'\"
'\" Copyright (c) 1996 Sun Microsystems, Inc.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\"
'\" SCCS: @(#) CrtChannel.3 1.24 96/05/08 18:10:56
.so man.macros
.TH Tcl_CreateChannel 3 7.5 Tcl "Tcl Library Procedures"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
Tcl_CreateChannel, Tcl_GetChannelInstanceData, Tcl_GetChannelType, Tcl_GetChannelName, Tcl_GetChannelFile, Tcl_GetChannelMode, Tcl_GetChannelBufferSize, Tcl_SetDefaultTranslation, Tcl_SetChannelBufferSize \- procedures for creating and manipulating channels
.SH SYNOPSIS
.nf
\fB#include <tcl.h>\fR
.sp
Tcl_Channel
\fBTcl_CreateChannel\fR(\fItypePtr, channelName, instanceData, mask\fR)
.sp
ClientData
\fBTcl_GetChannelInstanceData\fR(\fIchannel\fR)
.sp
Tcl_ChannelType *
\fBTcl_GetChannelType\fR(\fIchannel\fR)
.sp
char *
\fBTcl_GetChannelName\fR(\fIchannel\fR)
.sp
Tcl_File
\fBTcl_GetChannelFile\fR(\fIchannel, direction\fR)
.sp
int
\fBTcl_GetChannelFlags\fR(\fIchannel\fR)
.sp
void
\fBTcl_SetDefaultTranslation\fR(\fIchannel, transMode\fR)
.sp
int
\fBTcl_GetChannelBufferSize\fR(\fIchannel\fR)
.sp
void
\fBTcl_SetChannelBufferSize\fR(\fIchannel, size\fR)
.sp
.SH ARGUMENTS
.AS Tcl_FileHandle pipelineSpec in
.AP Tcl_ChannelType *typePtr in
Points to a structure containing the addresses of procedures that
can be called to perform I/O and other functions on the channel.
.AP char *channelName in
The name of this channel, such as \fBfile3\fR; must not be in use
by any other channel. Can be NULL, in which case the channel is
created without a name.
.AP ClientData instanceData in
Arbitrary one-word value to be associated with this channel.  This
value is passed to procedures in \fItypePtr\fR when they are invoked.
.AP int mask in
OR-ed combination of \fBTCL_READABLE\fR and \fBTCL_WRITABLE\fR to indicate
whether a channel is readable and writable.
.AP Tcl_Channel channel in
The channel to operate on.
.AP int direction in
\fBTCL_READABLE\fR means the input file is wanted; \fBTCL_WRITABLE\fR
means the output file is wanted.
.AP Tcl_EolTranslation transMode in
The translation mode; one of the constants \fBTCL_TRANSLATE_AUTO\fR,
\fBTCL_TRANSLATE_CR\fR, \fBTCL_TRANSLATE_LF\fR and \fBTCL_TRANSLATE_CRLF\fR.
.AP int size in
The size, in bytes, of buffers to allocate in this channel.
.BE

