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
(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()
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
, said function will complain, and return an error, instead of attempting to use the deleted resource object.
The Functions Used In The Example
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
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
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
, and set start_pos
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()
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()
The following are the possible enumerated values returned by gl_return_status()
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 gl_io_mode(3TECLA).
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 gl_watch_fd().
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.
An unexpected error caused gl_get_line() to abort (consult errno and/or gl_error_message() for details.
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.
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 interpreted as follows:
Display subsequent characters with a bold font.
Stop displaying characters with the bold font.
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 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.
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.
, 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
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",
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 over riden 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
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.
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_configure_getline()
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
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()
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 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_completion_action()
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 shouldcall 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.
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)
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.
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 application and subsequently restore them when you next start the application, the gl_save_history()
functions are provided.
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
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.
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-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()
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.
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 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:
The date on which the line was originally entered, formatted like 2001-11-20.
The time of day when the line was entered, formatted like 23:59:59.
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 string.
Looking Up History
function allows the calling application to look up lines in the history list.
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
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()
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.
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.
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 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()
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()
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()
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.
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
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
-1. Otherwise they are both zero.
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
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.
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
), 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
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
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.
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().
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.
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:
Watch for the arrival of data to be read.
Watch for the ability to write to the file descriptor without blocking.
Watch for the arrival of urgent out-of-band data on the file descriptor.
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.
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.
Redraw the input line then continue waiting for input. Return this if your callback wrote to the terminal.
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
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
arguments, but with a callback
argument of 0. The data
argument is ignored in this case.
Setting An 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
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.
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 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
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
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.
This signal is generated both by the keyboard interrupt key (usually ^C), and the keyboard break key. The errno value is EINTR.
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 ^\). 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()
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.
This signal is generated when a suspended process is resumed.
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.
This signal is generated when a power failure occurs (presumably 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 setitimer(2).
This signal is generated when a process exceeds its soft CPU time limit.
This signal is generated when a process exceeds its soft file-size limit.
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 background.
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
, which by definition cannot be caught, or any of the hardware generated exception signals, such as SIGSEGV
, 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()
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.
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.
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
what to set errno
to if told to abort.
argument is a bitwise OR of zero or more of the following enumerators:
Restore the caller's signal environment while handling the signal.
Restore the caller's terminal settings while handling the signal.
Move the cursor to the start of the line following the input line before invoking the application's signal handler.
Redraw the input line when the application's signal handler returns.
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 program.
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 terminal, nor suspends or terminates the calling program, then you can safely set the flags
argument to 0.
The cursor does not get left in the middle of the input line.
So that the user can type in input and have it echoed.
So that you do not need to end each output line with \r\n, instead of just \n.
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 writting to the terminal.
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.
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.
If needed, use the gl_trap_signal() function to configure gl_get_line() to abort when important signals are caught.
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.
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).
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.
Eventually gl_get_line() returns, either because a signal was caught, an error occurred, or the user finished entering their input line.
Now one would check the global signal flag again, and if it is set, respond to it, and zero the flag.
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.
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.
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
returns zero, gl_get_line()
then does the following.
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.
It records the number of the signal that was received in a file-scope variable.
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.
Now that the signal handler has been set up, gl_get_line() unblocks all of the signals that it is configured to catch.
It then calls the read() or select() function to wait for keyboard input.
If this function returns (that is, no signal is received), gl_get_line() blocks delivery of the signals of interest again.
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()
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.
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.
It then reinstates the application's signal handlers.
Then it uses the C standard-library raise(3C) function to re-send the application the signal that was caught.
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.
If the signal handler returns (that is, it does not terminate the process), gl_get_line() blocks delivery of the above signal again.
It then undoes any actions performed in the first of the above steps and redisplays the line, if the signal configuration calls for this.
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
environment variables. If these are not found, or they contain unusable values, then if a terminal information database like terminfo
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.
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()
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.
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()
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
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()
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.
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.
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.
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.
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()
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
(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
, 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) 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.