NAME¶
gl_get_line, new_GetLine, del_GetLine, gl_customize_completion,
gl_change_terminal, gl_configure_getline, gl_load_history, gl_save_history,
gl_group_history, gl_show_history, gl_watch_fd, gl_inactivity_timeout,
gl_terminal_size, gl_set_term_size, gl_resize_history, gl_limit_history,
gl_clear_history, gl_toggle_history, gl_lookup_history, gl_state_of_history,
gl_range_of_history, gl_size_of_history, gl_echo_mode, gl_replace_prompt,
gl_prompt_style, gl_ignore_signal, gl_trap_signal, gl_last_signal,
gl_completion_action, gl_display_text, gl_return_status, gl_error_message,
gl_catch_blocked, gl_list_signals, gl_bind_keyseq, gl_erase_terminal,
gl_automatic_history, gl_append_history, gl_query_char, gl_read_char - allow
the user to compose an input line
SYNOPSIS¶
#include <stdio.h>
#include <libtecla.h>
GetLine *new_GetLine(size_t linelen, size_t histlen);
GetLine *del_GetLine(GetLine *gl);
char *gl_get_line(GetLine *gl, const char *prompt,
const char *start_line, int start_pos);
int gl_query_char(GetLine *gl, const char *prompt,
char defchar);
int gl_read_char(GetLine *gl);
int gl_customize_completion(GetLine *gl, void *data,
CplMatchFn *match_fn);
int gl_change_terminal(GetLine *gl, FILE *input_fp,
FILE *output_fp, const char *term);
int gl_configure_getline(GetLine *gl,
const char *app_string,
const char *app_file,
const char *user_file);
int gl_bind_keyseq(GetLine *gl, GlKeyOrigin origin,
const char *keyseq, const char *action);
int gl_save_history(GetLine *gl, const char *filename,
const char *comment, int max_lines);
int gl_load_history(GetLine *gl, const char *filename,
const char *comment);
int gl_watch_fd(GetLine *gl, int fd, GlFdEvent event,
GlFdEventFn *callback, void *data);
int gl_inactivity_timeout(GetLine *gl, GlTimeoutFn *callback,
void *data, unsigned long sec,
unsigned long nsec);
int gl_group_history(GetLine *gl, unsigned stream);
int gl_show_history(GetLine *gl, FILE *fp,
const char *fmt, int all_groups,
int max_lines);
int gl_resize_history(GetLine *gl, size_t bufsize);
void gl_limit_history(GetLine *gl, int max_lines);
void gl_clear_history(GetLine *gl, int all_groups);
void gl_toggle_history(GetLine *gl, int enable);
GlTerminalSize gl_terminal_size(GetLine *gl,
int def_ncolumn,
int def_nline);
int gl_set_term_size(GetLine *gl, int ncolumn, int nline);
int gl_lookup_history(GetLine *gl, unsigned long id,
GlHistoryLine *hline);
void gl_state_of_history(GetLine *gl,
GlHistoryState *state);
void gl_range_of_history(GetLine *gl,
GlHistoryRange *range);
void gl_size_of_history(GetLine *gl, GlHistorySize *size);
void gl_echo_mode(GetLine *gl, int enable);
void gl_replace_prompt(GetLine *gl, const char *prompt);
void gl_prompt_style(GetLine *gl, GlPromptStyle style);
int gl_ignore_signal(GetLine *gl, int signo);
int gl_trap_signal(GetLine *gl, int signo, unsigned flags,
GlAfterSignal after, int errno_value);
int gl_last_signal(GetLine *gl);
int gl_completion_action(GetLine *gl,
void *data, CplMatchFn *match_fn,
int list_only, const char *name,
const char *keyseq);
int gl_register_action(GetLine *gl, void *data,
GlActionFn *fn, const char *name,
const char *keyseq);
int gl_display_text(GetLine *gl, int indentation,
const char *prefix,
const char *suffix, int fill_char,
int def_width, int start,
const char *string);
GlReturnStatus gl_return_status(GetLine *gl);
const char *gl_error_message(GetLine *gl, char *buff,
size_t n);
void gl_catch_blocked(GetLine *gl);
int gl_list_signals(GetLine *gl, sigset_t *set);
int gl_append_history(GetLine *gl, const char *line);
int gl_automatic_history(GetLine *gl, int enable);
DESCRIPTION¶
The
gl_get_line() function is part of the tecla library (see the
libtecla(3) man page). If the user is typing at a terminal, each call
prompts them for an line of input, then provides interactive editing
facilities, similar to those of the unix
tcsh shell. In addition to
simple command-line editing, it supports recall of previously entered command
lines, TAB completion of file names, and in-line wild-card expansion of
filenames. Documentation of both the user-level command-line editing features
and all user configuration options, can be found in the
tecla(7) man
page. This man page concerns itself with documentation for programmers
interested in using this library in their application.
AN EXAMPLE¶
The following shows a complete example of how to use the
gl_get_line()
function to get input from the user:
#include <stdio.h>
#include <locale.h>
#include <libtecla.h>
int main(int argc, char *argv[])
{
char *line; /* The line that the user typed */
GetLine *gl; /* The gl_get_line() resource object */
setlocale(LC_CTYPE, ""); /* Adopt the user's choice */
/* of character set. */
gl = new_GetLine(1024, 2048);
if(!gl)
return 1;
while((line=gl_get_line(gl, "$ ", NULL, -1)) != NULL &&
strcmp(line, "exit\n") != 0)
printf("You typed: %s\n", line);
gl = del_GetLine(gl);
return 0;
}
In the example, first the resources needed by the
gl_get_line() function
are created by calling
new_GetLine(). This allocates the memory used in
subsequent calls to the
gl_get_line() function, including the history
buffer for recording previously entered lines. Then one or more lines are read
from the user, until either an error occurs, or the user types
exit.
Then finally the resources that were allocated by
new_GetLine(), are
returned to the system by calling
del_GetLine(). Note the use of the
NULL return value of
del_GetLine() to make
gl
NULL. This is a safety precaution. If the program subsequently attempts
to pass
gl to
gl_get_line(), said function will complain, and
return an error, instead of attempting to use the deleted resource object.
THE FUNCTIONS USED IN THE EXAMPLE¶
The descriptions of the functions used in the example are as follows:
GetLine *new_GetLine(size_t linelen, size_t histlen)
This function creates the resources used by the
gl_get_line() function
and returns an opaque pointer to the object that contains them. The maximum
length of an input line is specified via the
linelen argument, and the
number of bytes to allocate for storing history lines is set by the
histlen argument. History lines are stored back-to-back in a single
buffer of this size. Note that this means that the number of history lines
that can be stored at any given time, depends on the lengths of the individual
lines. If you want to place an upper limit on the number of lines that can be
stored, see the
gl_limit_history() function described later. If you
don't want history at all, specify
histlen as zero, and no history
buffer will be allocated.
On error, a message is printed to
stderr and
NULL is returned.
GetLine *del_GetLine(GetLine *gl)
This function deletes the resources that were returned by a previous call to
new_GetLine(). It always returns
NULL (ie a deleted object). It
does nothing if the
gl argument is
NULL.
char *gl_get_line(GetLine *gl, const char *prompt,
const char *start_line, int start_pos);
The
gl_get_line() function can be called any number of times to read
input from the user. The
gl argument must have been previously returned
by a call to
new_GetLine(). The
prompt argument should be a
normal
NUL terminated string, specifying the prompt to present the user
with. By default prompts are displayed literally, but if enabled with the
gl_prompt_style() function (see later), prompts can contain directives
to do underlining, switch to and from bold fonts, or turn highlighting on and
off.
If you want to specify the initial contents of the line, for the user to edit,
pass the desired string via the
start_line argument. You can then
specify which character of this line the cursor is initially positioned over,
using the
start_pos argument. This should be -1 if you want the cursor
to follow the last character of the start line. If you don't want to preload
the line in this manner, send
start_line as
NULL, and set
start_pos to -1. Note that the line pointer returned by one call to
gl_get_line() can be passed back to the next call to
gl_get_line() via the
start_line. This allows the application to
take the last entered line, and if it contains an error, to then present it
back to the user for re-editing, with the cursor initially positioned where
the error was encountered.
The
gl_get_line() function returns a pointer to the line entered by the
user, or
NULL on error or at the end of the input. The returned pointer
is part of the specified
gl resource object, and thus should not be
free'd by the caller, or assumed to be unchanging from one call to the next.
