gl_load_history

gl_get_line(3)             Library Functions Manual             gl_get_line(3)



NAME
       gl_get_line, new_GetLine, del_GetLine, gl_customize_completion,
       gl_change_terminal, gl_configure_getline, gl_load_history,
       gl_save_history, gl_group_history, gl_show_history, gl_watch_fd,
       gl_inactivity_timeout, gl_terminal_size, gl_set_term_size,
       gl_resize_history, gl_limit_history, gl_clear_history,
       gl_toggle_history, gl_lookup_history, gl_state_of_history,
       gl_range_of_history, gl_size_of_history, gl_echo_mode,
       gl_replace_prompt, gl_prompt_style, gl_ignore_signal, gl_trap_signal,
       gl_last_signal, gl_completion_action, gl_display_text,
       gl_return_status, gl_error_message, gl_catch_blocked, gl_list_signals,
       gl_bind_keyseq, gl_erase_terminal, gl_automatic_history,
       gl_append_history, gl_query_char, gl_read_char - allow the user to
       compose an input line

SYNOPSIS
       #include <stdio.h>
       #include <libtecla.h>

       GetLine *new_GetLine(size_t linelen, size_t histlen);

       GetLine *del_GetLine(GetLine *gl);

       char *gl_get_line(GetLine *gl, const char *prompt,
                         const char *start_line, int start_pos);

       int gl_query_char(GetLine *gl, const char *prompt,
                         char defchar);

       int gl_read_char(GetLine *gl);

       int gl_customize_completion(GetLine *gl, void *data,
                                   CplMatchFn *match_fn);

       int gl_change_terminal(GetLine *gl, FILE *input_fp,
                              FILE *output_fp, const char *term);

       int gl_configure_getline(GetLine *gl,
                                const char *app_string,
                                const char *app_file,
                                const char *user_file);

       int gl_bind_keyseq(GetLine *gl, GlKeyOrigin origin,
                          const char *keyseq, const char *action);

       int gl_save_history(GetLine *gl, const char *filename,
                           const char *comment, int max_lines);

       int gl_load_history(GetLine *gl, const char *filename,
                           const char *comment);

       int gl_watch_fd(GetLine *gl, int fd, GlFdEvent event,
                       GlFdEventFn *callback, void *data);

       int gl_inactivity_timeout(GetLine *gl, GlTimeoutFn *callback,
                          void *data, unsigned long sec,
                          unsigned long nsec);

       int gl_group_history(GetLine *gl, unsigned stream);

       int gl_show_history(GetLine *gl, FILE *fp,
                           const char *fmt, int all_groups,
                           int max_lines);

       int gl_resize_history(GetLine *gl, size_t bufsize);

       void gl_limit_history(GetLine *gl, int max_lines);

       void gl_clear_history(GetLine *gl, int all_groups);

       void gl_toggle_history(GetLine *gl, int enable);

       GlTerminalSize gl_terminal_size(GetLine *gl,
                                       int def_ncolumn,
                                       int def_nline);

       int gl_set_term_size(GetLine *gl, int ncolumn, int nline);

       int gl_lookup_history(GetLine *gl, unsigned long id,
                             GlHistoryLine *hline);

       void gl_state_of_history(GetLine *gl,
                                GlHistoryState *state);

       void gl_range_of_history(GetLine *gl,
                                GlHistoryRange *range);

       void gl_size_of_history(GetLine *gl, GlHistorySize *size);

       void gl_echo_mode(GetLine *gl, int enable);

       void gl_replace_prompt(GetLine *gl, const char *prompt);

       void gl_prompt_style(GetLine *gl, GlPromptStyle style);

       int gl_ignore_signal(GetLine *gl, int signo);

       int gl_trap_signal(GetLine *gl, int signo, unsigned flags,
                          GlAfterSignal after, int errno_value);

       int gl_last_signal(GetLine *gl);

       int gl_completion_action(GetLine *gl,
                                void *data, CplMatchFn *match_fn,
                                int list_only, const char *name,
                                const char *keyseq);

       int gl_register_action(GetLine *gl, void *data,
                              GlActionFn *fn, const char *name,
                              const char *keyseq);

       int gl_display_text(GetLine *gl, int indentation,
                           const char *prefix,
                           const char *suffix, int fill_char,
                           int def_width, int start,
                           const char *string);

       GlReturnStatus gl_return_status(GetLine *gl);

       const char *gl_error_message(GetLine *gl, char *buff,
                                    size_t n);

       void gl_catch_blocked(GetLine *gl);

       int gl_list_signals(GetLine *gl, sigset_t *set);

       int gl_append_history(GetLine *gl, const char *line);

       int gl_automatic_history(GetLine *gl, int enable);



DESCRIPTION
       The gl_get_line() function is part of the tecla library (see the
       libtecla(3) man page). If the user is typing at a terminal, each call
       prompts them for an line of input, then provides interactive editing
       facilities, similar to those of the unix tcsh shell. In addition to
       simple command-line editing, it supports recall of previously entered
       command lines, TAB completion of file names, and in-line wild-card
       expansion of filenames. Documentation of both the user-level command-
       line editing features and all user configuration options, can be found
       in the tecla(7) man page. This man page concerns itself with
       documentation for programmers interested in using this library in their
       application.


AN EXAMPLE
       The following shows a complete example of how to use the gl_get_line()
       function to get input from the user:

         #include <stdio.h>
         #include <locale.h>
         #include <libtecla.h>

         int main(int argc, char *argv[])
         {
           char *line;    /* The line that the user typed */
           GetLine *gl;   /* The gl_get_line() resource object */

           setlocale(LC_CTYPE, ""); /* Adopt the user's choice */
                                    /* of character set. */

           gl = new_GetLine(1024, 2048);
           if(!gl)
             return 1;

           while((line=gl_get_line(gl, "$ ", NULL, -1)) != NULL &&
                  strcmp(line, "exit\n") != 0)
             printf("You typed: %s\n", line);

           gl = del_GetLine(gl);
           return 0;
         }

       In the example, first the resources needed by the gl_get_line()
       function are created by calling new_GetLine(). This allocates the
       memory used in subsequent calls to the gl_get_line() function,
       including the history buffer for recording previously entered lines.
       Then one or more lines are read from the user, until either an error
       occurs, or the user types exit. Then finally the resources that were
       allocated by new_GetLine(), are returned to the system by calling
       del_GetLine(). Note the use of the NULL return value of del_GetLine()
       to make gl NULL. This is a safety precaution. If the program
       subsequently attempts to pass gl to gl_get_line(), said function will
       complain, and return an error, instead of attempting to use the deleted
       resource object.



THE FUNCTIONS USED IN THE EXAMPLE
       The descriptions of the functions used in the example are as follows:

         GetLine *new_GetLine(size_t linelen, size_t histlen)

       This function creates the resources used by the gl_get_line() function
       and returns an opaque pointer to the object that contains them.  The
       maximum length of an input line is specified via the linelen argument,
       and the number of bytes to allocate for storing history lines is set by
       the histlen argument. History lines are stored back-to-back in a single
       buffer of this size. Note that this means that the number of history
       lines that can be stored at any given time, depends on the lengths of
       the individual lines.  If you want to place an upper limit on the
       number of lines that can be stored, see the gl_limit_history() function
       described later. If you don't want history at all, specify histlen as
       zero, and no history buffer will be allocated.

       On error, a message is printed to stderr and NULL is returned.

         GetLine *del_GetLine(GetLine *gl)

       This function deletes the resources that were returned by a previous
       call to new_GetLine(). It always returns NULL (ie a deleted object). It
       does nothing if the gl argument is NULL.

         char *gl_get_line(GetLine *gl, const char *prompt,
                          const char *start_line, int start_pos);

       The gl_get_line() function can be called any number of times to read
       input from the user. The gl argument must have been previously returned
       by a call to new_GetLine(). The prompt argument should be a normal NUL
       terminated string, specifying the prompt to present the user with. By
       default prompts are displayed literally, but if enabled with the
       gl_prompt_style() function (see later), prompts can contain directives
       to do underlining, switch to and from bold fonts, or turn highlighting
       on and off.

       If you want to specify the initial contents of the line, for the user
       to edit, pass the desired string via the start_line argument. You can
       then specify which character of this line the cursor is initially
       positioned over, using the start_pos argument. This should be -1 if you
       want the cursor to follow the last character of the start line. If you
       don't want to preload the line in this manner, send start_line as NULL,
       and set start_pos to -1. Note that the line pointer returned by one
       call to gl_get_line() can be passed back to the next call to
       gl_get_line() via the start_line. This allows the application to take
       the last entered line, and if it contains an error, to then present it
       back to the user for re-editing, with the cursor initially positioned
       where the error was encountered.