.SH DESCRIPTION
.PP
Tcl uses a two-layered channel architecture. It provides a generic upper
layer to enable C and Tcl programs to perform input and output using the
same APIs for a variety of files, devices, sockets etc. The generic C APIs
are described in the manual entry for \fBTcl_OpenFileChannel\fR.
.PP
The lower layer provides type-specific channel drivers for each type of
file, socket and device supported on each platform.
This manual entry describes the C APIs
used by the generic layer to communicate with type-specific channel drivers
to perform the input and output operations. It also explains how new types
of channels can be added by providing new channel drivers.
.PP
Channel drivers consist of a number of components: First, each channel
driver provides a \fBTcl_ChannelType\fR structure containing pointers to
functions implementing the various operations used by the generic layer to
communicate with the channel driver. The \fBTcl_ChannelType\fR structure
and the functions referenced by it are described in the section
TCL_CHANNELTYPE, below.
.PP
Second, channel drivers usually provide a Tcl command to create instances
of that type of channel. For example, the Tcl \fBopen\fR command creates
channels that use the \fBfile\fR and \fBcommand\fR channel drivers, and
the Tcl \fBsocket\fR command creates channels that use TCP sockets for
network communication.
.PP
Third, a channel driver optionally provides a C function to open channel
instances of that type. For example, \fBTcl_OpenFileChannel\fR opens a
channel that uses the \fBfile\fR channel driver, and
\fBTcl_OpenTcpClient\fR opens a channel that uses the TCP network protocol.
These creation functions typically use
\fBTcl_CreateChannel\fR internally to open the channel.
.PP
To add a new type of channel you must implement a C API or a Tcl command
that opens a channel by invoking \fBTcl_CreateChannel\fR.
When your driver calls \fBTcl_CreateChannel\fR it passes in
a \fBTcl_ChannelType\fR structure describing the driver's I/O
procedures.
The generic layer will then invoke the functions referenced in that
structure to perform operations on the channel.
.PP
\fBTcl_CreateChannel\fR opens a new channel and associates the supplied
\fItypePtr\fR and \fIinstanceData\fR with it. The channel is opened in the
mode indicated by \fImask\fR.
For a discussion of channel drivers, their operations and the
\fBTcl_ChannelType\fR structure, see the section TCL_CHANNELTYPE, below.
.PP
\fBTcl_GetChannelInstanceData\fR returns the instance data associated with
the channel in \fIchannel\fR. This is the same as the \fIinstanceData\fR
argument in the call to \fBTcl_CreateChannel\fR that created this channel.
.PP
\fBTcl_GetChannelType\fR returns a pointer to the \fBTcl_ChannelType\fR
structure used by the channel in the \fIchannel\fR argument. This is
the same as the \fItypePtr\fR argument in the call to
\fBTcl_CreateChannel\fR that created this channel.
.PP
\fBTcl_GetChannelName\fR returns a string containing the name associated
with the channel, or NULL if the \fIchannelName\fR argument to
\fBTcl_CreateChannel\fR was NULL.
.PP
\fBTcl_GetChannelFile\fR returns the \fIinFile\fR associated with
\fIchannel\fR if \fIdirection\fR is \fBTCL_READABLE\fR, or the
\fIoutFile\fR if \fIdirection\fR is \fBTCL_WRITABLE\fR. The operation
returns NULL if the channel is not based on \fBTcl_File\fRs or if the
channel is not open for the specified direction.
.PP
\fBTcl_GetChannelMode\fR returns an OR-ed combination of \fBTCL_READABLE\fR
and \fBTCL_WRITABLE\fR, indicating whether the channel is open for input
and output.
.PP
\fBTcl_SetDefaultTranslation\fR sets the default end of line translation
mode. This mode will be installed as the translation mode for the channel
if an attempt is made to output on the channel while it is still in
\fBTCL_TRANSLATE_AUTO\fR mode. For a description of end of line translation
modes, see the manual entry for \fBfconfigure\fR.
.PP
\fBTcl_GetChannelBufferSize\fR returns the size, in bytes, of buffers
allocated to store input or output in \fIchan\fR. If the value was not set
by a previous call to \fBTcl_SetChannelBufferSize\fR, described below, then
the default value of 4096 is returned.
.PP
\fBTcl_SetChannelBufferSize\fR sets the size, in bytes, of buffers that
will be allocated in subsequent operations on the channel to store input or
output. The \fIsize\fR argument should be between ten and one million,
allowing buffers of ten bytes to one million bytes. If \fIsize\fR is
outside this range, \fBTcl_SetChannelBufferSize\fR sets the buffer size to
4096.

.SH TCL_CHANNELTYPE
.PP
A channel driver provides a \fBTcl_ChannelType\fR structure that contains
pointers to functions that implement the various operations on a channel;
these operations are invoked as needed by the generic layer. The
\fBTcl_ChannelType\fR structure contains the following fields:
.PP
.CS
typedef struct Tcl_ChannelType {
	char *\fItypeName\fR;
	Tcl_DriverBlockModeProc *\fIblockModeProc\fR;	
	Tcl_DriverCloseProc *\fIcloseProc\fR;
	Tcl_DriverInputProc *\fIinputProc\fR;
	Tcl_DriverOutputProc *\fIoutputProc\fR;
	Tcl_DriverSeekProc *\fIseekProc\fR;
	Tcl_DriverSetOptionProc *\fIsetOptionProc\fR;
	Tcl_DriverGetOptionProc *\fIgetOptionProc\fR;
	Tcl_DriverWatchChannelProc *\fIwatchChannelProc\fR;
	Tcl_DriverChannelReadyProc *\fIchannelReadyProc\fR;
	Tcl_DriverGetFileProc *\fIgetFileProc\fR;
} Tcl_ChannelType;
.CE
.PP
The driver must provide implementations for all functions except
\fIblockModeProc\fR, \fIseekProc\fR, \fIsetOptionProc\fR, and
\fIgetOptionProc\fR, which may be specified as NULL to indicate that the
channel does not support seeking.  Other functions that can not be
implemented for this type of device should return \fBEINVAL\fR when invoked
to indicate that they are not implemented.