When reading from a user at a terminal, there will always be a newline
character at the end of the returned line. When standard input is being taken
from a pipe or a file, there will similarly be a newline unless the input line
was too long to store in the internal buffer. In the latter case you should
call
gl_get_line() again to read the rest of the line. Note that this
behavior makes
gl_get_line() similar to
fgets(). In fact when
stdin isn't connected to a terminal,
gl_get_line() just calls
fgets().
THE RETURN STATUS OF GL_GET_LINE¶
As described above, the
gl_get_line() function has two possible return
values; a pointer to the completed input line, or
NULL. Extra
information about what caused
gl_get_line() to return is available both
by inspecting
errno, and by calling the
gl_return_status()
function.
GlReturnStatus gl_return_status(GetLine *gl);
The following are the possible enumerated values that this function returns.
GLR_NEWLINE - The last call to gl_get_line()
successfully returned a completed
input line.
GLR_BLOCKED - gl_get_line() was in non-blocking
server mode, and returned early to
avoid blocking the process while
waiting for terminal I/O. The
gl_pending_io() function can be
used to see what type of I/O
gl_get_line() was waiting for.
(see the gl_io_mode(3) man page
for details).
GLR_SIGNAL - A signal was caught by
gl_get_line() that had an
after-signal disposition of
GLS_ABORT (See gl_trap_signal()).
GLR_TIMEOUT - The inactivity timer expired while
gl_get_line() was waiting for
input, and the timeout callback
function returned GLTO_ABORT.
See gl_inactivity_timeout() for
information about timeouts.
GLR_FDABORT - An application I/O callack returned
GLFD_ABORT (see gl_watch_fd()).
GLR_EOF - End of file reached. This can happen
when input is coming from a file or a
pipe, instead of the terminal. It also
occurs if the user invokes the
list-or-eof or del-char-or-list-or-eof
actions at the start of a new line.
GLR_ERROR - An unexpected error caused
gl_get_line() to abort (consult
errno and/or
gl_error_message() for details.
When
gl_return_status() returns
GLR_ERROR, and the value of
errno isn't sufficient to explain what happened, you can use the
gl_error_message() function to request a description of the last error
that occurred.
const char *gl_error_message(GetLine *gl, char *buff,
size_t n);
The return value is a pointer to the message that occurred. If the
buff
argument is
NULL, this will be a pointer to a buffer within
gl,
who's value will probably change on the next call to any function associated
with
gl_get_line(). Otherwise, if a non-
NULL buff
argument is provided, the error message, including a
'\0' terminator,
will be written within the first
n elements of this buffer, and the
return value will be a pointer to the first element of this buffer. If the
message won't fit in the provided buffer, it will be truncated to fit.
Whereas by default the prompt string that you specify is displayed literally,
without any special interpretation of the characters within it, the
gl_prompt_style() function can be used to enable optional formatting
directives within the prompt.
void gl_prompt_style(GetLine *gl, GlPromptStyle style);
The
style argument, which specifies the formatting style, can take any of
the following values:
GL_FORMAT_PROMPT - In this style, the formatting
directives described below, when
included in prompt strings, are
interpreted as follows:
%B - Display subsequent
characters with a bold
font.
%b - Stop displaying characters
with the bold font.
%F - Make subsequent characters
flash.
%f - Turn off flashing
characters.
%U - Underline subsequent
characters.
%u - Stop underlining
characters.
%P - Switch to a pale (half
brightness) font.
%p - Stop using the pale font.
%S - Highlight subsequent
characters (also known as
standout mode).
%s - Stop highlighting
characters.
%V - Turn on reverse video.
%v - Turn off reverse video.
%% - Display a single %
character.
For example, in this mode, a prompt
string like "%UOK%u$ " would
display the prompt "OK$ ",
but with the OK part
underlined.
Note that although a pair of
characters that starts with a %
character, but doesn't match any of
the above directives is displayed
literally, if a new directive is
subsequently introduced which does
match, the displayed prompt will
change, so it is better to always
use %% to display a literal %.
Also note that not all terminals
support all of these text
attributes, and that some substitute
a different attribute for missing
ones.
GL_LITERAL_PROMPT - In this style, the prompt string is
printed literally. This is the
default style.
ALTERNATE CONFIGURATION SOURCES¶
As mentioned above, by default users have the option of configuring the behavior
of
gl_get_line() via a configuration file called
.teclarc in
their home directories. The fact that all applications share this same
configuration file is both an advantage and a disadvantage. In most cases it
is an advantage, since it encourages uniformity, and frees the user from
having to configure each application separately. In some applications,
however, this single means of configuration is a problem. This is particularly
true of embedded software, where there's no filesystem to read a configuration
file from, and also in applications where a radically different choice of
keybindings is needed to emulate a legacy keyboard interface. To cater for
such cases, the following function allows the application to control where
configuration information is read from.
int gl_configure_getline(GetLine *gl,
const char *app_string,
const char *app_file,
const char *user_file);
It allows the configuration commands that would normally be read from a user's
~/.teclarc file, to be read from any or none of, a string, an
application specific configuration file, and/or a user-specific configuration
file. If this function is called before the first call to
gl_get_line(), the default behavior of reading
~/.teclarc on the
first call to
gl_get_line() is disabled, so all configuration must be
achieved using the configuration sources specified with this function.
If
app_string != NULL, then it is interpreted as a string containing one
or more configuration commands, separated from each other in the string by
embedded newline characters. If
app_file != NULL then it is
interpreted as the full pathname of an application-specific configuration
file. If
user_file != NULL then it is interpreted as the full pathname
of a user-specific configuration file, such as
~/.teclarc. For example,
in the following call,
gl_configure_getline(gl, "edit-mode vi \n nobeep",
"/usr/share/myapp/teclarc",
"~/.teclarc");
the
app_string argument causes the calling application to start in vi
edit-mode, instead of the default emacs mode, and turns off the use of the
terminal bell by the library. It then attempts to read system-wide
configuration commands from an optional file called
/usr/share/myapp/teclarc, then finally reads user-specific
configuration commands from an optional
.teclarc file in the user's
home directory. Note that the arguments are listed in ascending order of
priority, with the contents of
app_string being potentially overriden
by commands in
app_file, and commands in
app_file potentially
being overriden by commands in
user_file.
You can call this function as many times as needed, the results being
cumulative, but note that copies of any filenames specified via the
app_file and
user_file arguments are recorded internally for
subsequent use by the
read-init-files key-binding function, so if you
plan to call this function multiple times, be sure that the last call
specifies the filenames that you want re-read when the user requests that the
configuration files be re-read.
Individual key sequences can also be bound and unbound using the
gl_bind_keyseq() function.
int gl_bind_keyseq(GetLine *gl, GlKeyOrigin origin,
const char *keyseq,
const char *action);
The
origin argument specifies the priority of the binding, according to
who it is being established for, and must be one of the following two values.
GL_USER_KEY - The user requested this key-binding.
GL_APP_KEY - This is a default binding set by the
application.
When both user and application bindings for a given key-sequence have been
specified, the user binding takes precedence. The application's binding is
subsequently reinstated if the user's binding is later unbound via either
another to this function, or a call to
gl_configure_getline().
The
keyseq argument specifies the key-sequence to be bound or unbound,
and is expressed in the same way as in a
~/.teclarc configuration file.
The
action argument must either be a string containing the name of the
action to bind the key-sequence to, or it must be
NULL or ""
to unbind the key-sequence.
CUSTOMIZED WORD COMPLETION¶
If in your application, you would like to have TAB completion complete other
things in addition to or instead of filenames, you can arrange this by
registering an alternate completion callback function, via a call to the
gl_customize_completion() function.
int gl_customize_completion(GetLine *gl, void *data,
CplMatchFn *match_fn);
The
data argument provides a way for your application to pass arbitrary,
application-specific information to the callback function. This is passed to
the callback every time that it is called. It might for example, point to the
symbol table from which possible completions are to be sought. The
match_fn argument specifies the callback function to be called. The
CplMatchFn function type is defined in
libtecla.h, as is a
CPL_MATCH_FN() macro that you can use to declare and prototype callback
functions. The declaration and responsibilities of callback functions are
described in depth in the
cpl_complete_word(3) man page.
