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# Programming with GNU History

This chapter describes how to interface programs that you write with the GNU History Library. It should be considered a technical guide. For information on the interactive use of GNU History, see section Using History Interactively.

- Introduction to History: What is the GNU History library for?
- History Storage: How information is stored.
- History Functions: Functions that you can use.
- History Variables: Variables that control behaviour.
- History Programming Example: Example of using the GNU History Library.

## Introduction to History

Many programs read input from the user a line at a time. The GNU History library is able to keep track of those lines, associate arbitrary data with each line, and utilize information from previous lines in composing new ones.

The programmer using the History library has available functions for remembering lines on a history list, associating arbitrary data with a line, removing lines from the list, searching through the list for a line containing an arbitrary text string, and referencing any line in the list directly. In addition, a history expansion function is available which provides for a consistent user interface across different programs.

The user using programs written with the History library has the benefit of a consistent user interface with a set of well-known commands for manipulating the text of previous lines and using that text in new commands. The basic history manipulation commands are similar to the history substitution provided by `csh`.

If the programmer desires, he can use the Readline library, which includes some history manipulation by default, and has the added advantage of command line editing.

## History Storage

The history list is an array of history entries. A history entry is declared as follows:

    typedef struct _hist_entry {
      char *line;
      char *data;
    } HIST_ENTRY;

The history list itself might therefore be declared as

    HIST_ENTRY **the_history_list;

The state of the History library is encapsulated into a single structure:

    /* A structure used to pass the current state of the history stuff around. */
    typedef struct _hist_state {
      HIST_ENTRY **entries;         /* Pointer to the entries themselves. */
      int offset;                   /* The location pointer within this array. */
      int length;                   /* Number of elements within this array. */
      int size;                     /* Number of slots allocated to this array. */
      int flags;
    } HISTORY_STATE;

If the flags member includes `HS_STIFLED`, the history has been stifled.

## History Functions

This section describes the calling sequence for the various functions present in GNU History.

- Initializing History and State Management: Functions to call when you want to use history in a program.
- History List Management: Functions used to manage the list of history entries.
- Information About the History List: Functions returning information about the history list.
- Moving Around the History List: Functions used to change the position in the history list.
- Searching the History List: Functions to search the history list for entries containing a string.
- Managing the History File: Functions that read and write a file containing the history list.
- History Expansion: Functions to perform csh-like history expansion.

### Initializing History and State Management

This section describes functions used to initialize and manage the state of the History library when you want to use the history functions in your program.

Function: void using_history ()
  Begin a session in which the history functions might be used. This initializes the interactive variables.

Function: HISTORY_STATE * history_get_history_state ()
  Return a structure describing the current state of the input history.

Function: void history_set_history_state (HISTORY_STATE *state)
  Set the state of the history list according to state.

### History List Management

These functions manage individual entries on the history list, or set parameters managing the list itself.

Function: void add_history (char *string)
  Place string at the end of the history list. The associated data field (if any) is set to `NULL`.

Function: HIST_ENTRY * remove_history (int which)
  Remove history entry at offset which from the history. The removed element is returned so you can free the line, data, and containing structure.

Function: HIST_ENTRY * replace_history_entry (int which, char *line, char *data)
  Make the history entry at offset which have line and data. This returns the old entry so you can dispose of the data. In the case of an invalid which, a `NULL` pointer is returned.

Function: void stifle_history (int max)
  Stifle the history list, remembering only the last max entries.

Function: int unstifle_history ()
  Stop stifling the history. This returns the previous amount the history was stifled. The value is positive if the history was stifled, negative if it wasn't.

Function: int history_is_stifled ()
  Returns non-zero if the history is stifled, zero if it is not.

### Information About the History List

These functions return information about the entire history list or individual list entries.

Function: HIST_ENTRY ** history_list ()
  Return a `NULL` terminated array of `HIST_ENTRY` which is the current input history. Element 0 of this list is the beginning of time. If there is no history, return `NULL`.

Function: int where_history ()
  Returns the offset of the current history element.

Function: HIST_ENTRY * current_history ()
  Return the history entry at the current position, as determined by `where_history ()`. If there is no entry there, return a `NULL` pointer.

Function: HIST_ENTRY * history_get (int offset)
  Return the history entry at position offset, starting from `history_base`. If there is no entry there, or if offset is greater than the history length, return a `NULL` pointer.

Function: int history_total_bytes ()
  Return the number of bytes that the primary history entries are using. This function returns the sum of the lengths of all the lines in the history.

### Moving Around the History List

These functions allow the current index into the history list to be set or changed.

Function: int history_set_pos (int pos)
  Set the position in the history list to pos, an absolute index into the list.

Function: HIST_ENTRY * previous_history ()
  Back up the current history offset to the previous history entry, and return a pointer to that entry. If there is no previous entry, return a `NULL` pointer.

Function: HIST_ENTRY * next_history ()
  Move the current history offset forward to the next history entry, and return the a pointer to that entry. If there is no next entry, return a `NULL` pointer.

### Searching the History List

These functions allow searching of the history list for entries containing a specific string. Searching may be performed both forward and backward from the current history position. The search may be anchored, meaning that the string must match at the beginning of the history entry.

Function: int history_search (char *string, int direction)
  Search the history for string, starting at the current history offset. If direction < 0, then the search is through previous entries, else through subsequent. If string is found, then the current history index is set to that history entry, and the value returned is the offset in the line of the entry where string was found. Otherwise, nothing is changed, and a -1 is returned.

