gl_get_line, new_GetLine, del_GetLine, gl_customize_completion,
gl_change_terminal, gl_configure_getline, gl_load_history, gl_save_his‐
tory, gl_group_history, gl_show_history, gl_watch_fd, gl_inactiv‐
ity_timeout, gl_terminal_size, gl_set_term_size, gl_resize_history,
gl_limit_history, gl_clear_history, gl_toggle_history, gl_lookup_his‐
tory, gl_state_of_history, gl_range_of_history, gl_size_of_history,
gl_echo_mode, gl_replace_prompt, gl_prompt_style, gl_ignore_signal,
gl_trap_signal, gl_last_signal, gl_completion_action, gl_regis‐
ter_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
cc [ flag... ] file... -ltecla [ library... ]
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,
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 gl_set_term_size(GetLine *gl, int ncolumn, int nline);
int gl_lookup_history(GetLine *gl, unsigned long id,
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);
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 an line of
input, then provides interactive editing facilities, similar to those
of the UNIX tcsh shell. In addition to simple command-line editing, it
supports recall of previously entered command lines, TAB completion of
file names, and in-line wild-card expansion of filenames. Documentation
of both the user-level command-line editing features and all user con‐
figuration options can be found on the tecla(5) manual page.
The following shows a complete example of how to use the gl_get_line()
function to get input from the user:
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);
while((line=gl_get_line(gl, "$ ", NULL, -1)) != NULL &&
strcmp(line, "exit\n") != 0)
printf("You typed: %s\n", line);
gl = del_GetLine(gl);
In the example, first the resources needed by the gl_get_line() func‐
tion are created by calling new_GetLine(). This allocates the memory
used in subsequent calls to the gl_get_line() function, including the
history buffer for recording previously entered lines. Then one or more
lines are read from the user, until either an error occurs, or the user
types exit. Then finally the resources that were allocated by new_Get‐
Line(), are returned to the system by calling del_GetLine(). Note the
use of the NULL return value of del_GetLine() to make gl NULL. This is
a safety precaution. If the program subsequently attempts to pass gl to
gl_get_line(), said function will complain, and return an error,
instead of attempting to use the deleted resource object.
The Functions Used In The Example
The 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 his‐
tory 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 descrip‐
tion 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 under‐
lining, 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 posi‐
tioned 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
The last call to gl_get_line() successfully returned a
completed input line.
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
A signal was caught by gl_get_line() that had an after-
signal disposition of GLS_ABORT. See gl_trap_signal().
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.
An application I/O callback returned GLFD_ABORT. Ssee
End of file reached. This can happen when input is com‐
ing 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.
An unexpected error caused gl_get_line() to abort (con‐
sult 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' termina‐
tor, 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 trun‐
cated to fit.
Optional Prompt Formatting
Whereas by default the prompt string that you specify is displayed lit‐
erally without any special interpretation of the characters within it,
the gl_prompt_style() function can be used to enable optional format‐
ting directives within the prompt.
The style argument, which specifies the formatting style, can take any
of the following values:
In this style, the formatting directives described
below, when included in prompt strings, are inter‐
preted as follows:
Display subsequent characters with a bold
Stop displaying characters with the bold
Make subsequent characters flash.
Turn off flashing characters.
Underline subsequent characters.
Stop underlining characters.
Switch to a pale (half brightness) font.
Stop using the pale font.
Highlight subsequent characters (also known
as standout mode).
Stop highlighting characters.
Turn on reverse video.
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.
In this style, the prompt string is printed liter‐
ally. 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 configura‐
tion 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 par‐
ticularly true of embedded software, where there's no filesystem to
read a configuration file from, and also in applications where a radi‐
cally 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 informa‐
tion 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 configura‐
tion 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",
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 sys‐
tem-wide configuration commands from an optional file called
/usr/share/myapp/teclarc, then finally reads user-specific configura‐
tion commands from an optional .teclarc file in the user's home direc‐
tory. Note that the arguments are listed in ascending order of prior‐
ity, with the contents of app_string being potentially over riden by
commands in app_file, and commands in app_file potentially being over‐
riden by commands in user_file.
You can call this function as many times as needed, the results being
cumulative, but note that copies of any 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 config‐
uration 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.
The user requested this key-binding.
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_con‐
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 arbi‐
trary, application-specific information to the callback function. This
is passed to the callback every time that it is called. It might for
example point to the symbol table from which possible completions are
to be sought. The match_fn argument specifies the callback function to
be called. The CplMatchFn function type is defined in <libtecla.h>, as
is a CPL_MATCH_FN() macro that you can use to declare and prototype
callback functions. The declaration and responsibilities of callback
functions are described in depth on the cpl_complete_word(3TECLA) man‐
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 also is
documented on the cpl_complete_word(3TECLA) manual page.