In brief, the callback function is responsible for looking backwards in the
input line, back from the point at which the user pressed TAB, to find the
start of the word being completed. It then must lookup possible completions of
this word, and record them one by one in the
WordCompletion object that
is passed to it as an argument, by calling the
cpl_add_completion()
function. If the callback function wishes to provide filename completion in
addition to its own specific completions, it has the option of itself calling
the builtin file-name completion callback. This also, is documented in the
cpl_complete_word(3) man page.
Note that if you would like
gl_get_line() to return the current input
line when a successful completion is been made, you can arrange this when you
call
cpl_add_completion(), by making the last character of the
continuation suffix a newline character. If you do this, the input line will
be updated to display the completion, together with any contiuation suffix up
to the newline character, then
gl_get_line() will return this input
line.
If, for some reason, your callback function needs to write something to the
terminal, it must call
gl_normal_io() before doing so. This will start
a new line after the input line that is currently being edited, reinstate
normal terminal I/O, and tell
gl_get_line() that the input line will
need to be redrawn when the callback returns.
ADDING COMPLETION ACTIONS¶
In the previous section the ability to customize the behavior of the only
default completion action,
complete-word, was described. In this
section the ability to install additional action functions, so that different
types of word completion can be bound to different key-sequences, is
described. This is achieved by using the
gl_completion_action()
function.
int gl_completion_action(GetLine *gl,
void *data, CplMatchFn *match_fn,
int list_only, const char *name,
const char *keyseq);
The
data and
match_fn arguments are as described in the
cpl_complete_word man page, and specify the callback function that
should be invoked to identify possible completions. The
list_only
argument determines whether the action that is being defined should attempt to
complete the word as far as possible in the input line before displaying any
possible ambiguous completions, or whether it should simply display the list
of possible completions without touching the input line. The former option is
selected by specifying a value of
0, and the latter by specifying a
value of
1. The
name argument specifies the name by which
configuration files and future invokations of this function should refer to
the action. This must either be the name of an existing completion action to
be changed, or be a new unused name for a new action. Finally, the
keyseq argument specifies the default key-sequence to bind the action
to. If this is
NULL, no new keysequence will be bound to the action.
Beware that in order for the user to be able to change the key-sequence that is
bound to actions that are installed in this manner, when you call
gl_completion_action() to install a given action for the first time,
you should do this between calling
new_GetLine() and the first call to
gl_get_line(). Otherwise, when the user's configuration file is read on
the first call to
gl_get_line(), the name of the your additional action
won't be known, and any reference to it in the configuration file will
generate an error.
As discussed for
gl_customize_completion(), if your callback function,
for some reason, needs to write anything to the terminal, it must call
gl_normal_io() before doing so.
DEFINING CUSTOM ACTIONS¶
Although the built-in key-binding actions are sufficient for the needs of most
applications, occasionally a specialized application may need to define one or
more custom actions, bound to application-specific key-sequences. For example,
a sales application would benefit from having a key-sequence that displayed
the part name that corresponded to a part number preceding the cursor. Such a
feature is clearly beyond the scope of the built-in action functions. So for
such special cases, the
gl_register_action() function is provided.
int gl_register_action(GetLine *gl, void *data,
GlActionFn *fn, const char *name,
const char *keyseq);
This function lets the application register an external function,
fn,
that will thereafter be called whenever either the specified key-sequence,
keyseq, is entered by the user, or the user enters any other
key-sequence that the user subsequently binds to the specified action name,
name, in their configuration file. The
data argument can be a
pointer to anything that the application wishes to have passed to the action
function,
fn, whenever that function is invoked.
The action function,
fn, should be declared using the following macro,
which is defined in
libtecla.h.
#define GL_ACTION_FN(fn) GlAfterAction (fn)(GetLine *gl, \
void *data, int count, size_t curpos, \
const char *line)
The
gl and
data arguments are those that were previously passed to
gl_register_action() when the action function was registered. The
count argument is a numeric argument which the user has the option of
entering using the
digit-argument action, before invoking the action.
If the user doesn't enter a number, then the
count argument is set to
1. Nominally this argument is interpreted as a repeat count, meaning that the
action should be repeated that many times. In practice however, for some
actions a repeat count makes little sense. In such cases, actions can either
simply ignore the
count argument, or use its value for a different
purpose.
A copy of the current input line is passed in the read-only
line
argument. The current cursor position within this string is given by the index
contained in the
curpos argument. Note that direct manipulation of the
input line and the cursor position is not permitted. This is because the rules
dicated by various modes, such as vi mode versus emacs mode, no-echo mode, and
insert mode versus overstrike mode etc, make it too complex for an application
writer to write a conforming editing action, as well as constrain future
changes to the internals of
gl_get_line(). A potential solution to this
dilema would be to allow the action function to edit the line using the
existing editing actions. This is currently under consideration.
If the action function wishes to write text to the terminal, without this
getting mixed up with the displayed text of the input line, or read from the
terminal without having to handle raw terminal I/O, then before doing either
of these operations, it must temporarily suspend line editing by calling the
gl_normal_io() function. This function flushes any pending output to
the terminal, moves the cursor to the start of the line that follows the last
terminal line of the input line, then restores the terminal to a state that is
suitable for use with the C stdio facilities. The latter includes such things
as restoring the normal mapping of
\n to
\r\n, and, when in
server mode, restoring the normal blocking form of terminal I/O. Having called
this function, the action function can read from and write to the terminal
without the fear of creating a mess. It isn't necessary for the action
function to restore the original editing environment before it returns. This
is done automatically by
gl_get_line() after the action function
returns. The following is a simple example of an action function which writes
the sentence "Hello world" on a new terminal line after the line
being edited. When this function returns, the input line is redrawn on the
line that follows the "Hello world" line, and line editing resumes.
static GL_ACTION_FN(say_hello_fn)
{
if(gl_normal_io(gl)) /* Temporarily suspend editing */
return GLA_ABORT;
printf("Hello world\n");
return GLA_CONTINUE;
}
Action functions must return one of the following values, to tell
gl_get_line() how to procede.
GLA_ABORT - Cause gl_get_line() to return NULL.
GLA_RETURN - Cause gl_get_line() to return the
completed input line.
GLA_CONTINUE - Resume command-line editing.
Note that the
name argument of
gl_register_action() specifies the
name by which a user can refer to the action in their configuration file. This
allows them to re-bind the action to an alternate key-seqeunce. In order for
this to work, it is necessary to call
gl_register_action() between
calling
new_GetLine() and the first call to
gl_get_line().
HISTORY FILES¶
To save the contents of the history buffer before quitting your application, and
subsequently restore them when you next start the application, the following
functions are provided.
int gl_save_history(GetLine *gl, const char *filename,
const char *comment, int max_lines);
int gl_load_history(GetLine *gl, const char *filename,
const char *comment);
The
filename argument specifies the name to give the history file when
saving, or the name of an existing history file, when loading. This may
contain home-directory and environment variable expressions, such as
"~/.myapp_history" or "$HOME/.myapp_history".
Along with each history line, extra information about it, such as when it was
entered by the user, and what its nesting level is, is recorded as a comment
preceding the line in the history file. Writing this as a comment allows the
history file to double as a command file, just in case you wish to replay a
whole session using it. Since comment prefixes differ in different languages,
the
comment argument is provided for specifying the comment prefix. For
example, if your application were a unix shell, such as the bourne shell, you
would specify "#" here. Whatever you choose for the comment
character, you must specify the same prefix to
gl_load_history() that
you used when you called
gl_save_history() to write the history file.
The
max_lines must be either -1 to specify that all lines in the history
list be saved, or a positive number specifying a ceiling on how many of the
most recent lines should be saved.
Both fuctions return non-zero on error, after writing an error message to
stderr. Note that
gl_load_history() does not consider the non-existence
of a file to be an error.
MULTIPLE HISTORY LISTS¶
If your application uses a single
GetLine object for entering many
different types of input lines, you may wish
gl_get_line() to
distinguish the different types of lines in the history list, and only recall
lines that match the current type of line. To support this requirement,
gl_get_line() marks lines being recorded in the history list with an
integer identifier chosen by the application. Initially this identifier is set
to 0
by new_GetLine(), but it can be changed subsequently by
calling
gl_group_history().
int gl_group_history(GetLine *gl, unsigned id);
The integer identifier
id can be any number chosen by the application,
but note that
gl_save_history() and
gl_load_history() preserve
the association between identifiers and historical input lines between program
invokations, so you should choose fixed identifiers for the different types of
input line used by your application.
