Tribblix: manual page: gl_io

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

NAME

gl_io_mode, gl_raw_io, gl_normal_io, gl_tty_signals, gl_abandon_line,
gl_handle_signal, gl_pending_io - use gl_get_line() from an external
event loop

SYNOPSIS

cc [ flag... ] file... -ltecla [ library... ]
#include <libtecla.h>

int gl_io_mode(GetLine *gl, GlIOMode mode);

int gl_raw_io(GetLine *gl);

int gl_normal_io(GetLine *gl);

int gl_tty_signals(void (*term_handler)(int), void (*susp_handler)(int),
void (*cont_handler)(int), void (*size_handler)(int));

void gl_abandon_line(GetLine *gl);

void gl_handle_signal(int signo, GetLine *gl, int ngl);

GlPendingIO gl_pending_io(GetLine *gl);

DESCRIPTION

The gl_get_line(3TECLA) function supports two different I/O modes.
These are selected by calling the gl_io_mode() function. The mode
argument of gl_io_mode() specifies the new I/O mode and must be one
of the following.

GL_NORMAL_MODE
Select the normal blocking-I/O mode. In this mode
gl_get_line() does not return until either an error
occurs of the user finishes entering a new line.

GL_SERVER_MODE
Select non-blocking server I/O mode. In this mode,
since non-blocking terminal I/O is used, the entry
of each new input line typically requires many
calls to gl_get_line() from an external I/O-driven
event loop.

Newly created GetLine objects start in normal I/O mode, so to switch
to non-blocking server mode requires an initial call to gl_io_mode().

Server I/O Mode
In non-blocking server I/O mode, the application is required to have
an event loop that calls gl_get_line() whenever the terminal file
descriptor can perform the type of I/O that gl_get_line() is waiting
for. To determine which type of I/O gl_get_line() is waiting for, the
application calls the gl_pending_io() function. The return value is
one of the following two enumerated values.

GLP_READ
gl_get_line() is waiting to write a character to the
terminal.

GLP_WRITE
gl_get_line() is waiting to read a character from the
keyboard.

If the application is using either the select(3C) or poll(2) function
to watch for I/O on a group of file descriptors, then it should call
the gl_pending_io() function before each call to these functions to
determine which direction of I/O it should tell them to watch for,
and configure their arguments accordingly. In the case of the
select() function, this means using the FD_SET() macro to add the
terminal file descriptor either to the set of file descriptors to be
watched for readability or the set to be watched for writability.

As in normal I/O mode, the return value of gl_get_line() is either a
pointer to a completed input line or NULL. However, whereas in normal
I/O mode a NULL return value always means that an error occurred, in
non-blocking server mode, NULL is also returned when gl_get_line()
cannot read or write to the terminal without blocking. Thus in non-
blocking server mode, in order to determine when a NULL return value
signifies that an error occurred or not, it is necessary to call the
gl_return_status() function. If this function returns the enumerated
value GLR_BLOCKED, gl_get_line() is waiting for I/O and no error has
occurred.

When gl_get_line() returns NULL and gl_return_status() indicates that
this is due to blocked terminal I/O, the application should call
gl_get_line() again when the type of I/O reported by gl_pending_io()
becomes possible. The prompt, start_line and start_pos arguments of
gl_get_line() will be ignored on these calls. If you need to change
the prompt of the line that is currently being edited, you can call
the gl_replace_prompt(3TECLA) function between calls to
gl_get_line().

Giving Up The Terminal

A complication that is unique to non-blocking server mode is that it
requires that the terminal be left in raw mode between calls to
gl_get_line(). If this were not the case, the external event loop
would not be able to detect individual key-presses, and the basic
line editing implemented by the terminal driver would clash with the
editing provided by gl_get_line(). When the terminal needs to be used
for purposes other than entering a new input line with gl_get_line(),
it needs to be restored to a usable state. In particular, whenever
the process is suspended or terminated, the terminal must be returned
to a normal state. If this is not done, then depending on the
characteristics of the shell that was used to invoke the program, the
user could end up with a hung terminal. To this end, the
gl_normal_io() function is provided for switching the terminal back
to the state that it was in when raw mode was last established.

The gl_normal_io() function first flushes any pending output to the
terminal, then moves the cursor to the start of the terminal line
which follows the end of the incompletely entered input line. At this
point it is safe to suspend or terminate the process, and it is safe
for the application to read and write to the terminal. To resume
entry of the input line, the application should call the gl_raw_io()
function.

The gl_normal_io() function starts a new line, redisplays the
partially completed input line (if any), restores the cursor position
within this line to where it was when gl_normal_io() was called, then
switches back to raw, non-blocking terminal mode ready to continue
entry of the input line when gl_get_line() is next called.

