illumos: 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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