Whenever
gl_get_line() appends a new input line to the history list, the
current history identifier is recorded with it, and when it is asked to recall
a historical input line, it only recalls lines that are marked with the
current identifier.
DISPLAYING HISTORY¶
The history list can be displayed by calling
gl_show_history().
int gl_show_history(GetLine *gl, FILE *fp,
const char *fmt,
int all_groups,
int max_lines);
This displays the current contents of the history list to the stdio output
stream
fp. If the
max_lines argument is greater than or equal to
zero, then no more than this number of the most recent lines will be
displayed. If the
all_groups argument is non-zero, lines from all
history groups are displayed. Otherwise just those of the currently selected
history group are displayed. The format string argument,
fmt,
determines how the line is displayed. This can contain arbitrary characters
which are written verbatim, interleaved with any of the following format
directives:
%D - The date on which the line was originally
entered, formatted like 2001-11-20.
%T - The time of day when the line was entered,
formatted like 23:59:59.
%N - The sequential entry number of the line in
the history buffer.
%G - The number of the history group which the
line belongs to.
%% - A literal % character.
%H - The history line itself.
Thus a format string like
"%D %T %H0 would output something like:
2001-11-20 10:23:34 Hello world
Note the inclusion of an explicit newline character in the format string.
LOOKING UP HISTORY¶
The
gl_lookup_history() function allows the calling application to look
up lines in the history list.
typedef struct {
const char *line; /* The requested historical */
/* line. */
unsigned group; /* The history group to which */
/* the line belongs. */
time_t timestamp; /* The date and time at which */
/* the line was originally */
/* entered. */
} GlHistoryLine;
int gl_lookup_history(GetLine *gl, unsigned long id,
GlHistoryLine *hline);
The
id argument indicates which line to look up, where the first line
that was entered in the history list after
new_GetLine() was called, is
denoted by 0, and subsequently entered lines are denoted with successively
higher numbers. Note that the range of lines currently preserved in the
history list can be queried by calling the
gl_range_of_history()
function, described later. If the requested line is in the history list, the
details of the line are recorded in the variable pointed to by the
hline argument, and
1 is returned. Otherwise
0 is
returned, and the variable pointed to by
hline is left unchanged.
Beware that the string returned in
hline->line is part of the history
buffer, so it must not be modified by the caller, and will be recycled on the
next call to any function that takes
gl as its argument. Therefore you
should make a private copy of this string if you need to keep it around.
MANUAL HISTORY ARCHIVAL¶
By default, whenever a line is entered by the user, it is automatically appended
to the history list, just before
gl_get_line() returns the line to the
caller. This is convenient for the majority of applications, but there are
also applications that need finer grained control over what gets added to the
history list. In such cases, the automatic addition of entered lines to the
history list can be turned off by calling the
gl_automatic_history()
function.
int gl_automatic_history(GetLine *gl, int enable);
If this function is called with its
enable argument set to
0,
gl_get_line() won't automatically archive subsequently entered lines.
Automatic archiving can be reenabled at a later time, by calling this function
again, with its
enable argument set to 1. While automatic history
archiving is disabled, the calling application can use the
gl_append_history() to append lines to the history list as needed.
int gl_append_history(GetLine *gl, const char *line);
The
line argument specifies the line to be added to the history list.
This must be a normal
' ' terminated string. If this string
contains any newline characters, the line that gets archived in the history
list will be terminated by the first of these. Otherwise it will be terminated
by the
' ' terminator. If the line is longer than the maximum
input line length, that was specified when
new_GetLine() was called,
when the line is recalled, it will get truncated to the actual
gl_get_line() line length.
If successful,
gl_append_history() returns 0. Otherwise it returns
non-zero, and sets
errno to one of the following values.
EINVAL - One of the arguments passed to
gl_append_history() was NULL.
ENOMEM - The specified line was longer than the allocated
size of the history buffer (as specified when
new_GetLine() was called), so it couldn't be
archived.
A textual description of the error can optionally be obtained by calling
gl_error_message(). Note that after such an error, the history list
remains in a valid state to receive new history lines, so there is little harm
in simply ignoring the return status of
gl_append_history().
MISCELLANEOUS HISTORY CONFIGURATION¶
If you wish to change the size of the history buffer that was originally
specified in the call to
new_GetLine(), you can do so with the
gl_resize_history() function.
int gl_resize_history(GetLine *gl, size_t histlen);
The
histlen argument specifies the new size in bytes, and if you specify
this as 0, the buffer will be deleted.
As mentioned in the discussion of
new_GetLine(), the number of lines that
can be stored in the history buffer, depends on the lengths of the individual
lines. For example, a 1000 byte buffer could equally store 10 lines of average
length 100 bytes, or 2 lines of average length 50 bytes. Although the buffer
is never expanded when new lines are added, a list of pointers into the buffer
does get expanded when needed to accomodate the number of lines currently
stored in the buffer. To place an upper limit on the number of lines in the
buffer, and thus a ceiling on the amount of memory used in this list, you can
call the
gl_limit_history() function.
void gl_limit_history(GetLine *gl, int max_lines);
The
max_lines should either be a positive number
>= 0,
specifying an upper limit on the number of lines in the buffer, or be
-1 to cancel any previously specified limit. When a limit is in effect,
only the
max_lines most recently appended lines are kept in the buffer.
Older lines are discarded.
To discard lines from the history buffer, use the
gl_clear_history()
function.
void gl_clear_history(GetLine *gl, int all_groups);
The
all_groups argument tells the function whether to delete just the
lines associated with the current history group (see
gl_group_history()), or all historical lines in the buffer.
The
gl_toggle_history() function allows you to toggle history on and off
without losing the current contents of the history list.
void gl_toggle_history(GetLine *gl, int enable);
Setting the
enable argument to 0 turns off the history mechanism, and
setting it to 1 turns it back on. When history is turned off, no new lines
will be added to the history list, and history lookup key-bindings will act as
though there is nothing in the history buffer.
QUERYING HISTORY INFORMATION¶
The configured state of the history list can be queried with the
gl_history_state() function.
typedef struct {
int enabled; /* True if history is enabled */
unsigned group; /* The current history group */
int max_lines; /* The current upper limit on the */
/* number of lines in the history */
/* list, or -1 if unlimited. */
} GlHistoryState;
void gl_state_of_history(GetLine *gl,
GlHistoryState *state);
On return, the status information is recorded in the variable pointed to by the
state argument.
The
gl_range_of_history() function returns the number and range of lines
in the history list.
typedef struct {
unsigned long oldest; /* The sequential entry number */
/* of the oldest line in the */
/* history list. */
unsigned long newest; /* The sequential entry number */
/* of the newest line in the */
/* history list. */
int nlines; /* The number of lines in the */
/* history list. */
} GlHistoryRange;
void gl_range_of_history(GetLine *gl, GlHistoryRange *range);
The return values are recorded in the variable pointed to by the
range
argument. If the
nlines member of this structure is greater than zero,
then the
oldest and
newest members report the range of lines in
the list, and
newest=oldest+nlines-1. Otherwise they are both zero.
The
gl_size_of_history() function returns the total size of the history
buffer and the amount of the buffer that is currently occupied.
typedef struct {
size_t size; /* The size of the history buffer */
/* (bytes). */
size_t used; /* The number of bytes of the */
/* history buffer that are */
/* currently occupied. */
} GlHistorySize;
void gl_size_of_history(GetLine *gl, GlHistorySize *size);
On return, the size information is recorded in the variable pointed to by the
size argument.
CHANGING TERMINALS¶
The
new_GetLine() constructor function assumes that input is to be read
from
stdin, and output written to
stdout. The following function
allows you to switch to different input and output streams.
int gl_change_terminal(GetLine *gl, FILE *input_fp,
FILE *output_fp, const char *term);
The
gl argument is the object that was returned by
new_GetLine().
The
input_fp argument specifies the stream to read from, and
output_fp specifies the stream to be written to. Only if both of these
refer to a terminal, will interactive terminal input be enabled. Otherwise
gl_get_line() will simply call
fgets() to read command input. If
both streams refer to a terminal, then they must refer to the same terminal,
and the type of this terminal must be specified via the
term argument.
The value of the
term argument is looked up in the terminal information
database (terminfo or termcap), in order to determine which special control
sequences are needed to control various aspects of the terminal.
new_GetLine() for example, passes the return value of
getenv("TERM") in this argument. Note that if one or both of
input_fp and
output_fp don't refer to a terminal, then it is
legal to pass
NULL instead of a terminal type.
