@ignore
This file documents the user interface to the GNU History library.

Copyright (C) 1988, 1991 Free Software Foundation, Inc.
Authored by Brian Fox.

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 process this file through Tex and print the
results, provided the printed document carries copying permission notice
identical to this one except for the removal of this paragraph (this
paragraph not being relevant to the printed manual).

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that the
GNU Copyright statement is available to the distributee, 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.
@end ignore

@node Programming with GNU History
@chapter 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, @pxref{Using
History Interactively}.

@menu
* 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.
@end menu

@node Introduction to History
@section 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 @dfn{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 @code{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.

@node History Storage
@section History Storage

@example
typedef struct _hist_entry @{
  char *line;
  char *data;
@} HIST_ENTRY;
@end example

@node History Functions
@section History Functions

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

@defun {void using_history} ()
Begin a session in which the history functions might be used.  This
just initializes the interactive variables.
@end defun

@defun {void add_history} (char *string)
Place @var{string} at the end of the history list.  The associated data
field (if any) is set to @code{NULL}.
@end defun

@defun {int where_history} ()
Returns the number which says what history element we are now looking
at.
@end defun
  
@defun {int history_set_pos} (int pos)
Set the position in the history list to @var{pos}.
@end defun

@defun {int history_search_pos} (char *string, int direction, int pos)
Search for @var{string} in the history list, starting at @var{pos}, an
absolute index into the list.  @var{direction}, if negative, says to search
backwards from @var{pos}, else forwards.  Returns the absolute index of
the history element where @var{string} was found, or -1 otherwise.
@end defun

@defun {HIST_ENTRY *remove_history} ();
Remove history element @var{which} from the history.  The removed
element is returned to you so you can free the line, data,
and containing structure.
@end defun

@defun {void stifle_history} (int max)
Stifle the history list, remembering only @var{max} number of entries.
@end defun

@defun {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.
@end defun

@defun {int read_history} (char *filename)
Add the contents of @var{filename} to the history list, a line at a
time.  If @var{filename} is @code{NULL}, then read from
@file{~/.history}.  Returns 0 if successful, or errno if not.
@end defun

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

@defun {int write_history} (char *filename)
Append the current history to @var{filename}.  If @var{filename} is
@code{NULL}, then append the history list to @file{~/.history}.  Values
returned are as in @code{read_history ()}.
@end defun

@defun {int append_history} (int nelements, char *filename)
Append @var{nelement} entries to @var{filename}.  The entries appended
are from the end of the list minus @var{nelements} up to the end of the
list.
@end defun

@defun {HIST_ENTRY *replace_history_entry} ()
Make the history entry at @var{which} have @var{line} and @var{data}.
This returns the old entry so you can dispose of the data.  In the case
of an invalid @var{which}, a @code{NULL} pointer is returned.
@end defun

@defun {HIST_ENTRY *current_history} ()
Return the history entry at the current position, as determined by
@code{history_offset}.  If there is no entry there, return a @code{NULL}
pointer.
@end defun

@defun {HIST_ENTRY *previous_history} ()
Back up @var{history_offset} to the previous history entry, and return a
pointer to that entry.  If there is no previous entry, return a
@code{NULL} pointer.
@end defun

@defun {HIST_ENTRY *next_history} ()
Move @code{history_offset} forward to the next history entry, and return
the a pointer to that entry.  If there is no next entry, return a
@code{NULL} pointer.
@end defun

@defun {HIST_ENTRY **history_list} ()
Return a @code{NULL} terminated array of @code{HIST_ENTRY} which is the
current input history.  Element 0 of this list is the beginning of time.
If there is no history, return @code{NULL}.
@end defun

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

@defun {int history_expand} (char *string, char **output)
Expand @var{string}, placing the result into @var{output}, a pointer
to a string.  Returns:
@table @code
@item 0
If no expansions took place (or, if the only change in
the text was the de-slashifying of the history expansion
character),
@item 1
if expansions did take place, or
@item -1
if there was an error in expansion.
@end table

If an error ocurred in expansion, then @var{output} contains a descriptive
error message.
@end defun

@defun {char *history_arg_extract} (int first, int last, char *string)
Extract a string segment consisting of the @var{first} through @var{last}
arguments present in @var{string}.  Arguments are broken up as in
the GNU Bash shell.
@end defun

@defun {int history_total_bytes} ();
Return the number of bytes that the primary history entries are using.
This just adds up the lengths of @code{the_history->lines}.
@end defun

@node History Variables
@section History Variables

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

@defvar {int history_base}
For convenience only.  You set this when interpreting history commands.
It is the logical offset of the first history element.
@end defvar

@node History Programming Example
@section History Programming Example

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

@smallexample
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");
            @}
        @}
    @}
@}
@end smallexample
