

void * bsearch (const void *key, const void *array, size_t count, size_t size, comparison_fn_t compare)

'stdlib.h' (ANSI):  See Array Search Function.

int BUFSIZ

'stdio.h' (ANSI):  See Controlling Buffering.

void * bzero (void *block, size_t size)

'string.h' (BSD):  See Copying and Concatenation.

double cabs (struct { double real, imag; } z)

'math.h' (BSD):  See Absolute Value.

void * calloc (size_t count, size_t eltsize)

'malloc.h', 'stdlib.h' (ANSI):  See Allocating Cleared Space.

double cbrt (double x)

'math.h' (BSD):  See Exponents and Logarithms.

cc_t

'termios.h' (POSIX.1):  See Mode Data Types.

CCTS_OFLOW

'termios.h' (BSD):  See Control Modes.

double ceil (double x)

'math.h' (ANSI):  See Rounding and Remainders.

speed_t cfgetispeed (const struct termios *termios_p)

'termios.h' (POSIX.1):  See Line Speed.

speed_t cfgetospeed (const struct termios *termios_p)

'termios.h' (POSIX.1):  See Line Speed.

int cfmakeraw (struct termios *termios_p)

'termios.h' (BSD):  See Line Speed.

void cfree (void *ptr)

'stdlib.h' (Sun):  See Freeing after Malloc.

int cfsetispeed (struct termios *termios_p, speed_t speed)

'termios.h' (POSIX.1):  See Line Speed.

int cfsetospeed (struct termios *termios_p, speed_t speed)

'termios.h' (POSIX.1):  See Line Speed.

int cfsetspeed (struct termios *termios_p, speed_t speed)

'termios.h' (BSD):  See Line Speed.

CHAR_BIT

'limits.h' (ANSI):  See Width of Type.

CHAR_MAX

'limits.h' (ANSI):  See Range of Type.

CHAR_MIN

'limits.h' (ANSI):  See Range of Type.

int chdir (const char *filename)

'unistd.h' (POSIX.1):  See Working Directory.

int CHILD_MAX

'limits.h' (POSIX.1):  See General Limits.

int chmod (const char *filename, mode_t mode)

'sys/stat.h' (POSIX.1):  See Setting Permissions.

int chown (const char *filename, uid_t owner, gid_t group)

'unistd.h' (POSIX.1):  See File Owner.

void clearerr (FILE *stream)

'stdio.h' (ANSI):  See EOF and Errors.

int CLK_TCK

'time.h' (POSIX.1):  See Basic CPU Time.

CLOCAL

'termios.h' (POSIX.1):  See Control Modes.

clock_t clock (void)

'time.h' (ANSI):  See Basic CPU Time.

int CLOCKS_PER_SEC

'time.h' (ANSI):  See Basic CPU Time.

clock_t

'time.h' (ANSI):  See Basic CPU Time.

int closedir (DIR *dirstream)

'dirent.h' (POSIX.1):  See Reading/Closing Directory.

int close (int filedes)

'unistd.h' (POSIX.1):  See Opening and Closing Files.

int COLL_WEIGHTS_MAX

'limits.h' (POSIX.2):  See Utility Limits.

size_t confstr (int parameter, char *buf, size_t len)

'unistd.h' (POSIX.2):  See String Parameters.

int connect (int socket, struct sockaddr *addr, size_t length)

'sys/socket.h' (BSD):  See Connecting.

cookie_close_function

'stdio.h' (GNU):  See Hook Functions.

cookie_io_functions_t

'stdio.h' (GNU):  See Streams and Cookies.

cookie_read_function

'stdio.h' (GNU):  See Hook Functions.

cookie_seek_function

'stdio.h' (GNU):  See Hook Functions.

cookie_write_function

'stdio.h' (GNU):  See Hook Functions.

double copysign (double value, double sign)

'math.h' (BSD):  See Normalization Functions.

double cosh (double x)

'math.h' (ANSI):  See Hyperbolic Functions.

double cos (double x)

'math.h' (ANSI):  See Trig Functions.

CREAD

'termios.h' (POSIX.1):  See Control Modes.

int creat (const char *filename, mode_t mode)

'fcntl.h' (POSIX.1):  See Opening and Closing Files.

CRTS_IFLOW

'termios.h' (BSD):  See Control Modes.

CS5

'termios.h' (POSIX.1):  See Control Modes.

CS6

'termios.h' (POSIX.1):  See Control Modes.

CS7

'termios.h' (POSIX.1):  See Control Modes.

CS8

'termios.h' (POSIX.1):  See Control Modes.

CSIZE

'termios.h' (POSIX.1):  See Control Modes.

_CS_PATH

'unistd.h' (POSIX.2):  See String Parameters.

CSTOPB

'termios.h' (POSIX.1):  See Control Modes.

char * ctermid (char *string)

'stdio.h' (POSIX.1):  See Identifying the Terminal.

char * ctime (const time_t *time)

'time.h' (ANSI):  See Formatting Date and Time.

char * cuserid (char *string)

'stdio.h' (POSIX.1):  See Who Logged In.

int daylight

'time.h' (SVID):  See Time Zone Functions.

DBL_DIG

'float.h' (ANSI):  See Floating Point Parameters.

DBL_EPSILON

'float.h' (ANSI):  See Floating Point Parameters.

DBL_MANT_DIG

'float.h' (ANSI):  See Floating Point Parameters.

DBL_MAX_10_EXP

'float.h' (ANSI):  See Floating Point Parameters.

DBL_MAX_EXP

'float.h' (ANSI):  See Floating Point Parameters.

DBL_MAX

'float.h' (ANSI):  See Floating Point Parameters.

DBL_MIN_10_EXP

'float.h' (ANSI):  See Floating Point Parameters.

DBL_MIN_EXP

'float.h' (ANSI):  See Floating Point Parameters.

DBL_MIN

'float.h' (ANSI):  See Floating Point Parameters.

dev_t

'sys/types.h' (POSIX.1):  See Attribute Meanings.

double difftime (time_t time1, time_t time0)

'time.h' (ANSI):  See Simple Calendar Time.

DIR

'dirent.h' (POSIX.1):  See Opening a Directory.

div_t div (int numerator, int denominator)

'stdlib.h' (ANSI):  See Integer Division.

div_t

'stdlib.h' (ANSI):  See Integer Division.

double drem (double numerator, double denominator)

'math.h' (BSD):  See Rounding and Remainders.

int dup2 (int old, int new)

'unistd.h' (POSIX.1):  See Duplicating Descriptors.

int dup (int old)

'unistd.h' (POSIX.1):  See Duplicating Descriptors.

int E2BIG

'errno.h' (POSIX.1: Argument list too long):  See Error Codes.

int EACCES

'errno.h' (POSIX.1: Permission denied):  See Error Codes.

int EADDRINUSE

'errno.h' (BSD: Address already in use):  See Error Codes.

int EADDRNOTAVAIL

'errno.h' (BSD: Can't assign requested address):  See Error Codes.

int EAFNOSUPPORT

'errno.h' (BSD: Address family not supported by protocol family):  See Error Codes.

int EAGAIN

'errno.h' (POSIX.1: Resource temporarily unavailable):  See Error Codes.

int EALREADY

'errno.h' (BSD: Operation already in progress):  See Error Codes.

int EBACKGROUND

'errno.h' (GNU: Inappropriate operation for background process):  See Error Codes.

int EBADF

'errno.h' (POSIX.1: Bad file descriptor):  See Error Codes.

int EBUSY

'errno.h' (POSIX.1: Device busy):  See Error Codes.

int ECHILD

'errno.h' (POSIX.1: No child processes):  See Error Codes.

ECHOCTL

'termios.h' (BSD):  See Local Modes.

ECHOE

'termios.h' (POSIX.1):  See Local Modes.

ECHO

'termios.h' (POSIX.1):  See Local Modes.

ECHOKE

'termios.h' (BSD):  See Local Modes.

ECHOK

'termios.h' (POSIX.1):  See Local Modes.

ECHONL

'termios.h' (POSIX.1):  See Local Modes.

ECHOPRT

'termios.h' (BSD):  See Local Modes.

int ECONNABORTED

'errno.h' (BSD: Software caused connection abort):  See Error Codes.

int ECONNREFUSED

'errno.h' (BSD: Connection refused):  See Error Codes.

int ECONNRESET

'errno.h' (BSD: Connection reset by peer):  See Error Codes.

int EDEADLK

'errno.h' (POSIX.1: Resource deadlock avoided):  See Error Codes.

int EDESTADDRREQ

'errno.h' (BSD: Destination address required):  See Error Codes.

int ED

'errno.h' (GNU: ?):  See Error Codes.

int EDOM

'errno.h' (ANSI: Numerical argument out of domain):  See Error Codes.

int EDQUOT

'errno.h' (BSD: Disc quota exceeded):  See Error Codes.

int EEXIST

'errno.h' (POSIX.1: File exists):  See Error Codes.

int EFAULT

'errno.h' (POSIX.1: Bad address):  See Error Codes.

int EFBIG

'errno.h' (POSIX.1: File too large):  See Error Codes.

int EGRATUITOUS

'errno.h' (GNU: Gratuitous error):  See Error Codes.

int EGREGIOUS

'errno.h' (GNU: You really blew it this time):  See Error Codes.

int EHOSTDOWN

'errno.h' (BSD: Host is down):  See Error Codes.

int EHOSTUNREACH

'errno.h' (BSD: No route to host):  See Error Codes.

int EIEIO

'errno.h' (GNU: Computer bought the farm):  See Error Codes.

int EINPROGRESS

'errno.h' (BSD: Operation now in progress):  See Error Codes.

int EINTR

'errno.h' (POSIX.1: Interrupted system call):  See Error Codes.

int EINVAL

'errno.h' (POSIX.1: Invalid argument):  See Error Codes.

int EIO

'errno.h' (POSIX.1: Input/output error):  See Error Codes.

int EISCONN

'errno.h' (BSD: Socket is already connected):  See Error Codes.

int EISDIR

'errno.h' (POSIX.1: Is a directory):  See Error Codes.

int ELOOP

'errno.h' (BSD: Too many levels of symbolic links):  See Error Codes.

int EMFILE

'errno.h' (POSIX.1: Too many open files):  See Error Codes.

int EMLINK

'errno.h' (POSIX.1: Too many links):  See Error Codes.

int EMSGSIZE

'errno.h' (BSD: Message too long):  See Error Codes.

int ENAMETOOLONG

'errno.h' (POSIX.1: File name too long):  See Error Codes.

void endgrent (void)

'grp.h' (SVID, BSD):  See Scanning All Groups.

void endhostent ()

'netdb.h' (BSD):  See Host Names.

void endnetent (void)

'netdb.h' (BSD):  See Networks Database.

void endprotoent (void)

'netdb.h' (BSD):  See Protocols Database.

void endpwent (void)

'pwd.h' (SVID, BSD):  See Scanning All Users.

void endservent (void)

'netdb.h' (BSD):  See Services Database.

int ENETDOWN

'errno.h' (BSD: Network is down):  See Error Codes.

int ENETRESET

'errno.h' (BSD: Network dropped connection on reset):  See Error Codes.

int ENETUNREACH

'errno.h' (BSD: Network is unreachable):  See Error Codes.

int ENFILE

'errno.h' (POSIX.1: Too many open files in system):  See Error Codes.

int ENOBUFS

'errno.h' (BSD: No buffer space available):  See Error Codes.

int ENODEV

'errno.h' (POSIX.1: Operation not supported by device):  See Error Codes.

int ENOENT

'errno.h' (POSIX.1: No such file or directory):  See Error Codes.

int ENOEXEC

'errno.h' (POSIX.1: Exec format error):  See Error Codes.

int ENOLCK

'errno.h' (POSIX.1: No locks available):  See Error Codes.

int ENOMEM

'errno.h' (POSIX.1: Cannot allocate memory):  See Error Codes.

int ENOPROTOOPT

'errno.h' (BSD: Protocol not available):  See Error Codes.

int ENOSPC

'errno.h' (POSIX.1: No space left on device):  See Error Codes.

int ENOSYS

'errno.h' (POSIX.1: Function not implemented):  See Error Codes.

int ENOTBLK

'errno.h' (BSD: Block device required):  See Error Codes.

int ENOTCONN

'errno.h' (BSD: Socket is not connected):  See Error Codes.

int ENOTDIR

'errno.h' (POSIX.1: Not a directory):  See Error Codes.

int ENOTEMPTY

'errno.h' (POSIX.1: Directory not empty):  See Error Codes.

int ENOTSOCK

'errno.h' (BSD: Socket operation on non-socket):  See Error Codes.

int ENOTTY

'errno.h' (POSIX.1: Inappropriate ioctl for device):  See Error Codes.

char ** environ

'unistd.h' (POSIX.1):  See Environment Access.

int ENXIO

'errno.h' (POSIX.1: Device not configured):  See Error Codes.

int EOF

'stdio.h' (ANSI):  See EOF and Errors.

int EOPNOTSUPP

'errno.h' (BSD: Operation not supported):  See Error Codes.

int EPERM

'errno.h' (POSIX.1: Operation not permitted):  See Error Codes.

int EPFNOSUPPORT

'errno.h' (BSD: Protocol family not supported):  See Error Codes.

int EPIPE

'errno.h' (POSIX.1: Broken pipe):  See Error Codes.

int EPROTONOSUPPORT

'errno.h' (BSD: Protocol not supported):  See Error Codes.

int EPROTOTYPE

'errno.h' (BSD: Protocol wrong type for socket):  See Error Codes.

int EQUIV_CLASS_MAX

'limits.h' (POSIX.2):  See Utility Limits.

int ERANGE

'errno.h' (ANSI: Numerical result out of range):  See Error Codes.

int EREMOTE

'errno.h' (BSD: Too many levels of remote in path):  See Error Codes.

int EROFS

'errno.h' (POSIX.1: Read-only file system):  See Error Codes.

volatile int errno

'errno.h' (ANSI):  See Checking for Errors.

int ESHUTDOWN

'errno.h' (BSD: Can't send after socket shutdown):  See Error Codes.

int ESOCKTNOSUPPORT

'errno.h' (BSD: Socket type not supported):  See Error Codes.

int ESPIPE

'errno.h' (POSIX.1: Illegal seek):  See Error Codes.

int ESRCH

'errno.h' (POSIX.1: No such process):  See Error Codes.

int ESTALE

'errno.h' (BSD: Stale NFS file handle):  See Error Codes.

int ETIMEDOUT

'errno.h' (BSD: Connection timed out):  See Error Codes.

int ETXTBSY

'errno.h' (BSD: Text file busy):  See Error Codes.

int EUSERS

'errno.h' (BSD: Too many users):  See Error Codes.

int EWOULDBLOCK

'errno.h' (BSD: Operation would block):  See Error Codes.

int EXDEV

'errno.h' (POSIX.1: Invalid cross-device link):  See Error Codes.

int execle (const char *filename, const char *arg0, char *const env[], ...)

'unistd.h' (POSIX.1):  See Executing a File.

int execl (const char *filename, const char *arg0, ...)

'unistd.h' (POSIX.1):  See Executing a File.

int execlp (const char *filename, const char *arg0, ...)

'unistd.h' (POSIX.1):  See Executing a File.

int execve (const char *filename, char *const argv[], char *const env[])

'unistd.h' (POSIX.1):  See Executing a File.

int execv (const char *filename, char *const argv[])

'unistd.h' (POSIX.1):  See Executing a File.

int execvp (const char *filename, char *const argv[])

'unistd.h' (POSIX.1):  See Executing a File.

int EXIT_FAILURE

'stdlib.h' (ANSI):  See Exit Status.

void exit (int status)

'stdlib.h' (ANSI):  See Normal Termination.

void _exit (int status)

'unistd.h' (POSIX.1):  See Termination Internals.

int EXIT_SUCCESS

'stdlib.h' (ANSI):  See Exit Status.

double exp (double x)

'math.h' (ANSI):  See Exponents and Logarithms.

double expm1 (double x)

'math.h' (BSD):  See Exponents and Logarithms.

int EXPR_NEST_MAX

'limits.h' (POSIX.2):  See Utility Limits.

double fabs (double number)

'math.h' (ANSI):  See Absolute Value.

int fchmod (int filedes, int mode)

'sys/stat.h' (BSD):  See Setting Permissions.

int fchown (int filedes, int owner, int group)

'unistd.h' (BSD):  See File Owner.

int fclean (FILE *stream)

'stdio.h' (GNU):  See Cleaning Streams.

int fclose (FILE *stream)

'stdio.h' (ANSI):  See Closing Streams.

int fcntl (int filedes, int command, ...)

'fcntl.h' (POSIX.1):  See Control Operations.

int FD_CLOEXEC

'fcntl.h' (POSIX.1):  See Descriptor Flags.

void FD_CLR (int filedes, fd_set *set)

'sys/types.h' (BSD):  See Waiting for I/O.

int FD_ISSET (int filedes, fd_set *set)

'sys/types.h' (BSD):  See Waiting for I/O.

FILE * fdopen (int filedes, const char *opentype)

'stdio.h' (POSIX.1):  See Descriptors and Streams.

void FD_SET (int filedes, fd_set *set)

'sys/types.h' (BSD):  See Waiting for I/O.

fd_set

'sys/types.h' (BSD):  See Waiting for I/O.

int FD_SETSIZE

'sys/types.h' (BSD):  See Waiting for I/O.

int F_DUPFD

'fcntl.h' (POSIX.1):  See Duplicating Descriptors.

void FD_ZERO (fd_set *set)

'sys/types.h' (BSD):  See Waiting for I/O.

int feof (FILE *stream)

'stdio.h' (ANSI):  See EOF and Errors.

int ferror (FILE *stream)

'stdio.h' (ANSI):  See EOF and Errors.

int fflush (FILE *stream)

'stdio.h' (ANSI):  See Flushing Buffers.

int fgetc (FILE *stream)

'stdio.h' (ANSI):  See Character Input.

int F_GETFD

'fcntl.h' (POSIX.1):  See Descriptor Flags.

int F_GETFL

'fcntl.h' (POSIX.1):  See File Status Flags.

struct group * fgetgrent (FILE *stream)

'grp.h' (SVID):  See Scanning All Groups.

int F_GETLK

'fcntl.h' (POSIX.1):  See File Locks.

int F_GETOWN

'fcntl.h' (BSD):  See Interrupt Input.

int fgetpos (FILE *stream, fpos_t *position)

'stdio.h' (ANSI):  See Portable Positioning.

struct passwd * fgetpwent (FILE *stream)

'pwd.h' (SVID):  See Scanning All Users.

char * fgets (char *s, int count, FILE *stream)

'stdio.h' (ANSI):  See Line Input.

FILE

'stdio.h' (ANSI):  See Streams.

int FILENAME_MAX

'stdio.h' (ANSI):  See Limits for Files.

int fileno (FILE *stream)

'stdio.h' (POSIX.1):  See Descriptors and Streams.

int finite (double x)

'math.h' (BSD):  See Predicates on Floats.

double floor (double x)

'math.h' (ANSI):  See Rounding and Remainders.

FLT_DIG

'float.h' (ANSI):  See Floating Point Parameters.

FLT_EPSILON

'float.h' (ANSI):  See Floating Point Parameters.

FLT_MANT_DIG

'float.h' (ANSI):  See Floating Point Parameters.

FLT_MAX_10_EXP

'float.h' (ANSI):  See Floating Point Parameters.

FLT_MAX_EXP

'float.h' (ANSI):  See Floating Point Parameters.

FLT_MAX

'float.h' (ANSI):  See Floating Point Parameters.

FLT_MIN_10_EXP

'float.h' (ANSI):  See Floating Point Parameters.

FLT_MIN_EXP

'float.h' (ANSI):  See Floating Point Parameters.

FLT_MIN

'float.h' (ANSI):  See Floating Point Parameters.

FLT_RADIX

'float.h' (ANSI):  See Floating Point Parameters.

FLT_ROUNDS

'float.h' (ANSI):  See Floating Point Parameters.

FLUSHO

'termios.h' (BSD):  See Local Modes.

FILE * fmemopen (void *buf, size_t size, const char *opentype)

'stdio.h' (GNU):  See String Streams.

double fmod (double numerator, double denominator)

'math.h' (ANSI):  See Rounding and Remainders.

int fnmatch (const char *pattern, const char *string, int flags)

'fnmatch.h' (POSIX.2):  See Wildcard Matching.

FNM_CASEFOLD

'fnmatch.h' (GNU):  See Wildcard Matching.

FNM_FILE_NAME

'fnmatch.h' (GNU):  See Wildcard Matching.

FNM_LEADING_DIR

'fnmatch.h' (GNU):  See Wildcard Matching.

FNM_NOESCAPE

'fnmatch.h' (POSIX.2):  See Wildcard Matching.

FNM_PATHNAME

'fnmatch.h' (POSIX.2):  See Wildcard Matching.

FNM_PERIOD

'fnmatch.h' (POSIX.2):  See Wildcard Matching.

int F_OK

'unistd.h' (POSIX.1):  See Testing File Access.

FILE * fopencookie (void *cookie, const char *opentype, cookie_io_functions_t io_functions)

'stdio.h' (GNU):  See Streams and Cookies.

FILE * fopen (const char *filename, const char *opentype)

'stdio.h' (ANSI):  See Opening Streams.

int FOPEN_MAX

'stdio.h' (ANSI):  See Opening Streams.

pid_t fork (void)

'unistd.h' (POSIX.1):  See Creating a Process.

long int fpathconf (int filedes, int parameter)

'unistd.h' (POSIX.1):  See Pathconf.

FPE_DECOVF_TRAP

'signal.h' (BSD):  See Program Error Signals.

FPE_FLTDIV_FAULT

'signal.h' (BSD):  See Program Error Signals.

FPE_FLTDIV_TRAP

'signal.h' (BSD):  See Program Error Signals.

FPE_FLTOVF_FAULT

'signal.h' (BSD):  See Program Error Signals.

FPE_FLTOVF_TRAP

'signal.h' (BSD):  See Program Error Signals.

FPE_FLTUND_FAULT

'signal.h' (BSD):  See Program Error Signals.

FPE_FLTUND_TRAP

'signal.h' (BSD):  See Program Error Signals.

FPE_INTDIV_TRAP

'signal.h' (BSD):  See Program Error Signals.

FPE_INTOVF_TRAP

'signal.h' (BSD):  See Program Error Signals.

FPE_SUBRNG_TRAP

'signal.h' (BSD):  See Program Error Signals.

fpos_t

'stdio.h' (ANSI):  See Portable Positioning.

int fprintf (FILE *stream, const char *template, ...)

'stdio.h' (ANSI):  See Formatted Output Functions.

int fputc (int c, FILE *stream)

'stdio.h' (ANSI):  See Simple Output.

int fputs (const char *s, FILE *stream)

'stdio.h' (ANSI):  See Simple Output.

F_RDLCK

'fcntl.h' (POSIX.1):  See File Locks.

size_t fread (void *data, size_t size, size_t count, FILE *stream)

'stdio.h' (ANSI):  See Block Input/Output.

__free_hook

'malloc.h' (GNU):  See Hooks for Malloc.

void free (void *ptr)

'malloc.h', 'stdlib.h' (ANSI):  See Freeing after Malloc.

FILE * freopen (const char *filename, const char *opentype, FILE *stream)

'stdio.h' (ANSI):  See Opening Streams.

double frexp (double value, int *exponent)

'math.h' (ANSI):  See Normalization Functions.

int fscanf (FILE *stream, const char *template, ...)

'stdio.h' (ANSI):  See Formatted Input Functions.

int fseek (FILE *stream, long int offset, int whence)

'stdio.h' (ANSI):  See File Positioning.

int F_SETFD

'fcntl.h' (POSIX.1):  See Descriptor Flags.

int F_SETFL

'fcntl.h' (POSIX.1):  See File Status Flags.

int F_SETLK

'fcntl.h' (POSIX.1):  See File Locks.

int F_SETLKW

'fcntl.h' (POSIX.1):  See File Locks.

int F_SETOWN

'fcntl.h' (BSD):  See Interrupt Input.

int fsetpos (FILE *stream, const fpos_t position)

'stdio.h' (ANSI):  See Portable Positioning.

int fstat (int filedes, struct stat *buf)

'sys/stat.h' (POSIX.1):  See Reading Attributes.

long int ftell (FILE *stream)

'stdio.h' (ANSI):  See File Positioning.

F_UNLCK

'fcntl.h' (POSIX.1):  See File Locks.

size_t fwrite (const void *data, size_t size, size_t count, FILE *stream)

'stdio.h' (ANSI):  See Block Input/Output.

F_WRLCK

'fcntl.h' (POSIX.1):  See File Locks.

int getchar (void)

'stdio.h' (ANSI):  See Character Input.

int getc (FILE *stream)

'stdio.h' (ANSI):  See Character Input.

char * getcwd (char *buffer, size_t size)

'unistd.h' (POSIX.1):  See Working Directory.

ssize_t getdelim (char **lineptr, size_t *n, int delimiter, FILE *stream)

'stdio.h' (GNU):  See Line Input.

gid_t getegid (void)

'unistd.h' (POSIX.1):  See Reading Persona.

char * getenv (const char *name)

'stdlib.h' (ANSI):  See Environment Access.

uid_t geteuid (void)

'unistd.h' (POSIX.1):  See Reading Persona.

gid_t getgid (void)

'unistd.h' (POSIX.1):  See Reading Persona.

struct group * getgrent (void)

'grp.h' (SVID, BSD):  See Scanning All Groups.

struct group * getgrgid (gid_t gid)

'grp.h' (POSIX.1):  See Lookup Group.

struct group * getgrnam (const char *name)

'grp.h' (SVID, BSD):  See Lookup Group.

int getgroups (int count, gid_t *groups)

'unistd.h' (POSIX.1):  See Reading Persona.

struct hostent * gethostbyaddr (const char *addr, int length, int format)

'netdb.h' (BSD):  See Host Names.

struct hostent * gethostbyname (const char *name)

'netdb.h' (BSD):  See Host Names.

struct hostent * gethostent ()

'netdb.h' (BSD):  See Host Names.

long int gethostid (void)

'unistd.h' (BSD):  See Host Identification.

int gethostname (char *name, size_t size)

'unistd.h' (BSD):  See Host Identification.

int getitimer (int which, struct itimerval *old)

'sys/time.h' (BSD):  See Setting an Alarm.

ssize_t getline (char **lineptr, size_t *n, FILE *stream)

'stdio.h' (GNU):  See Line Input.

char * getlogin (void)

'unistd.h' (POSIX.1):  See Who Logged In.

struct netent * getnetbyaddr (long net, int type)

'netdb.h' (BSD):  See Networks Database.

struct netent * getnetbyname (const char *name)

'netdb.h' (BSD):  See Networks Database.

struct netent * getnetent (void)

'netdb.h' (BSD):  See Networks Database.

int getopt (int argc, char **argv, const char *options)

'unistd.h' (POSIX.2):  See Parsing Options.

int getopt_long (int argc, char **argv, const char *shortopts, struct option *longopts, int *indexptr)

'getopt.h' (GNU):  See Long Options.

int getpeername (int socket, struct sockaddr *addr, size_t *length_ptr)

'sys/socket.h' (BSD):  See Who is Connected.

pid_t getpgrp (pid_t pid)

'unistd.h' (BSD):  See Process Group Functions.

pid_t getpgrp (void)

'unistd.h' (POSIX.1):  See Process Group Functions.

pid_t getpid (void)

'unistd.h' (POSIX.1):  See Process Identification.

pid_t getppid (void)

'unistd.h' (POSIX.1):  See Process Identification.

int getpriority (int class, int id)

'sys/resource.h' (BSD):  See Priority.

struct protoent * getprotobyname (const char *name)

'netdb.h' (BSD):  See Protocols Database.

struct protoent * getprotobynumber (int protocol)

'netdb.h' (BSD):  See Protocols Database.

struct protoent * getprotoent (void)

'netdb.h' (BSD):  See Protocols Database.

struct passwd * getpwent (void)

'pwd.h' (POSIX.1):  See Scanning All Users.

struct passwd * getpwnam (const char *name)

'pwd.h' (POSIX.1):  See Lookup User.

struct passwd * getpwuid (uid_t uid)

'pwd.h' (POSIX.1):  See Lookup User.

int getrlimit (int resource, struct rlimit *rlp)

'sys/resource.h' (BSD):  See Limits on Resources.

int getrusage (int processes, struct rusage *rusage)

'sys/resource.h' (BSD):  See Resource Usage.

struct servent * getservbyname (const char *name, const char *proto)

'netdb.h' (BSD):  See Services Database.

struct servent * getservbyport (int port, const char *proto)

'netdb.h' (BSD):  See Services Database.

struct servent * getservent (void)

'netdb.h' (BSD):  See Services Database.

char * gets (char *s)

'stdio.h' (ANSI):  See Line Input.

int getsockname (int socket, struct sockaddr *addr, size_t *length_ptr)

'sys/socket.h' (BSD):  See Reading Address.

int getsockopt (int socket, int level, int optname, void *optval, size_t *optlen_ptr)

'sys/socket.h' (BSD):  See Socket Option Functions.

int gettimeofday (struct timeval *tp, struct timezone *tzp)

'sys/time.h' (BSD):  See High-Resolution Calendar.

uid_t getuid (void)

'unistd.h' (POSIX.1):  See Reading Persona.

mode_t getumask (void)

'sys/stat.h' (GNU):  See Setting Permissions.

char * getwd (char *buffer)

'unistd.h' (BSD):  See Working Directory.

int getw (FILE *stream)

'stdio.h' (SVID):  See Character Input.

gid_t

'sys/types.h' (POSIX.1):  See Reading Persona.