Note that if you want to pass file descriptors to
gl_change_terminal(),
you can do this by creating stdio stream wrappers using the POSIX
fdopen() function.
EXTERNAL EVENT HANDLING¶
By default,
gl_get_line() doesn't return until either a complete input
line has been entered by the user, or an error occurs. In programs that need
to watch for I/O from other sources than the terminal, there are two options.
1. Use the functions described in the
gl_io_mode(3) man page to switch
gl_get_line() into non-blocking server mode. In this mode,
gl_get_line() becomes a non-blocking, incremental
line-editing function that can safely be called from
an external event loop. Although this is a very
versatile method, it involves taking on some
responsibilities that are normally performed behind
the scenes by gl_get_line().
2. While gl_get_line() is waiting for keyboard
input from the user, you can ask it to also watch for
activity on arbitrary file descriptors, such as
network sockets, pipes etc, and have it call functions
of your choosing when activity is seen. This works on
any system that has the select() system call,
which is most, if not all flavors of unix.
Registering a file descriptor to be watched by
gl_get_line() involves
calling the
gl_watch_fd() function.
int gl_watch_fd(GetLine *gl, int fd, GlFdEvent event,
GlFdEventFn *callback, void *data);
If this returns non-zero, then it means that either your arguments are invalid,
or that this facility isn't supported on the host system.
The
fd argument is the file descriptor to be watched. The
event
argument specifies what type of activity is of interest, chosen from the
following enumerated values:
GLFD_READ - Watch for the arrival of data to be read.
GLFD_WRITE - Watch for the ability to write to the file
descriptor without blocking.
GLFD_URGENT - Watch for the arrival of urgent
out-of-band data on the file descriptor.
The
callback argument is the function to call when the selected activity
is seen. It should be defined with the following macro, which is defined in
libtecla.h.
#define GL_FD_EVENT_FN(fn) GlFdStatus (fn)(GetLine *gl, \
void *data, int fd, \
GlFdEvent event)
The
data argument of the
gl_watch_fd() function is passed to the
callback function for its own use, and can point to anything you like,
including
NULL. The file descriptor and the event argument are also
passed to the callback function, and this potentially allows the same callback
function to be registered to more than one type of event and/or more than one
file descriptor. The return value of the callback function should be one of
the following values.
GLFD_ABORT - Tell gl_get_line() to abort. When this
happens, gl_get_line() returns
NULL, and a following call to
gl_return_status() will return
GLR_FDABORT. Note that if the
application needs errno always to
have a meaningful value when
gl_get_line() returns NULL,
the callback function should set
errno appropriately.
GLFD_REFRESH - Redraw the input line then continue
waiting for input. Return this if
your callback wrote to the terminal.
GLFD_CONTINUE - Continue to wait for input, without
redrawing the line.
Note that before calling the callback,
gl_get_line() blocks most signals,
and leaves its own signal handlers installed, so if you need to catch a
particular signal you will need to both temporarily install your own signal
handler, and unblock the signal. Be sure to re-block the signal (if it was
originally blocked) and reinstate the original signal handler, if any, before
returning.
If the callback function needs to read or write to the terminal, it should
ideally first call
gl_normal_io(gl) to temporarily suspend line
editing. This will restore the terminal to canonical, blocking-I/O, mode, and
move the cursor to the start of a new terminal line. Later, when the callback
returns,
gl_get_line() will notice that
gl_normal_io() was
called, redisplay the input line and resume editing. Note that in this case
the return values,
GLFD_REFRESH and
GLFD_CONTINUE are
equivalent.
To support cases where the callback function calls a third-party function which
occasionally and unpredictably writes to the terminal, the automatic
conversion of
"0 to
"r0 is re-enabled before the
callback function is called. If the callack knows that the third-party
function wrote to the terminal, it should then return the
GLFD_REFRESH
return value, to tell
gl_get_line() to redisplay the input line.
To remove a callback function that you previously registered for a given file
descriptor and event, simply call
gl_watch_fd() with the same file
descriptor and
event arguments, but with a
callback argument of
0. The
data argument is ignored in this case.
SETTING AN INACTIVITY TIMEOUT¶
On systems with the
select() system call, the
gl_inactivity_timeout() function can be used to set or cancel an
inactivity timeout. Inactivity in this case refers both to keyboard input, and
to I/O on any file descriptors registered by prior and subsequent calls to
gl_watch_fd(). On oddball systems that don't have
select(), this
call has no effect.
int gl_inactivity_timeout(GetLine *gl, GlTimeoutFn *callback,
void *data, unsigned long sec,
unsigned long nsec);
The timeout is specified in the form of an integral number of seconds and an
integral number of nanoseconds, via the
sec and
nsec arguments
respectively. Subsequently, whenever no activity is seen for this time period,
the function specified via the
callback argument is called. The
data argument of
gl_inactivity_timeout() is passed verbatim to
this callback function whenever it is invoked, and can thus be used to pass
arbitrary application-specific information to the callback. The following
macro is provided in
libtecla.h for applications to use to declare and
prototype timeout callback functions.
#define GL_TIMEOUT_FN(fn) \
GlAfterTimeout (fn)(GetLine *gl, void *data)
On returning, the application's callback is expected to return one of the
following enumerators to tell
gl_get_line() how to procede after the
timeout has been handled by the callback.
GLTO_ABORT - Tell gl_get_line() to abort. When
this happens, gl_get_line() will
return NULL, and a following call
to gl_return_status() will return
GLR_TIMEOUT. Note that if the
application needs errno always to
have a meaningful value when
gl_get_line() returns NULL,
the callback function should set
errno appropriately.
GLTO_REFRESH - Redraw the input line, then continue
waiting for input. You should return
this value if your callback wrote to the
terminal without having first called
gl_normal_io(gl).
GLTO_CONTINUE - In normal blocking-I/O mode, continue to
wait for input, without redrawing the
user's input line.
In non-blocking server I/O mode (see
gl_io_mode(3)), cause gl_get_line()
to act as though I/O blocked. This means
that gl_get_line() will immediately
return NULL, and a following call
to gl_return_status() will return
GLR_BLOCKED.
Note that before calling the callback,
gl_get_line() blocks most signals,
and leaves its own signal handlers installed, so if you need to catch a
particular signal you will need to both temporarily install your own signal
handler, and unblock the signal. Be sure to re-block the signal (if it was
originally blocked) and reinstate the original signal handler, if any, before
returning.
If the callback function needs to read or write to the terminal, it should
ideally first call
gl_normal_io(gl) to temporarily suspend line
editing. This will restore the terminal to canonical, blocking-I/O, mode, and
move the cursor to the start of a new terminal line. Later, when the callback
returns,
gl_get_line() will notice that
gl_normal_io() was
called, redisplay the input line and resume editing. Note that in this case
the return values,
GLTO_REFRESH and
GLTO_CONTINUE are
equivalent.
To support cases where the callback function calls a third-party function which
occasionally and unpredictably writes to the terminal, the automatic
conversion of
"0 to
"r0 is re-enabled before the
callback function is called. If the callack knows that the third-party
function wrote to the terminal, it should then return the
GLTO_REFRESH
return value, to tell
gl_get_line() to redisplay the input line.
Note that although the timeout argument includes a nano-second component, few
computer clocks presently have resolutions that are finer than a few
milliseconds, so asking for less than a few milliseconds is equivalent to
requesting zero seconds on a lot of systems. If this would be a problem, you
should base your timeout selection on the actual resolution of the host clock
(eg. by calling
sysconf(_SC_CLK_TCK)).
To turn off timeouts, simply call
gl_inactivity_timeout() with a
callback argument of
0. The
data argument is ignored in
this case.
SIGNAL HANDLING DEFAULTS¶
By default, the
gl_get_line() function intercepts a number of signals.
This is particularly important for signals which would by default terminate
the process, since the terminal needs to be restored to a usable state before
this happens. In this section, the signals that are trapped by default, and
how
gl_get_line() responds to them, is described. Changing these
defaults is the topic of the following section.
When the following subset of signals are caught,
gl_get_line() first
restores the terminal settings and signal handling to how they were before
gl_get_line() was called, resends the signal, to allow the calling
application's signal handlers to handle it, then if the process still exists,
gl_get_line() returns
NULL and sets
errno as specified
below.
