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# Programming with GNU History

This chapter describes how to interface the GNU History Library with programs that you write. 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 making up new ones.

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

The end-user using programs written with the History library has the benifit 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 used 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 Emacs style command line editing.

## History Storage

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

## History Functions

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

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

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: int where_history ()
  Returns the number which says what history element we are now looking at.

Function: int history_set_pos (int pos)
  Set the position in the history list to pos.

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. direction, if negative, says to search backwards from pos, else forwards. Returns the absolute index of the history element where string was found, or -1 otherwise.

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

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

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

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 the from'th line and end at the to'th. 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)
  Append the current history to filename. If filename is `NULL`, then append the history list to ``~/.history'`. Values returned are as in `read_history ()`.

Function: int append_history (int nelements, char *filename)
  Append nelement entries to filename. The entries appended are from the end of the list minus nelements up to the end of the list.

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

Function: HIST_ENTRY *previous_history ()
  Back up 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 `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.

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 history_search (char *string, int direction)
  Search the history for string, starting at `history_offset`. If direction < 0, then the search is through previous entries, else through subsequent. If string is found, then `current_history ()` is the history entry, and the value of this function is the offset in the line of that history entry that the string was found in. Otherwise, nothing is changed, and a -1 is returned.

Function: int history_expand (char *string, char **output)
  Expand string, placing the result into output, a pointer to a string. 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, or
  `-1`
    if there was an error in expansion.

  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 the GNU Bash shell.

Function: int history_total_bytes ();
  Return the number of bytes that the primary history entries are using. This just adds up the lengths of `the_history->lines`.

## History Variables

This section describes the variables in GNU History that are externally visible.

Variable: int history_base
  For convenience only. You set this when interpreting history commands. It is the logical offset of the first history element.

## History Programming Example

The following snippet of code demonstrates simple use of the GNU History Library.

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

      line[0] = 0;

      while (!done)
        {
          fprintf (stdout, "history%% ");
          t = gets (line);

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

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

              using_history ();

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

              if (result < 0)
                continue;

              add_history (line);
            }

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

              if (the_list)
                for (i = 0; the_list[i]; i++)
                  fprintf (stdout, "%d: %s\n",
                     i + history_base, the_list[i]->line);
            }
          if (strncmp (line, "delete", strlen ("delete")) == 0)
            {
              int which;
              if ((sscanf (line + strlen ("delete"), "%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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