       The gl_get_line() function returns a pointer to the line entered by the
       user, or NULL on error or at the end of the input. The returned pointer
       is part of the specified gl resource object, and thus should not be
       free'd by the caller, or assumed to be unchanging from one call to the
       next. When reading from a user at a terminal, there will always be a
       newline character at the end of the returned line.  When standard input
       is being taken from a pipe or a file, there will similarly be a newline
       unless the input line was too long to store in the internal buffer. In
       the latter case you should call gl_get_line() again to read the rest of
       the line. Note that this behavior makes gl_get_line() similar to
       fgets().  In fact when stdin isn't connected to a
       terminal,gl_get_line() just calls fgets().


THE RETURN STATUS OF GL_GET_LINE
       As described above, the gl_get_line() function has two possible return
       values; a pointer to the completed input line, or NULL. Extra
       information about what caused gl_get_line() to return is available both
       by inspecting errno, and by calling the gl_return_status() function.


         GlReturnStatus gl_return_status(GetLine *gl);


       The following are the possible enumerated values that this function
       returns.


         GLR_NEWLINE     -  The last call to gl_get_line()
                            successfully returned a completed
                            input line.

         GLR_BLOCKED     -  gl_get_line() was in non-blocking
                            server mode, and returned early to
                            avoid blocking the process while
                            waiting for terminal I/O. The
                            gl_pending_io() function can be
                            used to see what type of I/O
                            gl_get_line() was waiting for.
                            (see the gl_io_mode(3) man page
                            for details).

         GLR_SIGNAL      -  A signal was caught by
                            gl_get_line() that had an
                            after-signal disposition of
                            GLS_ABORT (See gl_trap_signal()).

         GLR_TIMEOUT     -  The inactivity timer expired while
                            gl_get_line() was waiting for
                            input, and the timeout callback
                            function returned GLTO_ABORT.
                            See gl_inactivity_timeout() for
                            information about timeouts.

         GLR_FDABORT     -  An application I/O callack returned
                            GLFD_ABORT (see gl_watch_fd()).

         GLR_EOF         -  End of file reached. This can happen
                            when input is coming from a file or a
                            pipe, instead of the terminal. It also
                            occurs if the user invokes the
                            list-or-eof or del-char-or-list-or-eof
                            actions at the start of a new line.

         GLR_ERROR       -  An unexpected error caused
                            gl_get_line() to abort (consult
                            errno and/or
                            gl_error_message() for details.


       When gl_return_status() returns GLR_ERROR, and the value of errno isn't
       sufficient to explain what happened, you can use the gl_error_message()
       function to request a description of the last error that occurred.


         const char *gl_error_message(GetLine *gl, char *buff,
                                      size_t n);


       The return value is a pointer to the message that occurred. If the buff
       argument is NULL, this will be a pointer to a buffer within gl, who's
       value will probably change on the next call to any function associated
       with gl_get_line(). Otherwise, if a non-NULL buff argument is provided,
       the error message, including a '\0' terminator, will be written within
       the first n elements of this buffer, and the return value will be a
       pointer to the first element of this buffer. If the message won't fit
       in the provided buffer, it will be truncated to fit.


OPTIONAL PROMPT FORMATTING
       Whereas by default the prompt string that you specify is displayed
       literally, without any special interpretation of the characters within
       it, the gl_prompt_style() function can be used to enable optional
       formatting directives within the prompt.

         void gl_prompt_style(GetLine *gl, GlPromptStyle style);

       The style argument, which specifies the formatting style, can take any
       of the following values:

         GL_FORMAT_PROMPT   -  In this style, the formatting
                               directives described below, when
                               included in prompt strings, are
                               interpreted as follows:

                                 %B  -  Display subsequent
                                        characters with a bold
                                        font.
                                 %b  -  Stop displaying characters
                                        with the bold font.
                                 %F  -  Make subsequent characters
                                        flash.
                                 %f  -  Turn off flashing
                                        characters.
                                 %U  -  Underline subsequent
                                        characters.
                                 %u  -  Stop underlining
                                        characters.
                                 %P  -  Switch to a pale (half
                                        brightness) font.
                                 %p  -  Stop using the pale font.
                                 %S  -  Highlight subsequent
                                        characters (also known as
                                        standout mode).
                                 %s  -  Stop highlighting
                                        characters.
                                 %V  -  Turn on reverse video.
                                 %v  -  Turn off reverse video.
                                 %%  -  Display a single %
                                        character.

                               For example, in this mode, a prompt
                               string like "%UOK%u$ " would
                               display the prompt "OK$ ",
                               but with the OK part
                               underlined.

                               Note that although a pair of
                               characters that starts with a %
                               character, but doesn't match any of
                               the above directives is displayed
                               literally, if a new directive is
                               subsequently introduced which does
                               match, the displayed prompt will
                               change, so it is better to always
                               use %% to display a literal %.

                               Also note that not all terminals
                               support all of these text
                               attributes, and that some substitute
                               a different attribute for missing
                               ones.

         GL_LITERAL_PROMPT  -  In this style, the prompt string is
                               printed literally. This is the
                               default style.


ALTERNATE CONFIGURATION SOURCES
       As mentioned above, by default users have the option of configuring the
       behavior of gl_get_line() via a configuration file called .teclarc in
       their home directories. The fact that all applications share this same
       configuration file is both an advantage and a disadvantage.  In most
       cases it is an advantage, since it encourages uniformity, and frees the
       user from having to configure each application separately.  In some
       applications, however, this single means of configuration is a problem.
       This is particularly true of embedded software, where there's no
       filesystem to read a configuration file from, and also in applications
       where a radically different choice of keybindings is needed to emulate
       a legacy keyboard interface.  To cater for such cases, the following
       function allows the application to control where configuration
       information is read from.


         int gl_configure_getline(GetLine *gl,
                                  const char *app_string,
                                  const char *app_file,
                                  const char *user_file);


       It allows the configuration commands that would normally be read from a
       user's ~/.teclarc file, to be read from any or none of, a string, an
       application specific configuration file, and/or a user-specific
       configuration file. If this function is called before the first call to
       gl_get_line(), the default behavior of reading ~/.teclarc on the first
       call to gl_get_line() is disabled, so all configuration must be
       achieved using the configuration sources specified with this function.

       If app_string != NULL, then it is interpreted as a string containing
       one or more configuration commands, separated from each other in the
       string by embedded newline characters. If app_file != NULL then it is
       interpreted as the full pathname of an application-specific
       configuration file. If user_file != NULL then it is interpreted as the
       full pathname of a user-specific configuration file, such as
       ~/.teclarc. For example, in the following call,


         gl_configure_getline(gl, "edit-mode vi \n nobeep",
                                  "/usr/share/myapp/teclarc",
                                  "~/.teclarc");


       the app_string argument causes the calling application to start in vi
       edit-mode, instead of the default emacs mode, and turns off the use of
       the terminal bell by the library. It then attempts to read system-wide
       configuration commands from an optional file called
       /usr/share/myapp/teclarc, then finally reads user-specific
       configuration commands from an optional .teclarc file in the user's
       home directory. Note that the arguments are listed in ascending order
       of priority, with the contents of app_string being potentially
       overriden by commands in app_file, and commands in app_file potentially
       being overriden by commands in user_file.

       You can call this function as many times as needed, the results being
       cumulative, but note that copies of any filenames specified via the
       app_file and user_file arguments are recorded internally for subsequent
       use by the read-init-files key-binding function, so if you plan to call
       this function multiple times, be sure that the last call specifies the
       filenames that you want re-read when the user requests that the
       configuration files be re-read.

       Individual key sequences can also be bound and unbound using the
       gl_bind_keyseq() function.


         int gl_bind_keyseq(GetLine *gl, GlKeyOrigin origin,
                            const char *keyseq,
                            const char *action);


       The origin argument specifies the priority of the binding, according to
       who it is being established for, and must be one of the following two
       values.

         GL_USER_KEY   -   The user requested this key-binding.
         GL_APP_KEY    -   This is a default binding set by the
                           application.

       When both user and application bindings for a given key-sequence have
       been specified, the user binding takes precedence. The application's
       binding is subsequently reinstated if the user's binding is later
       unbound via either another to this function, or a call to
       gl_configure_getline().

       The keyseq argument specifies the key-sequence to be bound or unbound,
       and is expressed in the same way as in a ~/.teclarc configuration file.
       The action argument must either be a string containing the name of the
       action to bind the key-sequence to, or it must be NULL or "" to unbind
       the key-sequence.


CUSTOMIZED WORD COMPLETION
       If in your application, you would like to have TAB completion complete
       other things in addition to or instead of filenames, you can arrange
       this by registering an alternate completion callback function, via a
       call to the gl_customize_completion() function.

         int gl_customize_completion(GetLine *gl, void *data,
                                     CplMatchFn *match_fn);

       The data argument provides a way for your application to pass
       arbitrary, application-specific information to the callback function.
       This is passed to the callback every time that it is called. It might
       for example, point to the symbol table from which possible completions
       are to be sought. The match_fn argument specifies the callback function
       to be called. The CplMatchFn function type is defined in libtecla.h, as
       is a CPL_MATCH_FN() macro that you can use to declare and prototype
       callback functions. The declaration and responsibilities of callback
       functions are described in depth in the cpl_complete_word(3) man page.