If you would like gl_get_line() to return the current input line when a
successful completion is been made, you can arrange this when you call
cpl_add_completion() by making the last character of the continuation
suffix a newline character. The input line will be updated to display
the completion, together with any contiuation 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_comple‐
The data and match_fn arguments are as described on the cpl_com‐
plete_word(3TECLA) manual page, and specify the callback function that
should be invoked to identify possible completions. The list_only argu‐
ment determines whether the action that is being defined should attempt
to complete the word as far as possible in the input line before dis‐
playing 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 func‐
tion 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
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
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 enter‐
ing 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
A copy of the current input line is passed in the read-only line argu‐
ment. The current cursor position within this string is given by the
index contained in the curpos argument. Note that direct manipulation
of the input line and the cursor position is not permitted 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 poten‐
tial solution to this dilemma would be to allow the action function to
edit the line using the existing editing actions. This is currently
If the action function wishes to write text to the terminal without
this getting mixed up with the displayed text of the input line, or
read from the terminal without having to handle raw terminal I/O, then
before doing either of these operations, it must temporarily suspend
line editing by calling the gl_normal_io() function. This function
flushes any pending output to the terminal, moves the cursor to the
start of the line that follows the last terminal line of the input
line, then restores the terminal to a state that is suitable for use
with the C stdio facilities. The latter includes such things as restor‐
ing the normal mapping of \n to \r\n, and, when in server mode, restor‐
ing the normal blocking form of terminal I/O. Having called this func‐
tion, the action function can read from and write to the terminal with‐
out the fear of creating a mess. It 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.
if(gl_normal_io(gl)) /* Temporarily suspend editing */
Action functions must return one of the following values, to tell
gl_get_line() how to proceed.
Cause gl_get_line() to return NULL.
Cause gl_get_line() to return the completed input line
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().
To save the contents of the history buffer before quitting your appli‐
cation and subsequently restore them when you next start the applica‐
tion, the gl_save_history() and gl_load_history() functions are pro‐
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 com‐
ment 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 fuctions return non-zero on error, after writing an error message
to stderr. Note that gl_load_history() does not consider the non-exis‐
tence of a file to be an error.
Multiple History Lists
If your application uses a single GetLine object for entering many dif‐
ferent types of input lines, you might want gl_get_line() to distin‐
guish 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
The integer identifier ID can be any number chosen by the application,
but note that gl_save_history() and gl_load_history() preserve the
association between identifiers and historical input lines between pro‐
gram invocations, so you should choose fixed identifiers for the dif‐
ferent types of input line used by your application.
Whenever gl_get_line() appends a new input line to the history list,
the current history identifier is recorded with it, and when it is
asked to recall a historical input line, it only recalls lines that are
marked with the current identifier.
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 his‐
tory 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 char‐
acters which are written verbatim, interleaved with any of the follow‐
ing format directives:
The date on which the line was originally entered, formatted like
The time of day when the line was entered, formatted like
The sequential entry number of the line in the history buffer.
The number of the history group which the line belongs to.
A literal % character.
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
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 succes‐
sively higher numbers. Note that the range of lines currently preserved
in the history list can be queried by calling the gl_range_of_history()
function. If the requested line is in the history list, the details of
the line are recorded in the variable pointed to by the hline argument,
and 1 is returned. Otherwise 0 is returned, and the variable pointed
to by hline is left unchanged.
Beware that the string returned in hline->line is part of the history
buffer, so it must not be modified by the caller, and will be recycled
on the next call to any function that takes gl as its argument. There‐
fore you should make a private copy of this string if you need to keep
Manual History Archival
By default, whenever a line is entered by the user, it is automatically
appended to the history list, just before gl_get_line() returns the
line to the caller. This is convenient for the majority of applica‐
tions, but there are also applications that need finer-grained control
over what gets added to the history list. In such cases, the automatic
addition of entered lines to the history list can be turned off by
calling the gl_automatic_history() function.
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 termi‐
nated 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
If successful, gl_append_history() returns 0. Otherwise it returns non-
zero and sets errno to one of the following values.
One of the arguments passed to gl_append_history() was NULL.
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 call‐
ing gl_error_message(). Note that after such an error, the history list
remains in a valid state to receive new history lines, so there is lit‐
tle harm in simply ignoring the return status of gl_append_history().