GLOB_ABORTED

'glob.h' (POSIX.2):  See Calling Glob.

GLOB_APPEND

'glob.h' (POSIX.2):  See Flags for Globbing.

GLOB_DOOFFS

'glob.h' (POSIX.2):  See Flags for Globbing.

GLOB_ERR

'glob.h' (POSIX.2):  See Flags for Globbing.

int glob (const char *pattern, int flags, int (*errfunc) (const char *filename, int error-code), glob_t *vector_ptr)

'glob.h' (POSIX.2):  See Calling Glob.

GLOB_MARK

'glob.h' (POSIX.2):  See Flags for Globbing.

GLOB_NOCHECK

'glob.h' (POSIX.2):  See Flags for Globbing.

GLOB_NOESCAPE

'glob.h' (POSIX.2):  See Flags for Globbing.

GLOB_NOMATCH

'glob.h' (POSIX.2):  See Calling Glob.

GLOB_NOSORT

'glob.h' (POSIX.2):  See Flags for Globbing.

GLOB_NOSPACE

'glob.h' (POSIX.2):  See Calling Glob.

glob_t

'glob.h' (POSIX.2):  See Calling Glob.

struct tm * gmtime (const time_t *time)

'time.h' (ANSI):  See Broken-down Time.

_GNU_SOURCE

(GNU):  See Feature Test Macros.

int gsignal (int signum)

'signal.h' (SVID):  See Signaling Yourself.

HOST_NOT_FOUND

'netdb.h' (BSD):  See Host Names.

unsigned long int htonl (unsigned long int hostlong)

'netinet/in.h' (BSD):  See Byte Order.

unsigned short int htons (unsigned short int hostshort)

'netinet/in.h' (BSD):  See Byte Order.

double HUGE_VAL

'math.h' (ANSI):  See Domain and Range Errors.

HUPCL

'termios.h' (POSIX.1):  See Control Modes.

double hypot (double x, double y)

'math.h' (BSD):  See Exponents and Logarithms.

ICANON

'termios.h' (POSIX.1):  See Local Modes.

ICRNL

'termios.h' (POSIX.1):  See Input Modes.

IEXTEN

'termios.h' (POSIX.1):  See Local Modes.

IGNBRK

'termios.h' (POSIX.1):  See Input Modes.

IGNCR

'termios.h' (POSIX.1):  See Input Modes.

IGNPAR

'termios.h' (POSIX.1):  See Input Modes.

IMAXBEL

'termios.h' (BSD):  See Input Modes.

unsigned long int INADDR_ANY

'netinet/in.h' (BSD):  See Host Address Data Type.

unsigned long int INADDR_BROADCAST

'netinet/in.h' (BSD):  See Host Address Data Type.

unsigned long int INADDR_LOOPBACK

'netinet/in.h' (BSD):  See Host Address Data Type.

unsigned long int INADDR_NONE

'netinet/in.h' (BSD):  See Host Address Data Type.

char * index (const char *string, int c)

'string.h' (BSD):  See Search Functions.

unsigned long int inet_addr (const char *name)

'arpa/inet.h' (BSD):  See Host Address Functions.

int inet_aton (const char *name, struct in_addr *addr)

'arpa/inet.h' (BSD):  See Host Address Functions.

int inet_lnaof (struct in_addr addr)

'arpa/inet.h' (BSD):  See Host Address Functions.

struct in_addr inet_makeaddr (int net, int local)

'arpa/inet.h' (BSD):  See Host Address Functions.

int inet_netof (struct in_addr addr)

'arpa/inet.h' (BSD):  See Host Address Functions.

unsigned long int inet_network (const char *name)

'arpa/inet.h' (BSD):  See Host Address Functions.

char * inet_ntoa (struct in_addr addr)

'arpa/inet.h' (BSD):  See Host Address Functions.

double infnan (int error)

'math.h' (BSD):  See Predicates on Floats.

int initgroups (const char *user, gid_t gid)

'grp.h' (BSD):  See Setting Groups.

void * initstate (unsigned int seed, void *state, size_t size)

'stdlib.h' (BSD):  See BSD Random.

INLCR

'termios.h' (POSIX.1):  See Input Modes.

ino_t

'sys/types.h' (POSIX.1):  See Attribute Meanings.

INPCK

'termios.h' (POSIX.1):  See Input Modes.

int RLIM_INFINITY

'sys/resource.h' (BSD):  See Limits on Resources.

INT_MAX

'limits.h' (ANSI):  See Range of Type.

INT_MIN

'limits.h' (ANSI):  See Range of Type.

int _IOFBF

'stdio.h' (ANSI):  See Controlling Buffering.

int _IOLBF

'stdio.h' (ANSI):  See Controlling Buffering.

int _IONBF

'stdio.h' (ANSI):  See Controlling Buffering.

int IPPORT_RESERVED

'netinet/in.h' (BSD):  See Ports.

int IPPORT_USERRESERVED

'netinet/in.h' (BSD):  See Ports.

int isalnum (int c)

'ctype.h' (ANSI):  See Classification of Characters.

int isalpha (int c)

'ctype.h' (ANSI):  See Classification of Characters.

int isascii (int c)

'ctype.h' (SVID, BSD):  See Classification of Characters.

int isatty (int filedes)

'unistd.h' (POSIX.1):  See Is It a Terminal.

int isblank (int c)

'ctype.h' (GNU):  See Classification of Characters.

int iscntrl (int c)

'ctype.h' (ANSI):  See Classification of Characters.

int isdigit (int c)

'ctype.h' (ANSI):  See Classification of Characters.

int isgraph (int c)

'ctype.h' (ANSI):  See Classification of Characters.

ISIG

'termios.h' (POSIX.1):  See Local Modes.

int isinf (double x)

'math.h' (BSD):  See Predicates on Floats.

int islower (int c)

'ctype.h' (ANSI):  See Classification of Characters.

int isnan (double x)

'math.h' (BSD):  See Predicates on Floats.

int isprint (int c)

'ctype.h' (ANSI):  See Classification of Characters.

int ispunct (int c)

'ctype.h' (ANSI):  See Classification of Characters.

int isspace (int c)

'ctype.h' (ANSI):  See Classification of Characters.

ISTRIP

'termios.h' (POSIX.1):  See Input Modes.

int isupper (int c)

'ctype.h' (ANSI):  See Classification of Characters.

int isxdigit (int c)

'ctype.h' (ANSI):  See Classification of Characters.

char * tzname [2]

'time.h' (POSIX.1):  See Time Zone Functions.

ITIMER_PROF

'sys/time.h' (BSD):  See Setting an Alarm.

ITIMER_REAL

'sys/time.h' (BSD):  See Setting an Alarm.

ITIMER_VIRTUAL

'sys/time.h' (BSD):  See Setting an Alarm.

IXANY

'termios.h' (BSD):  See Input Modes.

IXOFF

'termios.h' (POSIX.1):  See Input Modes.

IXON

'termios.h' (POSIX.1):  See Input Modes.

jmp_buf

'setjmp.h' (ANSI):  See Non-Local Details.

int kill (pid_t pid, int signum)

'signal.h' (POSIX.1):  See Signaling Another Process.

int killpg (int pgid, int signum)

'signal.h' (BSD):  See Signaling Another Process.

long int labs (long int number)

'stdlib.h' (ANSI):  See Absolute Value.

LANG

'locale.h' (ANSI):  See Locale Categories.

LC_ALL

'locale.h' (ANSI):  See Locale Categories.

LC_COLLATE

'locale.h' (ANSI):  See Locale Categories.

LC_CTYPE

'locale.h' (ANSI):  See Locale Categories.

LC_MONETARY

'locale.h' (ANSI):  See Locale Categories.

LC_NUMERIC

'locale.h' (ANSI):  See Locale Categories.

LC_RESPONSE

'locale.h' (GNU):  See Locale Categories.

int L_ctermid

'stdio.h' (POSIX.1):  See Identifying the Terminal.

LC_TIME

'locale.h' (ANSI):  See Locale Categories.

int L_cuserid

'stdio.h' (POSIX.1):  See Who Logged In.

double ldexp (double value, int exponent)

'math.h' (ANSI):  See Normalization Functions.

ldiv_t ldiv (long int numerator, long int denominator)

'stdlib.h' (ANSI):  See Integer Division.

ldiv_t

'stdlib.h' (ANSI):  See Integer Division.

L_INCR

'sys/file.h' (BSD):  See File Positioning.

int LINE_MAX

'limits.h' (POSIX.2):  See Utility Limits.

int link (const char *oldname, const char *newname)

'unistd.h' (POSIX.1):  See Hard Links.

int LINK_MAX

'limits.h' (POSIX.1):  See Limits for Files.

int listen (int socket, unsigned int n)

'sys/socket.h' (BSD):  See Listening.

struct lconv * localeconv (void)

'locale.h' (ANSI):  See Numeric Formatting.

struct tm * localtime (const time_t *time)

'time.h' (ANSI):  See Broken-down Time.

double log10 (double x)

'math.h' (ANSI):  See Exponents and Logarithms.

double log1p (double x)

'math.h' (BSD):  See Exponents and Logarithms.

double logb (double x)

'math.h' (BSD):  See Normalization Functions.

double log (double x)

'math.h' (ANSI):  See Exponents and Logarithms.

void longjmp (jmp_buf state, int value)

'setjmp.h' (ANSI):  See Non-Local Details.

LONG_LONG_MAX

'limits.h' (GNU):  See Range of Type.

LONG_LONG_MIN

'limits.h' (GNU):  See Range of Type.

LONG_MAX

'limits.h' (ANSI):  See Range of Type.

LONG_MIN

'limits.h' (ANSI):  See Range of Type.

off_t lseek (int filedes, off_t offset, int whence)

'unistd.h' (POSIX.1):  See File Position Primitive.

L_SET

'sys/file.h' (BSD):  See File Positioning.

int lstat (const char *filename, struct stat *buf)

'sys/stat.h' (BSD):  See Reading Attributes.

int L_tmpnam

'stdio.h' (ANSI):  See Temporary Files.

L_XTND

'sys/file.h' (BSD):  See File Positioning.

__malloc_hook

'malloc.h' (GNU):  See Hooks for Malloc.

void * malloc (size_t size)

'malloc.h', 'stdlib.h' (ANSI):  See Basic Allocation.

int MAX_CANON

'limits.h' (POSIX.1):  See Limits for Files.

int MAX_INPUT

'limits.h' (POSIX.1):  See Limits for Files.

int MAXNAMLEN

'dirent.h' (BSD):  See Limits for Files.

int MB_CUR_MAX

'stdlib.h' (ANSI):  See Multibyte Char Intro.

int mblen (const char *string, size_t size)

'stdlib.h' (ANSI):  See Length of Char.

int MB_LEN_MAX

'limits.h' (ANSI):  See Multibyte Char Intro.

size_t mbstowcs (wchar_t *wstring, const char *string, size_t size)

'stdlib.h' (ANSI):  See Wide String Conversion.

int mbtowc (wchar_t *result, const char *string, size_t size)

'stdlib.h' (ANSI):  See Converting One Char.

int mcheck (void (*abortfn) (void))

'malloc.h' (GNU):  See Heap Consistency Checking.

MDMBUF

'termios.h' (BSD):  See Control Modes.

void * memalign (size_t size, size_t boundary)

'malloc.h', 'stdlib.h' (BSD):  See Aligned Memory Blocks.

void * memccpy (void *to, const void *from, int c, size_t size)

'string.h' (SVID):  See Copying and Concatenation.

void * memchr (const void *block, int c, size_t size)

'string.h' (ANSI):  See Search Functions.

int memcmp (const void *a1, const void *a2, size_t size)

'string.h' (ANSI):  See String/Array Comparison.

void * memcpy (void *to, const void *from, size_t size)

'string.h' (ANSI):  See Copying and Concatenation.

void * memmem (const void *needle, size_t needle_len,

const void *haystack, size_t haystack_len)

'string.h' (GNU):  See Search Functions.

void * memmove (void *to, const void *from, size_t size)

'string.h' (ANSI):  See Copying and Concatenation.

void memory_warnings (void *start, void (*warn_func) (const char *))

'malloc.h' (GNU):  See Memory Warnings.

void * memset (void *block, int c, size_t size)

'string.h' (ANSI):  See Copying and Concatenation.

int mkdir (const char *filename, mode_t mode)

'sys/stat.h' (POSIX.1):  See Creating Directories.

int mkfifo (const char *filename, mode_t mode)

'sys/stat.h' (POSIX.1):  See FIFO Special Files.

int mknod (const char *filename, int mode, int dev)

'sys/stat.h' (BSD):  See Making Special Files.

int mkstemp (char *template)

'unistd.h' (BSD):  See Temporary Files.

char * mktemp (char *template)

'unistd.h' (Unix):  See Temporary Files.

time_t mktime (struct tm *brokentime)

'time.h' (ANSI):  See Broken-down Time.

mode_t

'sys/types.h' (POSIX.1):  See Attribute Meanings.

double modf (double value, double *integer_part)

'math.h' (ANSI):  See Rounding and Remainders.

int MSG_DONTROUTE

'sys/socket.h' (BSD):  See Socket Data Options.

int MSG_OOB

'sys/socket.h' (BSD):  See Socket Data Options.

int MSG_PEEK

'sys/socket.h' (BSD):  See Socket Data Options.

struct mstats mstats (void)

'malloc.h' (GNU):  See Statistics of Malloc.

int NAME_MAX

'limits.h' (POSIX.1):  See Limits for Files.

double NAN

'math.h' (GNU):  See Not a Number.

int NCCS

'termios.h' (POSIX.1):  See Mode Data Types.

int NGROUPS_MAX

'limits.h' (POSIX.1):  See General Limits.

int nice (int increment)

'dunno.h' (dunno.h):  See Priority.

nlink_t

'sys/types.h' (POSIX.1):  See Attribute Meanings.

NO_ADDRESS

'netdb.h' (BSD):  See Host Names.

NOFLSH

'termios.h' (POSIX.1):  See Local Modes.

NOKERNINFO

'termios.h' (BSD):  See Local Modes.

NO_RECOVERY

'netdb.h' (BSD):  See Host Names.

int NSIG

'signal.h' (BSD):  See Standard Signals.

unsigned long int ntohl (unsigned long int netlong)

'netinet/in.h' (BSD):  See Byte Order.

unsigned short int ntohs (unsigned short int netshort)

'netinet/in.h' (BSD):  See Byte Order.

void * NULL

'stddef.h' (ANSI):  See Null Pointer Constant.

int O_ACCMODE

'fcntl.h' (POSIX.1):  See File Status Flags.

O_APPEND

'fcntl.h' (POSIX.1):  See File Status Flags.

O_APPEND

'fcntl.h' (POSIX.1):  See Opening and Closing Files.

void obstack_1grow_fast (struct obstack *obstack_ptr, char c)

'obstack.h' (GNU):  See Extra Fast Growing.

void obstack_1grow (struct obstack *obstack_ptr, char c)

'obstack.h' (GNU):  See Growing Objects.

int obstack_alignment_mask (struct obstack *obstack_ptr)

'obstack.h' (GNU):  See Obstacks Data Alignment.

void * obstack_alloc (struct obstack *obstack_ptr, size_t size)

'obstack.h' (GNU):  See Allocation in an Obstack.

void * obstack_base (struct obstack *obstack_ptr)

'obstack.h' (GNU):  See Status of an Obstack.

void obstack_blank_fast (struct obstack *obstack_ptr, size_t size)

'obstack.h' (GNU):  See Extra Fast Growing.

void obstack_blank (struct obstack *obstack_ptr, size_t size)

'obstack.h' (GNU):  See Growing Objects.

size_t obstack_chunk_size (struct obstack *obstack_ptr)

'obstack.h' (GNU):  See Obstack Chunks.

void * obstack_copy0 (struct obstack *obstack_ptr, void *address, size_t size)

'obstack.h' (GNU):  See Allocation in an Obstack.

void * obstack_copy (struct obstack *obstack_ptr, void *address, size_t size)

'obstack.h' (GNU):  See Allocation in an Obstack.

void * obstack_finish (struct obstack *obstack_ptr)

'obstack.h' (GNU):  See Growing Objects.

void obstack_free (struct obstack *obstack_ptr, void *object)

'obstack.h' (GNU):  See Freeing Obstack Objects.

void obstack_grow0 (struct obstack *obstack_ptr, void *data, size_t size)

'obstack.h' (GNU):  See Growing Objects.

void obstack_grow (struct obstack *obstack_ptr, void *data, size_t size)

'obstack.h' (GNU):  See Growing Objects.

int obstack_init (struct obstack *obstack_ptr)

'obstack.h' (GNU):  See Preparing for Obstacks.

void * obstack_next_free (struct obstack *obstack_ptr)

'obstack.h' (GNU):  See Status of an Obstack.

size_t obstack_object_size (struct obstack *obstack_ptr)

'obstack.h' (GNU):  See Growing Objects.

size_t obstack_object_size (struct obstack *obstack_ptr)

'obstack.h' (GNU):  See Status of an Obstack.

int obstack_printf (struct obstack *obstack, const char *template, ...)

'stdio.h' (GNU):  See Dynamic Output.

size_t obstack_room (struct obstack *obstack_ptr)

'obstack.h' (GNU):  See Extra Fast Growing.

int obstack_vprintf (struct obstack *obstack, const char *template, va_list ap)

'stdio.h' (GNU):  See Variable Arguments Output.

O_CREAT

'fcntl.h' (POSIX.1):  See Opening and Closing Files.

O_EXCL

'fcntl.h' (POSIX.1):  See Opening and Closing Files.

size_t offsetof (type, member)

'stddef.h' (ANSI):  See Structure Measurement.

off_t

'sys/types.h' (POSIX.1):  See File Position Primitive.

O_NDELAY

'fcntl.h' (BSD):  See File Status Flags.

int on_exit (void (*function)(int status, void *arg), void *arg)

'stdlib.h' (SunOS):  See Cleanups on Exit.

int ONLCR

'termios.h' (BSD):  See Output Modes.

O_NOCTTY

'fcntl.h' (POSIX.1):  See Opening and Closing Files.

int ONOEOT

'termios.h' (BSD):  See Output Modes.

O_NONBLOCK

'fcntl.h' (POSIX.1):  See File Status Flags.

O_NONBLOCK

'fcntl.h' (POSIX.1):  See Opening and Closing Files.

DIR * opendir (const char *dirname)

'dirent.h' (POSIX.1):  See Opening a Directory.

int open (const char *filename, int flags[, mode_t mode])

'fcntl.h' (POSIX.1):  See Opening and Closing Files.

int OPEN_MAX

'limits.h' (POSIX.1):  See General Limits.

FILE * open_memstream (char **ptr, size_t *sizeloc)

'stdio.h' (GNU):  See String Streams.

FILE * open_obstack_stream (struct obstack *obstack)

'stdio.h' (GNU):  See Obstack Streams.

int OPOST

'termios.h' (POSIX.1):  See Output Modes.

char * optarg

'unistd.h' (POSIX.2):  See Parsing Options.

int opterr

'unistd.h' (POSIX.2):  See Parsing Options.

int optind

'unistd.h' (POSIX.2):  See Parsing Options.

int optopt

'unistd.h' (POSIX.2):  See Parsing Options.

O_RDONLY

'fcntl.h' (POSIX.1):  See Opening and Closing Files.

O_RDWR

'fcntl.h' (POSIX.1):  See Opening and Closing Files.

O_TRUNC

'fcntl.h' (POSIX.1):  See Opening and Closing Files.

O_WRONLY

'fcntl.h' (POSIX.1):  See Opening and Closing Files.

int OXTABS

'termios.h' (BSD):  See Output Modes.

PA_CHAR

'printf.h' (GNU):  See Parsing a Template String.

PA_DOUBLE

'printf.h' (GNU):  See Parsing a Template String.

PA_FLAG_LONG_DOUBLE

'printf.h' (GNU):  See Parsing a Template String.

PA_FLAG_LONG

'printf.h' (GNU):  See Parsing a Template String.

PA_FLAG_LONG_LONG

'printf.h' (GNU):  See Parsing a Template String.

int PA_FLAG_MASK

'printf.h' (GNU):  See Parsing a Template String.

PA_FLAG_PTR

'printf.h' (GNU):  See Parsing a Template String.

PA_FLAG_SHORT

'printf.h' (GNU):  See Parsing a Template String.

PA_FLOAT

'printf.h' (GNU):  See Parsing a Template String.

PA_INT

'printf.h' (GNU):  See Parsing a Template String.

PA_LAST

'printf.h' (GNU):  See Parsing a Template String.

PA_POINTER

'printf.h' (GNU):  See Parsing a Template String.

PARENB

'termios.h' (POSIX.1):  See Control Modes.

PARMRK

'termios.h' (POSIX.1):  See Input Modes.

PARODD

'termios.h' (POSIX.1):  See Control Modes.

size_t parse_printf_format (const char *template, size_t n, int *argtypes)

'printf.h' (GNU):  See Parsing a Template String.

PA_STRING

'printf.h' (GNU):  See Parsing a Template String.

long int pathconf (const char *filename, int parameter)

'unistd.h' (POSIX.1):  See Pathconf.

int PATH_MAX

'limits.h' (POSIX.1):  See Limits for Files.

int pause ()

'unistd.h' (POSIX.1):  See Using Pause.

_PC_CHOWN_RESTRICTED

'unistd.h' (POSIX.1):  See Pathconf.

_PC_LINK_MAX

'unistd.h' (POSIX.1):  See Pathconf.

int pclose (FILE *stream)

'stdio.h' (POSIX.2, SVID, BSD):  See Pipe to a Subprocess.

_PC_MAX_CANON

'unistd.h' (POSIX.1):  See Pathconf.

_PC_MAX_INPUT

'unistd.h' (POSIX.1):  See Pathconf.

_PC_NAME_MAX

'unistd.h' (POSIX.1):  See Pathconf.

_PC_NO_TRUNC

'unistd.h' (POSIX.1):  See Pathconf.

_PC_PATH_MAX

'unistd.h' (POSIX.1):  See Pathconf.

_PC_PIPE_BUF

'unistd.h' (POSIX.1):  See Pathconf.

_PC_VDISABLE

'unistd.h' (POSIX.1):  See Pathconf.

PENDIN

'termios.h' (BSD):  See Local Modes.

void perror (const char *message)

'stdio.h' (ANSI):  See Error Messages.

int PF_FILE

'sys/socket.h' (GNU):  See File Namespace Details.

int PF_INET

'sys/socket.h' (BSD):  See Internet Namespace.

int PF_UNIX

'sys/socket.h' (BSD):  See File Namespace Details.

pid_t

'sys/types.h' (POSIX.1):  See Process Identification.

int PIPE_BUF

'limits.h' (POSIX.1):  See Limits for Files.

int pipe (int filedes[2])

'unistd.h' (POSIX.1):  See Creating a Pipe.

FILE * popen (const char *command, const char *mode)

'stdio.h' (POSIX.2, SVID, BSD):  See Pipe to a Subprocess.

_POSIX2_BC_BASE_MAX

'limits.h' (POSIX.2):  See Utility Minimums.

_POSIX2_BC_DIM_MAX

'limits.h' (POSIX.2):  See Utility Minimums.

_POSIX2_BC_SCALE_MAX

'limits.h' (POSIX.2):  See Utility Minimums.

_POSIX2_BC_STRING_MAX

'limits.h' (POSIX.2):  See Utility Minimums.

int _POSIX2_C_DEV

'unistd.h' (POSIX.2):  See System Options.

_POSIX2_COLL_WEIGHTS_MAX

'limits.h' (POSIX.2):  See Utility Minimums.

long int _POSIX2_C_VERSION

'unistd.h' (POSIX.2):  See Version Supported.

_POSIX2_EQUIV_CLASS_MAX

'limits.h' (POSIX.2):  See Utility Minimums.

_POSIX2_EXPR_NEST_MAX

'limits.h' (POSIX.2):  See Utility Minimums.

int _POSIX2_FORT_DEV

'unistd.h' (POSIX.2):  See System Options.

int _POSIX2_FORT_RUN

'unistd.h' (POSIX.2):  See System Options.

_POSIX2_LINE_MAX

'limits.h' (POSIX.2):  See Utility Minimums.

int _POSIX2_LOCALEDEF

'unistd.h' (POSIX.2):  See System Options.

_POSIX2_RE_DUP_MAX

'limits.h' (POSIX.2):  See Minimums.

int _POSIX2_SW_DEV

'unistd.h' (POSIX.2):  See System Options.

_POSIX_ARG_MAX

'limits.h' (POSIX.1):  See Minimums.

_POSIX_CHILD_MAX

'limits.h' (POSIX.1):  See Minimums.

int _POSIX_CHOWN_RESTRICTED

'unistd.h' (POSIX.1):  See Options for Files.

_POSIX_C_SOURCE

(POSIX.2):  See Feature Test Macros.

int _POSIX_JOB_CONTROL

'unistd.h' (POSIX.1):  See System Options.

_POSIX_LINK_MAX

'limits.h' (POSIX.1):  See File Minimums.

_POSIX_MAX_CANON

'limits.h' (POSIX.1):  See File Minimums.