       In brief, the callback function is responsible for looking backwards in
       the input line, back from the point at which the user pressed TAB, to
       find the start of the word being completed. It then must lookup
       possible completions of this word, and record them one by one in the
       WordCompletion object that is passed to it as an argument, by calling
       the cpl_add_completion() function. If the callback function wishes to
       provide filename completion in addition to its own specific
       completions, it has the option of itself calling the builtin file-name
       completion callback. This also, is documented in the
       cpl_complete_word(3) man page.

       Note that if you would like gl_get_line() to return the current input
       line when a successful completion is been made, you can arrange this
       when you call cpl_add_completion(), by making the last character of the
       continuation suffix a newline character. If you do this, the input line
       will be updated to display the completion, together with any
       contiuation suffix up to the newline character, then gl_get_line() will
       return this input line.


       If, for some reason, your callback function needs to write something to
       the terminal, it must call gl_normal_io() before doing so. This will
       start a new line after the input line that is currently being edited,
       reinstate normal terminal I/O, and tell gl_get_line() that the input
       line will need to be redrawn when the callback returns.


ADDING COMPLETION ACTIONS
       In the previous section the ability to customize the behavior of the
       only default completion action, complete-word, was described.  In this
       section the ability to install additional action functions, so that
       different types of word completion can be bound to different key-
       sequences, is described. This is achieved by using the
       gl_completion_action() function.


         int gl_completion_action(GetLine *gl,
                                  void *data, CplMatchFn *match_fn,
                                  int list_only, const char *name,
                                  const char *keyseq);


       The data and match_fn arguments are as described in the
       cpl_complete_word man page, and specify the callback function that
       should be invoked to identify possible completions.  The list_only
       argument determines whether the action that is being defined should
       attempt to complete the word as far as possible in the input line
       before displaying any possible ambiguous completions, or whether it
       should simply display the list of possible completions without touching
       the input line. The former option is selected by specifying a value of
       0, and the latter by specifying a value of 1. The name argument
       specifies the name by which configuration files and future invokations
       of this function should refer to the action. This must either be the
       name of an existing completion action to be changed, or be a new unused
       name for a new action. Finally, the keyseq argument specifies the
       default key-sequence to bind the action to. If this is NULL, no new
       keysequence will be bound to the action.

       Beware that in order for the user to be able to change the key-sequence
       that is bound to actions that are installed in this manner, when you
       call gl_completion_action() to install a given action for the first
       time, you should do this between calling new_GetLine() and the first
       call to gl_get_line().  Otherwise, when the user's configuration file
       is read on the first call to gl_get_line(), the name of the your
       additional action won't be known, and any reference to it in the
       configuration file will generate an error.

       As discussed for gl_customize_completion(), if your callback function,
       for some reason, needs to write anything to the terminal, it must call
       gl_normal_io() before doing so.


DEFINING CUSTOM ACTIONS
       Although the built-in key-binding actions are sufficient for the needs
       of most applications, occasionally a specialized application may need
       to define one or more custom actions, bound to application-specific
       key-sequences. For example, a sales application would benefit from
       having a key-sequence that displayed the part name that corresponded to
       a part number preceding the cursor. Such a feature is clearly beyond
       the scope of the built-in action functions. So for such special cases,
       the gl_register_action() function is provided.


         int gl_register_action(GetLine *gl, void *data,
                       GlActionFn *fn, const char *name,
                       const char *keyseq);


       This function lets the application register an external function, fn,
       that will thereafter be called whenever either the specified key-
       sequence, keyseq, is entered by the user, or the user enters any other
       key-sequence that the user subsequently binds to the specified action
       name, name, in their configuration file. The data argument can be a
       pointer to anything that the application wishes to have passed to the
       action function, fn, whenever that function is invoked.

       The action function, fn, should be declared using the following macro,
       which is defined in libtecla.h.


         #define GL_ACTION_FN(fn) GlAfterAction (fn)(GetLine *gl, \
                     void *data, int count, size_t curpos, \
                     const char *line)


       The gl and data arguments are those that were previously passed to
       gl_register_action() when the action function was registered. The count
       argument is a numeric argument which the user has the option of
       entering using the digit-argument action, before invoking the action.
       If the user doesn't enter a number, then the count argument is set to
       1. Nominally this argument is interpreted as a repeat count, meaning
       that the action should be repeated that many times. In practice
       however, for some actions a repeat count makes little sense. In such
       cases, actions can either simply ignore the count argument, or use its
       value for a different purpose.

       A copy of the current input line is passed in the read-only line
       argument. The current cursor position within this string is given by
       the index contained in the curpos argument. Note that direct
       manipulation of the input line and the cursor position is not
       permitted. This is because the rules dicated by various modes, such as
       vi mode versus emacs mode, no-echo mode, and insert mode versus
       overstrike mode etc, make it too complex for an application writer to
       write a conforming editing action, as well as constrain future changes
       to the internals of gl_get_line(). A potential solution to this dilema
       would be to allow the action function to edit the line using the
       existing editing actions. This is currently under consideration.

       If the action function wishes to write text to the terminal, without
       this getting mixed up with the displayed text of the input line, or
       read from the terminal without having to handle raw terminal I/O, then
       before doing either of these operations, it must temporarily suspend
       line editing by calling the gl_normal_io() function. This function
       flushes any pending output to the terminal, moves the cursor to the
       start of the line that follows the last terminal line of the input
       line, then restores the terminal to a state that is suitable for use
       with the C stdio facilities. The latter includes such things as
       restoring the normal mapping of \n to \r\n, and, when in server mode,
       restoring the normal blocking form of terminal I/O. Having called this
       function, the action function can read from and write to the terminal
       without the fear of creating a mess.  It isn't necessary for the action
       function to restore the original editing environment before it returns.
       This is done automatically by gl_get_line() after the action function
       returns.  The following is a simple example of an action function which
       writes the sentence "Hello world" on a new terminal line after the line
       being edited. When this function returns, the input line is redrawn on
       the line that follows the "Hello world" line, and line editing resumes.


         static GL_ACTION_FN(say_hello_fn)
         {
           if(gl_normal_io(gl))   /* Temporarily suspend editing */
             return GLA_ABORT;
           printf("Hello world\n");
           return GLA_CONTINUE;
         }


       Action functions must return one of the following values, to tell
       gl_get_line() how to procede.


         GLA_ABORT     -   Cause gl_get_line() to return NULL.
         GLA_RETURN    -   Cause gl_get_line() to return the
                           completed input line.
         GLA_CONTINUE  -   Resume command-line editing.


       Note that the name argument of gl_register_action() specifies the name
       by which a user can refer to the action in their configuration file.
       This allows them to re-bind the action to an alternate key-seqeunce. In
       order for this to work, it is necessary to call gl_register_action()
       between calling new_GetLine() and the first call to gl_get_line().


HISTORY FILES
       To save the contents of the history buffer before quitting your
       application, and subsequently restore them when you next start the
       application, the following functions are provided.


        int gl_save_history(GetLine *gl, const char *filename,
                            const char *comment, int max_lines);
        int gl_load_history(GetLine *gl, const char *filename,
                            const char *comment);


       The filename argument specifies the name to give the history file when
       saving, or the name of an existing history file, when loading. This may
       contain home-directory and environment variable expressions, such as
       "~/.myapp_history" or "$HOME/.myapp_history".

       Along with each history line, extra information about it, such as when
       it was entered by the user, and what its nesting level is, is recorded
       as a comment preceding the line in the history file. Writing this as a
       comment allows the history file to double as a command file, just in
       case you wish to replay a whole session using it. Since comment
       prefixes differ in different languages, the comment argument is
       provided for specifying the comment prefix. For example, if your
       application were a unix shell, such as the bourne shell, you would
       specify "#" here. Whatever you choose for the comment character, you
       must specify the same prefix to gl_load_history() that you used when
       you called gl_save_history() to write the history file.

       The max_lines must be either -1 to specify that all lines in the
       history list be saved, or a positive number specifying a ceiling on how
       many of the most recent lines should be saved.

       Both fuctions return non-zero on error, after writing an error message
       to stderr. Note that gl_load_history() does not consider the non-
       existence of a file to be an error.


MULTIPLE HISTORY LISTS
       If your application uses a single GetLine object for entering many
       different types of input lines, you may wish gl_get_line() to
       distinguish the different types of lines in the history list, and only
       recall lines that match the current type of line. To support this
       requirement, gl_get_line() marks lines being recorded in the history
       list with an integer identifier chosen by the application.  Initially
       this identifier is set to 0 by new_GetLine(), but it can be changed
       subsequently by calling gl_group_history().


         int gl_group_history(GetLine *gl, unsigned id);


       The integer identifier id can be any number chosen by the application,
       but note that gl_save_history() and gl_load_history() preserve the
       association between identifiers and historical input lines between
       program invokations, so you should choose fixed identifiers for the
       different types of input line used by your application.