Miscellaneous History Configuration
If you wish to change the size of the history buffer that was origi‐
nally specified in the call to new_GetLine(), you can do so with the
The histlen argument specifies the new size in bytes, and if you spec‐
ify this as 0, the buffer will be deleted.
As mentioned in the discussion of new_GetLine(), the number of lines
that can be stored in the history buffer, depends on the lengths of the
individual lines. For example, a 1000 byte buffer could equally store
10 lines of average length 100 bytes, or 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
accomodate the number of lines currently stored in the buffer. To place
an upper limit on the number of lines in the buffer, and thus a ceiling
on the amount of memory used in this list, you can call the
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()
The all_groups argument tells the function whether to delete just the
lines associated with the current history group (see gl_group_his‐
tory()) or all historical lines in the buffer.
The gl_toggle_history() function allows you to toggle history on and
off without losing the current contents of the history list.
Setting the enable argument to 0 turns off the history mechanism, and
setting it to 1 turns it back on. When history is turned off, no new
lines will be added to the history list, and history lookup key-bind‐
ings will act as though there is nothing in the history buffer.
Querying History Information
The configured state of the history list can be queried with the
gl_history_state() function. 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 vari‐
able 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. Oth‐
erwise 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.
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 termi‐
nal. new_GetLine() for example, passes the return value of
getenv("TERM") in this argument. Note that if one or both of input_fp
and output_fp 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
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
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, incre‐
mental 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 argu‐
ment specifies what type of activity is of interest, chosen from the
following enumerated values:
Watch for the arrival of data to be read.
Watch for the ability to write to the file descriptor
Watch for the arrival of urgent out-of-band data on the
The callback argument is the function to call when the selected activ‐
ity is seen. It should be defined with the following macro, which is
defined in libtecla.h.
#define GL_FD_EVENT_FN(fn) GlFdStatus (fn)(GetLine *gl, void *data, int fd, GlFdEvent event)
The data argument of the gl_watch_fd() function is passed to the call‐
back function for its own use, and can point to anything you like,
including NULL. The file descriptor and the event argument are also
passed to the callback function, and this potentially allows the same
callback function to be registered to more than one type of event
and/or more than one file descriptor. The return value of the callback
function should be one of the following values.
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 mean‐
ingful value when gl_get_line() returns NULL, the
callback function should set errno appropriately.
Redraw the input line then continue waiting for
input. Return this if your callback wrote to the ter‐
Continue to wait for input, without redrawing the
Note that before calling the callback, gl_get_line() blocks most sig‐
nals and leaves its own signal handlers installed, so if you need to
catch a particular signal you will need to both temporarily install
your own signal handler, and unblock the signal. Be sure to re-block
the signal (if it was originally blocked) and reinstate the original
signal handler, if any, before returning.
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 new‐
line first, and when your callback returns, tell gl_get_line() that the
input line needs to be redrawn, by returning the GLFD_REFRESH status
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 sub‐
sequent 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 follow‐
ing macro is provided in <libtecla.h> for applications to use to
declare and prototype timeout callback functions.
#define GL_TIMEOUT_FN(fn) GlAfterTimeout (fn)(GetLine *gl, void *data)
On returning, the application's callback is expected to return one of
the following enumerators to tell gl_get_line() how to procede after
the timeout has been handled by the callback.
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.
Redraw the input line, then continue waiting for
input. You should return this value if your callback
wrote to the terminal.
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
Note that before calling the callback, gl_get_line() blocks most sig‐
nals and leaves its own signal handlers installed, so if you need to
catch a particular signal you will need to both temporarily install
your own signal handler and unblock the signal. Be sure to re-block the
signal (if it was originally blocked) and reinstate the original signal
handler, if any, before returning.
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 car‐
riage-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 sta‐
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 millisec‐
onds 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
To turn off timeouts, simply call gl_inactivity_timeout() with a call‐
back argument of 0. The data argument is ignored in this case.
Signal Handling Defaults
By default, the gl_get_line() function intercepts a number of signals.
This is particularly important for signals that would by default termi‐
nate 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 call‐
ing application's signal handlers to handle it, then, if the process
still exists, returns NULL and sets errno as specified below.
This signal is generated both by the keyboard interrupt key
(usually ^C), and the keyboard break key. The errno value is
This signal is generated when the controlling terminal
exits. The errno value is ENOTTY.
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.
This signal is generated by the keyboard quit key (usually
^\fR). The errno value is EINTR.
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.
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 accomodate 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.
This signal is generated when a suspended process is
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
This signal is generated when a power failure occurs (pre‐
sumably when the system is on a UPS).
This signal is generated when a timer expires.
An application specific signal.
