Manual Page

    gl_get_line                          gl_get_line

## NAME

           gl_get_line,    new_GetLine,    del_GetLine,   gl_customize_completion,
           gl_change_terminal, gl_configure_getline, gl_load_history, gl_save_his-
           tory,   gl_group_history,   gl_show_history,  gl_watch_fd,  gl_inactiv-
           ity_timeout,  gl_terminal_size,  gl_set_term_size,   gl_resize_history,
           gl_limit_history,  gl_clear_history,  gl_toggle_history, gl_lookup_his-
           tory,  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_his-
           tory,  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 libte-
           cla(@LIBR_MANEXT@) man page). If the user is typing at a terminal, each
           call prompts them for an line of input, then provides interactive edit-
           ing 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 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() func-
           tion 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_Get-
           Line(),  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  num-
           ber  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 posi-
           tioned  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  termi-
           nal,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 informa-
           tion 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 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 lit-
           erally, without any special interpretation of the characters within it,
           the  gl_prompt_style()  function can be used to enable optional format-
           ting 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 informa-
           tion 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 config-
           uration 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 configura-
           tion 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  configura-
           tion  commands from an optional .teclarc file in the user's home direc-
           tory. Note that the arguments are listed in ascending order  of  prior-
           ity,  with  the  contents  of app_string being potentially overriden by
           commands in app_file, and commands in app_file potentially being  over-
           riden 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 config-
           uration 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_config-
           ure_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 arbi-
           trary, 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 call-
           back functions. The declaration and responsibilities of callback  func-
           tions  are  described  in depth in the cpl_complete_word
           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  possi-
           ble  completions  of this word, and record them one by one in the Word-
           Completion object that is passed to it as an argument, by  calling  the
           cpl_add_completion()  function. If the callback function wishes to pro-
           vide 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_com-
           plete_word(@FUNC_MANEXT@) 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_comple-
           tion_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_com-
           plete_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  display-
           ing  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 addi-
           tional action won't be known, and any reference to it in the configura-
           tion 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 hav-
           ing  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 enter-
           ing 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. Nomi-
           nally 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  argu-
           ment.  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 restor-
           ing the normal mapping of \n to \r\n, and, when in server mode, restor-
           ing the normal blocking form of terminal I/O. Having called this  func-
           tion, the action function can read from and write to the terminal with-
           out 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  appli-
           cation,  and subsequently restore them when you next start the applica-
           tion, 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 pre-
           fixes 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 his-
           tory 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-exis-
           tence of a file to be an error.

## MULTIPLE HISTORY LISTS

           If your application uses a single GetLine object for entering many dif-
           ferent 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  identi-
           fier  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 pro-
           gram  invokations,  so you should choose fixed identifiers for the dif-
           ferent 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  his-
           tory  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 char-
           acters which are written verbatim, interleaved with any of the  follow-
           ing 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 succes-
           sively 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. There-
           fore 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 applica-
           tions, 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 termi-
           nated 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  call-
           ing 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 lit-
           tle 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 origi-
           nally 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 spec-
           ify 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_his-
           tory()), 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-bind-
           ings 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 termi-
           nal.  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 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  argu-
           ment  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  call-
           back  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  sig-
           nals,  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_nor-
           mal_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  func-
           tion  which  occasionally and u0prisicre-enabledesbeforee themicallback
           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 follow-
           ing  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), 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  sig-
           nals,  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_nor-
           mal_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  func-
           tion  which  occasionally and u0prisicre-enabledesbeforee themicallback
           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 compo-
           nent, 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 call-
           back 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 ter-
           minate 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 sec-
           tion.

           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 call-
           ing  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 sig-
           nals 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-con-
           trol shells usually reset the terminal settings when a  process  relin-
           quishes  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 sig-
           nal 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 envi-
           ronment 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 termi-
           nal, 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  argu-
           ment 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 sig-
           nal 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 writ-
           ting 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  condi-
           tions.  In gl_get_line() a lot of care has been taken to allow applica-
           tions 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  hap-
           pens  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  func-
           tions,  such  as  sigsetjmp()  and sigsuspend(), and in particular, the
           former of these two functions can be  used  in  conjunction  with  sig-
           longjmp() 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 execut-
           ing. 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  sig-
           nals 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 SIG-
           WINCH 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 vari-
           ables.  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 sup-
           port 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 dimen-
           sions, 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.  Fur-
           thermore, 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 tog-
           gle on and off the display and archival of  any  text  that  is  subse-
           quently 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_tog-
           gle_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 set-
           ting  via the return value. In all cases, the return value of the func-
           tion 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  comple-
           tions 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 char-
           acter, 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_sta-
           tus(), 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),
           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() func-
           tion  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_termi-
           nal(). 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 intro-
           ductions. In those examples the advanced use of optional prefixes, suf-
           fixes  and  filled  lines  to draw a box around the text is also illus-
           trated.

             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  out-
           put of a program that uses gl_get_line() is being piped to another pro-
           gram 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 charac-
           ter 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 para-
           graph. 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 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  cur-
           rent  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 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 mod-
           ule 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, gl_io_mode, tecla, ef_expand_file,
           cpl_complete_word, pca_lookup_file

## AUTHOR

           Martin Shepherd  (mcs@astro.caltech.edu)

                                                        gl_get_line