.SH TYPENAME
.PP
The \fItypeName\fR field contains a null-terminated string that
identifies the type of the device implemented by this driver, e.g.
\fBfile\fR or \fBsocket\fR.

.SH BLOCKMODEPROC
.PP
The \fIblockModeProc\fR field contains the address of a function called by
the generic layer to set blocking and nonblocking mode on the device.
\fIBlockModeProc\fR should match the following prototype:
.PP
.CS
typedef int Tcl_DriverBlockModeProc(
	ClientData \fIinstanceData\fR,
	int \fImode\fR);
.CE
.PP
The \fIinstanceData\fR is the same as the value passed to
\fBTcl_CreateChannel\fR when this channel was created.  The \fImode\fR
argument is either \fBTCL_MODE_BLOCKING\fR or \fBTCL_MODE_NONBLOCKING\fR to
set the device into blocking or nonblocking mode. The function should
return zero if the operation was successful, or a nonzero POSIX error code
if the operation failed.
.PP
If the operation is successful, the function can modify the supplied
\fIinstanceData\fR to record that the channel entered blocking or
nonblocking mode and to implement the blocking or nonblocking behavior.
For some device types, the blocking and nonblocking behavior can be
implemented by the underlying operating system; for other device types, the
behavior must be emulated in the channel driver.

.SH CLOSEPROC
.PP
The \fIcloseProc\fR field contains the address of a function called by the
generic layer to clean up driver-related information when the channel is
closed. \fICloseProc\fR must match the following prototype:
.PP
.CS
typedef int Tcl_DriverCloseProc(
	ClientData \fIinstanceData\fR,
	Tcl_Interp *\fIinterp\fR);
.CE
.PP
The \fIinstanceData\fR argument is the same as the value provided to
\fBTcl_CreateChannel\fR when the channel was created. The function should
release any storage maintained by the channel driver for this channel, and
close the input and output devices encapsulated by this channel. All queued
output will have been flushed to the device before this function is called,
and no further driver operations will be invoked on this instance after
calling the \fIcloseProc\fR. If the close operation is successful, the
procedure should return zero; otherwise it should return a nonzero POSIX
error code. In addition, if an error occurs and \fIinterp\fR is not NULL,
the procedure should store an error message in \fIinterp->result\fR.

.SH INPUTPROC
.PP
The \fIinputProc\fR field contains the address of a function called by the
generic layer to read data from the file or device and store it in an
internal buffer. \fIInputProc\fR must match the following prototype:
.PP
.CS
typedef int Tcl_DriverInputProc(
	ClientData \fIinstanceData\fR,
	char *\fIbuf\fR,
	int \fIbufSize\fR,
	int *\fIerrorCodePtr\fR);
.CE
.PP
\fIInstanceData\fR is the same as the value passed to
\fBTcl_CreateChannel\fR when the channel was created.  The \fIbuf\fR
argument points to an array of bytes in which to store input from the
device, and the \fIbufSize\fR argument indicates how many bytes are
available at \fIbuf\fR.
.PP
The \fIerrorCodePtr\fR argument points to an integer variable provided by
the generic layer. If an error occurs, the function should set the variable
to a POSIX error code that identifies the error that occurred.
.PP
The function should read data from the input device encapsulated by the
channel and store it at \fIbuf\fR.  On success, the function should return
a nonnegative integer indicating how many bytes were read from the input
device and stored at \fIbuf\fR. On error, the function should return -1. If
an error occurs after some data has been read from the device, that data is
lost.
.PP
If \fIinputProc\fR can determine that the input device has some data
available but less than requested by the \fIbufSize\fR argument, the
function should only attempt to read as much data as is available and
return without blocking. If the input device has no data available
whatsoever and the channel is in nonblocking mode, the function should
return an \fBEAGAIN\fR error. If the input device has no data available
whatsoever and the channel is in blocking mode, the function should block
for the shortest possible time until at least one byte of data can be read
from the device; then, it should return as much data as it can read without
blocking.

