Tribblix: manual page: gl_get

GL_GET_LINE(3TECLA) Interactive Command-line Input Library Functions

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_register_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

cc [ flag... ] file... -ltecla [ library... ]
#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);

int gl_erase_terminal(GetLine *gl);

DESCRIPTION

The gl_get_line() function is part of the libtecla(3LIB) library. If
the user is typing at a terminal, each call prompts them for a 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 on the
tecla(7) manual page.

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 new_GetLine() 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
by 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 description of the gl_limit_history() function. If
you do not 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.

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

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 null-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, 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 with the start_line argument. You
can then specify which character of this line the cursor is initially
positioned over by 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 do not want to preload the line in this manner, send
start_line as NULL, and set start_pos to -1.

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 freed 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(3C). When stdin is not connected to a
terminal, gl_get_line() simply calls fgets().

The Return Status Of gl_get_line()
The gl_get_line() function has two possible return values: a pointer
to the completed input line, or NULL. Additional information about
what caused gl_get_line() to return is available both by inspecting
errno and by calling the gl_return_status() function.

The following are the possible enumerated values returned by
gl_return_status():

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

GLR_BLOCKED
The gl_get_line() function 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(3TECLA).

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 callback 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 is
not sufficient to explain what happened, you can use the
gl_error_message() function to request a description of the last
error that occurred.

The return value of gl_error_message() is a pointer to the message
that occurred. If the buff argument is NULL, this will be a pointer
to a buffer within gl whose 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 will not 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.

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 does not 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

By default users have the option of configuring the behavior of
gl_get_line() with 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 gl_configure_getline() function allows the application to
control where configuration information is read from.

The gl_configure_getline() function 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 configurations 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 path name of a user-specific configuration file, such as
~/.teclarc. For example, in the 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(1) 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
overridden by commands in app_file, and commands in app_file
potentially being overridden 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 file names specified with 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. The origin argument specifies the priority
of the binding, according to whom 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 with either another call 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 with a
call to the gl_customize_completion() function.

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 on the
cpl_complete_word(3TECLA) manual page.

The callback function is responsible for looking backwards in the
input line 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 wants to
provide filename completion in addition to its own specific
completions, it has the option of itself calling the builtin filename
completion callback. This is also documented in the
cpl_complete_word(3TECLA) manual page.

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

If 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 notify 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.

The data and match_fn arguments are as described on the
cpl_complete_word(3TECLA) manual 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 invocations 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 key sequence 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,
you should call gl_completion_action() to install a given action for
the first time 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 will not 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
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.

The gl_register_action() 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 wants
to have passed to the action function, fn, whenever that function is
invoked.

The action function, fn, should be declared using the GL_ACTION_FN()
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 does not 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 because the rules dictated by various modes (such as vi
mode versus emacs mode, no-echo mode, and insert mode versus
overstrike mode) 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 dilemma 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 is not 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 proceed.

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-
sequence. 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 gl_save_history() and gl_load_history() functions
are provided.

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, additional information about it, such
as its nesting level and when it was entered by the user, 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 argument 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 functions 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 might want 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().

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 invocations, 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(). This
function 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 only those of
the currently selected history group are displayed. The format string
argument, fmt, determines how the line is displayed. This can contain
arbitrary characters which are written verbatim, interleaved with any
of the following format directives:

%D
The date on which the line was originally entered, formatted
like 2001-11-20.

%T
The time of day when the line was entered, formatted like
23:59:59.

%N
The sequential entry number of the line in the history buffer.

%G
The number of the history group which the line belongs to.

%%
A literal % character.

%H
The history line itself.

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

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

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

Looking Up History

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

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. 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.

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.

If this function is called with its enable argument set to 0,
gl_get_line() will not 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.

The line argument specifies the line to be added to the history list.
This must be a normal '\0 ' 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 '\0 ' terminator. If the line is longer
than the maximum input line length that was specified when
new_GetLine() was called, it will be truncated to the actual
gl_get_line() line length when the line is recalled.

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 could not 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.

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 20 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 accommodate 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.

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.

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.

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. 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. 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.

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.

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 with 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 do not 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(3C) function.

External Event Handling

By default, gl_get_line() does not 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.

o Use the functions described in the gl_io_mode(3TECLA)
manual 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().

o 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 or
pipes, 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. If this returns non-zero, then it
means that either your arguments are invalid, or that this facility
is not 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.

Your callback should not try to read from the terminal, which is left
in raw mode as far as input is concerned. You can write to the
terminal as usual, since features like conversion of newline to
carriage-return/linefeed are re-enabled while the callback is
running. If your callback function does write to the terminal, be
sure to output a newline first, and when your callback returns, tell
gl_get_line() that the input line needs to be redrawn, by returning
the GLFD_REFRESH status code.

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

Setting An Inactivity Timeout

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().