_POSIX_MAX_INPUT

'limits.h' (POSIX.1):  See File Minimums.

_POSIX_NAME_MAX

'limits.h' (POSIX.1):  See File Minimums.

_POSIX_NGROUPS_MAX

'limits.h' (POSIX.1):  See Minimums.

int _POSIX_NO_TRUNC

'unistd.h' (POSIX.1):  See Options for Files.

_POSIX_OPEN_MAX

'limits.h' (POSIX.1):  See Minimums.

_POSIX_PATH_MAX

'limits.h' (POSIX.1):  See File Minimums.

_POSIX_PIPE_BUF

'limits.h' (POSIX.1):  See File Minimums.

int _POSIX_SAVED_IDS

'unistd.h' (POSIX.1):  See System Options.

_POSIX_SOURCE

(POSIX.1):  See Feature Test Macros.

_POSIX_SSIZE_MAX

'limits.h' (POSIX.1):  See Minimums.

_POSIX_STREAM_MAX

'limits.h' (POSIX.1):  See Minimums.

_POSIX_TZNAME_MAX

'limits.h' (POSIX.1):  See Minimums.

unsigned char _POSIX_VDISABLE

'unistd.h' (POSIX.1):  See Options for Files.

long int _POSIX_VERSION

'unistd.h' (POSIX.1):  See Version Supported.

double pow (double base, double power)

'math.h' (ANSI):  See Exponents and Logarithms.

printf_arginfo_function

'printf.h' (GNU):  See Defining the Output Handler.

printf_function

'printf.h' (GNU):  See Defining the Output Handler.

int printf (const char *template, ...)

'stdio.h' (ANSI):  See Formatted Output Functions.

PRIO_MAX

'sys/resource.h' (BSD):  See Priority.

PRIO_MIN

'sys/resource.h' (BSD):  See Priority.

PRIO_PGRP

'sys/resource.h' (BSD):  See Priority.

PRIO_PROCESS

'sys/resource.h' (BSD):  See Priority.

PRIO_USER

'sys/resource.h' (BSD):  See Priority.

char * program_invocation_name

'errno.h' (GNU):  See Error Messages.

char * program_invocation_short_name

'errno.h' (GNU):  See Error Messages.

void psignal (int signum, const char *message)

'signal.h' (BSD):  See Signal Messages.

char * P_tmpdir

'stdio.h' (SVID):  See Temporary Files.

ptrdiff_t

'stddef.h' (ANSI):  See Important Data Types.

int putchar (int c)

'stdio.h' (ANSI):  See Simple Output.

int putc (int c, FILE *stream)

'stdio.h' (ANSI):  See Simple Output.

int putenv (const char *string)

'stdlib.h' (SVID):  See Environment Access.

int putpwent (const struct passwd *p, FILE *stream)

'pwd.h' (SVID):  See Writing a User Entry.

int puts (const char *s)

'stdio.h' (ANSI):  See Simple Output.

int putw (int w, FILE *stream)

'stdio.h' (SVID):  See Simple Output.

void qsort (void *array, size_t count, size_t size, comparison_fn_t compare)

'stdlib.h' (ANSI):  See Array Sort Function.

int raise (int signum)

'signal.h' (ANSI):  See Signaling Yourself.

void r_alloc_free (void **handleptr)

'malloc.h' (GNU):  See Using Relocator.

void * r_alloc (void **handleptr, size_t size)

'malloc.h' (GNU):  See Using Relocator.

int rand ()

'stdlib.h' (ANSI):  See ANSI Random.

int RAND_MAX

'stdlib.h' (ANSI):  See ANSI Random.

long int random ()

'stdlib.h' (BSD):  See BSD Random.

struct dirent * readdir (DIR *dirstream)

'dirent.h' (POSIX.1):  See Reading/Closing Directory.

ssize_t read (int filedes, void *buffer, size_t size)

'unistd.h' (POSIX.1):  See I/O Primitives.

int readlink (const char *filename, char *buffer, size_t size)

'unistd.h' (BSD):  See Symbolic Links.

__realloc_hook

'malloc.h' (GNU):  See Hooks for Malloc.

void * realloc (void *ptr, size_t newsize)

'malloc.h', 'stdlib.h' (ANSI):  See Changing Block Size.

int recvfrom (int socket, void *buffer, size_t size, int flags, struct sockaddr *addr, size_t *length_ptr)

'sys/socket.h' (BSD):  See Receiving Datagrams.

int recv (int socket, void *buffer, size_t size, int flags)

'sys/socket.h' (BSD):  See Receiving Data.

int recvmsg (int socket, struct msghdr *message, int flags)

'sys/socket.h' (BSD):  See Receiving Datagrams.

int RE_DUP_MAX

'limits.h' (POSIX.2):  See General Limits.

REG_BADBR

'regex.h' (POSIX.2):  See POSIX Regexp Compilation.

REG_BADPAT

'regex.h' (POSIX.2):  See POSIX Regexp Compilation.

REG_BADRPT

'regex.h' (POSIX.2):  See POSIX Regexp Compilation.

int regcomp (regex_t *compiled, const char *pattern, int cflags)

'regex.h' (POSIX.2):  See POSIX Regexp Compilation.

REG_EBRACE

'regex.h' (POSIX.2):  See POSIX Regexp Compilation.

REG_EBRACK

'regex.h' (POSIX.2):  See POSIX Regexp Compilation.

REG_ECOLLATE

'regex.h' (POSIX.2):  See POSIX Regexp Compilation.

REG_ECTYPE

'regex.h' (POSIX.2):  See POSIX Regexp Compilation.

REG_EESCAPE

'regex.h' (POSIX.2):  See POSIX Regexp Compilation.

REG_EPAREN

'regex.h' (POSIX.2):  See POSIX Regexp Compilation.

REG_ERANGE

'regex.h' (POSIX.2):  See POSIX Regexp Compilation.

size_t regerror (int errcode, regex_t *compiled, char *buffer, size_t length)

'regex.h' (POSIX.2):  See Regexp Cleanup.

REG_ESPACE

'regex.h' (POSIX.2):  See Matching POSIX Regexps.

REG_ESPACE

'regex.h' (POSIX.2):  See POSIX Regexp Compilation.

REG_ESUBREG

'regex.h' (POSIX.2):  See POSIX Regexp Compilation.

int regexec (regex_t *compiled, char *string, size_t nmatch, regmatch_t matchptr [], int eflags)

'regex.h' (POSIX.2):  See Matching POSIX Regexps.

REG_EXTENDED

'regex.h' (POSIX.2):  See Flags for POSIX Regexps.

regex_t

'regex.h' (POSIX.2):  See POSIX Regexp Compilation.

void regfree (regex_t *compiled)

'regex.h' (POSIX.2):  See Regexp Cleanup.

REG_ICASE

'regex.h' (POSIX.2):  See Flags for POSIX Regexps.

int register_printf_function (int spec, printf_function handler_function, printf_arginfo_function arginfo_function)

'printf.h' (GNU):  See Registering New Conversions.

regmatch_t

'regex.h' (POSIX.2):  See Regexp Subexpressions.

REG_NEWLINE

'regex.h' (POSIX.2):  See Flags for POSIX Regexps.

REG_NOMATCH

'regex.h' (POSIX.2):  See Matching POSIX Regexps.

REG_NOSUB

'regex.h' (POSIX.2):  See Flags for POSIX Regexps.

REG_NOTBOL

'regex.h' (POSIX.2):  See Matching POSIX Regexps.

REG_NOTEOL

'regex.h' (POSIX.2):  See Matching POSIX Regexps.

regoff_t

'regex.h' (POSIX.2):  See Regexp Subexpressions.

int remove (const char *filename)

'stdio.h' (ANSI):  See Deleting Files.

int rename (const char *oldname, const char *newname)

'stdio.h' (ANSI):  See Renaming Files.

void rewinddir (DIR *dirstream)

'dirent.h' (POSIX.1):  See Random Access Directory.

void rewind (FILE *stream)

'stdio.h' (ANSI):  See File Positioning.

char * rindex (const char *string, int c)

'string.h' (BSD):  See Search Functions.

double rint (double x)

'math.h' (BSD):  See Rounding and Remainders.

RLIMIT_CORE

'sys/resource.h' (BSD):  See Limits on Resources.

RLIMIT_CPU

'sys/resource.h' (BSD):  See Limits on Resources.

RLIMIT_DATA

'sys/resource.h' (BSD):  See Limits on Resources.

RLIMIT_FSIZE

'sys/resource.h' (BSD):  See Limits on Resources.

RLIMIT_OPEN_FILES

'sys/resource.h' (BSD):  See Limits on Resources.

RLIMIT_RSS

'sys/resource.h' (BSD):  See Limits on Resources.

RLIMIT_STACK

'sys/resource.h' (BSD):  See Limits on Resources.

RLIM_NLIMITS

'sys/resource.h' (BSD):  See Limits on Resources.

int rmdir (const char *filename)

'unistd.h' (POSIX.1):  See Deleting Files.

int R_OK

'unistd.h' (POSIX.1):  See Testing File Access.

void * r_re_alloc (void **handleptr, size_t size)

'malloc.h' (GNU):  See Using Relocator.

RUSAGE_CHILDREN

'sys/resource.h' (BSD):  See Resource Usage.

RUSAGE_SELF

'sys/resource.h' (BSD):  See Resource Usage.

int SA_NOCLDSTOP

'signal.h' (POSIX.1):  See Flags for Sigaction.

int SA_ONSTACK

'signal.h' (BSD):  See Flags for Sigaction.

int SA_RESTART

'signal.h' (BSD):  See Flags for Sigaction.

_SC_2_C_DEV

'unistd.h' (POSIX.2):  See Constants for Sysconf.

_SC_2_FORT_DEV

'unistd.h' (POSIX.2):  See Constants for Sysconf.

_SC_2_FORT_RUN

'unistd.h' (POSIX.2):  See Constants for Sysconf.

_SC_2_LOCALEDEF

'unistd.h' (POSIX.2):  See Constants for Sysconf.

_SC_2_SW_DEV

'unistd.h' (POSIX.2):  See Constants for Sysconf.

_SC_2_VERSION

'unistd.h' (POSIX.2):  See Constants for Sysconf.

double scalb (double value, int exponent)

'math.h' (BSD):  See Normalization Functions.

int scanf (const char *template, ...)

'stdio.h' (ANSI):  See Formatted Input Functions.

_SC_ARG_MAX

'unistd.h' (POSIX.1):  See Constants for Sysconf.

_SC_BC_BASE_MAX

'unistd.h' (POSIX.2):  See Constants for Sysconf.

_SC_BC_DIM_MAX

'unistd.h' (POSIX.2):  See Constants for Sysconf.

_SC_BC_SCALE_MAX

'unistd.h' (POSIX.2):  See Constants for Sysconf.

_SC_BC_STRING_MAX

'unistd.h' (POSIX.2):  See Constants for Sysconf.

_SC_CHILD_MAX

'unistd.h' (POSIX.1):  See Constants for Sysconf.

_SC_CLK_TCK

'unistd.h' (POSIX.1):  See Constants for Sysconf.

_SC_COLL_WEIGHTS_MAX

'unistd.h' (POSIX.2):  See Constants for Sysconf.

_SC_EQUIV_CLASS_MAX

'unistd.h' (POSIX.2):  See Constants for Sysconf.

_SC_EXPR_NEST_MAX

'unistd.h' (POSIX.2):  See Constants for Sysconf.

SCHAR_MAX

'limits.h' (ANSI):  See Range of Type.

SCHAR_MIN

'limits.h' (ANSI):  See Range of Type.

_SC_JOB_CONTROL

'unistd.h' (POSIX.1):  See Constants for Sysconf.

_SC_LINE_MAX

'unistd.h' (POSIX.2):  See Constants for Sysconf.

_SC_NGROUPS_MAX

'unistd.h' (POSIX.1):  See Constants for Sysconf.

_SC_OPEN_MAX

'unistd.h' (POSIX.1):  See Constants for Sysconf.

_SC_PAGESIZE

'unistd.h' (GNU):  See Constants for Sysconf.

_SC_SAVED_IDS

'unistd.h' (POSIX.1):  See Constants for Sysconf.

_SC_STREAM_MAX

'unistd.h' (POSIX.1):  See Constants for Sysconf.

_SC_TZNAME_MAX

'unistd.h' (POSIX.1):  See Constants for Sysconf.

_SC_VERSION

'unistd.h' (POSIX.1):  See Constants for Sysconf.

_SC_VERSION

'unistd.h' (POSIX.2):  See Constants for Sysconf.

int SEEK_CUR

'stdio.h' (ANSI):  See File Positioning.

void seekdir (DIR *dirstream, off_t pos)

'dirent.h' (BSD):  See Random Access Directory.

int SEEK_END

'stdio.h' (ANSI):  See File Positioning.

int SEEK_SET

'stdio.h' (ANSI):  See File Positioning.

int select (int nfds, fd_set *read_fds, fd_set *write_fds, fd_set *except_fds, struct timeval *timeout)

'sys/types.h' (BSD):  See Waiting for I/O.

int send (int socket, void *buffer, size_t size, int flags)

'sys/socket.h' (BSD):  See Sending Data.

int sendmsg (int socket, const struct msghdr *message, int flags)

'sys/socket.h' (BSD):  See Receiving Datagrams.

int sendto (int socket, void *buffer. size_t size, int flags, struct sockaddr *addr, size_t length)

'sys/socket.h' (BSD):  See Sending Datagrams.

void setbuffer (FILE *stream, char *buf, size_t size)

'stdio.h' (BSD):  See Controlling Buffering.

void setbuf (FILE *stream, char *buf)

'stdio.h' (ANSI):  See Controlling Buffering.

int setgid (gid_t newgid)

'unistd.h' (POSIX.1):  See Setting Groups.

void setgrent (void)

'grp.h' (SVID, BSD):  See Scanning All Groups.

int setgroups (size_t count, gid_t *groups)

'grp.h' (BSD):  See Setting Groups.

void sethostent (int stayopen)

'netdb.h' (BSD):  See Host Names.

int sethostid (long int id)

'unistd.h' (BSD):  See Host Identification.

int sethostname (const char *name, size_t length)

'unistd.h' (BSD):  See Host Identification.

int setitimer (int which, struct itimerval *new, struct itimerval *old)

'sys/time.h' (BSD):  See Setting an Alarm.

int setjmp (jmp_buf state)

'setjmp.h' (ANSI):  See Non-Local Details.

void setlinebuf (FILE *stream)

'stdio.h' (BSD):  See Controlling Buffering.

char * setlocale (int category, const char *locale)

'locale.h' (ANSI):  See Setting the Locale.

void setnetent (int stayopen)

'netdb.h' (BSD):  See Networks Database.

int setpgid (pid_t pid, pid_t pgid)

'unistd.h' (POSIX.1):  See Process Group Functions.

int setpgrp (pid_t pid, pid_t pgid)

'unistd.h' (BSD):  See Process Group Functions.

int setpriority (int class, int id, int priority)

'sys/resource.h' (BSD):  See Priority.

void setprotoent (int stayopen)

'netdb.h' (BSD):  See Protocols Database.

void setpwent (void)

'pwd.h' (SVID, BSD):  See Scanning All Users.

int setregid (gid_t rgid, fid_t egid)

'unistd.h' (BSD):  See Setting Groups.

int setreuid (uid_t ruid, uid_t euid)

'unistd.h' (BSD):  See Setting User ID.

int setrlimit (int resource, struct rlimit *rlp)

'sys/resource.h' (BSD):  See Limits on Resources.

void setservent (int stayopen)

'netdb.h' (BSD):  See Services Database.

pid_t setsid (void)

'unistd.h' (POSIX.1):  See Process Group Functions.

int setsockopt (int socket, int level, int optname, void *optval, size_t optlen)

'sys/socket.h' (BSD):  See Socket Option Functions.

void * setstate (void *state)

'stdlib.h' (BSD):  See BSD Random.

int settimeofday (const struct timeval *tp, const struct timezone *tzp)

'sys/time.h' (BSD):  See High-Resolution Calendar.

int setuid (uid_t newuid)

'unistd.h' (POSIX.1):  See Setting User ID.

int setvbuf (FILE *stream, char *buf, int mode, size_t size)

'stdio.h' (ANSI):  See Controlling Buffering.

SHRT_MAX

'limits.h' (ANSI):  See Range of Type.

SHRT_MIN

'limits.h' (ANSI):  See Range of Type.

int shutdown (int socket, int how)

'sys/socket.h' (BSD):  See Closing a Socket.

S_IEXEC

'sys/stat.h' (BSD):  See Permission Bits.

S_IFBLK

'sys/stat.h' (BSD):  See Testing File Type.

S_IFCHR

'sys/stat.h' (BSD):  See Testing File Type.

S_IFDIR

'sys/stat.h' (BSD):  See Testing File Type.

S_IFIFO

'sys/stat.h' (BSD):  See Testing File Type.

S_IFLNK

'sys/stat.h' (BSD):  See Testing File Type.

int S_IFMT

'sys/stat.h' (BSD):  See Testing File Type.

S_IFREG

'sys/stat.h' (BSD):  See Testing File Type.

S_IFSOCK

'sys/stat.h' (BSD):  See Testing File Type.

int SIGABRT

'signal.h' (ANSI):  See Program Error Signals.

int sigaction (int signum, const struct sigaction *action, struct sigaction *old_action)

'signal.h' (POSIX.1):  See Advanced Signal Handling.

int sigaddset (sigset_t *set, int signum)

'signal.h' (POSIX.1):  See Signal Sets.

int SIGALRM

'signal.h' (POSIX.1):  See Alarm Signals.

int sigaltstack (const struct sigaltstack *stack, struct sigaltstack *oldstack)

'signal.h' (BSD):  See Signal Stack.

sig_atomic_t

'signal.h' (ANSI):  See Atomic Types.

int sigblock (int mask)

'signal.h' (BSD):  See Blocking in BSD.

SIG_BLOCK

'signal.h' (POSIX.1):  See Process Signal Mask.

int SIGBUS

'signal.h' (BSD):  See Program Error Signals.

int SIGCHLD

'signal.h' (POSIX.1):  See Job Control Signals.

int SIGCONT

'signal.h' (POSIX.1):  See Job Control Signals.

int sigdelset (sigset_t *set, int signum)

'signal.h' (POSIX.1):  See Signal Sets.

int sigemptyset (sigset_t *set)

'signal.h' (POSIX.1):  See Signal Sets.

sighandler_t SIG_ERR

'signal.h' (ANSI):  See Basic Signal Handling.

int sigfillset (sigset_t *set)

'signal.h' (POSIX.1):  See Signal Sets.

int SIGFPE

'signal.h' (ANSI):  See Program Error Signals.

sighandler_t

'signal.h' (GNU):  See Basic Signal Handling.

int SIGHUP

'signal.h' (POSIX.1):  See Termination Signals.

int SIGILL

'signal.h' (ANSI):  See Program Error Signals.

int siginterrupt (int signum, int failflag)

'signal.h' (BSD):  See BSD Handler.

int SIGINT

'signal.h' (ANSI):  See Termination Signals.

int SIGIO

'signal.h' (BSD):  See Asynchronous I/O Signals.

int sigismember (const sigset_t *set, int signum)

'signal.h' (POSIX.1):  See Signal Sets.

sigjmp_buf

'setjmp.h' (POSIX.1):  See Non-Local Exits and Signals.

int SIGKILL

'signal.h' (POSIX.1):  See Termination Signals.

void siglongjmp (sigjmp_buf state, int value)

'setjmp.h' (POSIX.1):  See Non-Local Exits and Signals.

int sigmask (int signum)

'signal.h' (BSD):  See Blocking in BSD.

sighandler_t signal (int signum, sighandler_t action)

'signal.h' (ANSI):  See Basic Signal Handling.

int sigpause (int mask)

'signal.h' (BSD):  See Blocking in BSD.

int sigpending (sigset_t *set)

'signal.h' (POSIX.1):  See Checking for Pending Signals.

int SIGPIPE

'signal.h' (POSIX.1):  See Miscellaneous Signals.

int sigprocmask (int how, const sigset_t *set, sigset_t *oldset)

'signal.h' (POSIX.1):  See Process Signal Mask.

int SIGPROF

'signal.h' (BSD):  See Alarm Signals.

int SIGQUIT

'signal.h' (POSIX.1):  See Termination Signals.

int SIGSEGV

'signal.h' (ANSI):  See Program Error Signals.

int sigsetjmp (sigjmp_buf state, int savesigs)

'setjmp.h' (POSIX.1):  See Non-Local Exits and Signals.

int sigsetmask (int mask)

'signal.h' (BSD):  See Blocking in BSD.

SIG_SETMASK

'signal.h' (POSIX.1):  See Process Signal Mask.

sigset_t

'signal.h' (POSIX.1):  See Signal Sets.

int sigstack (const struct sigstack *stack, struct sigstack *oldstack)

'signal.h' (BSD):  See Signal Stack.

int SIGSTOP

'signal.h' (POSIX.1):  See Job Control Signals.

int sigsuspend (const sigset_t *set)

'signal.h' (POSIX.1):  See Sigsuspend.

int SIGTERM

'signal.h' (ANSI):  See Termination Signals.

int SIGTSTP

'signal.h' (POSIX.1):  See Job Control Signals.

int SIGTTIN

'signal.h' (POSIX.1):  See Job Control Signals.

int SIGTTOU

'signal.h' (POSIX.1):  See Job Control Signals.

SIG_UNBLOCK

'signal.h' (POSIX.1):  See Process Signal Mask.

int SIGURG

'signal.h' (BSD):  See Asynchronous I/O Signals.

int SIGUSR1

'signal.h' (POSIX.1):  See Miscellaneous Signals.

int SIGUSR2

'signal.h' (POSIX.1):  See Miscellaneous Signals.

int sigvec (int signum, const struct sigvec *action,struct sigvec *old_action)

'signal.h' (BSD):  See BSD Handler.

int SIGVTALRM

'signal.h' (BSD):  See Alarm Signals.

double sinh (double x)

'math.h' (ANSI):  See Hyperbolic Functions.

double sin (double x)

'math.h' (ANSI):  See Trig Functions.

S_IREAD

'sys/stat.h' (BSD):  See Permission Bits.

S_IRGRP

'sys/stat.h' (POSIX.1):  See Permission Bits.

S_IROTH

'sys/stat.h' (POSIX.1):  See Permission Bits.

S_IRUSR

'sys/stat.h' (POSIX.1):  See Permission Bits.

S_IRWXG

'sys/stat.h' (POSIX.1):  See Permission Bits.

S_IRWXO

'sys/stat.h' (POSIX.1):  See Permission Bits.

S_IRWXU

'sys/stat.h' (POSIX.1):  See Permission Bits.

int S_ISBLK (mode_t m)

'sys/stat.h' (POSIX):  See Testing File Type.

int S_ISCHR (mode_t m)

'sys/stat.h' (POSIX):  See Testing File Type.

int S_ISDIR (mode_t m)

'sys/stat.h' (POSIX):  See Testing File Type.

int S_ISFIFO (mode_t m)

'sys/stat.h' (POSIX):  See Testing File Type.

S_ISGID

'sys/stat.h' (POSIX):  See Permission Bits.

int S_ISLNK (mode_t m)

'sys/stat.h' (GNU):  See Testing File Type.

int S_ISREG (mode_t m)

'sys/stat.h' (POSIX):  See Testing File Type.

int S_ISSOCK (mode_t m)

'sys/stat.h' (GNU):  See Testing File Type.

S_ISUID

'sys/stat.h' (POSIX):  See Permission Bits.

S_ISVTX

'sys/stat.h' (BSD):  See Permission Bits.

S_IWGRP

'sys/stat.h' (POSIX.1):  See Permission Bits.

S_IWOTH

'sys/stat.h' (POSIX.1):  See Permission Bits.

S_IWRITE

'sys/stat.h' (BSD):  See Permission Bits.

S_IWUSR

'sys/stat.h' (POSIX.1):  See Permission Bits.

S_IXGRP

'sys/stat.h' (POSIX.1):  See Permission Bits.

S_IXOTH

'sys/stat.h' (POSIX.1):  See Permission Bits.

S_IXUSR

'sys/stat.h' (POSIX.1):  See Permission Bits.

size_t

'stddef.h' (ANSI):  See Important Data Types.

unsigned int sleep (unsigned int seconds)

'unistd.h' (POSIX.1):  See Sleeping.

int snprintf (char *s, size_t size, const char *template, ...)

'stdio.h' (GNU):  See Formatted Output Functions.

SO_BROADCAST

'sys/socket.h' (BSD):  See Socket-Level Options.

int SOCK_DGRAM

'sys/socket.h' (BSD):  See Communication Styles.

int socket (int namespace, int style, int protocol)

'sys/socket.h' (BSD):  See Creating a Socket.

int socketpair (int namespace, int style, int protocol, int filedes[2])

'sys/socket.h' (BSD):  See Socket Pairs.

int SOCK_RAW

'sys/socket.h' (BSD):  See Communication Styles.

int SOCK_RDM

'sys/socket.h' (BSD):  See Communication Styles.

int SOCK_SEQPACKET

'sys/socket.h' (BSD):  See Communication Styles.

int SOCK_STREAM

'sys/socket.h' (BSD):  See Communication Styles.

SO_DEBUG

'sys/socket.h' (BSD):  See Socket-Level Options.

SO_DONTROUTE

'sys/socket.h' (BSD):  See Socket-Level Options.

SO_ERROR

'sys/socket.h' (BSD):  See Socket-Level Options.

SO_KEEPALIVE

'sys/socket.h' (BSD):  See Socket-Level Options.

SO_LINGER

'sys/socket.h' (BSD):  See Socket-Level Options.

int SOL_SOCKET

'sys/socket.h' (BSD):  See Socket-Level Options.

SO_OOBINLINE

'sys/socket.h' (BSD):  See Socket-Level Options.

SO_RCVBUF

'sys/socket.h' (BSD):  See Socket-Level Options.

SO_REUSEADDR

'sys/socket.h' (BSD):  See Socket-Level Options.

SO_SNDBUF

'sys/socket.h' (BSD):  See Socket-Level Options.

SO_STYLE

'sys/socket.h' (GNU):  See Socket-Level Options.

SO_TYPE

'sys/socket.h' (BSD):  See Socket-Level Options.

speed_t

'termios.h' (POSIX.1):  See Line Speed.

int sprintf (char *s, const char *template, ...)

'stdio.h' (ANSI):  See Formatted Output Functions.

double sqrt (double x)

'math.h' (ANSI):  See Exponents and Logarithms.

void srand (unsigned int seed)

'stdlib.h' (ANSI):  See ANSI Random.

void srandom (unsigned int seed)

'stdlib.h' (BSD):  See BSD Random.

int sscanf (const char *s, const char *template, ...)

'stdio.h' (ANSI):  See Formatted Input Functions.

sighandler_t ssignal (int signum, sighandler_t action)

'signal.h' (SVID):  See Basic Signal Handling.

int SSIZE_MAX

'limits.h' (POSIX.1):  See General Limits.

ssize_t

'unistd.h' (POSIX.1):  See I/O Primitives.

int stat (const char *filename, struct stat *buf)

'sys/stat.h' (POSIX.1):  See Reading Attributes.

STDERR_FILENO

'unistd.h' (POSIX.1):  See Descriptors and Streams.

FILE * stderr

'stdio.h' (ANSI):  See Standard Streams.