.SH OUTPUTPROC
.PP
The \fIoutputProc\fR field contains the address of a function called by the
generic layer to transfer data from an internal buffer to the output device.
\fIOutputProc\fR must match the following prototype:
.PP
.CS
typedef int Tcl_DriverOutputProc(
	ClientData \fIinstanceData\fR,
	char *\fIbuf\fR,
	int \fItoWrite\fR,
	int *\fIerrorCodePtr\fR);
.CE
.PP
\fIInstanceData\fR is the same as the value passed to
\fBTcl_CreateChannel\fR when the channel was created. The \fIbuf\fR
argument contains an array of bytes to be written to the device, and the
\fItoWrite\fR argument indicates how many bytes are to be written from the
\fIbuf\fR argument.
.PP
The \fIerrorCodePtr\fR argument points to an integer variable provided by
the generic layer. If an error occurs, the function should set this
variable to a POSIX error code that identifies the error.
.PP
The function should write the data at \fIbuf\fR to the output device
encapsulated by the channel. On success, the function should return a
nonnegative integer indicating how many bytes were written to the output
device.  The return value is normally the same as \fItoWrite\fR, but may be
less in some cases such as if the output operation is interrupted by a
signal. If an error occurs the function should return -1.  In case of
error, some data may have been written to the device.
.PP
If the channel is nonblocking and the output device is unable to absorb any
data whatsoever, the function should return -1 with an \fBEAGAIN\fR error
without writing any data.

.SH SEEKPROC
.PP
The \fIseekProc\fR field contains the address of a function called by the
generic layer to move the access point at which subsequent input or output
operations will be applied. \fISeekProc\fR must match the following
prototype:
.PP
.CS
typedef int Tcl_DriverSeekProc(
	ClientData \fIinstanceData\fR,
	long \fIoffset\fR,
	int \fIseekMode\fR,
	int *\fIerrorCodePtr\fR);
.CE
.PP
The \fIinstanceData\fR argument is the same as the value given to
\fBTcl_CreateChannel\fR when this channel was created.  \fIOffset\fR and
\fIseekMode\fR have the same meaning as for the \fBTcl_SeekChannel\fR
procedure (described in the manual entry for \fBTcl_OpenFileChannel\fR).
.PP
The \fIerrorCodePtr\fR argument points to an integer variable provided by
the generic layer for returning \fBerrno\fR values from the function.  The
function should set this variable to a POSIX error code if an error occurs.
The function should store an \fBEINVAL\fR error code if the channel type
does not implement seeking.
.PP
The return value is the new access point or -1 in case of error. If an
error occurred, the function should not move the access point.

.SH SETOPTIONPROC
.PP
The \fIsetOptionProc\fR field contains the address of a function called by
the generic layer to set a channel type specific option on a channel.
\fIsetOptionProc\fR must match the following prototype:
.PP
.CS
typedef int Tcl_DriverSetOptionProc(
	ClientData \fIinstanceData\fR,
	Tcl_Interp *\fIinterp\fR,
	char *\fIoptionName\fR,
	char *\fIoptionValue\fR);
.CE
.PP
\fIoptionName\fR is the name of an option to set, and \fIoptionValue\fR is
the new value for that option, as a string. The \fIinstanceData\fR is the
same as the value given to \fBTcl_CreateChannel\fR when this channel was
created. The function should do whatever channel type specific action is
required to implement the new value of the option.
.PP
Some options are handled by the generic code and this function is never
called to set them, e.g. \fB-blockmode\fR. Other options are specific to
each channel type and the \fIsetOptionProc\fR procedure of the channel
driver will get called to implement them. The \fIsetOptionProc\fR field can
be NULL, which indicates that this channel type supports no type specific
options. 
.PP
If the option value is successfully modified to the new value, the function
returns \fBTCL_OK\fR. It returns \fBTCL_ERROR\fR if the \fIoptionName\fR is
unrecognized or if \fIoptionValue\fR specifies a value for the option that
is not supported. In this case, the function leaves an error message in the
\fIresult\fR field of \fIinterp\fR if \fIinterp\fR is not NULL. The
function should also call \fBTcl_SetErrno\fR to store an appropriate POSIX
error code.