       Whenever gl_get_line() appends a new input line to the history list,
       the current history identifier is recorded with it, and when it is
       asked to recall a historical input line, it only recalls lines that are
       marked with the current identifier.


DISPLAYING HISTORY
       The history list can be displayed by calling gl_show_history().


         int gl_show_history(GetLine *gl, FILE *fp,
                             const char *fmt,
                             int all_groups,
                             int max_lines);


       This displays the current contents of the history list to the stdio
       output stream fp. If the max_lines argument is greater than or equal to
       zero, then no more than this number of the most recent lines will be
       displayed. If the all_groups argument is non-zero, lines from all
       history groups are displayed. Otherwise just those of the currently
       selected history group are displayed. The format string argument, fmt,
       determines how the line is displayed. This can contain arbitrary
       characters which are written verbatim, interleaved with any of the
       following format directives:

         %D  -  The date on which the line was originally
                entered, formatted like 2001-11-20.
         %T  -  The time of day when the line was entered,
                formatted like 23:59:59.
         %N  -  The sequential entry number of the line in
                the history buffer.
         %G  -  The number of the history group which the
                line belongs to.
         %%  -  A literal % character.
         %H  -  The history line itself.

       Thus a format string like "%D %T  %H0 would output something like:

         2001-11-20 10:23:34  Hello world

       Note the inclusion of an explicit newline character in the format
       string.


LOOKING UP HISTORY
       The gl_lookup_history() function allows the calling application to look
       up lines in the history list.


         typedef struct {
           const char *line;    /* The requested historical */
                                /*  line. */
           unsigned group;      /* The history group to which */
                                /*  the line belongs. */
           time_t timestamp;    /* The date and time at which */
                                /*  the line was originally */
                                /*  entered. */
         } GlHistoryLine;

         int gl_lookup_history(GetLine *gl, unsigned long id,
                               GlHistoryLine *hline);


       The id argument indicates which line to look up, where the first line
       that was entered in the history list after new_GetLine() was called, is
       denoted by 0, and subsequently entered lines are denoted with
       successively higher numbers. Note that the range of lines currently
       preserved in the history list can be queried by calling the
       gl_range_of_history() function, described later. If the requested line
       is in the history list, the details of the line are recorded in the
       variable pointed to by the hline argument, and 1 is returned. Otherwise
       0 is returned, and the variable pointed to by hline is left unchanged.

       Beware that the string returned in hline->line is part of the history
       buffer, so it must not be modified by the caller, and will be recycled
       on the next call to any function that takes gl as its argument.
       Therefore you should make a private copy of this string if you need to
       keep it around.


MANUAL HISTORY ARCHIVAL
       By default, whenever a line is entered by the user, it is automatically
       appended to the history list, just before gl_get_line() returns the
       line to the caller. This is convenient for the majority of
       applications, but there are also applications that need finer grained
       control over what gets added to the history list. In such cases, the
       automatic addition of entered lines to the history list can be turned
       off by calling the gl_automatic_history() function.


         int gl_automatic_history(GetLine *gl, int enable);


       If this function is called with its enable argument set to 0,
       gl_get_line() won't automatically archive subsequently entered lines.
       Automatic archiving can be reenabled at a later time, by calling this
       function again, with its enable argument set to 1.  While automatic
       history archiving is disabled, the calling application can use the
       gl_append_history() to append lines to the history list as needed.


         int gl_append_history(GetLine *gl, const char *line);


       The line argument specifies the line to be added to the history list.
       This must be a normal ' ' terminated string. If this string contains
       any newline characters, the line that gets archived in the history list
       will be terminated by the first of these. Otherwise it will be
       terminated by the ' ' terminator.  If the line is longer than the
       maximum input line length, that was specified when new_GetLine() was
       called, when the line is recalled, it will get truncated to the actual
       gl_get_line() line length.

       If successful, gl_append_history() returns 0. Otherwise it returns non-
       zero, and sets errno to one of the following values.


          EINVAL  -  One of the arguments passed to
                     gl_append_history() was NULL.
          ENOMEM  -  The specified line was longer than the allocated
                     size of the history buffer (as specified when
                     new_GetLine() was called), so it couldn't be
                     archived.


       A textual description of the error can optionally be obtained by
       calling gl_error_message(). Note that after such an error, the history
       list remains in a valid state to receive new history lines, so there is
       little harm in simply ignoring the return status of
       gl_append_history().


MISCELLANEOUS HISTORY CONFIGURATION
       If you wish to change the size of the history buffer that was
       originally specified in the call to new_GetLine(), you can do so with
       the gl_resize_history() function.


         int gl_resize_history(GetLine *gl, size_t histlen);


       The histlen argument specifies the new size in bytes, and if you
       specify this as 0, the buffer will be deleted.

       As mentioned in the discussion of new_GetLine(), the number of lines
       that can be stored in the history buffer, depends on the lengths of the
       individual lines. For example, a 1000 byte buffer could equally store
       10 lines of average length 100 bytes, or 2 lines of average length 50
       bytes. Although the buffer is never expanded when new lines are added,
       a list of pointers into the buffer does get expanded when needed to
       accomodate the number of lines currently stored in the buffer. To place
       an upper limit on the number of lines in the buffer, and thus a ceiling
       on the amount of memory used in this list, you can call the
       gl_limit_history() function.


         void gl_limit_history(GetLine *gl, int max_lines);


       The max_lines should either be a positive number >= 0, specifying an
       upper limit on the number of lines in the buffer, or be -1 to cancel
       any previously specified limit. When a limit is in effect, only the
       max_lines most recently appended lines are kept in the buffer. Older
       lines are discarded.

       To discard lines from the history buffer, use the gl_clear_history()
       function.

         void gl_clear_history(GetLine *gl, int all_groups);

       The all_groups argument tells the function whether to delete just the
       lines associated with the current history group (see
       gl_group_history()), or all historical lines in the buffer.

       The gl_toggle_history() function allows you to toggle history on and
       off without losing the current contents of the history list.


         void gl_toggle_history(GetLine *gl, int enable);


       Setting the enable argument to 0 turns off the history mechanism, and
       setting it to 1 turns it back on. When history is turned off, no new
       lines will be added to the history list, and history lookup key-
       bindings will act as though there is nothing in the history buffer.


QUERYING HISTORY INFORMATION
       The configured state of the history list can be queried with the
       gl_history_state() function.


         typedef struct {
           int enabled;     /* True if history is enabled */
           unsigned group;  /* The current history group */
           int max_lines;   /* The current upper limit on the */
                            /*  number of lines in the history */
                            /*  list, or -1 if unlimited. */
         } GlHistoryState;

         void gl_state_of_history(GetLine *gl,
                                  GlHistoryState *state);

       On return, the status information is recorded in the variable pointed
       to by the state argument.

       The gl_range_of_history() function returns the number and range of
       lines in the history list.


       typedef struct {
         unsigned long oldest;  /* The sequential entry number */
                                /*  of the oldest line in the */
                                /*  history list. */
         unsigned long newest;  /* The sequential entry number */
                                /*  of the newest line in the */
                                /*  history list. */
         int nlines;            /* The number of lines in the */
                                /*  history list. */
       } GlHistoryRange;

       void gl_range_of_history(GetLine *gl, GlHistoryRange *range);

       The return values are recorded in the variable pointed to by the range
       argument. If the nlines member of this structure is greater than zero,
       then the oldest and newest members report the range of lines in the
       list, and newest=oldest+nlines-1.  Otherwise they are both zero.

       The gl_size_of_history() function returns the total size of the history
       buffer and the amount of the buffer that is currently occupied.

         typedef struct {
           size_t size;      /* The size of the history buffer */
                             /*  (bytes). */
           size_t used;      /* The number of bytes of the */
                             /*  history buffer that are */
                             /*  currently occupied. */
         } GlHistorySize;

         void gl_size_of_history(GetLine *gl, GlHistorySize *size);

       On return, the size information is recorded in the variable pointed to
       by the size argument.


CHANGING TERMINALS
       The new_GetLine() constructor function assumes that input is to be read
       from stdin, and output written to stdout. The following function allows
       you to switch to different input and output streams.

         int gl_change_terminal(GetLine *gl, FILE *input_fp,
                                FILE *output_fp, const char *term);

       The gl argument is the object that was returned by new_GetLine().  The
       input_fp argument specifies the stream to read from, and output_fp
       specifies the stream to be written to. Only if both of these refer to a
       terminal, will interactive terminal input be enabled.  Otherwise
       gl_get_line() will simply call fgets() to read command input. If both
       streams refer to a terminal, then they must refer to the same terminal,
       and the type of this terminal must be specified via the term argument.
       The value of the term argument is looked up in the terminal information
       database (terminfo or termcap), in order to determine which special
       control sequences are needed to control various aspects of the
       terminal. new_GetLine() for example, passes the return value of
       getenv("TERM") in this argument. Note that if one or both of input_fp
       and output_fp don't refer to a terminal, then it is legal to pass NULL
       instead of a terminal type.