STDIN_FILENO

'unistd.h' (POSIX.1):  See Descriptors and Streams.

FILE * stdin

'stdio.h' (ANSI):  See Standard Streams.

STDOUT_FILENO

'unistd.h' (POSIX.1):  See Descriptors and Streams.

FILE * stdout

'stdio.h' (ANSI):  See Standard Streams.

char * stpcpy (char *to, const char *from)

'string.h' (Unknown origin):  See Copying and Concatenation.

int strcasecmp (const char *s1, const char *s2)

'string.h' (BSD):  See String/Array Comparison.

char * strcat (char *to, const char *from)

'string.h' (ANSI):  See Copying and Concatenation.

char * strchr (const char *string, int c)

'string.h' (ANSI):  See Search Functions.

int strcmp (const char *s1, const char *s2)

'string.h' (ANSI):  See String/Array Comparison.

int strcoll (const char *s1, const char *s2)

'string.h' (ANSI):  See Collation Functions.

char * strcpy (char *to, const char *from)

'string.h' (ANSI):  See Copying and Concatenation.

size_t strcspn (const char *string, const char *stopset)

'string.h' (ANSI):  See Search Functions.

char * strdup (const char *s)

'string.h' (SVID):  See Copying and Concatenation.

int STREAM_MAX

'limits.h' (POSIX.1):  See General Limits.

char * strerror (int errnum)

'string.h' (ANSI):  See Error Messages.

size_t strftime (char *s, size_t size, const char *template, const struct tm *brokentime)

'time.h' (ANSI):  See Formatting Date and Time.

size_t strlen (const char *s)

'string.h' (ANSI):  See String Length.

int strncasecmp (const char *s1, const char *s2, size_t n)

'string.h' (BSD):  See String/Array Comparison.

char * strncat (char *to, const char *from, size_t size)

'string.h' (ANSI):  See Copying and Concatenation.

int strncmp (const char *s1, const char *s2, size_t size)

'string.h' (ANSI):  See String/Array Comparison.

char * strncpy (char *to, const char *from, size_t size)

'string.h' (ANSI):  See Copying and Concatenation.

char * strpbrk (const char *string, const char *stopset)

'string.h' (ANSI):  See Search Functions.

char * strrchr (const char *string, int c)

'string.h' (ANSI):  See Search Functions.

char * strsignal (int signum)

'string.h' (GNU):  See Signal Messages.

size_t strspn (const char *string, const char *skipset)

'string.h' (ANSI):  See Search Functions.

char * strstr (const char *haystack, const char *needle)

'string.h' (ANSI):  See Search Functions.

double strtod (const char *string, char **tailptr)

'stdlib.h' (ANSI):  See Parsing of Floats.

char * strtok (char *newstring, const char *delimiters)

'string.h' (ANSI):  See Finding Tokens in a String.

long int strtol (const char *string, char **tailptr, int base)

'stdlib.h' (ANSI):  See Parsing of Integers.

unsigned long int strtoul (const char *string, char **tailptr, int base)

'stdlib.h' (ANSI):  See Parsing of Integers.

struct dirent

'dirent.h' (POSIX.1):  See Directory Entries.

struct flock

'fcntl.h' (POSIX.1):  See File Locks.

struct group

'grp.h' (POSIX.1):  See Group Data Structure.

struct hostent

'netdb.h' (BSD):  See Host Names.

struct in_addr

'netinet/in.h' (BSD):  See Host Address Data Type.

struct itimerval

'sys/time.h' (BSD):  See Setting an Alarm.

struct lconv

'locale.h' (ANSI):  See Numeric Formatting.

struct linger

'sys/socket.h' (BSD):  See Socket-Level Options.

struct msghdr

'sys/socket.h' (BSD):  See Receiving Datagrams.

struct mstats

'malloc.h' (GNU):  See Statistics of Malloc.

struct netent

'netdb.h' (BSD):  See Networks Database.

struct obstack

'obstack.h' (GNU):  See Creating Obstacks.

struct option

'getopt.h' (GNU):  See Long Options.

struct passwd

'pwd.h' (POSIX.1):  See User Data Structure.

struct printf_info

'printf.h' (GNU):  See Conversion Specifier Options.

struct protoent

'netdb.h' (BSD):  See Protocols Database.

struct rlimit

'sys/resource.h' (BSD):  See Limits on Resources.

struct rusage

'sys/resource.h' (BSD):  See Resource Usage.

struct servent

'netdb.h' (BSD):  See Services Database.

struct sigaction

'signal.h' (POSIX.1):  See Advanced Signal Handling.

struct sigaltstack

'signal.h' (BSD):  See Signal Stack.

struct sigstack

'signal.h' (BSD):  See Signal Stack.

struct sigvec

'signal.h' (BSD):  See BSD Handler.

struct sockaddr

'sys/socket.h' (BSD):  See Address Formats.

struct sockaddr_in

'netinet/in.h' (BSD):  See Internet Address Format.

struct sockaddr_un

'sys/un.h' (BSD):  See File Namespace Details.

struct stat

'sys/stat.h' (POSIX.1):  See Attribute Meanings.

struct termios

'termios.h' (POSIX.1):  See Mode Data Types.

struct timeval

'sys/time.h' (BSD):  See High-Resolution Calendar.

struct timezone

'sys/time.h' (BSD):  See High-Resolution Calendar.

struct tm

'time.h' (ANSI):  See Broken-down Time.

struct tms

'sys/times.h' (POSIX.1):  See Detailed CPU Time.

struct utimbuf

'time.h' (POSIX.1):  See File Times.

struct utsname

'sys/utsname.h' (POSIX.1):  See Hardware/Software Type ID.

size_t strxfrm (char *to, const char *from, size_t size)

'string.h' (ANSI):  See Collation Functions.

_SVID_SOURCE

(GNU):  See Feature Test Macros.

int SV_INTERRUPT

'signal.h' (BSD):  See BSD Handler.

int SV_ONSTACK

'signal.h' (BSD):  See BSD Handler.

int SV_RESETHAND

'signal.h' (Sun):  See BSD Handler.

int symlink (const char *oldname, const char *newname)

'unistd.h' (BSD):  See Symbolic Links.

long int sysconf (int parameter)

'unistd.h' (POSIX.1):  See Sysconf Definition.

int system (const char *command)

'stdlib.h' (ANSI):  See Running a Command.

double tanh (double x)

'math.h' (ANSI):  See Hyperbolic Functions.

double tan (double x)

'math.h' (ANSI):  See Trig Functions.

int tcdrain (int filedes)

'termios.h' (POSIX.1):  See Line Control.

tcflag_t

'termios.h' (POSIX.1):  See Mode Data Types.

int tcflow (int filedes, int action)

'termios.h' (POSIX.1):  See Line Control.

int tcflush (int filedes, int queue)

'termios.h' (POSIX.1):  See Line Control.

int tcgetattr (int filedes, struct termios *termios_p)

'termios.h' (POSIX.1):  See Mode Functions.

pid_t tcgetpgrp (int filedes)

'unistd.h' (POSIX.1):  See Terminal Access Functions.

TCSADRAIN

'termios.h' (POSIX.1):  See Mode Functions.

TCSAFLUSH

'termios.h' (POSIX.1):  See Mode Functions.

TCSANOW

'termios.h' (POSIX.1):  See Mode Functions.

TCSASOFT

'termios.h' (BSD):  See Mode Functions.

int tcsendbreak (int filedes, int duration)

'termios.h' (POSIX.1):  See Line Control.

int tcsetattr (int filedes, int when, const struct termios *termios_p)

'termios.h' (POSIX.1):  See Mode Functions.

int tcsetpgrp (int filedes, pid_t pgid)

'unistd.h' (POSIX.1):  See Terminal Access Functions.

off_t telldir (DIR *dirstream)

'dirent.h' (BSD):  See Random Access Directory.

TEMP_FAILURE_RETRY (expression)

'unistd.h' (GNU):  See Interrupted Primitives.

char * tempnam (const char *dir, const char *prefix)

'stdio.h' (SVID):  See Temporary Files.

time_t time (time_t *result)

'time.h' (ANSI):  See Simple Calendar Time.

clock_t times (struct tms *buffer)

'sys/times.h' (POSIX.1):  See Detailed CPU Time.

time_t

'time.h' (ANSI):  See Simple Calendar Time.

long int timezone

'time.h' (SVID):  See Time Zone Functions.

FILE * tmpfile (void)

'stdio.h' (ANSI):  See Temporary Files.

int TMP_MAX

'stdio.h' (ANSI):  See Temporary Files.

char * tmpnam (char *result)

'stdio.h' (ANSI):  See Temporary Files.

int toascii (int c)

'ctype.h' (SVID, BSD):  See Case Conversion.

int tolower (int c)

'ctype.h' (ANSI):  See Case Conversion.

int _tolower (int c)

'ctype.h' (SVID):  See Case Conversion.

TOSTOP

'termios.h' (POSIX.1):  See Local Modes.

int toupper (int c)

'ctype.h' (ANSI):  See Case Conversion.

int _toupper (int c)

'ctype.h' (SVID):  See Case Conversion.

TRY_AGAIN

'netdb.h' (BSD):  See Host Names.

char * ttyname (int filedes)

'unistd.h' (POSIX.1):  See Is It a Terminal.

int TZNAME_MAX

'limits.h' (POSIX.1):  See General Limits.

void tzset (void)

'time.h' (POSIX.1):  See Time Zone Functions.

UCHAR_MAX

'limits.h' (ANSI):  See Range of Type.

uid_t

'sys/types.h' (POSIX.1):  See Reading Persona.

UINT_MAX

'limits.h' (ANSI):  See Range of Type.

ULONG_LONG_MAX

'limits.h' (ANSI):  See Range of Type.

ULONG_MAX

'limits.h' (ANSI):  See Range of Type.

mode_t umask (mode_t mask)

'sys/stat.h' (POSIX.1):  See Setting Permissions.

int uname (struct utsname *info)

'sys/utsname.h' (POSIX.1):  See Hardware/Software Type ID.

int ungetc (int c, FILE *stream)

'stdio.h' (ANSI):  See How Unread.

union wait

'sys/wait.h' (BSD):  See BSD Wait Functions.

int unlink (const char *filename)

'unistd.h' (POSIX.1):  See Deleting Files.

USHRT_MAX

'limits.h' (ANSI):  See Range of Type.

int utime (const char *filename, const struct utimbuf *times)

'time.h' (POSIX.1):  See File Times.

int utimes (const char *filename, struct timeval tvp[2])

'sys/time.h' (BSD):  See File Times.

va_alist

'varargs.h' (Unix):  See Old Varargs.

type va_arg (va_list ap, type)

'stdarg.h' (ANSI):  See Argument Macros.

va_dcl

'varargs.h' (Unix):  See Old Varargs.

void va_end (va_list ap)

'stdarg.h' (ANSI):  See Argument Macros.

va_list

'stdarg.h' (ANSI):  See Argument Macros.

void * valloc (size_t size)

'malloc.h', 'stdlib.h' (BSD):  See Aligned Memory Blocks.

int vasprintf (char **ptr, const char *template, va_list ap)

'stdio.h' (GNU):  See Variable Arguments Output.

void va_start (va_list ap)

'varargs.h' (Unix):  See Old Varargs.

void va_start (va_list ap, last_required)

'stdarg.h' (ANSI):  See Argument Macros.

int VDISCARD

'termios.h' (BSD):  See Other Special.

int VDSUSP

'termios.h' (BSD):  See Signal Characters.

int VEOF

'termios.h' (POSIX.1):  See Editing Characters.

int VEOL2

'termios.h' (BSD):  See BSD Editing.

int VEOL

'termios.h' (POSIX.1):  See Editing Characters.

int VERASE

'termios.h' (POSIX.1):  See Editing Characters.

pid_t vfork (void)

'unistd.h' (BSD):  See Creating a Process.

int vfprintf (FILE *stream, const char *template, va_list ap)

'stdio.h' (ANSI):  See Variable Arguments Output.

int vfscanf (FILE *stream, const char *template, va_list ap)

'stdio.h' (GNU):  See Variable Arguments Input.

int VINTR

'termios.h' (POSIX.1):  See Signal Characters.

int VKILL

'termios.h' (POSIX.1):  See Editing Characters.

int VLNEXT

'termios.h' (BSD):  See BSD Editing.

int VMIN

'termios.h' (POSIX.1):  See Noncanonical Input.

int vprintf (const char *template, va_list ap)

'stdio.h' (ANSI):  See Variable Arguments Output.

int VQUIT

'termios.h' (POSIX.1):  See Signal Characters.

int VREPRINT

'termios.h' (BSD):  See BSD Editing.

int vscanf (const char *template, va_list ap)

'stdio.h' (GNU):  See Variable Arguments Input.

int vsnprintf (char *s, size_t size, const char *template, va_list ap)

'stdio.h' (GNU):  See Variable Arguments Output.

int vsprintf (char *s, const char *template, va_list ap)

'stdio.h' (ANSI):  See Variable Arguments Output.

int vsscanf (const char *s, const char *template, va_list ap)

'stdio.h' (GNU):  See Variable Arguments Input.

int VSTART

'termios.h' (POSIX.1):  See Start/Stop Characters.

int VSTATUS

'termios.h' (BSD):  See Other Special.

int VSTOP

'termios.h' (POSIX.1):  See Start/Stop Characters.

int VSUSP

'termios.h' (POSIX.1):  See Signal Characters.

int VTIME

'termios.h' (POSIX.1):  See Noncanonical Input.

int VWERASE

'termios.h' (BSD):  See BSD Editing.

pid_t wait3 (union wait *status_ptr, int options, struct rusage *usage)

'sys/wait.h' (BSD):  See BSD Wait Functions.

pid_t wait4 (pid_t pid, union wait *status_ptr, int options, struct rusage *usage)

'sys/wait.h' (BSD):  See BSD Wait Functions.

pid_t wait (int *status_ptr)

'sys/wait.h' (POSIX.1):  See Process Completion.

pid_t waitpid (pid_t pid, int *status_ptr, int options)

'sys/wait.h' (POSIX.1):  See Process Completion.

WCHAR_MAX

'limits.h' (GNU):  See Range of Type.

wchar_t

'stddef.h' (ANSI):  See Wide Char Intro.

int WCOREDUMP (int status)

'sys/wait.h' (BSD):  See Process Completion Status.

size_t wcstombs (char *string, const wchar_t wstring, size_t size)

'stdlib.h' (ANSI):  See Wide String Conversion.

int wctomb (char *string, wchar_t wchar)

'stdlib.h' (ANSI):  See Converting One Char.

int WEXITSTATUS (int status)

'sys/wait.h' (POSIX.1):  See Process Completion Status.

int WIFEXITED (int status)

'sys/wait.h' (POSIX.1):  See Process Completion Status.

int WIFSIGNALED (int status)

'sys/wait.h' (POSIX.1):  See Process Completion Status.

int WIFSTOPPED (int status)

'sys/wait.h' (POSIX.1):  See Process Completion Status.

int W_OK

'unistd.h' (POSIX.1):  See Testing File Access.

int wordexp (const char *words, wordexp_t *word-vector-ptr, int flags)

'wordexp.h' (POSIX.2):  See Calling Wordexp.

wordexp_t

'wordexp.h' (POSIX.2):  See Calling Wordexp.

void wordfree (wordexp_t *word-vector-ptr)

'wordexp.h' (POSIX.2):  See Calling Wordexp.

WRDE_APPEND

'wordexp.h' (POSIX.2):  See Flags for Wordexp.

WRDE_BADCHAR

'wordexp.h' (POSIX.2):  See Calling Wordexp.

WRDE_BADVAL

'wordexp.h' (POSIX.2):  See Calling Wordexp.

WRDE_CMDSUB

'wordexp.h' (POSIX.2):  See Calling Wordexp.

WRDE_DOOFFS

'wordexp.h' (POSIX.2):  See Flags for Wordexp.

WRDE_NOCMD

'wordexp.h' (POSIX.2):  See Flags for Wordexp.

WRDE_NOSPACE

'wordexp.h' (POSIX.2):  See Calling Wordexp.

WRDE_REUSE

'wordexp.h' (POSIX.2):  See Flags for Wordexp.

WRDE_SHOWERR

'wordexp.h' (POSIX.2):  See Flags for Wordexp.

WRDE_SYNTAX

'wordexp.h' (POSIX.2):  See Calling Wordexp.

WRDE_UNDEF

'wordexp.h' (POSIX.2):  See Flags for Wordexp.

ssize_t write (int filedes, const void *buffer, size_t size)

'unistd.h' (POSIX.1):  See I/O Primitives.

int WSTOPSIG (int status)

'sys/wait.h' (POSIX.1):  See Process Completion Status.

int WTERMSIG (int status)

'sys/wait.h' (POSIX.1):  See Process Completion Status.

int X_OK

'unistd.h' (POSIX.1):  See Testing File Access.


Library Maintenance
Node: Maintenance, Next: Copying, Prev: Library Summary, Up: Top

Library Maintenance

* Menu:

Installation	How to configure, compile and install the GNU C library.

Reporting Bugs	How to report bugs (if you want to get them fixed) and other troubles you may have with the GNU C library.

Source Layout	How to add new functions or header files to the GNU C library.

Porting	How to port the GNU C library to a new machine or operating system.

Contributors	Contributors to the GNU C Library.


How to Install the GNU C Library
Node: Installation, Next: Reporting Bugs, Prev: , Up: Maintenance

How to Install the GNU C Library

Installation of the GNU C library is relatively simple.

You need the latest version of GNU make.  Modifying the GNU C Library to work with other make programs would be so hard that we recommend you port GNU make instead.  Really.

To configure the GNU C library for your system, run the shell script 'configure' with sh.  Use an argument which is the conventional GNU name for your system configuration--for example, 'sparc-sun-sunos4.1', for a Sun 4 running Sunos 4.1.  See Installation, for a full description of standard GNU configuration names.  If you omit the configuration name, 'configure' will try to guess one for you by inspecting the system it is running on.  It may or may not be able to come up with a guess, and the its guess might be wrong.  'configure' will tell you the canonical name of the chosen configuration before proceeding.

The GNU C Library currently supports configurations that match the following patterns:

alpha-dec-osf1
i386-anything-bsd4.3
i386-anything-gnu
i386-anything-sco3.2
i386-anything-sco3.2v4
i386-anything-sysv
i386-anything-sysv4
i386-sequent-bsd
m68k-hp-bsd4.3
m68k-sony-newsos3
m68k-sony-newsos4
m68k-sun-sunos4.n
mips-dec-ultrix4.n
mips-sgi-irix4.n
sparc-sun-solaris2.n
sparc-sun-sunos4.n

While no other configurations are supported, there are handy aliases for these few.  (These aliases work in other GNU software as well.)

decstation
hp320-bsd4.3 hp300bsd
i386-sco
i386-sco3.2v4
i386-sequent-dynix
i386-svr4
news
sun3-sunos4.n sun3
sun4-solaris2.n sun4-sunos5.n
sun4-sunos4.n sun4

Here are some options that you should specify (if appropriate) when you run configure:

'--with-gnu-ld'

Use this option if you plan to use GNU ld to link programs with the GNU C Library.  (We strongly recommend that you do.)  This option enables use of features that exist only in GNU ld; so if you configure for GNU ld you must use GNU ld every time you link with the GNU C Library, and when building it.

'--with-gnu-as'

Use this option if you plan to use the GNU assembler, gas, when building the GNU C Library.  On some systems, the library may not build properly if you do not use gas.

'--nfp'

Use this option if your computer lacks hardware floating point support.

'--prefix=directory'

Install machine-independent data files in subdirectories of 'directory'.  (You can also set this in 'configparms'; see below.)

'--exec-prefix=directory'

Install the library and other machine-dependent files in subdirectories of 'directory'.  (You can also set this in 'configparms'; see below.)

The simplest way to run configure is to do it in the directory that contains the library sources.  This prepares to build the library in that very directory.

You can prepare to build the library in some other directory by going to that other directory to run configure.  In order to run configure, you will have to specify a directory for it, like this:

mkdir sun4
cd sun4
../configure sparc-sun-sunos4.1

configure looks for the sources in whatever directory you specified for finding configure itself.  It does not matter where in the file system the source and build directories are--as long as you specify the source directory when you run configure, you will get the proper results.

This feature lets you keep sources and binaries in different directories, and that makes it easy to build the library for several different machines from the same set of sources.  Simply create a build directory for each target machine, and run configure in that directory specifying the target machine's configuration name.

The library has a number of special-purpose configuration parameters.  These are defined in the file 'Makeconfig'; see the comments in that file for the details.

But don't edit the file 'Makeconfig' yourself--instead, create a file 'configparms' in the directory where you are building the library, and define in that file the parameters you want to specify.  'configparms' should not be an edited copy of 'Makeconfig'; specify only the parameters that you want to override.  To see how to set these parameters, find the section of 'Makeconfig' that says "These are the configuration variables." Then for each parameter that you want to change, copy the definition from 'Makeconfig' to your new 'configparms' file, and change the value as appropriate for your system.

Some of the machine-dependent code for some machines uses extensions in the GNU C compiler, so you may need to compile the library with GCC.  (In fact, all of the existing complete ports require GCC.)

The current release of the C library contains some header files that the compiler normally provides: 'stddef.h', 'stdarg.h', and several files with names of the form 'va-machine.h'.  The versions of these files that came with older releases of GCC do not work properly with the GNU C library.  The 'stddef.h' file in release 2.2 and later of GCC is correct.  If you have release 2.2 or later of GCC, use its version of 'stddef.h' instead of the C library's.  To do this, put the line 'override stddef.h =' in 'configparms'.  The other files are corrected in release 2.3 and later of GCC.  'configure' will automatically detect whether the installed 'stdarg.h' and 'va-machine.h' files are compatible with the C library, and use its own if not.

There is a potential problem with the size_t type and versions of GCC prior to release 2.4.  ANSI C requires that size_t always be an unsigned type.  For compatibility with existing systems' header files, GCC defines size_t in 'stddef.h' to be whatever type the system's 'sys/types.h' defines it to be.  Most Unix systems that define size_t in 'sys/types.h', define it to be a signed type.  Some code in the library depends on size_t being an unsigned type, and will not work correctly if it is signed.

The GNU C library code which expects size_t to be unsigned is correct.  The definition of size_t as a signed type is incorrect.  Versions 2.4 and later of GCC always define size_t as an unsigned type, and GCC's 'fixincludes' script massages the system's 'sys/types.h' so as not to conflict with this.

In the meantime, we work around this problem by telling GCC explicitly to use an unsigned type for size_t when compiling the GNU C library.  'configure' will automatically detect what type GCC uses for size_t arrange to override it if necessary.

To build the library, type make lib.  This will produce a lot of output, some of which looks like errors from make (but isn't).  Look for error messages from make containing '***'.  Those indicate that something is really wrong.

To build and run some test programs which exercise some of the library facilities, type make tests.  This will produce several files with names like 'program.out'.

To format the `GNU C Library Reference Manual' for printing, type make dvi.  To format the Info version of the manual for on line reading with ^C-h i in Emacs or with the info program, type make info.

To install the library and its header files, and the Info files of the manual, type make install, after setting the installation directories in 'configparms'.  This will build things if necessary, before installing them.


Reporting Bugs
Node: Reporting Bugs, Next: Source Layout, Prev: Installation, Up: Maintenance

Reporting Bugs

There are probably bugs in the GNU C library.  There are certainly errors and omissions in this manual.  If you report them, they will get fixed.  If you don't, no one will ever know about them and they will remain unfixed for all eternity, if not longer.

To report a bug, first you must find it.  Hopefully, this will be the hard part.  Once you've found a bug, make sure it's really a bug.  A good way to do this is to see if the GNU C library behaves the same way some other C library does.  If so, probably you are wrong and the libraries are right (but not necessarily).  If not, one of the libraries is probably wrong.

Once you're sure you've found a bug, try to narrow it down to the smallest test case that reproduces the problem.  In the case of a C library, you really only need to narrow it down to one library function call, if possible.  This should not be too difficult.

The final step when you have a simple test case is to report the bug.  When reporting a bug, send your test case, the results you got, the results you expected, what you think the problem might be (if you've thought of anything), your system type, and the version of the GNU C library which you are using.  Also include the files 'config.status' and 'config.make' which are created by running 'configure'; they will be in whatever directory was current when you ran 'configure'.

If you think you have found some way in which the GNU C library does not conform to the ANSI and POSIX standards (see Standards and Portability), that is definitely a bug.  Report it!

Send bug reports to the Internet address 'bug-glibc@prep.ai.mit.edu' or the UUCP path 'mit-eddie!prep.ai.mit.edu!bug-glibc'.  If you have other problems with installation or use, please report those as well.

If you are not sure how a function should behave, and this manual doesn't tell you, that's a bug in the manual.  Report that too!  If the function's behavior disagrees with the manual, then either the library or the manual has a bug, so report the disagreement.  If you find any errors or omissions in this manual, please report them to the Internet address 'bug-glibc-manual@prep.ai.mit.edu' or the UUCP path 'mit-eddie!prep.ai.mit.edu!bug-glibc-manual'.


Adding New Functions
Node: Source Layout, Next: Porting, Prev: Reporting Bugs, Up: Maintenance

Adding New Functions

The process of building the library is driven by the makefiles, which make heavy use of special features of GNU make.  The makefiles are very complex, and you probably don't want to try to understand them.  But what they do is fairly straightforward, and only requires that you define a few variables in the right places.

The library sources are divided into subdirectories, grouped by topic.  The 'string' subdirectory has all the string-manipulation functions, 'stdio' has all the standard I/O functions, etc.

Each subdirectory contains a simple makefile, called 'Makefile', which defines a few make variables and then includes the global makefile 'Rules' with a line like:

include ../Rules

The basic variables that a subdirectory makefile defines are:

subdir

The name of the subdirectory, for example 'stdio'.  This variable must be defined.

headers

The names of the header files in this section of the library, such as 'stdio.h'.

routines

aux

The names of the modules (source files) in this section of the library.  These should be simple names, such as 'strlen' (rather than complete file names, such as 'strlen.c').  Use routines for modules that define functions in the library, and aux for auxiliary modules containing things like data definitions.  But the values of routines and aux are just concatenated, so there really is no practical difference.

tests

The names of test programs for this section of the library.  These should be simple names, such as 'tester' (rather than complete file names, such as 'tester.c').  'make tests' will build and run all the test programs.  If a test program needs input, put the test data in a file called 'test-program.input'; it will be given to the test program on its standard input.  If a test program wants to be run with arguments, put the arguments (all on a single line) in a file called 'test-program.args'.

others

The names of "other" programs associated with this section of the library.  These are programs which are not tests per se, but are other small programs included with the library.  They are built by 'make others'.

install-lib

install-data

install

Files to be installed by 'make install'.  Files listed in 'install-lib' are installed in the directory specified by 'libdir' in 'configparms' or 'Makeconfig' (see Installation).  Files listed in install-data are installed in the directory specified by 'datadir' in 'configparms' or 'Makeconfig'.  Files listed in install are installed in the directory specified by 'bindir' in 'configparms' or 'Makeconfig'.

distribute

Other files from this subdirectory which should be put into a distribution tar file.  You need not list here the makefile itself or the source and header files listed in the other standard variables.  Only define distribute if there are files used in an unusual way that should go into the distribution.

generated

Files which are generated by 'Makefile' in this subdirectory.  These files will be removed by 'make clean', and they will never go into a distribution.

extra-objs

Extra object files which are built by 'Makefile' in this subdirectory.  This should be a list of file names like 'foo.o'; the files will actually be found in whatever directory object files are being built in.  These files will be removed by 'make clean'.  This variable is used for secondary object files needed to build others or tests.