SIGINT - This signal is generated both by the keyboard
interrupt key (usually ^C), and the keyboard
break key.
errno=EINTR
SIGHUP - This signal is generated when the controlling
terminal exits.
errno=ENOTTY
SIGPIPE - This signal is generated when a program attempts
to write to a pipe who's remote end isn't being
read by any process. This can happen for example
if you have called gl_change_terminal() to
redirect output to a pipe hidden under a pseudo
terminal.
errno=EPIPE
SIGQUIT - This signal is generated by the keyboard quit
key (usually ^\).
errno=EINTR
SIGABRT - This signal is generated by the standard C,
abort() function. By default it both
terminates the process and generates a core
dump.
errno=EINTR
SIGTERM - This is the default signal that the UN*X
kill command sends to processes.
errno=EINTR
Note that in the case of all of the above signals, POSIX mandates that by
default the process is terminated, with the addition of a core dump in the
case of the
SIGQUIT signal. In other words, if the calling application
doesn't override the default handler by supplying its own signal handler,
receipt of the corresponding signal will terminate the application before
gl_get_line() returns.
If gl_get_line() aborts with errno set to EINTR, you can find out what signal
caused it to abort, by calling the following function.
int gl_last_signal(const GetLine *gl);
This returns the numeric code (eg.
SIGINT) of the last signal that was
received during the most recent call to
gl_get_line(), or
-1 if
no signals were received.
On systems that support it, when a SIGWINCH (window change) signal is received,
gl_get_line() queries the terminal to find out its new size, redraws
the current input line to accomodate the new size, then returns to waiting for
keyboard input from the user. Unlike other signals, this signal isn't resent
to the application.
Finally, the following signals cause
gl_get_line() to first restore the
terminal and signal environment to that which prevailed before
gl_get_line() was called, then resend the signal to the application. If
the process still exists after the signal has been delivered, then
gl_get_line() then re-establishes its own signal handlers, switches the
terminal back to raw mode, redisplays the input line, and goes back to
awaiting terminal input from the user.
SIGCONT - This signal is generated when a suspended
process is resumed.
SIGPOLL - On SVR4 systems, this signal notifies the
process of an asynchronous I/O event. Note
that under 4.3+BSD, SIGIO and SIGPOLL are
the same. On other systems, SIGIO is ignored
by default, so gl_get_line() doesn't
trap it by default.
SIGPWR - This signal is generated when a power failure
occurs (presumably when the system is on a
UPS).
SIGALRM - This signal is generated when a timer
expires.
SIGUSR1 - An application specific signal.
SIGUSR2 - Another application specific signal.
SIGVTALRM - This signal is generated when a virtual
timer expires (see man setitimer(2)).
SIGXCPU - This signal is generated when a process
exceeds its soft CPU time limit.
SIGXFSZ - This signal is generated when a process
exceeds its soft file-size limit.
SIGTSTP - This signal is generated by the terminal
suspend key, which is usually ^Z, or the
delayed terminal suspend key, which is
usually ^Y.
SIGTTIN - This signal is generated if the program
attempts to read from the terminal while the
program is running in the background.
SIGTTOU - This signal is generated if the program
attempts to write to the terminal while the
program is running in the background.
Obviously not all of the above signals are supported on all systems, so code to
support them is conditionally compiled into the tecla library.
Note that if
SIGKILL or
SIGPOLL, which by definition can't be
caught, or any of the hardware generated exception signals, such as
SIGSEGV,
SIGBUS and
SIGFPE, are received and unhandled
while
gl_get_line() has the terminal in raw mode, the program will be
terminated without the terminal having been restored to a usable state. In
practice, job-control shells usually reset the terminal settings when a
process relinquishes the controlling terminal, so this is only a problem with
older shells.
CUSTOMIZED SIGNAL HANDLING¶
The previous section listed the signals that
gl_get_line() traps by
default, and described how it responds to them. This section describes how to
both add and remove signals from the list of trapped signals, and how to
specify how
gl_get_line() should respond to a given signal.
If you don't need
gl_get_line() to do anything in response to a signal
that it normally traps, you can tell to
gl_get_line() to ignore that
signal by calling
gl_ignore_signal().
int gl_ignore_signal(GetLine *gl, int signo);
The
signo argument is the number of the signal (eg.
SIGINT) that
you want to have ignored. If the specified signal isn't currently one of those
being trapped, this function does nothing.
The
gl_trap_signal() function allows you to either add a new signal to
the list that
gl_get_line() traps, or modify how it responds to a
signal that it already traps.
int gl_trap_signal(GetLine *gl, int signo, unsigned flags,
GlAfterSignal after, int errno_value);
The
signo argument is the number of the signal that you wish to have
trapped. The
flags argument is a set of flags which determine the
environment in which the application's signal handler is invoked, the
after argument tells
gl_get_line() what to do after the
application's signal handler returns, and
errno_value tells
gl_get_line() what to set
errno to if told to abort.
The
flags argument is a bitwise OR of zero or more of the following
enumerators:
GLS_RESTORE_SIG - Restore the caller's signal
environment while handling the
signal.
GLS_RESTORE_TTY - Restore the caller's terminal settings
while handling the signal.
GLS_RESTORE_LINE - Move the cursor to the start of the
line following the input line before
invoking the application's signal
handler.
GLS_REDRAW_LINE - Redraw the input line when the
application's signal handler returns.
GLS_UNBLOCK_SIG - Normally, if the calling program has
a signal blocked (man sigprocmask),
gl_get_line() does not trap that
signal. This flag tells gl_get_line()
to trap the signal and unblock it for
the duration of the call to
gl_get_line().
GLS_DONT_FORWARD - If this flag is included, the signal
will not be forwarded to the signal
handler of the calling program.
Two commonly useful flag combinations are also enumerated as follows:
GLS_RESTORE_ENV = GLS_RESTORE_SIG | GLS_RESTORE_TTY |
GLS_REDRAW_LINE
GLS_SUSPEND_INPUT = GLS_RESTORE_ENV | GLS_RESTORE_LINE
If your signal handler, or the default system signal handler for this signal, if
you haven't overridden it, never either writes to the terminal, nor suspends
or terminates the calling program, then you can safely set the
flags
argument to
0.
If your signal handler always writes to the terminal, reads from it, or suspends
or terminates the program, you should specify the
flags argument as
GL_SUSPEND_INPUT, so that:
1. The cursor doesn't get left in the middle of the input
line.
2. So that the user can type in input and have it echoed.
3. So that you don't need to end each output line with
\r\n, instead of just \n.
The
GL_RESTORE_ENV combination is the same as
GL_SUSPEND_INPUT,
except that it doesn't move the cursor, and if your signal handler doesn't
read or write anything to the terminal, the user won't see any visible
indication that a signal was caught. This can be useful if you have a signal
handler that only occasionally writes to the terminal, where using
GL_SUSPEND_LINE would cause the input line to be unnecessarily
duplicated when nothing had been written to the terminal. Such a signal
handler, when it does write to the terminal, should be sure to start a new
line at the start of its first write, by writing a
\n character, and
should be sure to leave the cursor on a new line before returning. If the
signal arrives while the user is entering a line that only occupies a signal
terminal line, or if the cursor is on the last terminal line of a longer input
line, this will have the same effect as
GL_SUSPEND_INPUT. Otherwise it
will start writing on a line that already contains part of the displayed input
line. This doesn't do any harm, but it looks a bit ugly, which is why the
GL_SUSPEND_INPUT combination is better if you know that you are always
going to be writting to the terminal.
The
after argument, which determines what
gl_get_line() does after
the application's signal handler returns (if it returns), can take any one of
the following values:
GLS_RETURN - Return the completed input line, just as
though the user had pressed the return
key.
GLS_ABORT - Cause gl_get_line() to abort. When
this happens, gl_get_line() returns
NULL, and a following call to
gl_return_status() will return
GLR_SIGNAL. Note that if the
application needs errno always to
have a meaningful value when
gl_get_line() returns NULL,
the callback function should set
errno appropriately.
GLS_CONTINUE - Resume command line editing.
The
errno_value argument is intended to be combined with the
GLS_ABORT option, telling
gl_get_line() what to set the standard
errno variable to before returning
NULL to the calling program.
It can also, however, be used with the
GL_RETURN option, in case you
wish to have a way to distinguish between an input line that was entered using
the return key, and one that was entered by the receipt of a signal.