       Note that if you want to pass file descriptors to gl_change_terminal(),
       you can do this by creating stdio stream wrappers using the POSIX
       fdopen() function.


EXTERNAL EVENT HANDLING
       By default, gl_get_line() doesn't return until either a complete input
       line has been entered by the user, or an error occurs. In programs that
       need to watch for I/O from other sources than the terminal, there are
       two options.


         1. Use the functions described in the
            gl_io_mode(3) man page to switch
            gl_get_line() into non-blocking server mode. In this mode,
            gl_get_line() becomes a non-blocking, incremental
            line-editing function that can safely be called from
            an external event loop. Although this is a very
            versatile method, it involves taking on some
            responsibilities that are normally performed behind
            the scenes by gl_get_line().

         2. While gl_get_line() is waiting for keyboard
            input from the user, you can ask it to also watch for
            activity on arbitrary file descriptors, such as
            network sockets, pipes etc, and have it call functions
            of your choosing when activity is seen. This works on
            any system that has the select() system call,
            which is most, if not all flavors of unix.


       Registering a file descriptor to be watched by gl_get_line() involves
       calling the gl_watch_fd() function.


         int gl_watch_fd(GetLine *gl, int fd, GlFdEvent event,
                         GlFdEventFn *callback, void *data);


       If this returns non-zero, then it means that either your arguments are
       invalid, or that this facility isn't supported on the host system.

       The fd argument is the file descriptor to be watched. The event
       argument specifies what type of activity is of interest, chosen from
       the following enumerated values:


         GLFD_READ   -  Watch for the arrival of data to be read.
         GLFD_WRITE  -  Watch for the ability to write to the file
                        descriptor without blocking.
         GLFD_URGENT -  Watch for the arrival of urgent
                        out-of-band data on the file descriptor.


       The callback argument is the function to call when the selected
       activity is seen. It should be defined with the following macro, which
       is defined in libtecla.h.


         #define GL_FD_EVENT_FN(fn) GlFdStatus (fn)(GetLine *gl, \
                                             void *data, int fd, \
                                             GlFdEvent event)

       The data argument of the gl_watch_fd() function is passed to the
       callback function for its own use, and can point to anything you like,
       including NULL. The file descriptor and the event argument are also
       passed to the callback function, and this potentially allows the same
       callback function to be registered to more than one type of event
       and/or more than one file descriptor. The return value of the callback
       function should be one of the following values.


         GLFD_ABORT    -  Tell gl_get_line() to abort. When this
                          happens, gl_get_line() returns
                          NULL, and a following call to
                          gl_return_status() will return
                          GLR_FDABORT. Note that if the
                          application needs errno always to
                          have a meaningful value when
                          gl_get_line() returns NULL,
                          the callback function should set
                          errno appropriately.
         GLFD_REFRESH  -  Redraw the input line then continue
                          waiting for input. Return this if
                          your callback wrote to the terminal.
         GLFD_CONTINUE -  Continue to wait for input, without
                          redrawing the line.

       Note that before calling the callback, gl_get_line() blocks most
       signals, and leaves its own signal handlers installed, so if you need
       to catch a particular signal you will need to both temporarily install
       your own signal handler, and unblock the signal. Be sure to re-block
       the signal (if it was originally blocked) and reinstate the original
       signal handler, if any, before returning.



       If the callback function needs to read or write to the terminal, it
       should ideally first call gl_normal_io(gl) to temporarily suspend line
       editing. This will restore the terminal to canonical, blocking-I/O,
       mode, and move the cursor to the start of a new terminal line. Later,
       when the callback returns, gl_get_line() will notice that
       gl_normal_io() was called, redisplay the input line and resume editing.
       Note that in this case the return values, GLFD_REFRESH and
       GLFD_CONTINUE are equivalent.



       To support cases where the callback function calls a third-party
       function which occasionally and unp0eiscre-enabledebeforehtheecallback
       the automatic conversion of "0 to "
       function is called. If the callack knows that the third-party function
       wrote to the terminal, it should then return the GLFD_REFRESH return
       value, to tell gl_get_line() to redisplay the input line.



       To remove a callback function that you previously registered for a
       given file descriptor and event, simply call gl_watch_fd() with the
       same file descriptor and event arguments, but with a callback argument
       of 0. The data argument is ignored in this case.


SETTING AN INACTIVITY TIMEOUT
       On systems with the select() system call, the gl_inactivity_timeout()
       function can be used to set or cancel an inactivity timeout. Inactivity
       in this case refers both to keyboard input, and to I/O on any file
       descriptors registered by prior and subsequent calls to gl_watch_fd().
       On oddball systems that don't have select(), this call has no effect.


         int gl_inactivity_timeout(GetLine *gl, GlTimeoutFn *callback,
                            void *data, unsigned long sec,
                            unsigned long nsec);


       The timeout is specified in the form of an integral number of seconds
       and an integral number of nanoseconds, via the sec and nsec arguments
       respectively. Subsequently, whenever no activity is seen for this time
       period, the function specified via the callback argument is called. The
       data argument of gl_inactivity_timeout() is passed verbatim to this
       callback function whenever it is invoked, and can thus be used to pass
       arbitrary application-specific information to the callback. The
       following macro is provided in libtecla.h for applications to use to
       declare and prototype timeout callback functions.


         #define GL_TIMEOUT_FN(fn) \
                      GlAfterTimeout (fn)(GetLine *gl, void *data)


       On returning, the application's callback is expected to return one of
       the following enumerators to tell gl_get_line() how to procede after
       the timeout has been handled by the callback.


         GLTO_ABORT    -  Tell gl_get_line() to abort. When
                          this happens, gl_get_line() will
                          return NULL, and a following call
                          to gl_return_status() will return
                          GLR_TIMEOUT. Note that if the
                          application needs errno always to
                          have a meaningful value when
                          gl_get_line() returns NULL,
                          the callback function should set
                          errno appropriately.
         GLTO_REFRESH  -  Redraw the input line, then continue
                          waiting for input. You should return
                          this value if your callback wrote to the
                          terminal without having first called
                          gl_normal_io(gl).
         GLTO_CONTINUE -  In normal blocking-I/O mode, continue to
                          wait for input, without redrawing the
                          user's input line.
                          In non-blocking server I/O mode (see
                          gl_io_mode(3)), cause gl_get_line()
                          to act as though I/O blocked. This means
                          that gl_get_line() will immediately
                          return NULL, and a following call
                          to gl_return_status() will return
                          GLR_BLOCKED.


       Note that before calling the callback, gl_get_line() blocks most
       signals, and leaves its own signal handlers installed, so if you need
       to catch a particular signal you will need to both temporarily install
       your own signal handler, and unblock the signal. Be sure to re-block
       the signal (if it was originally blocked) and reinstate the original
       signal handler, if any, before returning.



       If the callback function needs to read or write to the terminal, it
       should ideally first call gl_normal_io(gl) to temporarily suspend line
       editing. This will restore the terminal to canonical, blocking-I/O,
       mode, and move the cursor to the start of a new terminal line. Later,
       when the callback returns, gl_get_line() will notice that
       gl_normal_io() was called, redisplay the input line and resume editing.
       Note that in this case the return values, GLTO_REFRESH and
       GLTO_CONTINUE are equivalent.



       To support cases where the callback function calls a third-party
       function which occasionally and unp0eiscre-enabledebeforehtheecallback
       the automatic conversion of "0 to "
       function is called. If the callack knows that the third-party function
       wrote to the terminal, it should then return the GLTO_REFRESH return
       value, to tell gl_get_line() to redisplay the input line.



       Note that although the timeout argument includes a nano-second
       component, few computer clocks presently have resolutions that are
       finer than a few milliseconds, so asking for less than a few
       milliseconds is equivalent to requesting zero seconds on a lot of
       systems. If this would be a problem, you should base your timeout
       selection on the actual resolution of the host clock (eg. by calling
       sysconf(_SC_CLK_TCK)).



       To turn off timeouts, simply call gl_inactivity_timeout() with a
       callback argument of 0. The data argument is ignored in this case.


SIGNAL HANDLING DEFAULTS
       By default, the gl_get_line() function intercepts a number of signals.
       This is particularly important for signals which would by default
       terminate the process, since the terminal needs to be restored to a
       usable state before this happens. In this section, the signals that are
       trapped by default, and how gl_get_line() responds to them, is
       described. Changing these defaults is the topic of the following
       section.