Porting the GNU C Library
Node: Porting, Next: Contributors, Prev: Source Layout, Up: Maintenance

Porting the GNU C Library

The GNU C library is written to be easily portable to a variety of machines and operating systems.  Machine- and operating system-dependent functions are well separated to make it easy to add implementations for new machines or operating systems.  This section describes the layout of the library source tree and explains the mechanisms used to select machine-dependent code to use.

All the machine-dependent and operating system-dependent files in the library are in the subdirectory 'sysdeps' under the top-level library source directory.  This directory contains a hierarchy of subdirectories (see Hierarchy Conventions).

Each subdirectory of 'sysdeps' contains source files for a particular machine or operating system, or for a class of machine or operating system (for example, systems by a particular vendor, or all machines that use IEEE 754 floating-point format).  A configuration specifies an ordered list of these subdirectories.  Each subdirectory implicitly appends its parent directory to the list.  For example, specifying the list 'unix/bsd/vax' is equivalent to specifying the list 'unix/bsd/vax unix/bsd unix'.  A subdirectory can also specify that it implies other subdirectories which are not directly above it in the directory hierarchy.  If the file 'Implies' exists in a subdirectory, it lists other subdirectories of 'sysdeps' which are appended to the list, appearing after the subdirectory containing the 'Implies' file.  Lines in an 'Implies' file that begin with a '#' character are ignored as comments.  For example, 'unix/bsd/Implies' contains:

# BSD has Internet-related things.
unix/inet

and 'unix/Implies' contains:

posix

So the final list is 'unix/bsd/vax unix/bsd unix/inet unix posix'.

'sysdeps' has two "special" subdirectories, called 'generic' and 'stub'.  These two are always implicitly appended to the list of subdirectories (in that order), so you needn't put them in an 'Implies' file, and you should not create any subdirectories under them.  'generic' is for things that can be implemented in machine-independent C, using only other machine-independent functions in the C library.  'stub' is for "stub" versions of functions which cannot be implemented on a particular machine or operating system.  The stub functions always return an error, and set errno to ENOSYS (Function not implemented).  See Error Reporting.

A source file is known to be system-dependent by its having a version in 'generic' or 'stub'; every system-dependent function should have either a generic or stub implementation (there is no point in having both).

If you come across a file that is in one of the main source directories ('string', 'stdio', etc.), and you want to write a machine- or operating system-dependent version of it, move the file into 'sysdeps/generic' and write your new implementation in the appropriate system-specific subdirectory.  Note that if a file is to be system-dependent, it must not appear in one of the main source directories.

There are a few special files that may exist in each subdirectory of 'sysdeps':

'Makefile'

A makefile for this machine or operating system, or class of machine or operating system.  This file is included by the library makefile 'Makerules', which is used by the top-level makefile and the subdirectory makefiles.  It can change the variables set in the including makefile or add new rules.  It can use GNU make conditional directives based on the variable 'subdir' (see above) to select different sets of variables and rules for different sections of the library.  It can also set the make variable 'sysdep-routines', to specify extra modules to be included in the library.  You should use 'sysdep-routines' rather than adding modules to 'routines' because the latter is used in determining what to distribute for each subdirectory of the main source tree.

Each makefile in a subdirectory in the ordered list of subdirectories to be searched is included in order.  Since several system-dependent makefiles may be included, each should append to 'sysdep-routines' rather than simply setting it:

sysdep-routines := $(sysdep-routines) foo bar

'Subdirs'

This file contains the names of new whole subdirectories under the top-level library source tree that should be included for this system.  These subdirectories are treated just like the system-independent subdirectories in the library source tree, such as 'stdio' and 'math'.

Use this when there are completely new sets of functions and header files that should go into the library for the system this subdirectory of 'sysdeps' implements.  For example, 'sysdeps/unix/inet/Subdirs' contains 'inet'; the 'inet' directory contains various network-oriented operations which only make sense to put in the library on systems that support the Internet.

'Dist'

This file contains the names of files (relative to the subdirectory of 'sysdeps' in which it appears) which should be included in the distribution.  List any new files used by rules in the 'Makefile' in the same directory, or header files used by the source files in that directory.  You don't need to list files that are implementations (either C or assembly source) of routines whose names are given in the machine-independent makefiles in the main source tree.

'configure'

This file is a shell script fragment to be run at configuration time.  The top-level 'configure' script uses the shell . command to read the 'configure' file in each system-dependent directory chosen, in order.  The 'configure' files are often generated from 'configure.in' files using Autoconf.

A system-dependent 'configure' script will usually add things to the shell variables 'DEFS' and 'config_vars'; see the top-level 'configure' script for details.  The script can check for '--with-package' options that were passed to the top-level 'configure'.  For an option '--with-package=value' 'configure' sets the shell variable 'with_package' (with any dashes in package converted to underscores) to value; if the option is just '--with-package' (no argument), then it sets 'with_package' to 'yes'.

'configure.in'

This file is an Autoconf input fragment to be processed into the file 'configure' in this subdirectory.  See Introduction, for a description of Autoconf.  You should write either 'configure' or 'configure.in', but not both.  The first line of 'configure.in' should invoke the m4 macro 'GLIBC_PROVIDES'.  This macro does several AC_PROVIDE calls for Autoconf macros which are used by the top-level 'configure' script; without this, those macros might be invoked again unnecessarily by Autoconf.

That is the general system for how system-dependencies are isolated.

* Menu:

Hierarchy Conventions	The layout of the 'sysdeps' hierarchy.

Porting to Unix	Porting the library to an average Unix-like system.


Layout of the 'sysdeps' Directory Hierarchy
Node: Hierarchy Conventions, Next: Porting to Unix, Prev: , Up: Porting

Layout of the 'sysdeps' Directory Hierarchy

A GNU configuration name has three parts: the CPU type, the manufacturer's name, and the operating system.  'configure' uses these to pick the list of system-dependent directories to look for.  If the '--nfp' option is not passed to 'configure', the directory 'machine/fpu' is also used.  The operating system often has a "base operating system"; for example, if the operating system is 'sunos4.1', the base operating system is 'unix/bsd'.  The algorithm used to pick the list of directories is simple: 'configure' makes a list of the base operating system, manufacturer, CPU type, and operating system, in that order.  It then concatenates all these together with slashes in between, to produce a directory name; for example, the configuration 'sparc-sun-sunos4.1' results in 'unix/bsd/sun/sparc/sunos4.1'.  'configure' then tries removing each element of the list in turn, so 'unix/bsd/sparc' and 'sun/sparc' are also tried, among others.  Since the precise version number of the operating system is often not important, and it would be very inconvenient, for example, to have identical 'sunos4.1.1' and 'sunos4.1.2' directories, 'configure' tries successively less specific operating system names by removing trailing suffixes starting with a period.

As an example, here is the complete list of directories that would be tried for the configuration 'sparc-sun-sunos4.1' (without the '--nfp' option):

sparc/fpu
unix/bsd/sun/sunos4.1/sparc
unix/bsd/sun/sunos4.1
unix/bsd/sun/sunos4/sparc
unix/bsd/sun/sunos4
unix/bsd/sun/sunos/sparc
unix/bsd/sun/sunos
unix/bsd/sun/sparc
unix/bsd/sun
unix/bsd/sunos4.1/sparc
unix/bsd/sunos4.1
unix/bsd/sunos4/sparc
unix/bsd/sunos4
unix/bsd/sunos/sparc
unix/bsd/sunos
unix/bsd/sparc
unix/bsd
unix/sun/sunos4.1/sparc
unix/sun/sunos4.1
unix/sun/sunos4/sparc
unix/sun/sunos4
unix/sun/sunos/sparc
unix/sun/sunos
unix/sun/sparc
unix/sun
unix/sunos4.1/sparc
unix/sunos4.1
unix/sunos4/sparc
unix/sunos4
unix/sunos/sparc
unix/sunos
unix/sparc
unix
sun/sunos4.1/sparc
sun/sunos4.1
sun/sunos4/sparc
sun/sunos4
sun/sunos/sparc
sun/sunos
sun/sparc
sun
sunos4.1/sparc
sunos4.1
sunos4/sparc
sunos4
sunos/sparc
sunos
sparc

Different machine architectures are conventionally subdirectories at the top level of the 'sysdeps' directory tree.  For example, 'sysdeps/sparc' and 'sysdeps/m68k'.  These contain files specific to those machine architectures, but not specific to any particular operating system.  There might be subdirectories for specializations of those architectures, such as 'sysdeps/m68k/68020'. Code which is specific to the floating-point coprocessor used with a particular machine should go in 'sysdeps/machine/fpu'.

There are a few directories at the top level of the 'sysdeps' hierarchy that are not for particular machine architectures.

'generic'

'stub'

As described above (see Porting), these are the two subdirectories that every configuration implicitly uses after all others.

'ieee754'

This directory is for code using the IEEE 754 floating-point format, where the C type float is IEEE 754 single-precision format, and double is IEEE 754 double-precision format.  Usually this directory is referred to in the 'Implies' file in a machine architecture-specific directory, such as 'm68k/Implies'.

'posix'

This directory contains implementations of things in the library in terms of POSIX.1 functions.  This includes some of the POSIX.1 functions themselves.  Of course, POSIX.1 cannot be completely implemented in terms of itself, so a configuration using just 'posix' cannot be complete.

'unix'

This is the directory for Unix-like things.  See Porting to Unix.  'unix' implies 'posix'.  There are some special-purpose subdirectories of 'unix':

'unix/common'

This directory is for things common to both BSD and System V release 4.  Both 'unix/bsd' and 'unix/sysv/sysv4' imply 'unix/common'.

'unix/inet'

This directory is for socket and related functions on Unix systems.  The 'inet' top-level subdirectory is enabled by 'unix/inet/Subdirs'.  'unix/common' implies 'unix/inet'.

'mach'

This is the directory for things based on the Mach microkernel from CMU (including the GNU operating system).  Other basic operating systems (VMS, for example) would have their own directories at the top level of the 'sysdeps' hierarchy, parallel to 'unix' and 'mach'.


Porting the GNU C Library to Unix Systems
Node: Porting to Unix, Next: , Prev: Hierarchy Conventions, Up: Porting

Porting the GNU C Library to Unix Systems

Most Unix systems are fundamentally very similar.  There are variations between different machines, and variations in what facilities are provided by the kernel.  But the interface to the operating system facilities is, for the most part, pretty uniform and simple.

The code for Unix systems is in the directory 'unix', at the top level of the 'sysdeps' hierarchy.  This directory contains subdirectories (and subdirectory trees) for various Unix variants.

The functions which are system calls in most Unix systems are implemented in assembly code in files in 'sysdeps/unix'.  These files are named with a suffix of '.S'; for example, '__open.S'.  Files ending in '.S' are run through the C preprocessor before being fed to the assembler.

These files all use a set of macros that should be defined in 'sysdep.h'.  The 'sysdep.h' file in 'sysdeps/unix' partially defines them; a 'sysdep.h' file in another directory must finish defining them for the particular machine and operating system variant.  See 'sysdeps/unix/sysdep.h' and the machine-specific 'sysdep.h' implementations to see what these macros are and what they should do.

The system-specific makefile for the 'unix' directory (that is, the file 'sysdeps/unix/Makefile') gives rules to generate several files from the Unix system you are building the library on (which is assumed to be the target system you are building the library for).  All the generated files are put in the directory where the object files are kept; they should not affect the source tree itself.  The files generated are 'ioctls.h', 'errnos.h', 'sys/param.h', and 'errlist.c' (for the 'stdio' section of the library).


Contributors to the GNU C Library
Node: Contributors, Next: , Prev: Porting, Up: Maintenance

Contributors to the GNU C Library

The GNU C library was written almost entirely by Roland McGrath, who now maintains it.  Some parts of the library were contributed or worked on by other people.

•	The getopt function and related code were written by Richard Stallman, David J. MacKenzie, and Roland McGrath.

•	Most of the math functions are taken from 4.4 BSD; they have been modified only slightly to work with the GNU C library.  The Internet-related code (most of the 'inet' subdirectory) and several other miscellaneous functions and header files have been included with little or no modification.

All code incorporated from 4.4 BSD is under the following copyright:

Copyright (C) 1991 Regents of the University of California.
All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. 	Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

2. 	Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

3. 	All advertising materials mentioning features or use of this software must display the following acknowledgement:

This product includes software developed by the University of California, Berkeley and its contributors.

4. 	Neither the name of the University nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

•	The random number generation functions random, srandom, setstate and initstate, which are also the basis for the rand and srand functions, were written by Earl T. Cohen for the University of California at Berkeley and are copyrighted by the Regents of the University of California.  They have undergone minor changes to fit into the GNU C library and to fit the ANSI C standard, but the functional code is Berkeley's.

•	The merge sort function qsort was written by Michael J. Haertel.

•	The quick sort function used as a fallback by qsort was written by Douglas C. Schmidt.

•	The memory allocation functions malloc, realloc and free and related code were written by Michael J. Haertel.

•	Fast implementations of many of the string functions (memcpy, strlen, etc.) were written by Torbjorn Granlund.

•	Some of the support code for Mach is taken from Mach 3.0 by CMU, and is under the following copyright terms:

Mach Operating System
Copyright (C) 1991,1990,1989 Carnegie Mellon University
All Rights Reserved.

Permission to use, copy, modify and distribute this software and its documentation is hereby granted, provided that both the copyright notice and this permission notice appear in all copies of the software, derivative works or modified versions, and any portions thereof, and that both notices appear in supporting documentation.

CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS" CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.

Carnegie Mellon requests users of this software to return to

 Software Distribution Coordinator
 School of Computer Science
 Carnegie Mellon University
 Pittsburgh PA 15213-3890

or 'Software.Distribution@CS.CMU.EDU' any improvements or extensions that they make and grant Carnegie Mellon the rights to redistribute these changes.

•	The 'tar.h' header file was written by David J. MacKenzie.

•	The port to the MIPS DECStation running Ultrix 4 (mips-dec-ultrix4) was contributed by Brendan Kehoe and Ian Lance Taylor.

•	The DES encryption function crypt and related functions were contributed by Michael Glad.

•	The ftw function was contributed by Ian Lance Taylor.

•	The code to support SunOS shared libraries was contributed by Tom Quinn.

•	The mktime function was contributed by Noel Cragg.

•	The port to the Sequent Symmetry running Dynix version 3 (i386-sequent-bsd) was contributed by Jason Merrill.

•	The timezone support code is derived from the public-domain timezone package by Arthur David Olson.

•	The Internet resolver code is taken directly from BIND 4.9.1, which is under both the Berkeley copyright above and also:

Portions Copyright (C) 1993 by Digital Equipment Corporation.

Permission to use, copy, modify, and distribute this software for any purpose with or without fee is hereby granted, provided that the above copyright notice and this permission notice appear in all copies, and that the name of Digital Equipment Corporation not be used in advertising or publicity pertaining to distribution of the document or software without specific, written prior permission.

THE SOFTWARE IS PROVIDED "AS IS" AND DIGITAL EQUIPMENT CORP.  DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS.  IN NO EVENT SHALL DIGITAL EQUIPMENT CORPORATION BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

•	The port to the DEC Alpha running OSF/1 (alpha-dec-osf1) was contributed by Brendan Kehoe, using some code written by Roland McGrath.

•	The floating-point printing function used by printf and friends was written by Roland McGrath and Torbjorn Granlund.  The multi-precision integer functions used in that function are taken from GNU MP, which was contributed by Torbjorn Granlund.

•	The code to support Sun RPC is taken verbatim from Sun's RPCSRC-4.0 distribution, and is covered by this copyright:

Copyright (C) 1984, Sun Microsystems, Inc.

Sun RPC is a product of Sun Microsystems, Inc. and is provided for unrestricted use provided that this legend is included on all tape media and as a part of the software program in whole or part.  Users may copy or modify Sun RPC without charge, but are not authorized to license or distribute it to anyone else except as part of a product or program developed by the user.

SUN RPC IS PROVIDED AS IS WITH NO WARRANTIES OF ANY KIND INCLUDING THE WARRANTIES OF DESIGN, MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR ARISING FROM A COURSE OF DEALING, USAGE OR TRADE PRACTICE.

Sun RPC is provided with no support and without any obligation on the part of Sun Microsystems, Inc. to assist in its use, correction, modification or enhancement.

SUN MICROSYSTEMS, INC. SHALL HAVE NO LIABILITY WITH RESPECT TO THE INFRINGEMENT OF COPYRIGHTS, TRADE SECRETS OR ANY PATENTS BY SUN RPC OR ANY PART THEREOF.

In no event will Sun Microsystems, Inc. be liable for any lost revenue or profits or other special, indirect and consequential damages, even if Sun has been advised of the possibility of such damages.

Sun Microsystems, Inc.
2550 Garcia Avenue
Mountain View, California  94043

•	The port to SGI machines running Irix 4 (mips-sgi-irix4) was contributed by Tom Quinn.


GNU LIBRARY GENERAL PUBLIC LICENSE
Node: Copying, Next: Concept Index, Prev: Maintenance, Up: Top

GNU LIBRARY GENERAL PUBLIC LICENSE

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          Version 2, June 1991

Copyright (C) 1991 Free Software Foundation, Inc.
675 Mass Ave, Cambridge, MA 02139, USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

[This is the first released version of the library GPL.  It is
 numbered 2 because it goes with version 2 of the ordinary GPL.]

Preamble

The licenses for most software are designed to take away your freedom to share and change it.  By contrast, the GNU General Public Licenses are intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users.

This license, the Library General Public License, applies to some specially designated Free Software Foundation software, and to any other libraries whose authors decide to use it.  You can use it for your libraries, too.

When we speak of free software, we are referring to freedom, not price.  Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.

To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights.  These restrictions translate to certain responsibilities for you if you distribute copies of the library, or if you modify it.

For example, if you distribute copies of the library, whether gratis or for a fee, you must give the recipients all the rights that we gave you.  You must make sure that they, too, receive or can get the source code.  If you link a program with the library, you must provide complete object files to the recipients so that they can relink them with the library, after making changes to the library and recompiling it.  And you must show them these terms so they know their rights.

Our method of protecting your rights has two steps: (1) copyright the library, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the library.

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For an executable, the required form of the "work that uses the Library" must include any data and utility programs needed for reproducing the executable from it.  However, as a special exception, the source code distributed need not include anything that is normally distributed (in either source or binary form) with the major components (compiler, kernel, and so on) of the operating system on which the executable runs, unless that component itself accompanies the executable.

It may happen that this requirement contradicts the license restrictions of other proprietary libraries that do not normally accompany the operating system.  Such a contradiction means you cannot use both them and the Library together in an executable that you distribute.

7. 	You may place library facilities that are a work based on the Library side-by-side in a single library together with other library facilities not covered by this License, and distribute such a combined library, provided that the separate distribution of the work based on the Library and of the other library facilities is otherwise permitted, and provided that you do these two things:

a.	Accompany the combined library with a copy of the same work based on the Library, uncombined with any other library facilities.  This must be distributed under the terms of the Sections above.

b.	Give prominent notice with the combined library of the fact that part of it is a work based on the Library, and explaining where to find the accompanying uncombined form of the same work.

8. 	You may not copy, modify, sublicense, link with, or distribute the Library except as expressly provided under this License.  Any attempt otherwise to copy, modify, sublicense, link with, or distribute the Library is void, and will automatically terminate your rights under this License.  However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.

9. 	You are not required to accept this License, since you have not signed it.  However, nothing else grants you permission to modify or distribute the Library or its derivative works.  These actions are prohibited by law if you do not accept this License.  Therefore, by modifying or distributing the Library (or any work based on the Library), you indicate your acceptance of this License to do so, and all its terms and conditions for copying, distributing or modifying the Library or works based on it.

10. 	Each time you redistribute the Library (or any work based on the Library), the recipient automatically receives a license from the original licensor to copy, distribute, link with or modify the Library subject to these terms and conditions.  You may not impose any further restrictions on the recipients' exercise of the rights granted herein.  You are not responsible for enforcing compliance by third parties to this License.

11. 	If, as a consequence of a court judgment or allegation of patent infringement or for any other reason (not limited to patent issues), conditions are imposed on you (whether by court order, agreement or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License.  If you cannot distribute so as to satisfy simultaneously your obligations under this License and any other pertinent obligations, then as a consequence you may not distribute the Library at all.  For example, if a patent license would not permit royalty-free redistribution of the Library by all those who receive copies directly or indirectly through you, then the only way you could satisfy both it and this License would be to refrain entirely from distribution of the Library.

If any portion of this section is held invalid or unenforceable under any particular circumstance, the balance of the section is intended to apply, and the section as a whole is intended to apply in other circumstances.

It is not the purpose of this section to induce you to infringe any patents or other property right claims or to contest validity of any such claims; this section has the sole purpose of protecting the integrity of the free software distribution system which is implemented by public license practices.  Many people have made generous contributions to the wide range of software distributed through that system in reliance on consistent application of that system; it is up to the author/donor to decide if he or she is willing to distribute software through any other system and a licensee cannot impose that choice.

This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this License.

12. 	If the distribution and/or use of the Library is restricted in certain countries either by patents or by copyrighted interfaces, the original copyright holder who places the Library under this License may add an explicit geographical distribution limitation excluding those countries, so that distribution is permitted only in or among countries not thus excluded.  In such case, this License incorporates the limitation as if written in the body of this License.

13. 	The Free Software Foundation may publish revised and/or new versions of the Library General Public License from time to time.  Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.

Each version is given a distinguishing version number.  If the Library specifies a version number of this License which applies to it and "any later version", you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation.  If the Library does not specify a license version number, you may choose any version ever published by the Free Software Foundation.

14. 	If you wish to incorporate parts of the Library into other free programs whose distribution conditions are incompatible with these, write to the author to ask for permission.  For software which is copyrighted by the Free Software Foundation, write to the Free Software Foundation; we sometimes make exceptions for this.  Our decision will be guided by the two goals of preserving the free status of all derivatives of our free software and of promoting the sharing and reuse of software generally.

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     NO WARRANTY

15. 	BECAUSE THE LIBRARY IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE LIBRARY, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE LIBRARY "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE LIBRARY IS WITH YOU.  SHOULD THE LIBRARY PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.

16. 	IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE LIBRARY AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE LIBRARY (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE LIBRARY TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      END OF TERMS AND CONDITIONS

How to Apply These Terms to Your New Libraries

If you develop a new library, and you want it to be of the greatest possible use to the public, we recommend making it free software that everyone can redistribute and change.  You can do so by permitting redistribution under these terms (or, alternatively, under the terms of the ordinary General Public License).

To apply these terms, attach the following notices to the library.  It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the "copyright" line and a pointer to where the full notice is found.

one line to give the library's name and an idea of what it does.
Copyright (C) year  name of author

This library is free software; you can redistribute it and/or modify it
under the terms of the GNU Library General Public License as published
by the Free Software Foundation; either version 2 of the License, or (at
your option) any later version.

This library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Library General Public License for more details.

You should have received a copy of the GNU Library General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

Also add information on how to contact you by electronic and paper mail.

You should also get your employer (if you work as a programmer) or your school, if any, to sign a "copyright disclaimer" for the library, if necessary.  Here is a sample; alter the names:

Yoyodyne, Inc., hereby disclaims all copyright interest in the library
`Frob' (a library for tweaking knobs) written by James Random Hacker.

signature of Ty Coon, 1 April 1990
Ty Coon, President of Vice

That's all there is to it!


Concept Index
Node: Concept Index, Next: Type Index, Prev: Copying, Up: Top

Concept Index

4.n BSD Unix:	See Berkeley Unix.

alloca disadvantages:	See Disadvantages of Alloca.

alloca function:	See Variable Size Automatic.

exec functions:	See Executing a File.

fcntl function:	See Control Operations.

main function:	See Program Arguments.

malloc function:	See Unconstrained Allocation.

pause function:	See Waiting for a Signal.

setuid programs:	See How Change Persona.

sigaction flags:	See Flags for Sigaction.

sigaction function:	See Advanced Signal Handling.

SIGCHLD, handling of:	See Stopped and Terminated Jobs.

signal function:	See Basic Signal Handling.

SIGTTIN, from background job:	See Access to the Terminal.

SIGTTOU, from background job:	See Access to the Terminal.

volatile declarations:	See Nonreentrancy.

_POSIX_OPTION_ORDER environment variable.:	See Standard Environment.

abort signal:	See Program Error Signals.

aborting a program:	See Aborting a Program.

absolute file name:	See File Name Resolution.

absolute value functions:	See Absolute Value.

accepting connections:	See Accepting Connections.

access permission for a file:	See Access Permission.

access, testing for:	See Testing File Access.

accessing directories:	See Accessing Directories.

address of socket:	See Socket Addresses.

alarm signal:	See Alarm Signals.

alarms, setting:	See Setting an Alarm.

alignment (in obstacks):	See Obstacks Data Alignment.

alignment (with malloc):	See Aligned Memory Blocks.

allocation (obstacks):	See Allocation in an Obstack.

allocation hooks, for malloc:	See Hooks for Malloc.

allocation of memory with malloc:	See Basic Allocation.

allocation size of string:	See Representation of Strings.

allocation statistics:	See Statistics of Malloc.

alphabetic character:	See Classification of Characters.

alphanumeric character:	See Classification of Characters.

ANSI C:	See ANSI C.

append mode (file status flag):	See Opening and Closing Files.

append-access files:	See File Position.

argc (program argument count):	See Program Arguments.

argument promotion:	See Calling Variadics.

arguments (variadic functions):	See Receiving Arguments.

arguments, how many:	See How Many Arguments.

argv (program argument vector):	See Program Arguments.

arithmetic expansion:	See Expansion Stages.

array comparison functions:	See String/Array Comparison.

array copy functions:	See Copying and Concatenation.

array search function:	See Array Search Function.

array sort function:	See Array Sort Function.

ASCII character:	See Classification of Characters.

assertions:	See Consistency Checking.

attributes of a file:	See Attribute Meanings.

automatic allocation:	See Memory Concepts.

automatic freeing:	See Variable Size Automatic.

automatic storage with variable size:	See Variable Size Automatic.

background job:	See Concepts of Job Control.

background job, launching:	See Foreground and Background.

base (of floating point number):	See Floating Point Concepts.

basic byte sequence:	See Multibyte Char Intro.

baud rate:	See Line Speed.

Berkeley Unix:	See Berkeley Unix.

bias (of floating point number exponent):	See Floating Point Concepts.

big-endian:	See Byte Order.

binary I/O to a stream:	See Block Input/Output.

binary search function (for arrays):	See Array Search Function.

binary stream:	See Binary Streams.

binding a socket address:	See Socket Addresses.

blank character:	See Classification of Characters.

block I/O to a stream:	See Block Input/Output.

blocked signals:	See Delivery of Signal.

blocked signals, checking for:	See Checking for Pending Signals.

blocking signals:	See Blocking Signals.

blocking signals, in a handler:	See Blocking for Handler.

break condition, detecting:	See Input Modes.

break condition, generating:	See Line Control.

breaking a string into tokens:	See Finding Tokens in a String.

broken pipe signal:	See Miscellaneous Signals.

broken-down time <1>:	See Broken-down Time.

broken-down time:	See Calendar Time.