The timeout is specified in the form of an integral number of seconds
and an integral number of nanoseconds, specified by the sec and nsec
arguments, respectively. Subsequently, whenever no activity is seen
for this time period, the function specified by the callback argument
is called. The data argument of gl_inactivity_timeout() is passed 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 proceed 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.

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(3TECLA)), gl_get_line() acts 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.

Your callback should not try to read from the terminal, which is left
in raw mode as far as input is concerned. You can however write to
the terminal as usual, since features like conversion of newline to
carriage-return/linefeed are re-enabled while the callback is
running. If your callback function does write to the terminal, be
sure to output a newline first, and when your callback returns, tell
gl_get_line() that the input line needs to be redrawn, by returning
the GLTO_REFRESH status code.

Finally, note that although the timeout arguments include a
nanosecond 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 many
systems. If this would be a problem, you should base your timeout
selection on the actual resolution of the host clock (for example, 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 that would by
default terminate the process, since the terminal needs to be
restored to a usable state before this happens. This section
describes the signals that are trapped by default and how
gl_get_line() responds to them. 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, 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. The errno
value is EINTR.

SIGHUP
This signal is generated when the controlling terminal
exits. The errno value is ENOTTY.

SIGPIPE
This signal is generated when a program attempts to write
to a pipe whose remote end is not 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. The errno value is EPIPE.

SIGQUIT
This signal is generated by the keyboard quit key (usually
^\). The errno value is EINTR.

SIGABRT
This signal is generated by the standard C, abort
function. By default it both terminates the process and
generates a core dump. The errno value is EINTR.

SIGTERM
This is the default signal that the UNIX kill command
sends to processes. The errno value is 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 does not 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 gl_last_signal()
function. This returns the numeric code (for example, 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 accommodate the new size,
then returns to waiting for keyboard input from the user. Unlike
other signals, this signal is not 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() does not 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 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 cannot 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 do not 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().

The signo argument is the number of the signal (for example, SIGINT)
that you want to have ignored. If the specified signal is not
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.

The signo argument is the number of the signal that you want to have
trapped. The flags argument is a set of flags that 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. The 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 (see sigprocmask(2)), 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 have not 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.

o The cursor does not get left in the middle of the input
line.

o So that the user can type in input and have it echoed.

o So that you do not 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 does not move the cursor. If your signal handler does
not read or write anything to the terminal, the user will not see any
visible indication that a signal was caught. This can be useful if
you have a signal handler that only occasionally writes to the
terminal, where using GL_SUSPEND_LINE would cause the input line to
be unnecessarily duplicated when nothing had been written to the
terminal. Such a signal handler, when it does write to the terminal,
should be sure to start a new line at the start of its first write,
by writing a new line before returning. If the signal arrives while
the user is entering a line that only occupies a signal terminal
line, or if the cursor is on the last terminal line of a longer input
line, this will have the same effect as GL_SUSPEND_INPUT. Otherwise
it will start writing on a line that already contains part of the
displayed input line. This does not 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 writing 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
want 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 surprisingly 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 is
not 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
will not 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. 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. 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(2).

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(3C) and sigsuspend(2), and in
particular, the former of these two functions can be used in
conjunction with siglongjmp(3C) to implement race-condition free
signal handling around other long-running system calls. The
gl_get_line() function manages to reliably trap signals around calls
to functions like read(2) and select(3C) without race conditions.

The gl_get_line() function first uses 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 savemask argument.

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

1. It uses the POSIX sigaction(2) function to register a
temporary signal handler to all of the signals that it is
configured to catch. This signal handler does two things.

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

b. 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 do not tread on each other's
toes.

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

3. It then calls the read() or select() function to wait for
keyboard input.

4. If this function returns (that is, no signal is received),
gl_get_line() blocks delivery of the signals of interest
again.

5. 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 savemask argument of sigsetjmp() was non-
zero, 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(3C), 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(3C) function to
re-send the application the signal that was caught.

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

5. If the signal handler returns (that is, it does not
terminate the process), gl_get_line() blocks delivery of
the above signal again.

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

7. 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 is not available, at startup
new_GetLine() first looks for the LINES and COLUMNS environment
variables. If these are not 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) are not 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.

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 accommodate 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 is not available. Beware that the terminal size will
not 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.

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 arguments specify the default number of
terminal columns and lines to use if the terminal size cannot be
determined by 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().

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 has not been turned off
with 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 through 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.

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 '\n' 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, 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().

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 three characters each, so a single
call to gl_read_char() will not 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.

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.

If gl is not 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 output. The fill_char argument specifies the
character that will be used to perform this indentation.

The prefix argument can be either NULL or a string to place at the
beginning of each new line (after any indentation). Similarly, the
suffix argument can be either NULL or 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 by 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,
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().

International Character Sets

Since libtecla(3LIB) 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(3C), 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

Unfortunately neither terminfo nor termcap were designed to be
reentrant, so you cannot 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, it is safe
to use this module from a single thread of a multi-threaded program,
provided that your other threads do not use any termcap or terminfo
functions.

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