RELIABLE SIGNAL HANDLING¶
Signal handling is suprisingly hard to do reliably without race conditions. In
gl_get_line() a lot of care has been taken to allow applications to
perform reliable signal handling around
gl_get_line(). This section
explains how to make use of this.
As an example of the problems that can arise if the application isn't written
correctly, imagine that one's application has a SIGINT signal handler that
sets a global flag. Now suppose that the application tests this flag just
before invoking
gl_get_line(). If a SIGINT signal happens to be
received in the small window of time between the statement that tests the
value of this flag, and the statement that calls
gl_get_line(), then
gl_get_line() will not see the signal, and will not be interrupted. As
a result, the application won't be able to respond to the signal until the
user gets around to finishing entering the input line and
gl_get_line()
returns. Depending on the application, this might or might not be a disaster,
but at the very least it would puzzle the user.
The way to avoid such problems is to do the following.
1. If needed, use the
gl_trap_signal() function to
configure
gl_get_line() to abort when important
signals are caught.
2. Configure
gl_get_line() such that if any of the
signals that it catches are blocked when
gl_get_line() is called, they will be unblocked
automatically during times when
gl_get_line() is
waiting for I/O. This can be done either
on a per signal basis, by calling the
gl_trap_signal() function, and specifying the
GLS_UNBLOCK attribute of the signal, or globally by
calling the
gl_catch_blocked() function.
void gl_catch_blocked(GetLine *gl);
This function simply adds the
GLS_UNBLOCK attribute
to all of the signals that it is currently configured to
trap.
3. Just before calling
gl_get_line(), block delivery
of all of the signals that
gl_get_line() is
configured to trap. This can be done using the POSIX
sigprocmask() function in conjunction with the
gl_list_signals() function.
int gl_list_signals(GetLine *gl, sigset_t *set);
This function returns the set of signals that it is
currently configured to catch in the
set argument,
which is in the form required by
sigprocmask().
4. In the example, one would now test the global flag that
the signal handler sets, knowing that there is now no
danger of this flag being set again until
gl_get_line() unblocks its signals while performing
I/O.
5. Eventually
gl_get_line() returns, either because
a signal was caught, an error occurred, or the user
finished entering their input line.
6. Now one would check the global signal flag again, and if
it is set, respond to it, and zero the flag.
7. Use
sigprocmask() to unblock the signals that were
blocked in step 3.
The same technique can be used around certain POSIX signal-aware functions, such
as
sigsetjmp() and
sigsuspend(), and in particular, the former
of these two functions can be used in conjunction with
siglongjmp() to
implement race-condition free signal handling around other long-running system
calls. The way to do this, is explained next, by showing how
gl_get_line() manages to reliably trap signals around calls to
functions like
read() and
select() without race conditions.
The first thing that
gl_get_line() does, whenever it is called, is to use
the POSIX
sigprocmask() function to block the delivery of all of the
signals that it is currently configured to catch. This is redundant if the
application has already blocked them, but it does no harm. It undoes this step
just before returning.
Whenever
gl_get_line() needs to call
read() or
select() to
wait for input from the user, it first calls the POSIX
sigsetjmp()
function, being sure to specify a non-zero value for its
savesigs
argument. The reason for the latter argument will become clear shortly.
If
sigsetjmp() returns zero,
gl_get_line() then does the
following.
a. It uses the POSIX sigaction() function to register
a temporary signal handler to all of the signals that it
is configured to catch. This signal handler does two
things.
1. It records the number of the signal that was received
in a file-scope variable.
2. It then calls the POSIX siglongjmp()
function using the buffer that was passed to
sigsetjmp() for its first argument, and
a non-zero value for its second argument.
When this signal handler is registered, the sa_mask
member of the struct sigaction act argument of the
call to sigaction() is configured to contain all of
the signals that gl_get_line() is catching. This
ensures that only one signal will be caught at once by
our signal handler, which in turn ensures that multiple
instances of our signal handler don't tread on each
other's toes.
b. Now that the signal handler has been set up,
gl_get_line() unblocks all of the signals that it
is configured to catch.
c. It then calls the read() or select() system
calls to wait for keyboard input.
d. If this system call returns (ie. no signal is received),
gl_get_line() blocks delivery of the signals of
interest again.
e. It then reinstates the signal handlers that were
displaced by the one that was just installed.
Alternatively, if
sigsetjmp() returns non-zero, this means that one of
the signals being trapped was caught while the above steps were executing.
When this happens,
gl_get_line() does the following.
First, note that when a call to
siglongjmp() causes
sigsetjmp() to
return, provided that the
savesigs argument of
sigsetjmp() was
non-zero, as specified above, the signal process mask is restored to how it
was when
sigsetjmp() was called. This is the important difference
between
sigsetjmp() and the older problematic
setjmp(), and is
the essential ingredient that makes it possible to avoid signal handling race
conditions. Because of this we are guaranteed that all of the signals that we
blocked before calling
sigsetjmp() are blocked again as soon as any
signal is caught. The following statements, which are then executed, are thus
guaranteed to be executed without any further signals being caught.
1. If so instructed by the
gl_get_line() configuration
attributes of the signal that was caught,
gl_get_line() restores the terminal attributes to
the state that they had when
gl_get_line() was
called. This is particularly important for signals that
suspend or terminate the process, since otherwise the
terminal would be left in an unusable state.
2. It then reinstates the application's signal handlers.
3. Then it uses the C standard-library
raise()
function to re-send the application the signal that
was caught.
3. Next it unblocks delivery of the signal that we just
sent. This results in the signal that was just sent
via
raise(), being caught by the application's
original signal handler, which can now handle it as it
sees fit.
4. If the signal handler returns (ie. it doesn't terminate
the process),
gl_get_line() blocks delivery of the
above signal again.
5. It then undoes any actions performed in the first of the
above steps, and redisplays the line, if the signal
configuration calls for this.
6.
gl_get_line() then either resumes trying to
read a character, or aborts, depending on the
configuration of the signal that was caught.
What the above steps do in essence is to take asynchronously delivered signals
and handle them synchronously, one at a time, at a point in the code where
gl_get_line() has complete control over its environment.
THE TERMINAL SIZE¶
On most systems the combination of the
TIOCGWINSZ ioctl and the
SIGWINCH signal is used to maintain an accurate idea of the terminal
size. The terminal size is newly queried every time that
gl_get_line()
is called and whenever a
SIGWINCH signal is received.
On the few systems where this mechanism isn't available, at startup
new_GetLine() first looks for the
LINES and
COLUMNS
environment variables. If these aren't found, or they contain unusable values,
then if a terminal information database like terminfo or termcap is available,
the default size of the terminal is looked up in this database. If this too
fails to provide the terminal size, a default size of 80 columns by 24 lines
is used.
Even on systems that do support
ioctl(TIOCGWINSZ), if the terminal is on
the other end of a serial line, the terminal driver generally has no way of
detecting when a resize occurs or of querying what the current size is. In
such cases no
SIGWINCH is sent to the process, and the dimensions
returned by
ioctl(TIOCGWINSZ) aren't correct. The only way to handle
such instances is to provide a way for the user to enter a command that tells
the remote system what the new size is. This command would then call the
gl_set_term_size() function to tell
gl_get_line() about the
change in size.
int gl_set_term_size(GetLine *gl, int ncolumn, int nline);
The
ncolumn and
nline arguments are used to specify the new
dimensions of the terminal, and must not be less than 1. On systems that do
support
ioctl(TIOCGWINSZ), this function first calls
ioctl(TIOCSWINSZ) to tell the terminal driver about the change in size.
In non-blocking server-I/O mode, if a line is currently being input, the input
line is then redrawn to accomodate the changed size. Finally the new values
are recorded in
gl for future use by
gl_get_line().
The
gl_terminal_size() function allows you to query the current size of
the terminal, and install an alternate fallback size for cases where the size
isn't available. Beware that the terminal size won't be available if reading
from a pipe or a file, so the default values can be important even on systems
that do support ways of finding out the terminal size.
typedef struct {
int nline; /* The terminal has nline lines */
int ncolumn; /* The terminal has ncolumn columns */
} GlTerminalSize;
GlTerminalSize gl_terminal_size(GetLine *gl,
int def_ncolumn,
int def_nline);
This function first updates
gl_get_line()'s fallback terminal dimensions,
then records its findings in the return value.