BSD compatibility library:	See Process Group Functions.

BSD compatibility library.:	See Feature Test Macros.

BSD Unix:	See Berkeley Unix.

buffering of streams:	See Stream Buffering.

buffering, controlling:	See Controlling Buffering.

bus error:	See Program Error Signals.

byte order conversion, for socket:	See Byte Order.

byte stream:	See Socket Concepts.

calendar time:	See Calendar Time.

calendar time and broken-down time:	See Broken-down Time.

calling variadic functions:	See Calling Variadics.

canonical input processing:	See Canonical or Not.

capacity limits, POSIX:	See General Limits.

carrier detect:	See Control Modes.

case conversion of characters:	See Case Conversion.

catching signals:	See Delivery of Signal.

categories for locales:	See Locale Categories.

change working directory:	See Working Directory.

changing the locale:	See Setting the Locale.

changing the size of a block (malloc):	See Changing Block Size.

changing the size of a block (obstacks):	See Growing Objects.

channels:	See Stream/Descriptor Precautions.

character case conversion:	See Case Conversion.

character code:	See Wide Char Intro.

character predicates:	See Classification of Characters.

character testing:	See Classification of Characters.

checking for pending signals:	See Checking for Pending Signals.

child process <1>:	See Process Creation Concepts.

child process:	See Child Processes.

child process signal:	See Job Control Signals.

chunks:	See Obstack Chunks.

classification of characters:	See Classification of Characters.

cleaning up a stream:	See Linked Channels.

clearing terminal input queue:	See Line Control.

client:	See Connections.

clock ticks:	See Processor Time.

close-on-exec (file descriptor flag):	See Descriptor Flags.

closing a file descriptor:	See Opening and Closing Files.

closing a socket:	See Closing a Socket.

closing a stream:	See Closing Streams.

code, character:	See Wide Char Intro.

collating strings:	See Collation Functions.

combining locales:	See Choosing Locale.

command argument syntax:	See Argument Syntax.

command arguments, parsing:	See Parsing Options.

command line arguments:	See Program Arguments.

command substitution:	See Expansion Stages.

communication style (of a socket):	See Socket Concepts.

comparing strings and arrays:	See String/Array Comparison.

Comparison Function:	See Comparison Functions.

concatenating strings:	See Copying and Concatenation.

connecting a socket:	See Connecting.

connection:	See Connections.

consistency checking:	See Consistency Checking.

consistency checking, of heap:	See Heap Consistency Checking.

continue signal:	See Job Control Signals.

control character:	See Classification of Characters.

control operations on files:	See Control Operations.

controlling process:	See Controlling Terminal.

controlling terminal:	See Concepts of Job Control.

controlling terminal, access to:	See Access to the Terminal.

controlling terminal, determining:	See Identifying the Terminal.

conversion specifications (printf):	See Formatted Output Basics.

conversion specifications (scanf):	See Formatted Input Basics.

converting byte order:	See Byte Order.

converting case of characters:	See Case Conversion.

converting extended characters:	See Converting One Char.

converting extended strings:	See Wide String Conversion.

converting file descriptor to stream:	See Descriptors and Streams.

converting floats to integers:	See Rounding and Remainders.

converting group ID to group name:	See Lookup Group.

converting group name to group ID:	See Lookup Group.

converting host address to name:	See Host Names.

converting host name to address:	See Host Names.

converting network name to network number:	See Networks Database.

converting network number to network name:	See Networks Database.

converting port number to service name:	See Services Database.

converting service name to port number:	See Services Database.

converting string to collation order:	See Collation Functions.

converting strings to numbers:	See Parsing of Numbers.

converting user ID to user name:	See Lookup User.

converting user name to user ID:	See Lookup User.

cookie, for custom stream:	See Streams and Cookies.

copying strings and arrays:	See Copying and Concatenation.

CPU time:	See Processor Time.

create on open (file status flag):	See Opening and Closing Files.

creating a directory:	See Creating Directories.

creating a FIFO special file:	See FIFO Special Files.

creating a pipe:	See Creating a Pipe.

creating a pipe to a subprocess:	See Pipe to a Subprocess.

creating a process:	See Process Creation Concepts.

creating a socket:	See Creating a Socket.

creating a socket pair:	See Socket Pairs.

creating special files:	See Making Special Files.

cube root function:	See Exponents and Logarithms.

currency symbols:	See Currency Symbol.

current working directory:	See Working Directory.

custom streams:	See Custom Streams.

customizing printf:	See Customizing Printf.

data loss on sockets:	See Socket Concepts.

datagram socket:	See Datagrams.

datagrams, transmitting:	See Sending Datagrams.

date and time:	See Calendar Time.

Daylight Saving Time:	See Broken-down Time.

decimal digit character:	See Classification of Characters.

decimal-point separator:	See General Numeric.

declaration (compared to definition):	See Header Files.

declaring variadic functions:	See Calling Variadics.

default action (for a signal):	See Delivery of Signal.

default action for a signal:	See Basic Signal Handling.

default argument promotions:	See Calling Variadics.

defining new printf conversions:	See Customizing Printf.

definition (compared to declaration):	See Header Files.

delayed suspend character:	See Signal Characters.

deleting a directory:	See Deleting Files.

deleting a file:	See Deleting Files.

delivery of signals:	See Delivery of Signal.

descriptors and streams:	See Stream/Descriptor Precautions.

digit character:	See Classification of Characters.

directories, accessing:	See Accessing Directories.

directories, creating:	See Creating Directories.

directories, deleting:	See Deleting Files.

directory:	See Directories.

directory entry:	See Directories.

directory stream:	See Accessing Directories.

disadvantages of alloca:	See Disadvantages of Alloca.

DISCARD character:	See Other Special.

domain (of socket):	See Socket Concepts.

domain error:	See Domain and Range Errors.

dot notation, for Internet addresses:	See Abstract Host Addresses.

DSUSP character:	See Signal Characters.

duplicating file descriptors:	See Duplicating Descriptors.

dynamic allocation:	See Memory Concepts.

echo of terminal input:	See Local Modes.

effective group ID:	See Process Persona.

effective user ID:	See Process Persona.

efficiency and malloc:	See Efficiency and Malloc.

efficiency and obstacks:	See Extra Fast Growing.

efficiency of chunks:	See Obstack Chunks.

end of file, on a stream:	See EOF and Errors.

end-of-file, on a file descriptor:	See I/O Primitives.

environment:	See Environment Variables.

environment access:	See Environment Access.

environment representation:	See Environment Access.

environment variable:	See Environment Variables.

EOF character:	See Editing Characters.

EOL character:	See Editing Characters.

EOL2 character:	See BSD Editing.

epoch:	See Simple Calendar Time.

ERASE character:	See Editing Characters.

error codes:	See Error Reporting.

error reporting:	See Error Reporting.

establishing a handler:	See Signal Actions.

exception:	See Program Error Signals.

exclusive lock:	See File Locks.

executing a file:	See Executing a File.

exit status:	See Exit Status.

exit status value:	See Program Termination.

expansion of shell words:	See Word Expansion.

exponent (of floating point number):	See Floating Point Concepts.

exponentiation functions:	See Exponents and Logarithms.

extended character sets:	See Extended Characters.

extended characters, converting:	See Converting One Char.

extended strings, converting representations:	See Wide String Conversion.

extending printf:	See Customizing Printf.

extracting file descriptor from stream:	See Descriptors and Streams.

feature test macros:	See Feature Test Macros.

field splitting:	See Expansion Stages.

FIFO special file:	See Pipes and FIFOs.

file access permission:	See Access Permission.

file access time:	See File Times.

file attribute modification time:	See File Times.

file attributes:	See Attribute Meanings.

file creation mask:	See Setting Permissions.

file descriptor flags:	See Descriptor Flags.

file descriptor sets, for select:	See Waiting for I/O.

file descriptors, standard:	See Descriptors and Streams.

file locks:	See File Locks.

file modification time:	See File Times.

file name:	See File Names.

file name component:	See Directories.

file name resolution:	See File Name Resolution.

file name syntax errors:	See File Name Errors.

file names, multiple:	See Hard Links.

file namespace, for sockets:	See File Namespace.

file owner:	See File Owner.

file permission bits:	See Permission Bits.

file pointer:	See Streams.

file position:	See File Position.

file positioning on a file descriptor:	See File Position Primitive.

file positioning on a stream:	See File Positioning.

file status flags:	See File Status Flags.

filtering i/o through subprocess:	See Pipe to a Subprocess.

flag character (printf):	See Output Conversion Syntax.

flag character (scanf):	See Input Conversion Syntax.

flags for sigaction:	See Flags for Sigaction.

floating point, IEEE:	See IEEE Floating Point.

floating type measurements:	See Floating Type Macros.

floating-point exception:	See Program Error Signals.

flow control, terminal:	See Line Control.

flushing a stream:	See Flushing Buffers.

flushing terminal output queue:	See Line Control.

foreground job:	See Concepts of Job Control.

foreground job, launching:	See Foreground and Background.

forking a process:	See Process Creation Concepts.

format string, for printf:	See Formatted Output.

format string, for scanf:	See Formatted Input.

formatted input from a stream:	See Formatted Input.

formatted output to a stream:	See Formatted Output.

freeing (obstacks):	See Freeing Obstack Objects.

freeing memory allocated with malloc:	See Freeing after Malloc.

fully buffered stream:	See Buffering Concepts.

function prototypes (variadic):	See Variadic Prototypes.

generation of signals:	See Signal Generation.

globbing:	See Globbing.

graphic character:	See Classification of Characters.

Gregorian calendar:	See Calendar Time.

group database:	See Group Database.

group ID:	See User and Group IDs.

group name:	See User and Group IDs.

group owner of a file:	See File Owner.

grouping of digits:	See General Numeric.

growing objects (in obstacks):	See Growing Objects.

handle:	See Relocator Concepts.

handling multiple signals:	See Merged Signals.

hangup signal:	See Termination Signals.

hard link:	See Hard Links.

header files:	See Header Files.

heap consistency checking:	See Heap Consistency Checking.

heap, dynamic allocation from:	See Unconstrained Allocation.

heap, freeing memory from:	See Freeing after Malloc.

hexadecimal digit character:	See Classification of Characters.

hidden bit (of floating point number mantissa):	See Floating Point Concepts.

high-priority data:	See Out-of-Band Data.

high-resolution time:	See Calendar Time.

home directory:	See Standard Environment.

HOME environment variable:	See Standard Environment.

hook functions (of custom streams):	See Hook Functions.

host address, Internet:	See Abstract Host Addresses.

hosts database:	See Host Names.

how many arguments:	See How Many Arguments.

hyperbolic functions:	See Hyperbolic Functions.

identifying terminals:	See Is It a Terminal.

IEEE floating point:	See Not a Number.

IEEE floating point representation:	See IEEE Floating Point.

IEEE Std 1003.1:	See POSIX.

IEEE Std 1003.2:	See POSIX.

ignore action for a signal:	See Basic Signal Handling.

illegal instruction:	See Program Error Signals.

impossible events:	See Consistency Checking.

independent channels:	See Independent Channels.

initial signal actions:	See Initial Signal Actions.

inode number:	See Attribute Meanings.

input available signal:	See Asynchronous I/O Signals.

input conversions, for scanf:	See Table of Input Conversions.

input from multiple files:	See Waiting for I/O.

installing the library:	See Installation.

integer division functions:	See Integer Division.

integer type range:	See Range of Type.

integer type width:	See Width of Type.

interactive signals, from terminal:	See Local Modes.

interactive stop signal:	See Job Control Signals.

internationalization:	See Locales.

Internet host address:	See Abstract Host Addresses.

Internet namespace, for sockets:	See Internet Namespace.

interprocess communication, with FIFO:	See FIFO Special Files.

interprocess communication, with pipes:	See Creating a Pipe.

interprocess communication, with signals:	See Kill Example.

interprocess communication, with sockets:	See Sockets.

interrupt character:	See Signal Characters.

interrupt signal:	See Termination Signals.

interrupt-driven input:	See Interrupt Input.

interval timer, setting:	See Setting an Alarm.

INTR character:	See Signal Characters.

inverse hyperbolic functions:	See Hyperbolic Functions.

inverse trigonmetric functions:	See Inverse Trig Functions.

job:	See Job Control.

job control:	See Job Control.

job control functions:	See Functions for Job Control.

job control is optional:	See Job Control is Optional.

job control signals:	See Job Control Signals.

job control, enabling:	See Initializing the Shell.

Kermit the frog:	See Search/Sort Example.

KILL character:	See Editing Characters.

kill signal:	See Termination Signals.

killing a process:	See Signaling Another Process.

LANG environment variable:	See Standard Environment.

launching jobs:	See Launching Jobs.

LC_COLLATE environment variable:	See Standard Environment.

LC_CTYPE environment variable:	See Standard Environment.

LC_MONETARY environment variable:	See Standard Environment.

LC_NUMERIC environment variable:	See Standard Environment.

LC_TIME environment variable:	See Standard Environment.

leap second:	See Broken-down Time.

length of multibyte character:	See Length of Char.

length of string:	See Representation of Strings.

level, for socket options:	See Socket Options.

library:	See Introduction.

limits on resource usage:	See Limits on Resources.

limits, file name length:	See Limits for Files.

limits, floating types:	See Floating Type Macros.

limits, integer types:	See Range of Type.

limits, link count of files:	See Limits for Files.

limits, number of open files:	See General Limits.

limits, number of processes:	See General Limits.

limits, number of supplementary group IDs:	See General Limits.

limits, pipe buffer size:	See Limits for Files.

limits, POSIX:	See General Limits.

limits, program argument size:	See General Limits.

limits, terminal input queue:	See Limits for Files.

limits, time zone name length:	See General Limits.

line buffered stream:	See Buffering Concepts.

line speed:	See Line Speed.

lines (in a text file):	See Binary Streams.

link:	See Directories.

link, hard:	See Hard Links.

link, soft:	See Symbolic Links.

link, symbolic:	See Symbolic Links.

linked channels:	See Linked Channels.

listening (sockets):	See Listening.

little-endian:	See Byte Order.

LNEXT character:	See BSD Editing.

local network address number:	See Abstract Host Addresses.

local time:	See Calendar Time.

locale categories:	See Locale Categories.

locale, changing:	See Setting the Locale.

locales:	See Locales.

logarithm functions:	See Exponents and Logarithms.

login name:	See User and Group IDs.

login name, determining:	See Who Logged In.

LOGNAME environment variable:	See Standard Environment.

long jumps:	See Non-Local Exits.

long-named options:	See Argument Syntax.

longjmp:	See Advantages of Alloca.

loss of data on sockets:	See Socket Concepts.

lower-case character:	See Classification of Characters.

macros:	See Obstack Functions.

mantissa (of floating point number):	See Floating Point Concepts.

matching failure, in scanf:	See Formatted Input Basics.

maximum field width (scanf):	See Input Conversion Syntax.

measurements of floating types:	See Floating Type Macros.

memory allocation:	See Memory Allocation.

memory usage warnings:	See Memory Warnings.

merging of signals:	See Merged Signals.

MIN termios slot:	See Noncanonical Input.

minimum field width (printf):	See Output Conversion Syntax.

mixing descriptors and streams:	See Stream/Descriptor Precautions.

modem disconnect:	See Control Modes.

modem status lines:	See Control Modes.

monetary value formatting:	See Numeric Formatting.

multibyte character, length of:	See Length of Char.

multibyte characters:	See Multibyte Char Intro.

multiple names for one file:	See Hard Links.

multiplexing input:	See Waiting for I/O.

name of running program:	See Error Messages.

name of socket:	See Socket Addresses.

name space:	See Reserved Names.

names of signals:	See Standard Signals.

namespace (of socket):	See Socket Concepts.

NaN:	See Not a Number.

network byte order:	See Byte Order.

network number:	See Abstract Host Addresses.

network protocol:	See Socket Concepts.

networks database:	See Networks Database.

non-blocking mode (file status flag):	See Opening and Closing Files.

non-local exit, from signal handler:	See Longjmp in Handler.

non-local exits:	See Non-Local Exits.

noncanonical input processing:	See Canonical or Not.

normalization functions (floating-point):	See Normalization Functions.

normalized floating point number:	See Floating Point Concepts.

not a number:	See Not a Number.

null character:	See Representation of Strings.

null pointer constant:	See Null Pointer Constant.

number of arguments passed:	See How Many Arguments.

number syntax, parsing:	See Parsing of Numbers.

numeric value formatting:	See Numeric Formatting.

obstack status:	See Status of an Obstack.

obstacks:	See Obstacks.

opening a file:	See I/O Concepts.

opening a file descriptor:	See Opening and Closing Files.

opening a pipe:	See Creating a Pipe.

opening a socket:	See Creating a Socket.

opening a socket pair:	See Socket Pairs.

opening a stream:	See Opening Streams.

optional arguments:	See Variadic Functions.

optional POSIX features:	See System Options.

orphaned process group:	See Orphaned Process Groups.

out-of-band data:	See Out-of-Band Data.

output conversions, for printf:	See Table of Output Conversions.

output possible signal:	See Asynchronous I/O Signals.

owner of a file:	See File Owner.

packet:	See Socket Concepts.

page boundary:	See Aligned Memory Blocks.

parent directory:	See File Name Resolution.

parent process <1>:	See Child Processes.

parent process:	See Process Creation Concepts.

parity checking:	See Input Modes.

parsing a template string:	See Parsing a Template String.

parsing numbers (in formatted input):	See Parsing of Numbers.

parsing program arguments:	See Parsing Options.

parsing tokens from a string:	See Finding Tokens in a String.

password database:	See User Database.

PATH environment variable:	See Standard Environment.

peeking at input:	See Unreading.

pending signals:	See Delivery of Signal.

pending signals, checking for:	See Checking for Pending Signals.

permission to access a file:	See Access Permission.

persona:	See Process Persona.

pi (trigonometric constant):	See Trig Functions.

pipe:	See Pipes and FIFOs.

pipe signal:	See Miscellaneous Signals.

pipe to a subprocess:	See Pipe to a Subprocess.

port number:	See Ports.

positioning a file descriptor:	See File Position Primitive.

positioning a stream:	See File Positioning.

POSIX:	See POSIX.

POSIX capacity limits:	See General Limits.

POSIX optional features:	See System Options.

POSIX.1:	See POSIX.

POSIX.2:	See POSIX.

power functions:	See Exponents and Logarithms.

precision (printf):	See Output Conversion Syntax.

precision (of floating point number):	See Floating Point Concepts.

predicates on arrays:	See String/Array Comparison.

predicates on characters:	See Classification of Characters.

predicates on strings:	See String/Array Comparison.

printing character:	See Classification of Characters.

priority of a process:	See Priority.

process <1>:	See Process Startup.

process:	See Child Processes.

process completion:	See Process Completion.

process group functions:	See Functions for Job Control.

process group ID:	See Launching Jobs.

process group leader:	See Launching Jobs.

process groups:	See Job Control.

process ID:	See Process Creation Concepts.

process image:	See Process Creation Concepts.

process lifetime:	See Process Creation Concepts.

process priority:	See Priority.

process signal mask:	See Process Signal Mask.

process termination:	See Program Termination.

processor time:	See Processor Time.

profiling alarm signal:	See Alarm Signals.

profiling timer:	See Setting an Alarm.

program argument syntax:	See Argument Syntax.

program arguments:	See Program Arguments.

program arguments, parsing:	See Parsing Options.

program error signals:	See Program Error Signals.

program name:	See Error Messages.

program termination:	See Program Termination.

program termination signals:	See Termination Signals.

programming your own streams:	See Custom Streams.

protocol (of socket):	See Socket Concepts.

protocol family:	See Socket Concepts.

protocols database:	See Protocols Database.

prototypes for variadic functions:	See Variadic Prototypes.

pseudo-random numbers:	See Pseudo-Random Numbers.

punctuation character:	See Classification of Characters.

pushing input back:	See Unreading.

quick sort function (for arrays):	See Array Sort Function.

QUIT character:	See Signal Characters.

quit signal:	See Termination Signals.

quote removal:	See Expansion Stages.

race conditions, relating to job control:	See Launching Jobs.

race conditions, relating to signals:	See Signals in Handler.

radix (of floating point number):	See Floating Point Concepts.

raising signals:	See Generating Signals.

random numbers:	See Pseudo-Random Numbers.

random-access files:	See File Position.

range error:	See Domain and Range Errors.

range of integer type:	See Range of Type.

read lock:	See File Locks.

reading from a directory:	See Accessing Directories.

reading from a file descriptor:	See I/O Primitives.

reading from a socket:	See Transferring Data.

reading from a stream, by blocks:	See Block Input/Output.

reading from a stream, by characters:	See Character Input.

reading from a stream, formatted:	See Formatted Input.

real group ID:	See Process Persona.

real user ID:	See Process Persona.

real-time timer:	See Setting an Alarm.

receiving datagrams:	See Receiving Datagrams.

record locking:	See File Locks.

redirecting input and output:	See Duplicating Descriptors.

reentrant functions:	See Nonreentrancy.

relative file name:	See File Name Resolution.

relocating memory allocator:	See Relocating Allocator.

remainder functions:	See Rounding and Remainders.

removal of quotes:	See Expansion Stages.

removing a file:	See Deleting Files.

removing macros that shadow functions:	See Macro Definitions.

renaming a file:	See Renaming Files.

reporting bugs:	See Reporting Bugs.

reporting errors:	See Error Reporting.

REPRINT character:	See BSD Editing.

reserved names:	See Reserved Names.

resource limits:	See Limits on Resources.

restrictions on signal handler functions:	See Nonreentrancy.

root directory:	See File Name Resolution.

rounding functions:	See Rounding and Remainders.

running a command:	See Running a Command.

scanning the group list:	See Scanning All Groups.

scanning the user list:	See Scanning All Users.

search function (for arrays):	See Array Search Function.

search functions (for strings):	See Search Functions.

seed (for random numbers):	See Pseudo-Random Numbers.

seeking on a file descriptor:	See File Position Primitive.

seeking on a stream:	See File Positioning.

segmentation violation:	See Program Error Signals.

sending a datagram:	See Sending Datagrams.

sending signals:	See Generating Signals.

sequential-access files:	See File Position.

server:	See Connections.

services database:	See Services Database.

session <1>:	See Job Control.

session:	See Concepts of Job Control.

session leader:	See Concepts of Job Control.

setting an alarm:	See Setting an Alarm.

setuid programs and file access:	See Testing File Access.

shadowing functions with macros:	See Macro Definitions.

shared lock:	See File Locks.

shell:	See Concepts of Job Control.

shrinking objects:	See Growing Objects.

shutting down a socket:	See Closing a Socket.

sign (of floating point number):	See Floating Point Concepts.

signal:	See Signal Handling.

signal action:	See Delivery of Signal.

signal actions:	See Signal Actions.

signal flags:	See Flags for Sigaction.

signal handler function:	See Defining Handlers.

signal mask:	See Process Signal Mask.

signal messages:	See Signal Messages.

signal names:	See Standard Signals.

signal number:	See Standard Signals.

signal set:	See Signal Sets.

signal stack:	See POSIX vs BSD.

signals, generating:	See Generating Signals.

significand (of floating point number):	See Floating Point Concepts.

size of string:	See Representation of Strings.

socket:	See Sockets.

socket address (name) binding:	See Socket Addresses.

socket domain:	See Socket Concepts.

socket namespace:	See Socket Concepts.

socket option level:	See Socket Options.

socket options:	See Socket Options.

socket pair:	See Socket Pairs.

socket protocol:	See Socket Concepts.

socket shutdown:	See Closing a Socket.

socket, client actions:	See Connecting.

socket, closing:	See Closing a Socket.

socket, connecting:	See Connecting.

socket, creating:	See Creating a Socket.

socket, initiating a connection:	See Connecting.

sockets, accepting connections:	See Accepting Connections.

sockets, listening:	See Listening.

sockets, server actions:	See Listening.

soft link:	See Symbolic Links.

sort function (for arrays):	See Array Sort Function.

special files:	See Making Special Files.

specified action (for a signal):	See Delivery of Signal.

square root function:	See Exponents and Logarithms.

stable sorting:	See Array Sort Function.

standard dot notation, for Internet addresses:	See Abstract Host Addresses.

standard environment variables:	See Standard Environment.

standard error file descriptor:	See Descriptors and Streams.

standard error stream:	See Standard Streams.

standard file descriptors:	See Descriptors and Streams.

standard input file descriptor:	See Descriptors and Streams.

standard input stream:	See Standard Streams.

standard output file descriptor:	See Descriptors and Streams.

standard output stream:	See Standard Streams.

standard streams:	See Standard Streams.

standards:	See Standards and Portability.

START character:	See Start/Stop Characters.

static allocation:	See Memory Concepts.

STATUS character:	See Other Special.

status codes:	See Error Reporting.

status of a file:	See Attribute Meanings.

status of obstack:	See Status of an Obstack.

sticky bit:	See Permission Bits.

STOP character:	See Start/Stop Characters.

stop signal:	See Job Control Signals.

stopped job:	See Concepts of Job Control.

stopped jobs, continuing:	See Continuing Stopped Jobs.

stopped jobs, detecting:	See Stopped and Terminated Jobs.

storage allocation:	See Memory Allocation.

stream (sockets):	See Socket Concepts.

stream, for I/O to a string:	See String Streams.

streams and descriptors:	See Stream/Descriptor Precautions.

streams, and file descriptors:	See Descriptors and Streams.

streams, standard:	See Standard Streams.

string:	See Representation of Strings.

string allocation:	See Representation of Strings.

string collation functions:	See Collation Functions.

string comparison functions:	See String/Array Comparison.

string concatenation functions:	See Copying and Concatenation.

string copy functions:	See Copying and Concatenation.

string length:	See Representation of Strings.

string literal:	See Representation of Strings.

string search functions:	See Search Functions.

string stream:	See String Streams.

string, representation of:	See Representation of Strings.

style of communication (of a socket):	See Socket Concepts.

subshell:	See Initializing the Shell.

substitution of variables and commands:	See Expansion Stages.

successive signals:	See Merged Signals.

summer time:	See Broken-down Time.

SunOS:	See Berkeley Unix.

supplementary group IDs:	See Process Persona.

SUSP character:	See Signal Characters.

suspend character:	See Signal Characters.

SVID:	See SVID.

symbolic link:	See Symbolic Links.

syntax, for program arguments:	See Argument Syntax.

syntax, for reading numbers:	See Parsing of Numbers.

System V Unix:	See SVID.

TCP (Internet protocol):	See Protocols Database.

template, for printf:	See Formatted Output.

template, for scanf:	See Formatted Input.

TERM environment variable:	See Standard Environment.

terminal flow control:	See Line Control.

terminal identification:	See Is It a Terminal.

terminal input queue:	See I/O Queues.

terminal input queue, clearing:	See Line Control.

terminal input signal:	See Job Control Signals.

terminal line control functions:	See Line Control.

terminal line speed:	See Line Speed.

terminal mode data types:	See Mode Data Types.

terminal mode functions:	See Mode Functions.

terminal output queue:	See I/O Queues.

terminal output queue, flushing:	See Line Control.

terminal output signal:	See Job Control Signals.

terminated jobs, detecting:	See Stopped and Terminated Jobs.

termination signal:	See Termination Signals.

testing access permission:	See Testing File Access.

testing exit status of child process:	See Process Completion.

text stream:	See Binary Streams.

ticks, clock:	See Processor Time.

tilde expansion:	See Expansion Stages.