Another application specific signal.
This signal is generated when a virtual timer expires. See
This signal is generated when a process exceeds its soft
CPU time limit.
This signal is generated when a process exceeds its soft
This signal is generated by the terminal suspend key,
which is usually ^Z, or the delayed terminal suspend key,
which is usually ^Y.
This signal is generated if the program attempts to read
from the terminal while the program is running in the
This signal is generated if the program attempts to write
to the terminal while the program is running in the back‐
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-con‐
trol shells usually reset the terminal settings when a process relin‐
quishes the controlling terminal, so this is only a problem with older
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 envi‐
ronment in which the application's signal handler is invoked. The after
argument tells gl_get_line() what to do after the application's signal
handler returns. 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
Restore the caller's signal environment while han‐
dling the signal.
Restore the caller's terminal settings while han‐
dling the signal.
Move the cursor to the start of the line following
the input line before invoking the application's
Redraw the input line when the application's signal
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().
If this flag is included, the signal will not be
forwarded to the signal handler of the calling pro‐
Two commonly useful flag combinations are also enumerated as follows:
GLS_RESTORE_SIG | GLS_RESTORE_TTY |GLS_REDRAW_LINE
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 ter‐
minal, 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
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 combi‐
nation is better if you know that you are always going to be writting
to the terminal.
The after argument, which determines what gl_get_line() does after the
application's signal handler returns (if it returns), can take any one
of the following values:
Return the completed input line, just as though the
user had pressed the return key.
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.
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 suprisingly hard to do reliably without race condi‐
tions. In gl_get_line() a lot of care has been taken to allow applica‐
tions to perform reliable signal handling around gl_get_line(). This
section explains how to make use of this.
As an example of the problems that can arise if the application 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 hap‐
pens to be received in the small window of time between the statement
that tests the value of this flag, and the statement that calls
gl_get_line(), then gl_get_line() will not see the signal, and will not
be interrupted. As a result, the application 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() func‐
tion, and specifying the GLS_UNBLOCK attribute of the sig‐
nal, 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 conjunc‐
tion 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
4. In the example, one would now test the global flag that the
signal handler sets, knowing that there is now no danger of
this flag being set again until gl_get_line() unblocks its
signals while performing I/O.
5. Eventually gl_get_line() returns, either because a signal
was caught, an error occurred, or the user finished entering
their input line.
6. Now one would check the global signal flag again, and if it
is set, respond to it, and zero the flag.
7. Use sigprocmask() to unblock the signals that were blocked
in step 3.
The same technique can be used around certain POSIX signal-aware func‐
tions, such as sigsetjmp(3C) and sigsuspend(2), and in particular, the
former of these two functions can be used in conjunction with sig‐
longjmp(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 con‐
figured to catch. This is redundant if the application has already
blocked them, but it does no harm. It undoes this step just before
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 tempo‐
rary signal handler to all of the signals that it is config‐
ured 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 config‐
ured 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
4. If this function returns (that is, no signal is received),
gl_get_line() blocks delivery of the signals of interest
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 execut‐
ing. When this happens, gl_get_line() does the following.
First, note that when a call to siglongjmp() causes sigsetjmp() to
return, provided that the 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 condi‐
tions. 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
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 impor‐
tant for signals that suspend or terminate the process,
since otherwise the terminal would be left in an unusable
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 termi‐
nate 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 configu‐
ration calls for this.
7. gl_get_line() then either resumes trying to read a charac‐
ter, or aborts, depending on the configuration of the signal
that was caught.
What the above steps do in essence is to take asynchronously delivered
signals and handle them synchronously, one at a time, at a point in the
code where gl_get_line() has complete control over its environment.
The Terminal Size
On most systems the combination of the TIOCGWINSZ ioctl and the SIG‐
WINCH signal is used to maintain an accurate idea of the terminal size.
The terminal size is newly queried every time that gl_get_line() is
called and whenever a SIGWINCH signal is received.
On the few systems where this mechanism is not available, at startup
new_GetLine() first looks for the LINES and COLUMNS environment vari‐
ables. 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 sup‐
port ioctl(TIOCGWINSZ), this function first calls ioctl(TIOCSWINSZ) to
tell the terminal driver about the change in size. In non-blocking
server-I/O mode, if a line is currently being input, the input line is
then redrawn to accomodate the changed size. Finally the new values are
recorded in gl for future use by gl_get_line().
The gl_terminal_size() function allows you to query the current size of
the terminal, and install an alternate fallback size for cases where
the size 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
This function first updates gl_get_line()'s fallback terminal dimen‐
sions, then records its findings in the return value.
