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gl_get_line

gl_get_line

Section: C Library Functions (3)
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NAME

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

SYNOPSIS

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

GetLine *new_GetLine(size_t linelen, size_t histlen);

GetLine *del_GetLine(GetLine *gl);

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

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

int gl_read_char(GetLine *gl);

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

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

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

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

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

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

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

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

int gl_group_history(GetLine *gl, unsigned stream);

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

int gl_resize_history(GetLine *gl, size_t bufsize);

void gl_limit_history(GetLine *gl, int max_lines);

void gl_clear_history(GetLine *gl, int all_groups);

void gl_toggle_history(GetLine *gl, int enable);

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

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

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

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

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

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

void gl_echo_mode(GetLine *gl, int enable);

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

void gl_prompt_style(GetLine *gl, GlPromptStyle style);

int gl_ignore_signal(GetLine *gl, int signo);

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

int gl_last_signal(GetLine *gl);

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

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

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

GlReturnStatus gl_return_status(GetLine *gl);

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

void gl_catch_blocked(GetLine *gl);

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

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

int gl_automatic_history(GetLine *gl, int enable);

 

DESCRIPTION

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

 

AN EXAMPLE

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

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

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

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

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

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

    gl = del_GetLine(gl);
    return 0;
  }

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

 

THE FUNCTIONS USED IN THE EXAMPLE

The descriptions of the functions used in the example are as follows:

  GetLine *new_GetLine(size_t linelen, size_t histlen)

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

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

  GetLine *del_GetLine(GetLine *gl)

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

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

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

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

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

 

THE RETURN STATUS OF GL_GET_LINE

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

  GlReturnStatus gl_return_status(GetLine *gl);

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

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

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

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

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

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

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

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

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

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

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

 

OPTIONAL PROMPT FORMATTING

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

  void gl_prompt_style(GetLine *gl, GlPromptStyle style);

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

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

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

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

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

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

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

 

ALTERNATE CONFIGURATION SOURCES

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

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

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

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

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

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

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

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

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

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

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

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

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

 

CUSTOMIZED WORD COMPLETION

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

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

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

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

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

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

 

ADDING COMPLETION ACTIONS

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

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

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

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

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

 

DEFINING CUSTOM ACTIONS

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

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

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

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

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

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

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

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

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

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

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

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

 

HISTORY FILES

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

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

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

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

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

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

 

MULTIPLE HISTORY LISTS

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

  int gl_group_history(GetLine *gl, unsigned id);

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

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

 

DISPLAYING HISTORY

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

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

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

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

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

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

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

 

LOOKING UP HISTORY

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

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

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

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

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

 

MANUAL HISTORY ARCHIVAL

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

  int gl_automatic_history(GetLine *gl, int enable);

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

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

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

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

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

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

 

MISCELLANEOUS HISTORY CONFIGURATION

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

  int gl_resize_history(GetLine *gl, size_t histlen);

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

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

  void gl_limit_history(GetLine *gl, int max_lines);

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

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

  void gl_clear_history(GetLine *gl, int all_groups);

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

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

  void gl_toggle_history(GetLine *gl, int enable);

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

 

QUERYING HISTORY INFORMATION

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

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

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

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

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

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

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

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

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

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

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

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

 

CHANGING TERMINALS

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

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

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

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

 

EXTERNAL EVENT HANDLING

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

 

SETTING AN INACTIVITY TIMEOUT

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

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

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

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

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

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

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

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

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

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

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

 

SIGNAL HANDLING DEFAULTS

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

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

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

            errno=EINTR

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

            errno=ENOTTY

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

            errno=EPIPE

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

            errno=EINTR

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

            errno=EINTR

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

            errno=EINTR

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

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

  int gl_last_signal(const GetLine *gl);

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

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

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

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

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

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

 SIGALRM    -  This signal is generated when a timer
               expires.

 SIGUSR1    -  An application specific signal.

 SIGUSR2    -  Another application specific signal.

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

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

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

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

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

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

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

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

 

CUSTOMIZED SIGNAL HANDLING

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

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

  int gl_ignore_signal(GetLine *gl, int signo);

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

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

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

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

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

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

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

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

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

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

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

Two commonly useful flag combinations are also enumerated as follows:

  GLS_RESTORE_ENV   = GLS_RESTORE_SIG | GLS_RESTORE_TTY |
                      GLS_REDRAW_LINE

  GLS_SUSPEND_INPUT = GLS_RESTORE_ENV | GLS_RESTORE_LINE

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

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

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

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

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

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

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

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

 

RELIABLE SIGNAL HANDLING

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

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

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

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

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

     void gl_catch_blocked(GetLine *gl);


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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

 

THE TERMINAL SIZE

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

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

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

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

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

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

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

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

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

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

 

HIDING WHAT YOU TYPE

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

  int gl_echo_mode(GetLine *gl, int enable);

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

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

 

SINGLE CHARACTER QUERIES

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

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

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

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

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

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

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

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

 

READING RAW CHARACTERS

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

  int gl_read_char(GetLine *gl);

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

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

 

CLEARING THE TERMINAL

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

  int gl_erase_terminal(GetLine *gl);

 

DISPLAYING TEXT DYNAMICALLY

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

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

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

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

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

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

On error, gl_display_text() returns -1.

 

CALLBACK FUNCTION FACILITIES

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

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

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

 

INTERNATIONAL CHARACTER SETS

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

  setlocale(LC_CTYPE, "");

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

  locale -a

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

 

THREAD SAFETY

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

 

FILES

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

 

SEE ALSO

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

 

AUTHOR

Martin Shepherd (mcs@astro.caltech.edu)


 

Index

NAME
SYNOPSIS
DESCRIPTION
AN EXAMPLE
THE FUNCTIONS USED IN THE EXAMPLE
THE RETURN STATUS OF GL_GET_LINE
OPTIONAL PROMPT FORMATTING
ALTERNATE CONFIGURATION SOURCES
CUSTOMIZED WORD COMPLETION
ADDING COMPLETION ACTIONS
DEFINING CUSTOM ACTIONS
HISTORY FILES
MULTIPLE HISTORY LISTS
DISPLAYING HISTORY
LOOKING UP HISTORY
MANUAL HISTORY ARCHIVAL
MISCELLANEOUS HISTORY CONFIGURATION
QUERYING HISTORY INFORMATION
CHANGING TERMINALS
EXTERNAL EVENT HANDLING
SETTING AN INACTIVITY TIMEOUT
SIGNAL HANDLING DEFAULTS
CUSTOMIZED SIGNAL HANDLING
RELIABLE SIGNAL HANDLING
THE TERMINAL SIZE
HIDING WHAT YOU TYPE
SINGLE CHARACTER QUERIES
READING RAW CHARACTERS
CLEARING THE TERMINAL
DISPLAYING TEXT DYNAMICALLY
CALLBACK FUNCTION FACILITIES
INTERNATIONAL CHARACTER SETS
THREAD SAFETY
FILES
SEE ALSO
AUTHOR

This document was created by man2html, using the manual pages.
Time: 07:56:16 GMT, June 15, 2011