TIME termios slot:	See Noncanonical Input.

time zone:	See TZ Variable.

time, calendar:	See Calendar Time.

time, elapsed CPU:	See Processor Time.

timers, setting:	See Setting an Alarm.

timing error in signal handling:	See Remembering a Signal.

TMPDIR environment variable:	See Temporary Files.

tokenizing strings:	See Finding Tokens in a String.

transmitting datagrams:	See Sending Datagrams.

trigonometric functions:	See Trig Functions.

type measurements, floating:	See Floating Type Macros.

type measurements, integer:	See Width of Type.

type modifier character (printf):	See Output Conversion Syntax.

type modifier character (scanf):	See Input Conversion Syntax.

typeahead buffer:	See I/O Queues.

TZ environment variable:	See Standard Environment.

umask:	See Setting Permissions.

unbuffered stream:	See Buffering Concepts.

unconstrained storage allocation:	See Unconstrained Allocation.

undefining macros that shadow functions:	See Macro Definitions.

Unix, Berkeley:	See Berkeley Unix.

Unix, System V:	See SVID.

unlinking a file:	See Deleting Files.

unreading characters:	See Unreading.

upper-case character:	See Classification of Characters.

urgent data signal:	See Asynchronous I/O Signals.

urgent socket condition:	See Out-of-Band Data.

usage limits:	See Limits on Resources.

user database:	See User Database.

user ID:	See User and Group IDs.

user ID, determining:	See Who Logged In.

user name:	See User and Group IDs.

user signals:	See Miscellaneous Signals.

usual file name syntax errors:	See File Name Errors.

variable number of arguments:	See Variadic Functions.

variable substitution:	See Expansion Stages.

variable-sized arrays:	See GNU C Variable-Size Arrays.

variadic function argument access:	See Receiving Arguments.

variadic function prototypes:	See Variadic Prototypes.

variadic functions:	See Variadic Functions.

variadic functions, calling:	See Calling Variadics.

virtual time alarm signal:	See Alarm Signals.

virtual timer:	See Setting an Alarm.

waiting for a signal:	See Waiting for a Signal.

waiting for completion of child process:	See Process Completion.

waiting for input or output:	See Waiting for I/O.

warnings of memory almost full:	See Memory Warnings.

WERASE character:	See BSD Editing.

whitespace character:	See Classification of Characters.

wide characters:	See Extended Char Intro.

width of integer type:	See Width of Type.

wildcard expansion:	See Expansion Stages.

word expansion:	See Word Expansion.

working directory:	See Working Directory.

write lock:	See File Locks.

writing to a file descriptor:	See I/O Primitives.

writing to a socket:	See Transferring Data.

writing to a stream, by blocks:	See Block Input/Output.

writing to a stream, by characters:	See Simple Output.

writing to a stream, formatted:	See Formatted Output.


Type Index
Node: Type Index, Next: Function Index, Prev: Concept Index, Up: Top

Type Index

cc_t:	See Mode Data Types.

clock_t:	See Basic CPU Time.

comparison_fn_t:	See Comparison Functions.

cookie_close_function:	See Hook Functions.

cookie_io_functions_t:	See Streams and Cookies.

cookie_read_function:	See Hook Functions.

cookie_seek_function:	See Hook Functions.

cookie_write_function:	See Hook Functions.

dev_t:	See Attribute Meanings.

DIR:	See Opening a Directory.

div_t:	See Integer Division.

fd_set:	See Waiting for I/O.

FILE:	See Streams.

fpos_t:	See Portable Positioning.

gid_t:	See Reading Persona.

glob_t:	See Calling Glob.

ino_t:	See Attribute Meanings.

jmp_buf:	See Non-Local Details.

ldiv_t:	See Integer Division.

mode_t:	See Attribute Meanings.

nlink_t:	See Attribute Meanings.

off_t:	See File Position Primitive.

pid_t:	See Process Identification.

printf_arginfo_function:	See Defining the Output Handler.

printf_function:	See Defining the Output Handler.

ptrdiff_t:	See Important Data Types.

regex_t:	See POSIX Regexp Compilation.

regmatch_t:	See Regexp Subexpressions.

regoff_t:	See Regexp Subexpressions.

sig_atomic_t:	See Atomic Types.

sighandler_t:	See Basic Signal Handling.

sigjmp_buf:	See Non-Local Exits and Signals.

sigset_t:	See Signal Sets.

size_t:	See Important Data Types.

speed_t:	See Line Speed.

ssize_t:	See I/O Primitives.

struct dirent:	See Directory Entries.

struct flock:	See File Locks.

struct group:	See Group Data Structure.

struct hostent:	See Host Names.

struct in_addr:	See Host Address Data Type.

struct itimerval:	See Setting an Alarm.

struct lconv:	See Numeric Formatting.

struct linger:	See Socket-Level Options.

struct mstats:	See Statistics of Malloc.

struct netent:	See Networks Database.

struct obstack:	See Creating Obstacks.

struct option:	See Long Options.

struct passwd:	See User Data Structure.

struct printf_info:	See Conversion Specifier Options.

struct protoent:	See Protocols Database.

struct rlimit:	See Limits on Resources.

struct rusage:	See Resource Usage.

struct servent:	See Services Database.

struct sigaction:	See Advanced Signal Handling.

struct sigaltstack:	See Signal Stack.

struct sigstack:	See Signal Stack.

struct sigvec:	See BSD Handler.

struct sockaddr:	See Address Formats.

struct sockaddr_in:	See Internet Address Format.

struct sockaddr_un:	See File Namespace Details.

struct stat:	See Attribute Meanings.

struct termios:	See Mode Data Types.

struct timeval:	See High-Resolution Calendar.

struct timezone:	See High-Resolution Calendar.

struct tm:	See Broken-down Time.

struct tms:	See Detailed CPU Time.

struct utimbuf:	See File Times.

struct utsname:	See Hardware/Software Type ID.

tcflag_t:	See Mode Data Types.

time_t:	See Simple Calendar Time.

uid_t:	See Reading Persona.

union wait:	See BSD Wait Functions.

va_list:	See Argument Macros.

wchar_t:	See Wide Char Intro.

wordexp_t:	See Calling Wordexp.


Function and Macro Index
Node: Function Index, Next: Variable Index, Prev: Type Index, Up: Top

Function and Macro Index

_exit:	See Termination Internals.

_tolower:	See Case Conversion.

_toupper:	See Case Conversion.

abort:	See Aborting a Program.

abs:	See Absolute Value.

accept:	See Accepting Connections.

access:	See Testing File Access.

acos:	See Inverse Trig Functions.

acosh:	See Hyperbolic Functions.

adjtime:	See High-Resolution Calendar.

alarm:	See Setting an Alarm.

alloca:	See Variable Size Automatic.

asctime:	See Formatting Date and Time.

asin:	See Inverse Trig Functions.

asinh:	See Hyperbolic Functions.

asprintf:	See Dynamic Output.

assert:	See Consistency Checking.

atan:	See Inverse Trig Functions.

atan2:	See Inverse Trig Functions.

atanh:	See Hyperbolic Functions.

atexit:	See Cleanups on Exit.

atof:	See Parsing of Floats.

atoi:	See Parsing of Integers.

atol:	See Parsing of Integers.

bcmp:	See String/Array Comparison.

bcopy:	See Copying and Concatenation.

bind:	See Setting Address.

bsearch:	See Array Search Function.

bzero:	See Copying and Concatenation.

cabs:	See Absolute Value.

calloc:	See Allocating Cleared Space.

cbrt:	See Exponents and Logarithms.

ceil:	See Rounding and Remainders.

cfgetispeed:	See Line Speed.

cfgetospeed:	See Line Speed.

cfmakeraw:	See Line Speed.

cfree:	See Freeing after Malloc.

cfsetispeed:	See Line Speed.

cfsetospeed:	See Line Speed.

cfsetspeed:	See Line Speed.

chdir:	See Working Directory.

chmod:	See Setting Permissions.

chown:	See File Owner.

clearerr:	See EOF and Errors.

clock:	See Basic CPU Time.

close:	See Opening and Closing Files.

closedir:	See Reading/Closing Directory.

confstr:	See String Parameters.

connect:	See Connecting.

copysign:	See Normalization Functions.

cos:	See Trig Functions.

cosh:	See Hyperbolic Functions.

creat:	See Opening and Closing Files.

ctermid:	See Identifying the Terminal.

ctime:	See Formatting Date and Time.

cuserid:	See Who Logged In.

difftime:	See Simple Calendar Time.

div:	See Integer Division.

drem:	See Rounding and Remainders.

dup:	See Duplicating Descriptors.

dup2:	See Duplicating Descriptors.

endgrent:	See Scanning All Groups.

endhostent:	See Host Names.

endnetent:	See Networks Database.

endprotoent:	See Protocols Database.

endpwent:	See Scanning All Users.

endservent:	See Services Database.

execl:	See Executing a File.

execle:	See Executing a File.

execlp:	See Executing a File.

execv:	See Executing a File.

execve:	See Executing a File.

execvp:	See Executing a File.

exit:	See Normal Termination.

exp:	See Exponents and Logarithms.

expm1:	See Exponents and Logarithms.

fabs:	See Absolute Value.

fchmod:	See Setting Permissions.

fchown:	See File Owner.

fclean:	See Cleaning Streams.

fclose:	See Closing Streams.

fcntl:	See Control Operations.

FD_CLR:	See Waiting for I/O.

FD_ISSET:	See Waiting for I/O.

FD_SET:	See Waiting for I/O.

FD_ZERO:	See Waiting for I/O.

fdopen:	See Descriptors and Streams.

feof:	See EOF and Errors.

ferror:	See EOF and Errors.

fflush:	See Flushing Buffers.

fgetc:	See Character Input.

fgetgrent:	See Scanning All Groups.

fgetpos:	See Portable Positioning.

fgetpwent:	See Scanning All Users.

fgets:	See Line Input.

fileno:	See Descriptors and Streams.

finite:	See Predicates on Floats.

floor:	See Rounding and Remainders.

fmemopen:	See String Streams.

fmod:	See Rounding and Remainders.

fnmatch:	See Wildcard Matching.

fopen:	See Opening Streams.

fopencookie:	See Streams and Cookies.

fork:	See Creating a Process.

fpathconf:	See Pathconf.

fprintf:	See Formatted Output Functions.

fputc:	See Simple Output.

fputs:	See Simple Output.

fread:	See Block Input/Output.

free:	See Freeing after Malloc.

freopen:	See Opening Streams.

frexp:	See Normalization Functions.

fscanf:	See Formatted Input Functions.

fseek:	See File Positioning.

fsetpos:	See Portable Positioning.

fstat:	See Reading Attributes.

ftell:	See File Positioning.

fwrite:	See Block Input/Output.

getc:	See Character Input.

getchar:	See Character Input.

getcwd:	See Working Directory.

getdelim:	See Line Input.

getegid:	See Reading Persona.

getenv:	See Environment Access.

geteuid:	See Reading Persona.

getgid:	See Reading Persona.

getgrent:	See Scanning All Groups.

getgrgid:	See Lookup Group.

getgrnam:	See Lookup Group.

getgroups:	See Reading Persona.

gethostbyaddr:	See Host Names.

gethostbyname:	See Host Names.

gethostent:	See Host Names.

gethostid:	See Host Identification.

gethostname:	See Host Identification.

getitimer:	See Setting an Alarm.

getline:	See Line Input.

getlogin:	See Who Logged In.

getnetbyaddr:	See Networks Database.

getnetbyname:	See Networks Database.

getnetent:	See Networks Database.

getopt:	See Parsing Options.

getopt_long:	See Long Options.

getpeername:	See Who is Connected.

getpgrp:	See Process Group Functions.

getpid:	See Process Identification.

getppid:	See Process Identification.

getpriority:	See Priority.

getprotobyname:	See Protocols Database.

getprotobynumber:	See Protocols Database.

getprotoent:	See Protocols Database.

getpwent:	See Scanning All Users.

getpwnam:	See Lookup User.

getpwuid:	See Lookup User.

getrlimit:	See Limits on Resources.

getrusage:	See Resource Usage.

gets:	See Line Input.

getservbyname:	See Services Database.

getservbyport:	See Services Database.

getservent:	See Services Database.

getsockname:	See Reading Address.

getsockopt:	See Socket Option Functions.

gettimeofday:	See High-Resolution Calendar.

getuid:	See Reading Persona.

getumask:	See Setting Permissions.

getw:	See Character Input.

getwd:	See Working Directory.

glob:	See Calling Glob.

gmtime:	See Broken-down Time.

gsignal:	See Signaling Yourself.

htonl:	See Byte Order.

htons:	See Byte Order.

hypot:	See Exponents and Logarithms.

index:	See Search Functions.

inet_addr:	See Host Address Functions.

inet_aton:	See Host Address Functions.

inet_lnaof:	See Host Address Functions.

inet_makeaddr:	See Host Address Functions.

inet_netof:	See Host Address Functions.

inet_network:	See Host Address Functions.

inet_ntoa:	See Host Address Functions.

infnan:	See Predicates on Floats.

initgroups:	See Setting Groups.

initstate:	See BSD Random.

isalnum:	See Classification of Characters.

isalpha:	See Classification of Characters.

isascii:	See Classification of Characters.

isatty:	See Is It a Terminal.

isblank:	See Classification of Characters.

iscntrl:	See Classification of Characters.

isdigit:	See Classification of Characters.

isgraph:	See Classification of Characters.

isinf:	See Predicates on Floats.

islower:	See Classification of Characters.

isnan:	See Predicates on Floats.

isprint:	See Classification of Characters.

ispunct:	See Classification of Characters.

isspace:	See Classification of Characters.

isupper:	See Classification of Characters.

isxdigit:	See Classification of Characters.

ITIMER_PROF:	See Setting an Alarm.

ITIMER_REAL:	See Setting an Alarm.

ITIMER_VIRTUAL:	See Setting an Alarm.

kill:	See Signaling Another Process.

killpg:	See Signaling Another Process.

labs:	See Absolute Value.

ldexp:	See Normalization Functions.

ldiv:	See Integer Division.

link:	See Hard Links.

listen:	See Listening.

localeconv:	See Numeric Formatting.

localtime:	See Broken-down Time.

log:	See Exponents and Logarithms.

log10:	See Exponents and Logarithms.

log1p:	See Exponents and Logarithms.

logb:	See Normalization Functions.

longjmp:	See Non-Local Details.

lseek:	See File Position Primitive.

lstat:	See Reading Attributes.

malloc:	See Basic Allocation.

mblen:	See Length of Char.

mbstowcs:	See Wide String Conversion.

mbtowc:	See Converting One Char.

mcheck:	See Heap Consistency Checking.

memalign:	See Aligned Memory Blocks.

memccpy:	See Copying and Concatenation.

memchr:	See Search Functions.

memcmp:	See String/Array Comparison.

memcpy:	See Copying and Concatenation.

memmem:	See Search Functions.

memmove:	See Copying and Concatenation.

memory_warnings:	See Memory Warnings.

memset:	See Copying and Concatenation.

mkdir:	See Creating Directories.

mkfifo:	See FIFO Special Files.

mknod:	See Making Special Files.

mkstemp:	See Temporary Files.

mktemp:	See Temporary Files.

mktime:	See Broken-down Time.

modf:	See Rounding and Remainders.

mstats:	See Statistics of Malloc.

nice:	See Priority.

ntohl:	See Byte Order.

ntohs:	See Byte Order.

obstack_1grow:	See Growing Objects.

obstack_1grow_fast:	See Extra Fast Growing.

obstack_alignment_mask:	See Obstacks Data Alignment.

obstack_alloc:	See Allocation in an Obstack.

obstack_base:	See Status of an Obstack.

obstack_blank:	See Growing Objects.

obstack_blank_fast:	See Extra Fast Growing.

obstack_chunk_alloc:	See Preparing for Obstacks.

obstack_chunk_free:	See Preparing for Obstacks.

obstack_chunk_size:	See Obstack Chunks.

obstack_copy:	See Allocation in an Obstack.

obstack_copy0:	See Allocation in an Obstack.

obstack_finish:	See Growing Objects.

obstack_free:	See Freeing Obstack Objects.

obstack_grow:	See Growing Objects.

obstack_grow0:	See Growing Objects.

obstack_init:	See Preparing for Obstacks.

obstack_next_free:	See Status of an Obstack.

obstack_object_size <1>:	See Growing Objects.

obstack_object_size:	See Status of an Obstack.

obstack_printf:	See Dynamic Output.

obstack_room:	See Extra Fast Growing.

obstack_vprintf:	See Variable Arguments Output.

offsetof:	See Structure Measurement.

on_exit:	See Cleanups on Exit.

open:	See Opening and Closing Files.

open_memstream:	See String Streams.

open_obstack_stream:	See Obstack Streams.

opendir:	See Opening a Directory.

parse_printf_format:	See Parsing a Template String.

pathconf:	See Pathconf.

pause:	See Using Pause.

pclose:	See Pipe to a Subprocess.

perror:	See Error Messages.

pipe:	See Creating a Pipe.

popen:	See Pipe to a Subprocess.

pow:	See Exponents and Logarithms.

printf:	See Formatted Output Functions.

psignal:	See Signal Messages.

putc:	See Simple Output.

putchar:	See Simple Output.

putenv:	See Environment Access.

putpwent:	See Writing a User Entry.

puts:	See Simple Output.

putw:	See Simple Output.

qsort:	See Array Sort Function.

r_alloc:	See Using Relocator.

r_alloc_free:	See Using Relocator.

r_re_alloc:	See Using Relocator.

raise:	See Signaling Yourself.

rand:	See ANSI Random.

random:	See BSD Random.

read:	See I/O Primitives.

readdir:	See Reading/Closing Directory.

readlink:	See Symbolic Links.

realloc:	See Changing Block Size.

recv:	See Receiving Data.

recvfrom:	See Receiving Datagrams.

regcomp:	See POSIX Regexp Compilation.

regerror:	See Regexp Cleanup.

regexec:	See Matching POSIX Regexps.

regfree:	See Regexp Cleanup.

register_printf_function:	See Registering New Conversions.

remove:	See Deleting Files.

rename:	See Renaming Files.

rewind:	See File Positioning.

rewinddir:	See Random Access Directory.

rindex:	See Search Functions.

rint:	See Rounding and Remainders.

rmdir:	See Deleting Files.

S_ISBLK:	See Testing File Type.

S_ISCHR:	See Testing File Type.

S_ISDIR:	See Testing File Type.

S_ISFIFO:	See Testing File Type.

S_ISLNK:	See Testing File Type.

S_ISREG:	See Testing File Type.

S_ISSOCK:	See Testing File Type.

scalb:	See Normalization Functions.

scanf:	See Formatted Input Functions.

seekdir:	See Random Access Directory.

select:	See Waiting for I/O.

send:	See Sending Data.

sendto:	See Sending Datagrams.

setbuf:	See Controlling Buffering.

setbuffer:	See Controlling Buffering.

setgid:	See Setting Groups.

setgrent:	See Scanning All Groups.

setgroups:	See Setting Groups.

sethostent:	See Host Names.

sethostid:	See Host Identification.

sethostname:	See Host Identification.

setitimer:	See Setting an Alarm.

setjmp:	See Non-Local Details.

setlinebuf:	See Controlling Buffering.

setlocale:	See Setting the Locale.

setnetent:	See Networks Database.

setpgid:	See Process Group Functions.

setpgrp:	See Process Group Functions.

setpriority:	See Priority.

setprotoent:	See Protocols Database.

setpwent:	See Scanning All Users.

setregid:	See Setting Groups.

setreuid:	See Setting User ID.

setrlimit:	See Limits on Resources.

setservent:	See Services Database.

setsid:	See Process Group Functions.

setsockopt:	See Socket Option Functions.

setstate:	See BSD Random.

settimeofday:	See High-Resolution Calendar.

setuid:	See Setting User ID.

setvbuf:	See Controlling Buffering.

shutdown:	See Closing a Socket.

sigaction:	See Advanced Signal Handling.

sigaddset:	See Signal Sets.

sigaltstack:	See Signal Stack.

sigblock:	See Blocking in BSD.

sigdelset:	See Signal Sets.

sigemptyset:	See Signal Sets.

sigfillset:	See Signal Sets.

siginterrupt:	See BSD Handler.

sigismember:	See Signal Sets.

siglongjmp:	See Non-Local Exits and Signals.

sigmask:	See Blocking in BSD.

signal:	See Basic Signal Handling.

sigpause:	See Blocking in BSD.

sigpending:	See Checking for Pending Signals.

sigprocmask:	See Process Signal Mask.

sigsetjmp:	See Non-Local Exits and Signals.

sigsetmask:	See Blocking in BSD.

sigstack:	See Signal Stack.

sigsuspend:	See Sigsuspend.

sigvec:	See BSD Handler.

sin:	See Trig Functions.

sinh:	See Hyperbolic Functions.

sleep:	See Sleeping.

snprintf:	See Formatted Output Functions.

socket:	See Creating a Socket.

socketpair:	See Socket Pairs.

sprintf:	See Formatted Output Functions.

sqrt:	See Exponents and Logarithms.

srand:	See ANSI Random.

srandom:	See BSD Random.

sscanf:	See Formatted Input Functions.

ssignal:	See Basic Signal Handling.

stat:	See Reading Attributes.

stpcpy:	See Copying and Concatenation.

strcasecmp:	See String/Array Comparison.

strcat:	See Copying and Concatenation.

strchr:	See Search Functions.

strcmp:	See String/Array Comparison.

strcoll:	See Collation Functions.

strcpy:	See Copying and Concatenation.

strcspn:	See Search Functions.

strdup:	See Copying and Concatenation.

strerror:	See Error Messages.

strftime:	See Formatting Date and Time.

strlen:	See String Length.

strncasecmp:	See String/Array Comparison.

strncat:	See Copying and Concatenation.

strncmp:	See String/Array Comparison.

strncpy:	See Copying and Concatenation.

strpbrk:	See Search Functions.

strrchr:	See Search Functions.

strsignal:	See Signal Messages.

strspn:	See Search Functions.

strstr:	See Search Functions.

strtod:	See Parsing of Floats.

strtok:	See Finding Tokens in a String.

strtol:	See Parsing of Integers.

strtoul:	See Parsing of Integers.

strxfrm:	See Collation Functions.

symlink:	See Symbolic Links.

sysconf:	See Sysconf Definition.

system:	See Running a Command.

tan:	See Trig Functions.

tanh:	See Hyperbolic Functions.

tcdrain:	See Line Control.

tcflow:	See Line Control.

tcflush:	See Line Control.

tcgetattr:	See Mode Functions.

tcgetpgrp:	See Terminal Access Functions.

tcsendbreak:	See Line Control.

tcsetattr:	See Mode Functions.

tcsetpgrp:	See Terminal Access Functions.

telldir:	See Random Access Directory.

TEMP_FAILURE_RETRY:	See Interrupted Primitives.

tempnam:	See Temporary Files.

time:	See Simple Calendar Time.

times:	See Detailed CPU Time.

tmpfile:	See Temporary Files.

tmpnam:	See Temporary Files.

toascii:	See Case Conversion.

tolower:	See Case Conversion.

toupper:	See Case Conversion.

ttyname:	See Is It a Terminal.

tzset:	See Time Zone Functions.

umask:	See Setting Permissions.

uname:	See Hardware/Software Type ID.

ungetc:	See How Unread.

unlink:	See Deleting Files.

utime:	See File Times.

utimes:	See File Times.

va_alist:	See Old Varargs.

va_arg:	See Argument Macros.

va_dcl:	See Old Varargs.

va_end:	See Argument Macros.

va_start <1>:	See Argument Macros.

va_start:	See Old Varargs.

valloc:	See Aligned Memory Blocks.

vasprintf:	See Variable Arguments Output.

vfork:	See Creating a Process.

vfprintf:	See Variable Arguments Output.

vfscanf:	See Variable Arguments Input.

vprintf:	See Variable Arguments Output.

vscanf:	See Variable Arguments Input.

vsnprintf:	See Variable Arguments Output.

vsprintf:	See Variable Arguments Output.

vsscanf:	See Variable Arguments Input.

wait:	See Process Completion.

wait3:	See BSD Wait Functions.

wait4:	See BSD Wait Functions.

waitpid:	See Process Completion.

WCOREDUMP:	See Process Completion Status.

wcstombs:	See Wide String Conversion.

wctomb:	See Converting One Char.

WEXITSTATUS:	See Process Completion Status.

WIFEXITED:	See Process Completion Status.

WIFSIGNALED:	See Process Completion Status.

WIFSTOPPED:	See Process Completion Status.

wordexp:	See Calling Wordexp.

wordfree:	See Calling Wordexp.

write:	See I/O Primitives.

WSTOPSIG:	See Process Completion Status.

WTERMSIG:	See Process Completion Status.


Variable and Constant Macro Index
Node: Variable Index, Next: File Index, Prev: Function Index, Up: Top

Variable and Constant Macro Index

*:	See Time Zone Functions.

__free_hook:	See Hooks for Malloc.

__malloc_hook:	See Hooks for Malloc.

__realloc_hook:	See Hooks for Malloc.

_BSD_SOURCE:	See Feature Test Macros.

_GNU_SOURCE:	See Feature Test Macros.

_IOFBF:	See Controlling Buffering.

_IOLBF:	See Controlling Buffering.

_IONBF:	See Controlling Buffering.

_POSIX2_C_DEV:	See System Options.

_POSIX2_C_VERSION:	See Version Supported.

_POSIX2_FORT_DEV:	See System Options.

_POSIX2_FORT_RUN:	See System Options.

_POSIX2_LOCALEDEF:	See System Options.

_POSIX2_SW_DEV:	See System Options.

_POSIX_C_SOURCE:	See Feature Test Macros.

_POSIX_CHOWN_RESTRICTED:	See Options for Files.

_POSIX_JOB_CONTROL:	See System Options.

_POSIX_NO_TRUNC:	See Options for Files.

_POSIX_SAVED_IDS:	See System Options.

_POSIX_SOURCE:	See Feature Test Macros.

_POSIX_VDISABLE <1>:	See Options for Files.

_POSIX_VDISABLE:	See Special Characters.

_POSIX_VERSION:	See Version Supported.

_SVID_SOURCE:	See Feature Test Macros.

AF_FILE:	See Address Formats.

AF_INET:	See Address Formats.

AF_UNIX:	See Address Formats.

AF_UNSPEC:	See Address Formats.

ALTWERASE:	See Local Modes.

ARG_MAX:	See General Limits.

B0:	See Line Speed.

B110:	See Line Speed.

B1200:	See Line Speed.

B134:	See Line Speed.

B150:	See Line Speed.

B1800:	See Line Speed.

B19200:	See Line Speed.

B200:	See Line Speed.

B2400:	See Line Speed.

B300:	See Line Speed.

B38400:	See Line Speed.

B4800:	See Line Speed.

B50:	See Line Speed.

B600:	See Line Speed.

B75:	See Line Speed.

B9600:	See Line Speed.

BC_BASE_MAX:	See Utility Limits.

BC_DIM_MAX:	See Utility Limits.

BC_SCALE_MAX:	See Utility Limits.

BC_STRING_MAX:	See Utility Limits.

BRKINT:	See Input Modes.