       When the following subset of signals are caught, gl_get_line() first
       restores the terminal settings and signal handling to how they were
       before gl_get_line() was called, resends the signal, to allow the
       calling application's signal handlers to handle it, then if the process
       still exists, gl_get_line() returns NULL and sets errno as specified
       below.


        SIGINT  -  This signal is generated both by the keyboard
                   interrupt key (usually ^C), and the keyboard
                   break key.

                   errno=EINTR

        SIGHUP  -  This signal is generated when the controlling
                   terminal exits.

                   errno=ENOTTY

        SIGPIPE -  This signal is generated when a program attempts
                   to write to a pipe who's remote end isn't being
                   read by any process. This can happen for example
                   if you have called gl_change_terminal() to
                   redirect output to a pipe hidden under a pseudo
                   terminal.

                   errno=EPIPE

        SIGQUIT -  This signal is generated by the keyboard quit
                   key (usually ^\).

                   errno=EINTR

        SIGABRT -  This signal is generated by the standard C,
                   abort() function. By default it both
                   terminates the process and generates a core
                   dump.

                   errno=EINTR

        SIGTERM -  This is the default signal that the UN*X
                   kill command sends to processes.

                   errno=EINTR

       Note that in the case of all of the above signals, POSIX mandates that
       by default the process is terminated, with the addition of a core dump
       in the case of the SIGQUIT signal. In other words, if the calling
       application doesn't override the default handler by supplying its own
       signal handler, receipt of the corresponding signal will terminate the
       application before gl_get_line() returns.

       If gl_get_line() aborts with errno set to EINTR, you can find out what
       signal caused it to abort, by calling the following function.

         int gl_last_signal(const GetLine *gl);

       This returns the numeric code (eg. SIGINT) of the last signal that was
       received during the most recent call to gl_get_line(), or -1 if no
       signals were received.

       On systems that support it, when a SIGWINCH (window change) signal is
       received, gl_get_line() queries the terminal to find out its new size,
       redraws the current input line to accomodate the new size, then returns
       to waiting for keyboard input from the user. Unlike other signals, this
       signal isn't resent to the application.

       Finally, the following signals cause gl_get_line() to first restore the
       terminal and signal environment to that which prevailed before
       gl_get_line() was called, then resend the signal to the application. If
       the process still exists after the signal has been delivered, then
       gl_get_line() then re-establishes its own signal handlers, switches the
       terminal back to raw mode, redisplays the input line, and goes back to
       awaiting terminal input from the user.

        SIGCONT    -  This signal is generated when a suspended
                      process is resumed.

        SIGPOLL    -  On SVR4 systems, this signal notifies the
                      process of an asynchronous I/O event. Note
                      that under 4.3+BSD, SIGIO and SIGPOLL are
                      the same. On other systems, SIGIO is ignored
                      by default, so gl_get_line() doesn't
                      trap it by default.

        SIGPWR     -  This signal is generated when a power failure
                      occurs (presumably when the system is on a
                      UPS).

        SIGALRM    -  This signal is generated when a timer
                      expires.

        SIGUSR1    -  An application specific signal.

        SIGUSR2    -  Another application specific signal.

        SIGVTALRM  -  This signal is generated when a virtual
                      timer expires (see man setitimer(2)).

        SIGXCPU    -  This signal is generated when a process
                      exceeds its soft CPU time limit.

        SIGXFSZ    -  This signal is generated when a process
                      exceeds its soft file-size limit.

        SIGTSTP    -  This signal is generated by the terminal
                      suspend key, which is usually ^Z, or the
                      delayed terminal suspend key, which is
                      usually ^Y.

        SIGTTIN    -  This signal is generated if the program
                      attempts to read from the terminal while the
                      program is running in the background.

        SIGTTOU    -  This signal is generated if the program
                      attempts to write to the terminal while the
                      program is running in the background.


       Obviously not all of the above signals are supported on all systems, so
       code to support them is conditionally compiled into the tecla library.

       Note that if SIGKILL or SIGPOLL, which by definition can't be caught,
       or any of the hardware generated exception signals, such as SIGSEGV,
       SIGBUS and SIGFPE, are received and unhandled while gl_get_line() has
       the terminal in raw mode, the program will be terminated without the
       terminal having been restored to a usable state. In practice, job-
       control shells usually reset the terminal settings when a process
       relinquishes the controlling terminal, so this is only a problem with
       older shells.


CUSTOMIZED SIGNAL HANDLING
       The previous section listed the signals that gl_get_line() traps by
       default, and described how it responds to them. This section describes
       how to both add and remove signals from the list of trapped signals,
       and how to specify how gl_get_line() should respond to a given signal.

       If you don't need gl_get_line() to do anything in response to a signal
       that it normally traps, you can tell to gl_get_line() to ignore that
       signal by calling gl_ignore_signal().

         int gl_ignore_signal(GetLine *gl, int signo);

       The signo argument is the number of the signal (eg. SIGINT) that you
       want to have ignored. If the specified signal isn't currently one of
       those being trapped, this function does nothing.

       The gl_trap_signal() function allows you to either add a new signal to
       the list that gl_get_line() traps, or modify how it responds to a
       signal that it already traps.

         int gl_trap_signal(GetLine *gl, int signo, unsigned flags,
                            GlAfterSignal after, int errno_value);

       The signo argument is the number of the signal that you wish to have
       trapped. The flags argument is a set of flags which determine the
       environment in which the application's signal handler is invoked, the
       after argument tells gl_get_line() what to do after the application's
       signal handler returns, and errno_value tells gl_get_line() what to set
       errno to if told to abort.

       The flags argument is a bitwise OR of zero or more of the following
       enumerators:

         GLS_RESTORE_SIG  -  Restore the caller's signal
                             environment while handling the
                             signal.

         GLS_RESTORE_TTY  -  Restore the caller's terminal settings
                             while handling the signal.

         GLS_RESTORE_LINE -  Move the cursor to the start of the
                             line following the input line before
                             invoking the application's signal
                             handler.

         GLS_REDRAW_LINE  -  Redraw the input line when the
                             application's signal handler returns.

         GLS_UNBLOCK_SIG  -  Normally, if the calling program has
                             a signal blocked (man sigprocmask),
                             gl_get_line() does not trap that
                             signal. This flag tells gl_get_line()
                             to trap the signal and unblock it for
                             the duration of the call to
                             gl_get_line().

         GLS_DONT_FORWARD -  If this flag is included, the signal
                             will not be forwarded to the signal
                             handler of the calling program.

       Two commonly useful flag combinations are also enumerated as follows:

         GLS_RESTORE_ENV   = GLS_RESTORE_SIG | GLS_RESTORE_TTY |
                             GLS_REDRAW_LINE

         GLS_SUSPEND_INPUT = GLS_RESTORE_ENV | GLS_RESTORE_LINE


       If your signal handler, or the default system signal handler for this
       signal, if you haven't overridden it, never either writes to the
       terminal, nor suspends or terminates the calling program, then you can
       safely set the flags argument to 0.

       If your signal handler always writes to the terminal, reads from it, or
       suspends or terminates the program, you should specify the flags
       argument as GL_SUSPEND_INPUT, so that:

       1. The cursor doesn't get left in the middle of the input
          line.
       2. So that the user can type in input and have it echoed.
       3. So that you don't need to end each output line with
          \r\n, instead of just \n.

       The GL_RESTORE_ENV combination is the same as GL_SUSPEND_INPUT, except
       that it doesn't move the cursor, and if your signal handler doesn't
       read or write anything to the terminal, the user won't see any visible
       indication that a signal was caught. This can be useful if you have a
       signal handler that only occasionally writes to the terminal, where
       using GL_SUSPEND_LINE would cause the input line to be unnecessarily
       duplicated when nothing had been written to the terminal.  Such a
       signal handler, when it does write to the terminal, should be sure to
       start a new line at the start of its first write, by writing a new line
       before returning. If the signal arrives while the user is entering a
       line that only occupies a signal terminal line, or if the cursor is on
       the last terminal line of a longer input line, this will have the same
       effect as GL_SUSPEND_INPUT. Otherwise it will start writing on a line
       that already contains part of the displayed input line.  This doesn't
       do any harm, but it looks a bit ugly, which is why the GL_SUSPEND_INPUT
       combination is better if you know that you are always going to be
       writting to the terminal.

       The after argument, which determines what gl_get_line() does after the
       application's signal handler returns (if it returns), can take any one
       of the following values:

         GLS_RETURN   - Return the completed input line, just as
                        though the user had pressed the return
                        key.

         GLS_ABORT    - Cause gl_get_line() to abort. When
                        this happens, gl_get_line() returns
                        NULL, and a following call to
                        gl_return_status() will return
                        GLR_SIGNAL. Note that if the
                        application needs errno always to
                        have a meaningful value when
                        gl_get_line() returns NULL,
                        the callback function should set
                        errno appropriately.
         GLS_CONTINUE - Resume command line editing.