Function: int history_search_prefix (char *string, int direction)
  Search the history for string, starting at the current history offset. The search is anchored: matching lines must begin with string. If direction < 0, then the search is through previous entries, else through subsequent. If string is found, then the current history index is set to that entry, and the return value is 0. Otherwise, nothing is changed, and a -1 is returned.

Function: int history_search_pos (char *string, int direction, int pos)
  Search for string in the history list, starting at pos, an absolute index into the list. If direction is negative, the search proceeds backward from pos, otherwise forward. Returns the absolute index of the history element where string was found, or -1 otherwise.

### Managing the History File

The History library can read the history from and write it to a file. This section documents the functions for managing a history file.

Function: int read_history (char *filename)
  Add the contents of filename to the history list, a line at a time. If filename is `NULL`, then read from ``~/.history'`. Returns 0 if successful, or errno if not.

Function: int read_history_range (char *filename, int from, int to)
  Read a range of lines from filename, adding them to the history list. Start reading at line from and end at to. If from is zero, start at the beginning. If to is less than from, then read until the end of the file. If filename is `NULL`, then read from ``~/.history'`. Returns 0 if successful, or `errno` if not.

Function: int write_history (char *filename)
  Write the current history to filename, overwriting filename if necessary. If filename is `NULL`, then write the history list to ``~/.history'`. Values returned are as in `read_history ()`.

Function: int append_history (int nelements, char *filename)
  Append the last nelements of the history list to filename.

Function: int history_truncate_file (char *filename, int nlines)
  Truncate the history file filename, leaving only the last nlines lines.

### History Expansion

These functions implement `csh`-like history expansion.

Function: int history_expand (char *string, char **output)
  Expand string, placing the result into output, a pointer to a string (see section History Interaction). Returns:
  `0`
    If no expansions took place (or, if the only change in the text was the de-slashifying of the history expansion character);
  `1`
    if expansions did take place;
  `-1`
    if there was an error in expansion;
  `2`
    if the returned line should only be displayed, but not executed, as with the `:p` modifier (see section Modifiers).

  If an error ocurred in expansion, then output contains a descriptive error message.

Function: char * history_arg_extract (int first, int last, char *string)
  Extract a string segment consisting of the first through last arguments present in string. Arguments are broken up as in Bash.

Function: char * get_history_event (char *string, int *cindex, int qchar)
  Returns the text of the history event beginning at string + *cindex. *cindex is modified to point to after the event specifier. At function entry, cindex points to the index into string where the history event specification begins. qchar is a character that is allowed to end the event specification in addition to the "normal" terminating characters.

Function: char ** history_tokenize (char *string)
  Return an array of tokens parsed out of string, much as the shell might. The tokens are split on white space and on the characters `()<>;&|$`, and shell quoting conventions are obeyed.

## History Variables

This section describes the externally visible variables exported by the GNU History Library.

Variable: int history_base
  The logical offset of the first entry in the history list.

Variable: int history_length
  The number of entries currently stored in the history list.

Variable: int max_input_history
  The maximum number of history entries. This must be changed using `stifle_history ()`.

Variable: char history_expansion_char
  The character that starts a history event. The default is ``!'`.

Variable: char history_subst_char
  The character that invokes word substitution if found at the start of a line. The default is ``^'`.

Variable: char history_comment_char
  During tokenization, if this character is seen as the first character of a word, then it and all subsequent characters up to a newline are ignored, suppressing history expansion for the remainder of the line. This is disabled by default.

Variable: char * history_no_expand_chars
  The list of characters which inhibit history expansion if found immediately following history_expansion_char. The default is whitespace and ``='`.

## History Programming Example

The following program demonstrates simple use of the GNU History Library.

    main ()
    {
      char line[1024], *t;
      int len, done = 0;

      line[0] = 0;

      using_history ();
      while (!done)
        {
          printf ("history$ ");
          fflush (stdout);
          t = fgets (line, sizeof (line) - 1, stdin);
          if (t && *t)
            {
              len = strlen (t);
              if (t[len - 1] == '\n')
                t[len - 1] = '\0';
            }

          if (!t)
            strcpy (line, "quit");

          if (line[0])
            {
              char *expansion;
              int result;

              result = history_expand (line, &expansion);
              if (result)
                fprintf (stderr, "%s\n", expansion);

              if (result < 0 || result == 2)
                {
                  free (expansion);
                  continue;
                }

              add_history (expansion);
              strncpy (line, expansion, sizeof (line) - 1);
              free (expansion);
            }

          if (strcmp (line, "quit") == 0)
            done = 1;
          else if (strcmp (line, "save") == 0)
            write_history ("history_file");
          else if (strcmp (line, "read") == 0)
            read_history ("history_file");
          else if (strcmp (line, "list") == 0)
            {
              register HIST_ENTRY **the_list;
              register int i;

              the_list = history_list ();
              if (the_list)
                for (i = 0; the_list[i]; i++)
                  printf ("%d: %s\n", i + history_base, the_list[i]->line);
            }
          else if (strncmp (line, "delete", 6) == 0)
            {
              int which;
              if ((sscanf (line + 6, "%d", &which)) == 1)
                {
                  HIST_ENTRY *entry = remove_history (which);
                  if (!entry)
                    fprintf (stderr, "No such entry %d\n", which);
                  else
                    {
                      free (entry->line);
                      free (entry);
                    }
                }
              else
                {
                  fprintf (stderr, "non-numeric arg given to `delete'\n");
                }
            }
        }
    }

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