BUFSIZ:	See Controlling Buffering.

CCTS_OFLOW:	See Control Modes.

CHILD_MAX:	See General Limits.

CLK_TCK:	See Basic CPU Time.

CLOCAL:	See Control Modes.

CLOCKS_PER_SEC:	See Basic CPU Time.

COLL_WEIGHTS_MAX:	See Utility Limits.

COREFILE:	See Program Error Signals.

CREAD:	See Control Modes.

CRTS_IFLOW:	See Control Modes.

CS5:	See Control Modes.

CS6:	See Control Modes.

CS7:	See Control Modes.

CS8:	See Control Modes.

CSIZE:	See Control Modes.

CSTOPB:	See Control Modes.

daylight:	See Time Zone Functions.

E2BIG:	See Error Codes.

EACCES:	See Error Codes.

EADDRINUSE:	See Error Codes.

EADDRNOTAVAIL:	See Error Codes.

EAFNOSUPPORT:	See Error Codes.

EAGAIN:	See Error Codes.

EALREADY:	See Error Codes.

EBACKGROUND:	See Error Codes.

EBADF <1>:	See Error Codes.

EBADF:	See Line Control.

EBUSY:	See Error Codes.

ECHILD:	See Error Codes.

ECHO:	See Local Modes.

ECHOCTL:	See Local Modes.

ECHOE:	See Local Modes.

ECHOK:	See Local Modes.

ECHOKE:	See Local Modes.

ECHONL:	See Local Modes.

ECHOPRT:	See Local Modes.

ECONNABORTED:	See Error Codes.

ECONNREFUSED:	See Error Codes.

ECONNRESET:	See Error Codes.

ED:	See Error Codes.

EDEADLK:	See Error Codes.

EDESTADDRREQ:	See Error Codes.

EDOM:	See Error Codes.

EDQUOT:	See Error Codes.

EEXIST:	See Error Codes.

EFAULT:	See Error Codes.

EFBIG:	See Error Codes.

EGRATUITOUS:	See Error Codes.

EGREGIOUS:	See Error Codes.

EHOSTDOWN:	See Error Codes.

EHOSTUNREACH:	See Error Codes.

EIEIO:	See Error Codes.

EINPROGRESS:	See Error Codes.

EINTR:	See Error Codes.

EINVAL <1>:	See Error Codes.

EINVAL:	See Line Control.

EIO:	See Error Codes.

EISCONN:	See Error Codes.

EISDIR:	See Error Codes.

ELOOP:	See Error Codes.

EMFILE:	See Error Codes.

EMLINK:	See Error Codes.

EMSGSIZE:	See Error Codes.

ENAMETOOLONG:	See Error Codes.

ENETDOWN:	See Error Codes.

ENETRESET:	See Error Codes.

ENETUNREACH:	See Error Codes.

ENFILE:	See Error Codes.

ENOBUFS:	See Error Codes.

ENODEV:	See Error Codes.

ENOENT:	See Error Codes.

ENOEXEC:	See Error Codes.

ENOLCK:	See Error Codes.

ENOMEM:	See Error Codes.

ENOPROTOOPT:	See Error Codes.

ENOSPC:	See Error Codes.

ENOSYS:	See Error Codes.

ENOTBLK:	See Error Codes.

ENOTCONN:	See Error Codes.

ENOTDIR:	See Error Codes.

ENOTEMPTY:	See Error Codes.

ENOTSOCK:	See Error Codes.

ENOTTY <1>:	See Line Control.

ENOTTY:	See Error Codes.

environ:	See Environment Access.

ENXIO:	See Error Codes.

EOF:	See EOF and Errors.

EOPNOTSUPP:	See Error Codes.

EPERM:	See Error Codes.

EPFNOSUPPORT:	See Error Codes.

EPIPE:	See Error Codes.

EPROTONOSUPPORT:	See Error Codes.

EPROTOTYPE:	See Error Codes.

EQUIV_CLASS_MAX:	See Utility Limits.

ERANGE:	See Error Codes.

EREMOTE:	See Error Codes.

EROFS:	See Error Codes.

errno:	See Checking for Errors.

ESHUTDOWN:	See Error Codes.

ESOCKTNOSUPPORT:	See Error Codes.

ESPIPE:	See Error Codes.

ESRCH:	See Error Codes.

ESTALE:	See Error Codes.

ETIMEDOUT:	See Error Codes.

ETXTBSY:	See Error Codes.

EUSERS:	See Error Codes.

EWOULDBLOCK:	See Error Codes.

EXDEV:	See Error Codes.

EXIT_FAILURE:	See Exit Status.

EXIT_SUCCESS:	See Exit Status.

EXPR_NEST_MAX:	See Utility Limits.

EXTA:	See Line Speed.

EXTB:	See Line Speed.

F_DUPFD:	See Duplicating Descriptors.

F_GETFD:	See Descriptor Flags.

F_GETFL:	See File Status Flags.

F_GETLK:	See File Locks.

F_GETOWN:	See Interrupt Input.

F_OK:	See Testing File Access.

F_RDLCK:	See File Locks.

F_SETFD:	See Descriptor Flags.

F_SETFL:	See File Status Flags.

F_SETLK:	See File Locks.

F_SETLKW:	See File Locks.

F_SETOWN:	See Interrupt Input.

F_UNLCK:	See File Locks.

F_WRLCK:	See File Locks.

FD_CLOEXEC:	See Descriptor Flags.

FD_SETSIZE:	See Waiting for I/O.

FILENAME_MAX:	See Limits for Files.

FLUSHO:	See Local Modes.

FOPEN_MAX:	See Opening Streams.

FPE_DECOVF_TRAP:	See Program Error Signals.

FPE_FLTDIV_TRAP:	See Program Error Signals.

FPE_FLTOVF_TRAP:	See Program Error Signals.

FPE_FLTUND_TRAP:	See Program Error Signals.

FPE_INTDIV_TRAP:	See Program Error Signals.

FPE_INTOVF_TRAP:	See Program Error Signals.

FPE_SUBRNG_TRAP:	See Program Error Signals.

h_errno:	See Host Names.

HOST_NOT_FOUND:	See Host Names.

HUGE_VAL:	See Domain and Range Errors.

HUPCL:	See Control Modes.

ICANON:	See Local Modes.

ICRNL:	See Input Modes.

IEXTEN:	See Local Modes.

IGNBRK:	See Input Modes.

IGNCR:	See Input Modes.

IGNPAR:	See Input Modes.

INADDR_ANY:	See Host Address Data Type.

INADDR_BROADCAST:	See Host Address Data Type.

INADDR_LOOPBACK:	See Host Address Data Type.

INADDR_NONE:	See Host Address Data Type.

INLCR:	See Input Modes.

INPCK:	See Input Modes.

int:	See Limits on Resources.

IPPORT_RESERVED:	See Ports.

IPPORT_USERRESERVED:	See Ports.

ISIG:	See Local Modes.

ISTRIP:	See Input Modes.

IXOFF:	See Input Modes.

IXON:	See Input Modes.

L_ctermid:	See Identifying the Terminal.

L_cuserid:	See Who Logged In.

L_INCR:	See File Positioning.

L_SET:	See File Positioning.

L_tmpnam:	See Temporary Files.

L_XTND:	See File Positioning.

LANG:	See Locale Categories.

LC_ALL:	See Locale Categories.

LC_COLLATE:	See Locale Categories.

LC_CTYPE:	See Locale Categories.

LC_MONETARY:	See Locale Categories.

LC_NUMERIC:	See Locale Categories.

LC_TIME:	See Locale Categories.

LINE_MAX:	See Utility Limits.

LINK_MAX:	See Limits for Files.

MAX_CANON:	See Limits for Files.

MAX_INPUT:	See Limits for Files.

MAXNAMLEN:	See Limits for Files.

MB_CUR_MAX:	See Multibyte Char Intro.

MB_LEN_MAX:	See Multibyte Char Intro.

MDMBUF:	See Control Modes.

MINSIGSTKSZ:	See Signal Stack.

MSG_DONTROUTE:	See Socket Data Options.

MSG_OOB:	See Socket Data Options.

MSG_PEEK:	See Socket Data Options.

NAME_MAX:	See Limits for Files.

NAN:	See Not a Number.

NCCS:	See Mode Data Types.

NDEBUG:	See Consistency Checking.

NGROUPS_MAX:	See General Limits.

NO_ADDRESS:	See Host Names.

NO_RECOVERY:	See Host Names.

NOFLSH:	See Local Modes.

NOKERNINFO:	See Local Modes.

NSIG:	See Standard Signals.

NULL:	See Null Pointer Constant.

O_ACCMODE:	See File Status Flags.

O_APPEND:	See Opening and Closing Files.

O_CREAT:	See Opening and Closing Files.

O_EXCL:	See Opening and Closing Files.

O_NOCTTY:	See Opening and Closing Files.

O_NONBLOCK:	See Opening and Closing Files.

O_RDONLY:	See Opening and Closing Files.

O_RDWR:	See Opening and Closing Files.

O_TRUNC:	See Opening and Closing Files.

O_WRONLY:	See Opening and Closing Files.

ONLCR:	See Output Modes.

ONOEOT:	See Output Modes.

OPEN_MAX:	See General Limits.

OPOST:	See Output Modes.

optarg:	See Parsing Options.

opterr:	See Parsing Options.

optind:	See Parsing Options.

optopt:	See Parsing Options.

OXTABS:	See Output Modes.

P_tmpdir:	See Temporary Files.

PA_CHAR:	See Parsing a Template String.

PA_DOUBLE:	See Parsing a Template String.

PA_FLAG_LONG:	See Parsing a Template String.

PA_FLAG_LONG_DOUBLE:	See Parsing a Template String.

PA_FLAG_LONG_LONG:	See Parsing a Template String.

PA_FLAG_MASK:	See Parsing a Template String.

PA_FLAG_PTR:	See Parsing a Template String.

PA_FLAG_SHORT:	See Parsing a Template String.

PA_FLOAT:	See Parsing a Template String.

PA_INT:	See Parsing a Template String.

PA_LAST:	See Parsing a Template String.

PA_POINTER:	See Parsing a Template String.

PA_STRING:	See Parsing a Template String.

PARENB:	See Control Modes.

PARMRK:	See Input Modes.

PARODD:	See Control Modes.

PATH_MAX:	See Limits for Files.

PENDIN:	See Local Modes.

PF_CCITT:	See Misc Namespaces.

PF_FILE:	See File Namespace Details.

PF_IMPLINK:	See Misc Namespaces.

PF_INET:	See Internet Namespace.

PF_ISO:	See Misc Namespaces.

PF_NS:	See Misc Namespaces.

PF_ROUTE:	See Misc Namespaces.

PF_UNIX:	See File Namespace Details.

PIPE_BUF:	See Limits for Files.

PRIO_MAX:	See Priority.

PRIO_MIN:	See Priority.

PRIO_PGRP:	See Priority.

PRIO_PROCESS:	See Priority.

PRIO_USER:	See Priority.

program_invocation_name:	See Error Messages.

program_invocation_short_name:	See Error Messages.

R_OK:	See Testing File Access.

RAND_MAX:	See ANSI Random.

RE_DUP_MAX:	See General Limits.

RLIM_NLIMITS:	See Limits on Resources.

RLIMIT_CORE:	See Limits on Resources.

RLIMIT_CPU:	See Limits on Resources.

RLIMIT_DATA:	See Limits on Resources.

RLIMIT_FSIZE:	See Limits on Resources.

RLIMIT_OPEN_FILES:	See Limits on Resources.

RLIMIT_RSS:	See Limits on Resources.

RLIMIT_STACK:	See Limits on Resources.

S_IEXEC:	See Permission Bits.

S_IFBLK:	See Testing File Type.

S_IFCHR:	See Testing File Type.

S_IFDIR:	See Testing File Type.

S_IFIFO:	See Testing File Type.

S_IFLNK:	See Testing File Type.

S_IFMT:	See Testing File Type.

S_IFREG:	See Testing File Type.

S_IFSOCK:	See Testing File Type.

S_IREAD:	See Permission Bits.

S_IRGRP:	See Permission Bits.

S_IROTH:	See Permission Bits.

S_IRUSR:	See Permission Bits.

S_IRWXG:	See Permission Bits.

S_IRWXO:	See Permission Bits.

S_IRWXU:	See Permission Bits.

S_ISGID:	See Permission Bits.

S_ISUID:	See Permission Bits.

S_ISVTX:	See Permission Bits.

S_IWGRP:	See Permission Bits.

S_IWOTH:	See Permission Bits.

S_IWRITE:	See Permission Bits.

S_IWUSR:	See Permission Bits.

S_IXGRP:	See Permission Bits.

S_IXOTH:	See Permission Bits.

S_IXUSR:	See Permission Bits.

SA_DISABLE:	See Signal Stack.

SA_NOCLDSTOP:	See Flags for Sigaction.

SA_ONSTACK <1>:	See Flags for Sigaction.

SA_ONSTACK:	See Signal Stack.

SA_RESTART:	See Flags for Sigaction.

SEEK_CUR:	See File Positioning.

SEEK_END:	See File Positioning.

SEEK_SET:	See File Positioning.

SIG_BLOCK:	See Process Signal Mask.

SIG_DFL:	See Basic Signal Handling.

SIG_ERR:	See Basic Signal Handling.

SIG_IGN:	See Basic Signal Handling.

SIG_SETMASK:	See Process Signal Mask.

SIG_UNBLOCK:	See Process Signal Mask.

SIGABRT:	See Program Error Signals.

SIGALRM:	See Alarm Signals.

SIGBUS:	See Program Error Signals.

SIGCHLD:	See Job Control Signals.

SIGCONT:	See Job Control Signals.

SIGFPE:	See Program Error Signals.

SIGHUP:	See Termination Signals.

SIGILL:	See Program Error Signals.

SIGINT:	See Termination Signals.

SIGIO:	See Asynchronous I/O Signals.

SIGKILL:	See Termination Signals.

SIGPIPE:	See Miscellaneous Signals.

SIGPROF:	See Alarm Signals.

SIGQUIT:	See Termination Signals.

SIGSEGV:	See Program Error Signals.

SIGSTKSZ:	See Signal Stack.

SIGSTOP:	See Job Control Signals.

SIGTERM:	See Termination Signals.

SIGTSTP:	See Job Control Signals.

SIGTTIN:	See Job Control Signals.

SIGTTOU:	See Job Control Signals.

SIGURG:	See Asynchronous I/O Signals.

SIGUSR1:	See Miscellaneous Signals.

SIGUSR2:	See Miscellaneous Signals.

SIGVTALRM:	See Alarm Signals.

SOCK_DGRAM:	See Communication Styles.

SOCK_RAW:	See Communication Styles.

SOCK_STREAM:	See Communication Styles.

SOL_SOCKET:	See Socket-Level Options.

SSIZE_MAX:	See General Limits.

stderr:	See Standard Streams.

STDERR_FILENO:	See Descriptors and Streams.

stdin:	See Standard Streams.

STDIN_FILENO:	See Descriptors and Streams.

stdout:	See Standard Streams.

STDOUT_FILENO:	See Descriptors and Streams.

STREAM_MAX:	See General Limits.

SV_INTERRUPT:	See BSD Handler.

SV_ONSTACK:	See BSD Handler.

SV_RESETHAND:	See BSD Handler.

sys_siglist:	See Signal Messages.

TCIFLUSH:	See Line Control.

TCIOFF:	See Line Control.

TCIOFLUSH:	See Line Control.

TCION:	See Line Control.

TCOFLUSH:	See Line Control.

TCOOFF:	See Line Control.

TCOON:	See Line Control.

TCSADRAIN:	See Mode Functions.

TCSAFLUSH:	See Mode Functions.

TCSANOW:	See Mode Functions.

TCSASOFT:	See Mode Functions.

timezone:	See Time Zone Functions.

TMP_MAX:	See Temporary Files.

TOSTOP:	See Local Modes.

TRY_AGAIN:	See Host Names.

TZNAME_MAX:	See General Limits.

VDISCARD:	See Other Special.

VDSUSP:	See Signal Characters.

VEOF:	See Editing Characters.

VEOL:	See Editing Characters.

VEOL2:	See BSD Editing.

VERASE:	See Editing Characters.

VINTR:	See Signal Characters.

VKILL:	See Editing Characters.

VLNEXT:	See BSD Editing.

VMIN:	See Noncanonical Input.

VQUIT:	See Signal Characters.

VREPRINT:	See BSD Editing.

VSTART:	See Start/Stop Characters.

VSTATUS:	See Other Special.

VSTOP:	See Start/Stop Characters.

VSUSP:	See Signal Characters.

VTIME:	See Noncanonical Input.

VWERASE:	See BSD Editing.

W_OK:	See Testing File Access.

X_OK:	See Testing File Access.


Program and File Index
Node: File Index, Next: , Prev: Variable Index, Up: Top

Program and File Index

-lbsd-compat <1>:	See Process Group Functions.

-lbsd-compat:	See Feature Test Macros.

/etc/group:	See Group Database.

/etc/hosts:	See Host Names.

/etc/networks:	See Networks Database.

/etc/passwd:	See User Database.

/etc/protocols:	See Protocols Database.

/etc/services:	See Services Database.

arpa/inet.h:	See Host Address Functions.

assert.h:	See Consistency Checking.

bsd-compat <1>:	See Feature Test Macros.

bsd-compat:	See Process Group Functions.

cd:	See Working Directory.

chgrp:	See File Owner.

chmod:	See Setting Permissions.

chown:	See File Owner.

ctype.h <1>:	See Case Conversion.

ctype.h <1>:	See Character Handling.

ctype.h:	See Classification of Characters.

dirent.h <1>:	See Reserved Names.

dirent.h <1>:	See Random Access Directory.

dirent.h <1>:	See Reading/Closing Directory.

dirent.h <1>:	See Opening a Directory.

dirent.h:	See Directory Entries.

errno.h <1>:	See Error Reporting.

errno.h <1>:	See Checking for Errors.

errno.h:	See Error Codes.

fcntl.h <1>:	See Interrupt Input.

fcntl.h <1>:	See Opening and Closing Files.

fcntl.h <1>:	See Control Operations.

fcntl.h <1>:	See Reserved Names.

fcntl.h <1>:	See File Status Flags.

fcntl.h <1>:	See Duplicating Descriptors.

fcntl.h <1>:	See Descriptor Flags.

fcntl.h:	See File Locks.

float.h:	See Floating Point Parameters.

fnmatch.h:	See Wildcard Matching.

gcc:	See ANSI C.

grp.h <1>:	See Setting Groups.

grp.h <1>:	See Reserved Names.

grp.h:	See Group Data Structure.

hostid:	See Host Identification.

hostname:	See Host Identification.

kill:	See Termination Signals.

limits.h <1>:	See Reserved Names.

limits.h <1>:	See Limits for Files.

limits.h <1>:	See General Limits.

limits.h <1>:	See Width of Type.

limits.h:	See Multibyte Char Intro.

locale.h <1>:	See Setting the Locale.

locale.h:	See Numeric Formatting.

ls:	See File Attributes.

malloc.c:	See Memory Warnings.

malloc.h <1>:	See Statistics of Malloc.

malloc.h <1>:	See Hooks for Malloc.

malloc.h <1>:	See Using Relocator.

malloc.h:	See Heap Consistency Checking.

math.h <1>:	See Rounding and Remainders.

math.h <1>:	See Normalization Functions.

math.h <1>:	See Absolute Value.

math.h <1>:	See Mathematics.

math.h:	See Predicates on Floats.

mkdir:	See Creating Directories.

netdb.h <1>:	See Services Database.

netdb.h <1>:	See Host Names.

netdb.h <1>:	See Protocols Database.

netdb.h:	See Networks Database.

netinet/in.h <1>:	See Internet Address Format.

netinet/in.h <1>:	See Host Address Data Type.

netinet/in.h <1>:	See Ports.

netinet/in.h:	See Byte Order.

obstack.h:	See Creating Obstacks.

printf.h <1>:	See Registering New Conversions.

printf.h:	See Conversion Specifier Options.

pwd.h <1>:	See Reserved Names.

pwd.h:	See User Data Structure.

setjmp.h <1>:	See Non-Local Details.

setjmp.h:	See Non-Local Exits and Signals.

sh:	See Running a Command.

signal.h <1>:	See Signaling Another Process.

signal.h <1>:	See Standard Signals.

signal.h <1>:	See Reserved Names.

signal.h <1>:	See Process Signal Mask.

signal.h <1>:	See POSIX vs BSD.

signal.h <1>:	See Signaling Yourself.

signal.h <1>:	See Checking for Pending Signals.

signal.h <1>:	See Flags for Sigaction.

signal.h <1>:	See Basic Signal Handling.

signal.h <1>:	See Signal Sets.

signal.h:	See Advanced Signal Handling.

stdarg.h <1>:	See Receiving Arguments.

stdarg.h:	See Argument Macros.

stddef.h <1>:	See Wide Char Intro.

stddef.h:	See Important Data Types.

stdio.h <1>:	See Signal Messages.

stdio.h <1>:	See Descriptors and Streams.

stdio.h <1>:	See Temporary Files.

stdio.h <1>:	See Who Logged In.

stdio.h <1>:	See Streams.

stdio.h <1>:	See Deleting Files.

stdio.h <1>:	See Standard Streams.

stdio.h <1>:	See Opening Streams.

stdio.h <1>:	See Streams and Cookies.

stdio.h <1>:	See String Streams.

stdio.h <1>:	See Controlling Buffering.

stdio.h <1>:	See Flushing Buffers.

stdio.h <1>:	See Portable Positioning.

stdio.h <1>:	See File Positioning.

stdio.h <1>:	See Identifying the Terminal.

stdio.h <1>:	See EOF and Errors.

stdio.h <1>:	See Block Input/Output.

stdio.h <1>:	See Formatted Input Functions.

stdio.h <1>:	See Character Input.

stdio.h <1>:	See Simple Output.

stdio.h <1>:	See Formatted Output Functions.

stdio.h:	See Variable Arguments Output.

stdlib.h <1>:	See Integer Division.

stdlib.h <1>:	See Basic Allocation.

stdlib.h <1>:	See Aborting a Program.

stdlib.h <1>:	See Length of Char.

stdlib.h <1>:	See Absolute Value.

stdlib.h <1>:	See Parsing of Floats.

stdlib.h <1>:	See Converting One Char.

stdlib.h <1>:	See Aligned Memory Blocks.

stdlib.h <1>:	See Parsing of Integers.

stdlib.h <1>:	See Allocating Cleared Space.

stdlib.h <1>:	See Changing Block Size.

stdlib.h <1>:	See Freeing after Malloc.

stdlib.h <1>:	See BSD Random.

stdlib.h <1>:	See Array Sort Function.

stdlib.h <1>:	See Array Search Function.

stdlib.h <1>:	See Environment Access.

stdlib.h <1>:	See Running a Command.

stdlib.h <1>:	See ANSI Random.

stdlib.h <1>:	See Variable Size Automatic.

stdlib.h <1>:	See Exit Status.

stdlib.h <1>:	See Wide String Conversion.

stdlib.h:	See Multibyte Char Intro.

string.h <1>:	See Collation Functions.

string.h <1>:	See String/Array Comparison.

string.h <1>:	See Copying and Concatenation.

string.h <1>:	See Search Functions.

string.h <1>:	See String Length.

string.h <1>:	See Signal Messages.

string.h:	See Finding Tokens in a String.

sys/param.h:	See Host Identification.

sys/resource.h <1>:	See Priority.

sys/resource.h <1>:	See Limits on Resources.

sys/resource.h:	See Resource Usage.

sys/socket.h <1>:	See Socket Option Functions.

sys/socket.h <1>:	See Sending Datagrams.

sys/socket.h <1>:	See Communication Styles.

sys/socket.h <1>:	See Address Formats.

sys/socket.h <1>:	See Setting Address.

sys/socket.h <1>:	See Reading Address.

sys/socket.h <1>:	See File Namespace Details.

sys/socket.h <1>:	See Internet Namespace.

sys/socket.h <1>:	See Closing a Socket.

sys/socket.h <1>:	See Socket-Level Options.

sys/socket.h <1>:	See Receiving Data.

sys/socket.h <1>:	See Creating a Socket.

sys/socket.h <1>:	See Sending Data.

sys/socket.h <1>:	See Socket Pairs.

sys/socket.h:	See Socket Data Options.

sys/stat.h <1>:	See FIFO Special Files.

sys/stat.h <1>:	See Permission Bits.

sys/stat.h <1>:	See Testing File Type.

sys/stat.h <1>:	See Setting Permissions.

sys/stat.h <1>:	See Making Special Files.

sys/stat.h <1>:	See Creating Directories.

sys/stat.h <1>:	See Attribute Meanings.

sys/stat.h:	See Reserved Names.

sys/time.h <1>:	See High-Resolution Calendar.

sys/time.h <1>:	See Setting an Alarm.

sys/time.h:	See File Times.

sys/times.h <1>:	See Reserved Names.

sys/times.h:	See Detailed CPU Time.

sys/types.h <1>:	See Waiting for I/O.

sys/types.h <1>:	See Terminal Access Functions.

sys/types.h <1>:	See Setting User ID.

sys/types.h <1>:	See Process Identification.

sys/types.h <1>:	See Reading Persona.

sys/types.h <1>:	See Setting Groups.

sys/types.h:	See Process Group Functions.

sys/un.h:	See File Namespace Details.

sys/utsname.h:	See Hardware/Software Type ID.

sys/wait.h <1>:	See BSD Wait Functions.

sys/wait.h <1>:	See Process Completion.

sys/wait.h:	See Process Completion Status.

termios.h <1>:	See Reserved Names.

termios.h:	See Terminal Modes.

time.h <1>:	See File Times.

time.h <1>:	See Formatting Date and Time.

time.h <1>:	See TZ Variable.

time.h <1>:	See Simple Calendar Time.

time.h:	See Basic CPU Time.

umask:	See Setting Permissions.

unistd.h <1>:	See Testing File Access.

unistd.h <1>:	See Deleting Files.

unistd.h <1>:	See I/O Primitives.

unistd.h <1>:	See Deleting Files.

unistd.h <1>:	See Testing File Access.

unistd.h <1>:	See File Owner.

unistd.h <1>:	See Setting Groups.

unistd.h <1>:	See Hard Links.

unistd.h <1>:	See Process Identification.

unistd.h <1>:	See Creating a Pipe.

unistd.h <1>:	See Termination Internals.

unistd.h <1>:	See Creating a Process.

unistd.h <1>:	See Parsing Options.

unistd.h <1>:	See Is It a Terminal.

unistd.h <1>:	See Executing a File.

unistd.h <1>:	See Symbolic Links.

unistd.h <1>:	See Working Directory.

unistd.h <1>:	See Host Identification.

unistd.h <1>:	See Who Logged In.

unistd.h <1>:	See Process Group Functions.

unistd.h <1>:	See System Options.

unistd.h <1>:	See Options for Files.

unistd.h <1>:	See Duplicating Descriptors.

unistd.h <1>:	See Terminal Access Functions.

unistd.h <1>:	See Descriptors and Streams.

unistd.h <1>:	See Opening and Closing Files.

unistd.h <1>:	See Reading Persona.

unistd.h <1>:	See Options for Files.

unistd.h <1>:	See Setting User ID.

unistd.h:	See Setting an Alarm.

utime.h:	See File Times.

varargs.h:	See Old Varargs.