       The errno_value argument is intended to be combined with the GLS_ABORT
       option, telling gl_get_line() what to set the standard errno variable
       to before returning NULL to the calling program. It can also, however,
       be used with the GL_RETURN option, in case you wish to have a way to
       distinguish between an input line that was entered using the return
       key, and one that was entered by the receipt of a signal.


RELIABLE SIGNAL HANDLING
       Signal handling is suprisingly hard to do reliably without race
       conditions. In gl_get_line() a lot of care has been taken to allow
       applications to perform reliable signal handling around gl_get_line().
       This section explains how to make use of this.

       As an example of the problems that can arise if the application isn't
       written correctly, imagine that one's application has a SIGINT signal
       handler that sets a global flag. Now suppose that the application tests
       this flag just before invoking gl_get_line(). If a SIGINT signal
       happens to be received in the small window of time between the
       statement that tests the value of this flag, and the statement that
       calls gl_get_line(), then gl_get_line() will not see the signal, and
       will not be interrupted. As a result, the application won't be able to
       respond to the signal until the user gets around to finishing entering
       the input line and gl_get_line() returns. Depending on the application,
       this might or might not be a disaster, but at the very least it would
       puzzle the user.

       The way to avoid such problems is to do the following.

       1. If needed, use the gl_trap_signal() function to
          configure gl_get_line() to abort when important
          signals are caught.

       2. Configure gl_get_line() such that if any of the
          signals that it catches are blocked when
          gl_get_line() is called, they will be unblocked
          automatically during times when gl_get_line() is
          waiting for I/O. This can be done either
          on a per signal basis, by calling the
          gl_trap_signal() function, and specifying the
          GLS_UNBLOCK attribute of the signal, or globally by
          calling the gl_catch_blocked() function.


            void gl_catch_blocked(GetLine *gl);


          This function simply adds the GLS_UNBLOCK attribute
          to all of the signals that it is currently configured to
          trap.

       3. Just before calling gl_get_line(), block delivery
          of all of the signals that gl_get_line() is
          configured to trap. This can be done using the POSIX
          sigprocmask() function in conjunction with the
          gl_list_signals() function.


             int gl_list_signals(GetLine *gl, sigset_t *set);


          This function returns the set of signals that it is
          currently configured to catch in the set argument,
          which is in the form required by sigprocmask().

       4. In the example, one would now test the global flag that
          the signal handler sets, knowing that there is now no
          danger of this flag being set again until
          gl_get_line() unblocks its signals while performing
          I/O.

       5. Eventually gl_get_line() returns, either because
          a signal was caught, an error occurred, or the user
          finished entering their input line.

       6. Now one would check the global signal flag again, and if
          it is set, respond to it, and zero the flag.

       7. Use sigprocmask() to unblock the signals that were
          blocked in step 3.

       The same technique can be used around certain POSIX signal-aware
       functions, such as sigsetjmp() and sigsuspend(), and in particular, the
       former of these two functions can be used in conjunction with
       siglongjmp() to implement race-condition free signal handling around
       other long-running system calls. The way to do this, is explained next,
       by showing how gl_get_line() manages to reliably trap signals around
       calls to functions like read() and select() without race conditions.

       The first thing that gl_get_line() does, whenever it is called, is to
       use the POSIX sigprocmask() function to block the delivery of all of
       the signals that it is currently configured to catch. This is redundant
       if the application has already blocked them, but it does no harm. It
       undoes this step just before returning.

       Whenever gl_get_line() needs to call read() or select() to wait for
       input from the user, it first calls the POSIX sigsetjmp() function,
       being sure to specify a non-zero value for its savesigs argument.  The
       reason for the latter argument will become clear shortly.

       If sigsetjmp() returns zero, gl_get_line() then does the following.


       a. It uses the POSIX sigaction() function to register
          a temporary signal handler to all of the signals that it
          is configured to catch. This signal handler does two
          things.

          1. It records the number of the signal that was received
             in a file-scope variable.

          2. It then calls the POSIX siglongjmp()
             function using the buffer that was passed to
             sigsetjmp() for its first argument, and
             a non-zero value for its second argument.

          When this signal handler is registered, the sa_mask
          member of the struct sigaction act argument of the
          call to sigaction() is configured to contain all of
          the signals that gl_get_line() is catching. This
          ensures that only one signal will be caught at once by
          our signal handler, which in turn ensures that multiple
          instances of our signal handler don't tread on each
          other's toes.

       b. Now that the signal handler has been set up,
          gl_get_line() unblocks all of the signals that it
          is configured to catch.

       c. It then calls the read() or select() system
          calls to wait for keyboard input.

       d. If this system call returns (ie. no signal is received),
          gl_get_line() blocks delivery of the signals of
          interest again.

       e. It then reinstates the signal handlers that were
          displaced by the one that was just installed.


       Alternatively, if sigsetjmp() returns non-zero, this means that one of
       the signals being trapped was caught while the above steps were
       executing. When this happens, gl_get_line() does the following.

       First, note that when a call to siglongjmp() causes sigsetjmp() to
       return, provided that the savesigs argument of sigsetjmp() was non-
       zero, as specified above, the signal process mask is restored to how it
       was when sigsetjmp() was called. This is the important difference
       between sigsetjmp() and the older problematic setjmp(), and is the
       essential ingredient that makes it possible to avoid signal handling
       race conditions.  Because of this we are guaranteed that all of the
       signals that we blocked before calling sigsetjmp() are blocked again as
       soon as any signal is caught. The following statements, which are then
       executed, are thus guaranteed to be executed without any further
       signals being caught.

       1. If so instructed by the gl_get_line() configuration
          attributes of the signal that was caught,
          gl_get_line() restores the terminal attributes to
          the state that they had when gl_get_line() was
          called. This is particularly important for signals that
          suspend or terminate the process, since otherwise the
          terminal would be left in an unusable state.

       2. It then reinstates the application's signal handlers.

       3. Then it uses the C standard-library raise()
          function to re-send the application the signal that
          was caught.

       3. Next it unblocks delivery of the signal that we just
          sent. This results in the signal that was just sent
          via raise(), being caught by the application's
          original signal handler, which can now handle it as it
          sees fit.

       4. If the signal handler returns (ie. it doesn't terminate
          the process), gl_get_line() blocks delivery of the
          above signal again.

       5. It then undoes any actions performed in the first of the
          above steps, and redisplays the line, if the signal
          configuration calls for this.

       6. gl_get_line() then either resumes trying to
          read a character, or aborts, depending on the
          configuration of the signal that was caught.

       What the above steps do in essence is to take asynchronously delivered
       signals and handle them synchronously, one at a time, at a point in the
       code where gl_get_line() has complete control over its environment.


THE TERMINAL SIZE
       On most systems the combination of the TIOCGWINSZ ioctl and the
       SIGWINCH signal is used to maintain an accurate idea of the terminal
       size. The terminal size is newly queried every time that gl_get_line()
       is called and whenever a SIGWINCH signal is received.

       On the few systems where this mechanism isn't available, at startup
       new_GetLine() first looks for the LINES and COLUMNS environment
       variables.  If these aren't found, or they contain unusable values,
       then if a terminal information database like terminfo or termcap is
       available, the default size of the terminal is looked up in this
       database. If this too fails to provide the terminal size, a default
       size of 80 columns by 24 lines is used.

       Even on systems that do support ioctl(TIOCGWINSZ), if the terminal is
       on the other end of a serial line, the terminal driver generally has no
       way of detecting when a resize occurs or of querying what the current
       size is. In such cases no SIGWINCH is sent to the process, and the
       dimensions returned by ioctl(TIOCGWINSZ) aren't correct. The only way
       to handle such instances is to provide a way for the user to enter a
       command that tells the remote system what the new size is. This command
       would then call the gl_set_term_size() function to tell gl_get_line()
       about the change in size.


         int gl_set_term_size(GetLine *gl, int ncolumn, int nline);


       The ncolumn and nline arguments are used to specify the new dimensions
       of the terminal, and must not be less than 1. On systems that do
       support ioctl(TIOCGWINSZ), this function first calls ioctl(TIOCSWINSZ)
       to tell the terminal driver about the change in size. In non-blocking
       server-I/O mode, if a line is currently being input, the input line is
       then redrawn to accomodate the changed size. Finally the new values are
       recorded in gl for future use by gl_get_line().

       The gl_terminal_size() function allows you to query the current size of
       the terminal, and install an alternate fallback size for cases where
       the size isn't available.  Beware that the terminal size won't be
       available if reading from a pipe or a file, so the default values can
       be important even on systems that do support ways of finding out the
       terminal size.

         typedef struct {
           int nline;        /* The terminal has nline lines */
           int ncolumn;      /* The terminal has ncolumn columns */
         } GlTerminalSize;

         GlTerminalSize gl_terminal_size(GetLine *gl,
                                         int def_ncolumn,
                                         int def_nline);

       This function first updates gl_get_line()'s fallback terminal
       dimensions, then records its findings in the return value.