The def_ncolumn and def_nline arguments specify the default number of
terminal columns and lines to use if the terminal size cannot be deter‐
mined 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. Further‐
more, such text should not be recorded in the history list, since some‐
body 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_tog‐
gle_history(), then they will also be archived in the history list.
Finally, if the enable argument is -1, then the echoing mode is left
unchanged, which allows you to non-destructively query the current set‐
ting 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 comple‐
tions will not be displayed.
Single Character Queries
Using gl_get_line() to query the user for a single character reply, is
inconvenient for the user, since they must hit the enter or return key
before the character that they typed is returned to the program. Thus
the gl_query_char() function has been provided for single character
queries like this.
This function displays the specified prompt at the start of a new line,
and waits for the user to type a character. When the user types a char‐
acter, gl_query_char() displays it to the right of the prompt, starts a
newline, then returns the character to the calling program. The return
value of the function is the character that was typed. If the read had
to be aborted for some reason, EOF is returned instead. In the latter
case, the application can call the previously documented gl_return_sta‐
tus(), to find out what went wrong. This could, for example, have been
the reception of a signal, or the optional inactivity timer going off.
If the user simply hits enter, the value of the defchar argument is
substituted. This means that when the user hits either newline or
return, the character specified in defchar, is displayed after the
prompt, as though the user had typed it, as well as being returned to
the calling application. If such a replacement is not important, simply
pass '\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 par‐
tial 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() func‐
tion can be called to find out what went wrong. Possibilities include
the optional inactivity timer going off, the receipt of a signal that
is configured to abort gl_get_line(), or terminal I/O blocking, when in
non-blocking server-I/O mode.
Beware that certain keyboard keys, such as function keys, and cursor
keys, usually generate at least 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_termi‐
nal(). In non-blocking server-I/O mode, this function also arranges for
the current input line to be redrawn from scratch when gl_get_line() is
Displaying Text Dynamically
Between calls to gl_get_line(), the gl_display_text() function provides
a convenient way to display paragraphs of text, left-justified and
split over one or more terminal lines according to the constraints of
the current width of the terminal. Examples of the use of this function
may be found in the demo programs, where it is used to display intro‐
ductions. In those examples the advanced use of optional prefixes,
suffixes and filled lines to draw a box around the text is also illus‐
If gl is not currently connected to a terminal, for example if the out‐
put of a program that uses gl_get_line() is being piped to another pro‐
gram or redirected to a file, then the value of the def_width parameter
is used as the terminal width.
The indentation argument specifies the number of characters to use to
indent each line of ouput. The fill_char argument specifies the charac‐
ter that will be used to perform this indentation.
The prefix argument can be either NULL or a string to place at the
beginning of each new line (after any indentation). Similarly, the suf‐
fix 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 termi‐
nal, and any space between its first character and the last word on
that line is filled with the character specified by the fill_char argu‐
ment. Normally the fill-character is a space.
The start argument tells gl_display_text() how many characters have
already been written to the current terminal line, and thus tells it
the starting column index of the cursor. Since the return value of
gl_display_text() is the ending column index of the cursor, by passing
the return value of one call to the start argument of the next call, a
paragraph that is broken between more than one string can be composed
by calling gl_display_text() for each successive portion of the para‐
graph. Note that literal newline characters are necessary at the end
of each paragraph to force a new line to be started.
On error, gl_display_text() returns -1.
Callback Function Facilities
Unless otherwise stated, callback functions such as tab completion
callbacks and event callbacks should not call any functions in this
module. The following functions, however, are designed specifically to
be used by callback functions.
Calling the gl_replace_prompt() function from a callback tells
gl_get_line() to display a different prompt when the callback returns.
Except in non-blocking server mode, it has no effect if used between
calls to gl_get_line(). In non-blocking server mode, 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 fol‐
lowing 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,
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:
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(5)
Unfortunately neither terminfo nor termcap were designed to be reen‐
trant, so you cannot safely use the functions of the getline module in
multiple threads (you can use the separate file-expansion and word-com‐
pletion 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 sin‐
gle thread of a multi-threaded program, provided that your other
threads do not use any termcap or terminfo functions.
See attributes(5) for descriptions of the following attributes:
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
│Interface Stability │ Committed │
│MT-Level │ MT-Safe │
SEE ALSOcpl_complete_word(3TECLA), ef_expand_file(3TECLA), gl_io_mode(3TECLA),
libtecla(3LIB), pca_lookup_file(3TECLA), attributes(5), tecla(5)
Sep 10, 2013 GL_GET_LINE(3TECLA)