.SH GETOPTIONPROC
.PP
The \fIgetOptionProc\fR field contains the address of a function called by
the generic layer to get the value of a channel type specific option on a
channel. \fIgetOptionProc\fR must match the following prototype:
.PP
.CS
typedef int Tcl_DriverGetOptionProc(
	ClientData \fIinstanceData\fR,
	char *\fIoptionName\fR,
	Tcl_DString *\fIdsPtr\fR);
.CE
.PP
\fIOptionName\fR is the name of an option supported by this type of
channel. If the option name is not NULL, the function stores its current
value, as a string, in the Tcl dynamic string \fIdsPtr\fR.
If \fIoptionName\fR is NULL, the function stores in \fIdsPtr\fR an
alternating list of all supported options and their current values.
On success, the function returns \fBTCL_OK\fR. If an error occurs, the
function returns \fBTCL_ERROR\fR and calls \fBTcl_SetErrno\fR to store an
appropriate POSIX error code.
.PP
Some options are handled by the generic code and this function is never
called to retrieve their value, e.g. \fB-blockmode\fR. Other options are
specific to each channel type and the \fIgetOptionProc\fR procedure of the
channel driver will get called to implement them. The \fIgetOptionProc\fR
field can be NULL, which indicates that this channel type supports no type
specific options.

.SH WATCHCHANNELPROC
.PP
The \fIwatchChannelProc\fR field contains the address of a function called
by the generic layer to initialize the event notification mechanism to
notice events of interest on this channel.
\fIWatchChannelProc\fR should match the following prototype:
.PP
.CS
typeded void Tcl_DriverWatchChannelProc(
	ClientData \fIinstanceData\fR,
	int \fImask\fR);
.CE
.PP
The \fIinstanceData\fR is the same as the value passed to
\fBTcl_CreateChannel\fR when this channel was created. The \fImask\fR
argument is an OR-ed combination of \fBTCL_READABLE\fR, \fBTCL_WRITABLE\fR
and \fBTCL_EXCEPTION\fR; it indicates events the caller is interested in
noticing on this channel.
.PP
The function should initialize device type specific mechanisms to notice
when an event of interest is present on the channel. It may invoke
\fITcl_SetMaxBlockTime\fR to specify an upper limit on the length of time
to wait for an event, and it may invoke \fITcl_WatchFile\fR if the channel
implementation is based on \fBTcl_File\fRs.

.SH CHANNELREADYPROC
.PP
The \fIchannelReadyProc\fR field contains the address of a function called
by the generic layer to sense whether events of interest have occurred for
this channel.
\fIChannelReadyProc\fR should match the following prototype:
.PP
.CS
typedef int Tcl_DriverChannelReadyProc(
	ClientData \fIinstanceData\fR,
	int \fImask\fR);
.CE
.PP
\fIInstanceData\fR is the same as the value passed to
\fBTcl_CreateChannel\fR when this channel was created. The \fImask\fR
argument is an OR-ed combination of \fBTCL_READABLE\fR, \fBTCL_WRITABLE\fR
and \fBTCL_EXCEPTION\fR indicating what events are of interest. The
function returns a mask containing an OR-ed combination of a subset of the
flags in \fImask\fR to indicate what events have actually occurred on the
channel.
.PP
The function should use a device type dependent mechanism to detect whether
events of interest have occurred on the channel. It may invoke
\fBTcl_FileReady\fR if the channel implementation is based on
\fBTcl_File\fRs.

.SH GETFILEPROC
.PP
The \fIgetFileProc\fR field contains the address of a function called by
the generic layer to retrieve a \fBTcl_File\fR from the channel.
\fIGetFileProc\fR should match the following prototype:
.PP
.CS
typedef Tcl_File Tcl_DriverGetFileProc(
	ClientData \fIinstanceData\fR,
	int \fIdirection\fR);
.CE
.PP
\fIInstanceData is the same as the value passed to
\fBTcl_CreateChannel\fR when this channel was created. The \fIdirection\fR
argument is either \fBTCL_READABLE\fR to retrieve the \fBTcl_File\fR used
for input, or \fBTCL_WRITABLE\fR to retrieve the \fBTcl_File\fR used for
output.
.PP
If the channel implementation is based on \fBTcl_File\fRs, the function
should retrieve the appropriate \fBTcl_File\fR associated with the channel,
according the \fIdirection\fR argument; it can return NULL if the channel
is not open for that direction.  If the channel implementation does not use
\fBTcl_File\fRs, the function should always return NULL.

.SH "SEE ALSO"
Tcl_Close(3), Tcl_OpenFileChannel(3), Tcl_SetErrno(3),
Tcl_SetMaxBlockTime(3), Tcl_WatchFile(3), Tcl_FileReady(3)


.SH KEYWORDS
blocking, channel driver, channel registration, channel type, nonblocking