The
def_ncolumn and
def_nline specify the default number of
terminal columns and lines to use if the terminal size can't be determined via
ioctl(TIOCGWINSZ) or environment variables.
HIDING WHAT YOU TYPE¶
When entering sensitive information, such as passwords, it is best not to have
the text that you are entering echoed on the terminal. Furthermore, such text
should not be recorded in the history list, since somebody finding your
terminal unattended could then recall it, or somebody snooping through your
directories could see it in your history file. With this in mind, the
gl_echo_mode() function allows you to toggle on and off the display and
archival of any text that is subsequently entered in calls to
gl_get_line().
int gl_echo_mode(GetLine *gl, int enable);
The
enable argument specifies whether entered text should be visible or
not. If it is
0, then subsequently entered lines will not be visible on
the terminal, and will not be recorded in the history list. If it is
1,
then subsequent input lines will be displayed as they are entered, and
provided that history hasn't been turned off via a call to
gl_toggle_history(), then they will also be archived in the history
list. Finally, if the
enable argument is
-1, then the echoing
mode is left unchanged, which allows you to non-destructively query the
current setting via the return value. In all cases, the return value of the
function is
0 if echoing was disabled before the function was called,
and
1 if it was enabled.
When echoing is turned off, note that although tab completion will invisibly
complete your prefix as far as possible, ambiguous completions will not be
displayed.
SINGLE CHARACTER QUERIES¶
Using
gl_get_line() to query the user for a single character reply, is
inconvenient for the user, since they must hit the enter or return key before
the character that they typed is returned to the program. Thus the
gl_query_char() function has been provided for single character queries
like this.
int gl_query_char(GetLine *gl, const char *prompt,
char defchar);
This function displays the specified prompt at the start of a new line, and
waits for the user to type a character. When the user types a character,
gl_query_char() displays it to the right of the prompt, starts a
newline, then returns the character to the calling program. The return value
of the function is the character that was typed. If the read had to be aborted
for some reason,
EOF is returned instead. In the latter case, the
application can call the previously documented
gl_return_status(), to
find out what went wrong. This could, for example, have been the reception of
a signal, or the optional inactivity timer going off.
If the user simply hits enter, the value of the
defchar argument is
substituted. This means that when the user hits either newline or return, the
character specified in
defchar, is displayed after the prompt, as
though the user had typed it, as well as being returned to the calling
application. If such a replacement is not important, simply pass
'0 as
the value of
defchar.
If the entered character is an unprintable character, it is displayed
symbolically. For example, control-A is displayed as ^A, and characters beyond
127 are displayed in octal, preceded by a backslash.
As with
gl_get_line(), echoing of the entered character can be disabled
using the
gl_echo_mode() function.
If the calling process is suspended while waiting for the user to type their
response, the cursor is moved to the line following the prompt line, then when
the process resumes, the prompt is redisplayed, and
gl_query_char()
resumes waiting for the user to type a character.
Note that in non-blocking server mode, (see
gl_io_mode(3)), if an incomplete
input line is in the process of being read when
gl_query_char() is
called, the partial input line is discarded, and erased from the terminal,
before the new prompt is displayed. The next call to
gl_get_line() will
thus start editing a new line.
READING RAW CHARACTERS¶
Whereas the
gl_query_char() function visibly prompts the user for a
character, and displays what they typed, the
gl_read_char() function
reads a signal character from the user, without writing anything to the
terminal, or perturbing any incompletely entered input line. This means that
it can be called not only from between calls to
gl_get_line(), but also
from callback functions that the application has registered to be called by
gl_get_line().
int gl_read_char(GetLine *gl);
On success, the return value of
gl_read_char() is the character that was
read. On failure,
EOF is returned, and the
gl_return_status()
function can be called to find out what went wrong. Possibilities include the
optional inactivity timer going off, the receipt of a signal that is
configured to abort gl_get_line(), or terminal I/O blocking, when in
non-blocking server-I/O mode.
Beware that certain keyboard keys, such as function keys, and cursor keys,
usually generate at least 3 characters each, so a single call to
gl_read_char() won't be enough to identify such keystrokes.
CLEARING THE TERMINAL¶
The calling program can clear the terminal by calling
gl_erase_terminal(). In non-blocking server-I/O mode, this function
also arranges for the current input line to be redrawn from scratch when
gl_get_line() is next called.
int gl_erase_terminal(GetLine *gl);
DISPLAYING TEXT DYNAMICALLY¶
Between calls to
gl_get_line(), the
gl_display_text() function
provides a convenient way to display paragraphs of text, left-justified and
split over one or more terminal lines according to the constraints of the
current width of the terminal. Examples of the use of this function may be
found in the demo programs, where it is used to display introductions. In
those examples the advanced use of optional prefixes, suffixes and filled
lines to draw a box around the text is also illustrated.
int gl_display_text(GetLine *gl, int indentation,
const char *prefix,
const char *suffix, int fill_char,
int def_width, int start,
const char *string);
If
gl isn't currently connected to a terminal, for example if the output
of a program that uses
gl_get_line() is being piped to another program
or redirected to a file, then the value of the
def_width parameter is
used as the terminal width.
The
indentation argument specifies the number of characters to use to
indent each line of ouput. The
fill_char argument specifies the
character that will be used to perform this indentation.
The
prefix argument can either be
NULL, or be a string to place at
the beginning of each new line (after any indentation). Similarly, the
suffix argument can either be
NULL, or be a string to place at
the end of each line. The suffix is placed flush against the right edge of the
terminal, and any space between its first character and the last word on that
line is filled with the character specified via the
fill_char argument.
Normally the fill-character is a space.
The
start argument tells
gl_display_text() how many characters
have already been written to the current terminal line, and thus tells it the
starting column index of the cursor. Since the return value of
gl_display_text() is the ending column index of the cursor, by passing
the return value of one call to the
start argument of the next call, a
paragraph that is broken between more than one string can be composed by
calling
gl_display_text() for each successive portion of the paragraph.
Note that literal newline characters are necessary at the end of each
paragraph to force a new line to be started.
On error,
gl_display_text() returns -1.
CALLBACK FUNCTION FACILITIES¶
Unless otherwise stated, callback functions, such as tab completion callbacks
and event callbacks should not call any functions in this module. The
following functions, however, are designed specifically to be used by callback
functions.
Calling the
gl_replace_prompt() function from a callback tells
gl_get_line() to display a different prompt when the callback returns.
Except in non-blocking server mode, it has no effect if used between calls to
gl_get_line(). In non-blocking server mode (see the
gl_io_mode(3) man page, when used between two calls to
gl_get_line() that are operating on the same input line, the current
input line will be re-drawn with the new prompt on the following call to
gl_get_line().
void gl_replace_prompt(GetLine *gl, const char *prompt);
INTERNATIONAL CHARACTER SETS¶
Since libtecla version 1.4.0,
gl_get_line() has been 8-bit clean. This
means that all 8-bit characters that are printable in the user's current
locale are now displayed verbatim and included in the returned input line.
Assuming that the calling program correctly contains a call like the
following,
setlocale(LC_CTYPE, "");
then the current locale is determined by the first of the environment variables
LC_CTYPE,
LC_ALL, and
LANG, that is found to contain a
valid locale name. If none of these variables are defined, or the program
neglects to call setlocale, then the default
C locale is used, which is
US 7-bit ASCII. On most unix-like platforms, you can get a list of valid
locales by typing the command:
locale -a
at the shell prompt. Further documentation on how the user can make use of this
to enter international characters can be found in the
tecla(7) man
page.
THREAD SAFETY¶
In a multi-threaded program, you should use the libtecla_r.a version of the
library. This uses reentrant versions of system functions, where available.
Unfortunately neither terminfo nor termcap were designed to be reentrant, so
you can't safely use the functions of the getline module in multiple threads
(you can use the separate file-expansion and word-completion modules in
multiple threads, see the corresponding man pages for details). However due to
the use of POSIX reentrant functions for looking up home directories etc, it
is safe to use this module from a single thread of a multi-threaded program,
provided that your other threads don't use any termcap or terminfo functions.
FILES¶
libtecla.a - The tecla library
libtecla.h - The tecla header file.
~/.teclarc - The personal tecla customization file.
SEE ALSO¶
libtecla(3), gl_io_mode(3), tecla(7), ef_expand_file(3),
cpl_complete_word(3), pca_lookup_file(3)
AUTHOR¶
Martin Shepherd (mcs@astro.caltech.edu)