       The def_ncolumn and def_nline specify the default number of terminal
       columns and lines to use if the terminal size can't be determined via
       ioctl(TIOCGWINSZ) or environment variables.


HIDING WHAT YOU TYPE
       When entering sensitive information, such as passwords, it is best not
       to have the text that you are entering echoed on the terminal.
       Furthermore, such text should not be recorded in the history list,
       since somebody finding your terminal unattended could then recall it,
       or somebody snooping through your directories could see it in your
       history file. With this in mind, the gl_echo_mode() function allows you
       to toggle on and off the display and archival of any text that is
       subsequently entered in calls to gl_get_line().


         int gl_echo_mode(GetLine *gl, int enable);


       The enable argument specifies whether entered text should be visible or
       not. If it is 0, then subsequently entered lines will not be visible on
       the terminal, and will not be recorded in the history list. If it is 1,
       then subsequent input lines will be displayed as they are entered, and
       provided that history hasn't been turned off via a call to
       gl_toggle_history(), then they will also be archived in the history
       list. Finally, if the enable argument is -1, then the echoing mode is
       left unchanged, which allows you to non-destructively query the current
       setting via the return value. In all cases, the return value of the
       function is 0 if echoing was disabled before the function was called,
       and 1 if it was enabled.

       When echoing is turned off, note that although tab completion will
       invisibly complete your prefix as far as possible, ambiguous
       completions will not be displayed.


SINGLE CHARACTER QUERIES
       Using gl_get_line() to query the user for a single character reply, is
       inconvenient for the user, since they must hit the enter or return key
       before the character that they typed is returned to the program. Thus
       the gl_query_char() function has been provided for single character
       queries like this.


         int gl_query_char(GetLine *gl, const char *prompt,
                           char defchar);


       This function displays the specified prompt at the start of a new line,
       and waits for the user to type a character. When the user types a
       character, gl_query_char() displays it to the right of the prompt,
       starts a newline, then returns the character to the calling program.
       The return value of the function is the character that was typed. If
       the read had to be aborted for some reason, EOF is returned instead. In
       the latter case, the application can call the previously documented
       gl_return_status(), to find out what went wrong. This could, for
       example, have been the reception of a signal, or the optional
       inactivity timer going off.

       If the user simply hits enter, the value of the defchar argument is
       substituted. This means that when the user hits either newline or
       return, the character specified in defchar, is displayed after the
       prompt, as though the user had typed it, as well as being returned to
       the calling application. If such a replacement is not important, simply
       pass '0 as the value of defchar.

       If the entered character is an unprintable character, it is displayed
       symbolically. For example, control-A is displayed as ^A, and characters
       beyond 127 are displayed in octal, preceded by a backslash.

       As with gl_get_line(), echoing of the entered character can be disabled
       using the gl_echo_mode() function.

       If the calling process is suspended while waiting for the user to type
       their response, the cursor is moved to the line following the prompt
       line, then when the process resumes, the prompt is redisplayed, and
       gl_query_char() resumes waiting for the user to type a character.

       Note that in non-blocking server mode, (see gl_io_mode(3)), if an
       incomplete input line is in the process of being read when
       gl_query_char() is called, the partial input line is discarded, and
       erased from the terminal, before the new prompt is displayed. The next
       call to gl_get_line() will thus start editing a new line.


READING RAW CHARACTERS
       Whereas the gl_query_char() function visibly prompts the user for a
       character, and displays what they typed, the gl_read_char() function
       reads a signal character from the user, without writing anything to the
       terminal, or perturbing any incompletely entered input line. This means
       that it can be called not only from between calls to gl_get_line(), but
       also from callback functions that the application has registered to be
       called by gl_get_line().


         int gl_read_char(GetLine *gl);


       On success, the return value of gl_read_char() is the character that
       was read. On failure, EOF is returned, and the gl_return_status()
       function can be called to find out what went wrong. Possibilities
       include the optional inactivity timer going off, the receipt of a
       signal that is configured to abort gl_get_line(), or terminal I/O
       blocking, when in non-blocking server-I/O mode.

       Beware that certain keyboard keys, such as function keys, and cursor
       keys, usually generate at least 3 characters each, so a single call to
       gl_read_char() won't be enough to identify such keystrokes.


CLEARING THE TERMINAL
       The calling program can clear the terminal by calling
       gl_erase_terminal(). In non-blocking server-I/O mode, this function
       also arranges for the current input line to be redrawn from scratch
       when gl_get_line() is next called.


         int gl_erase_terminal(GetLine *gl);



DISPLAYING TEXT DYNAMICALLY
       Between calls to gl_get_line(), the gl_display_text() function provides
       a convenient way to display paragraphs of text, left-justified and
       split over one or more terminal lines according to the constraints of
       the current width of the terminal. Examples of the use of this function
       may be found in the demo programs, where it is used to display
       introductions. In those examples the advanced use of optional prefixes,
       suffixes and filled lines to draw a box around the text is also
       illustrated.


         int gl_display_text(GetLine *gl, int indentation,
                             const char *prefix,
                             const char *suffix, int fill_char,
                             int def_width, int start,
                             const char *string);

       If gl isn't currently connected to a terminal, for example if the
       output of a program that uses gl_get_line() is being piped to another
       program or redirected to a file, then the value of the def_width
       parameter is used as the terminal width.

       The indentation argument specifies the number of characters to use to
       indent each line of ouput. The fill_char argument specifies the
       character that will be used to perform this indentation.

       The prefix argument can either be NULL, or be a string to place at the
       beginning of each new line (after any indentation).  Similarly, the
       suffix argument can either be NULL, or be a string to place at the end
       of each line. The suffix is placed flush against the right edge of the
       terminal, and any space between its first character and the last word
       on that line is filled with the character specified via the fill_char
       argument.  Normally the fill-character is a space.

       The start argument tells gl_display_text() how many characters have
       already been written to the current terminal line, and thus tells it
       the starting column index of the cursor.  Since the return value of
       gl_display_text() is the ending column index of the cursor, by passing
       the return value of one call to the start argument of the next call, a
       paragraph that is broken between more than one string can be composed
       by calling gl_display_text() for each successive portion of the
       paragraph. Note that literal newline characters are necessary at the
       end of each paragraph to force a new line to be started.

       On error, gl_display_text() returns -1.


CALLBACK FUNCTION FACILITIES
       Unless otherwise stated, callback functions, such as tab completion
       callbacks and event callbacks should not call any functions in this
       module. The following functions, however, are designed specifically to
       be used by callback functions.

       Calling the gl_replace_prompt() function from a callback tells
       gl_get_line() to display a different prompt when the callback returns.
       Except in non-blocking server mode, it has no effect if used between
       calls to gl_get_line(). In non-blocking server mode (see the
       gl_io_mode(3) man page, when used between two calls to gl_get_line()
       that are operating on the same input line, the current input line will
       be re-drawn with the new prompt on the following call to gl_get_line().


         void gl_replace_prompt(GetLine *gl, const char *prompt);



INTERNATIONAL CHARACTER SETS
       Since libtecla version 1.4.0, gl_get_line() has been 8-bit clean. This
       means that all 8-bit characters that are printable in the user's
       current locale are now displayed verbatim and included in the returned
       input line.  Assuming that the calling program correctly contains a
       call like the following,

         setlocale(LC_CTYPE, "");

       then the current locale is determined by the first of the environment
       variables LC_CTYPE, LC_ALL, and LANG, that is found to contain a valid
       locale name. If none of these variables are defined, or the program
       neglects to call setlocale, then the default C locale is used, which is
       US 7-bit ASCII. On most unix-like platforms, you can get a list of
       valid locales by typing the command:

         locale -a

       at the shell prompt. Further documentation on how the user can make use
       of this to enter international characters can be found in the tecla(7)
       man page.


THREAD SAFETY
       In a multi-threaded program, you should use the libtecla_r.a version of
       the library. This uses reentrant versions of system functions, where
       available. Unfortunately neither terminfo nor termcap were designed to
       be reentrant, so you can't safely use the functions of the getline
       module in multiple threads (you can use the separate file-expansion and
       word-completion modules in multiple threads, see the corresponding man
       pages for details). However due to the use of POSIX reentrant functions
       for looking up home directories etc, it is safe to use this module from
       a single thread of a multi-threaded program, provided that your other
       threads don't use any termcap or terminfo functions.


FILES
       libtecla.a      -    The tecla library
       libtecla.h      -    The tecla header file.
       ~/.teclarc      -    The personal tecla customization file.


SEE ALSO
       libtecla(3), gl_io_mode(3), tecla(7), ef_expand_file(3),
       cpl_complete_word(3), pca_lookup_file(3)


AUTHOR
       Martin Shepherd  (mcs@astro.caltech.edu)




                                                                gl_get_line(3)