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 Using History Interactively.

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.

Before declaring any functions using any functionality the History library provides in other code, an application writer should include the file <readline/history.h> in any file that uses the History library's features.  It supplies extern declarations for all of the library's public functions and variables, and declares all of the public data structures.

History Storage
===============

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

     typedef void *histdata_t;

     typedef struct _hist_entry {
       char *line;
       char *timestamp;
       histdata_t 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 around the current state of the history.
      */
     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 exported by the GNU History library.

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

 -- Function: HISTORY_STATE * history_get_history_state (void)
     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 (const char *string)
     Place STRING at the end of the history list.  The associated data field (if any) is set to NULL.

 -- Function: void add_history_time (const char *string)
     Change the time stamp associated with the most recent history entry to STRING.

 -- 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: histdata_t free_history_entry (HIST_ENTRY *histent)
     Free the history entry HISTENT and any history library private data associated with it.  Returns the application-specific data so the caller can dispose of it.

 -- Function: HIST_ENTRY * replace_history_entry (int which, const char *line, histdata_t data)
     Make the history entry at offset WHICH have LINE and DATA. This returns the old entry so the caller can dispose of any application-specific data.  In the case of an invalid WHICH, a NULL pointer is returned.

 -- Function: void clear_history (void)
     Clear the history list by deleting all the entries.

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

 -- Function: int unstifle_history (void)
     Stop stifling the history.  This returns the previously-set maximum number of history entries (as set by stifle_history()). The value is positive if the history was stifled, negative if it wasn't.

 -- Function: int history_is_stifled (void)
     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 (void)
     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 (void)
     Returns the offset of the current history element.

 -- Function: HIST_ENTRY * current_history (void)
     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 (see History Variables). If there is no entry there, or if OFFSET is greater than the history length, return a NULL pointer.

 -- Function: time_t history_get_time (HIST_ENTRY *entry)
     Return the time stamp associated with the history entry ENTRY.

 -- Function: int history_total_bytes (void)
     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 current history offset to POS, an absolute index into the list. Returns 1 on success, 0 if POS is less than zero or greater than the number of history entries.

 -- Function: HIST_ENTRY * previous_history (void)
     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 (void)
     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 (const char *string, int direction)
     Search the history for STRING, starting at the current history offset. If DIRECTION is less than 0, then the search is through previous entries, otherwise through subsequent entries. 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 (const 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 is less than 0, then the search is through previous entries, otherwise through subsequent entries. 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 (const 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 (const 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 (const 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 (const char *filename)
     Write the current history to FILENAME, overwriting FILENAME if necessary. If FILENAME is NULL, then write the history list to ~/.history. Returns 0 on success, or errno on a read or write error.

 -- Function: int append_history (int nelements, const char *filename)
     Append the last NELEMENTS of the history list to FILENAME. If FILENAME is NULL, then append to ~/.history. Returns 0 on success, or errno on a read or write error.

 -- Function: int history_truncate_file (const char *filename, int nlines)
     Truncate the history file FILENAME, leaving only the last NLINES lines. If FILENAME is NULL, then ~/.history is truncated. Returns 0 on success, or errno on failure.

History Expansion
-----------------

These functions implement history expansion.

 -- Function: int history_expand (char *string, char **output)
     Expand STRING, placing the result into OUTPUT, a pointer to a string (see History Interaction).  Returns:

     0
          If no expansions took place (or, if the only change in the text was the removal of escape characters preceding the history expansion character);

     1
          if expansions did take place;

     -1
          if there was an error in expansion;

     2
          if the returned line should be displayed, but not executed, as with the :p modifier (see Modifiers).

     If an error occurred in expansion, then OUTPUT contains a descriptive error message.

 -- Function: char * get_history_event (const 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 (const char *string)
     Return an array of tokens parsed out of STRING, much as the shell might.  The tokens are split on the characters in the HISTORY_WORD_DELIMITERS variable, and shell quoting conventions are obeyed.

 -- Function: char * history_arg_extract (int first, int last, const char *string)
     Extract a string segment consisting of the FIRST through LAST arguments present in STRING.  Arguments are split using history_tokenize.

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 history_max_entries
     The maximum number of history entries.  This must be changed using stifle_history().

 -- Variable: int history_write_timestamps
     If non-zero, timestamps are written to the history file, so they can be preserved between sessions.  The default value is 0, meaning that timestamps are not saved.

     The current timestamp format uses the value of HISTORY_COMMENT_CHAR to delimit timestamp entries in the history file.  If that variable does not have a value (the default), timestamps will not be written.

 -- Variable: char history_expansion_char
     The character that introduces a history event.  The default is !. Setting this to 0 inhibits history expansion.

 -- 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_word_delimiters
     The characters that separate tokens for history_tokenize(). The default value is " \t\n()<>;&|".

 -- Variable: char * history_search_delimiter_chars
     The list of additional characters which can delimit a history search string, in addition to space, TAB, : and ? in the case of a substring search.  The default is empty.

 -- Variable: char * history_no_expand_chars
     The list of characters which inhibit history expansion if found immediately following HISTORY_EXPANSION_CHAR.  The default is space, tab, newline, carriage return, and =.

 -- Variable: int history_quotes_inhibit_expansion
     If non-zero, single-quoted words are not scanned for the history expansion character.  The default value is 0.

 -- Variable: rl_linebuf_func_t * history_inhibit_expansion_function
     This should be set to the address of a function that takes two arguments: a char * (STRING) and an int index into that string (I). It should return a non-zero value if the history expansion starting at STRING[I] should not be performed; zero if the expansion should be done. It is intended for use by applications like Bash that use the history expansion character for additional purposes. By default, this variable is set to NULL.

History Programming Example
===========================

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

     #include <stdio.h>
     #include <readline/history.h>

     main (argc, argv)
          int argc;
          char **argv;
     {
       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");
                 }
             }
         }
     }