Note that in non-blocking server mode, if gl_get_line() is called
after a call to gl_normal_io(), without an intervening call to
gl_raw_io(), gl_get_line() will call gl_raw_mode() itself, and the
terminal will remain in this mode when gl_get_line() returns.

Signal Handling

In the previous section it was pointed out that in non-blocking
server mode, the terminal must be restored to a sane state whenever a
signal is received that either suspends or terminates the process. In
normal I/O mode, this is done for you by gl_get_line(), but in non-
blocking server mode, since the terminal is left in raw mode between
calls to gl_get_line(), this signal handling has to be done by the
application. Since there are many signals that can suspend or
terminate a process, as well as other signals that are important to
gl_get_line(), such as the SIGWINCH signal, which tells it when the
terminal size has changed, the gl_tty_signals() function is provided
for installing signal handlers for all pertinent signals.

The gl_tty_signals() function uses gl_get_line()'s internal list of
signals to assign specified signal handlers to groups of signals. The
arguments of this function are as follows.

term_handler
This is the signal handler that is used to trap
signals that by default terminate any process that
receives them (for example, SIGINT or SIGTERM).

susp_handler
This is the signal handler that is used to trap
signals that by default suspend any process that
receives them, (for example, SIGTSTP or SIGTTOU).

cont_handler
This is the signal handler that is used to trap
signals that are usually sent when a process resumes
after being suspended (usually SIGCONT). Beware that
there is nothing to stop a user from sending one of
these signals at other times.

size_handler
This signal handler is used to trap signals that are
sent to processes when their controlling terminals
are resized by the user (for example, SIGWINCH).

These arguments can all be the same, if so desired, and SIG_IGN
(ignore this signal) or SIG_DFL (use the system-provided default
signal handler) can be specified instead of a function where
pertinent. In particular, it is rarely useful to trap SIGCONT, so the
cont_handler argument will usually be SIG_DFL or SIG_IGN.

The gl_tty_signals() function uses the POSIX sigaction(2) function to
install these signal handlers, and it is careful to use the sa_mask
member of each sigaction structure to ensure that only one of these
signals is ever delivered at a time. This guards against different
instances of these signal handlers from simultaneously trying to
write to common global data, such as a shared sigsetjmp(3C) buffer or
a signal-received flag. The signal handlers installed by this
function should call the gl_handle_signal().

The signo argument tells this function which signal it is being asked
to respond to, and the gl argument should be a pointer to the first
element of an array of ngl GetLine objects. If your application has
only one of these objects, pass its pointer as the gl argument and
specify ngl as 1.

Depending on the signal that is being handled, this function does
different things.

Process termination signals

If the signal that was caught is one of those that by default
terminates any process that receives it, then gl_handle_signal() does
the following steps.

1. First it blocks the delivery of all signals that can be
blocked (ie. SIGKILL and SIGSTOP cannot be blocked).

2. Next it calls gl_normal_io() for each of the ngl GetLine
objects. Note that this does nothing to any of the GetLine
objects that are not currently in raw mode.

3. Next it sets the signal handler of the signal to its
default, process-termination disposition.

4. Next it re-sends the process the signal that was caught.

5. Finally it unblocks delivery of this signal, which results
in the process being terminated.

Process suspension signals

If the default disposition of the signal is to suspend the process,
the same steps are executed as for process termination signals,
except that when the process is later resumed, gl_handle_signal()
continues, and does the following steps.

1. It re-blocks delivery of the signal.

2. It reinstates the signal handler of the signal to the one
that was displaced when its default disposition was
substituted.

3. For any of the GetLine objects that were in raw mode when
gl_handle_signal() was called, gl_handle_signal() then
calls gl_raw_io(), to resume entry of the input lines on
those terminals.

4. Finally, it restores the signal process mask to how it was
when gl_handle_signal() was called.

Note that the process is suspended or terminated using the original
signal that was caught, rather than using the uncatchable SIGSTOP and
SIGKILL signals. This is important, because when a process is
suspended or terminated, the parent of the process may wish to use
the status value returned by the wait system call to figure out which
signal was responsible. In particular, most shells use this
information to print a corresponding message to the terminal. Users
would be rightly confused if when their process received a SIGPIPE
signal, the program responded by sending itself a SIGKILL signal, and
the shell then printed out the provocative statement, "Killed!".

Interrupting The Event Loop

If a signal is caught and handled when the application's event loop
is waiting in select() or poll(), these functions will be aborted
with errno set to EINTR. When this happens the event loop should call
gl_pending_io() before calling select() or poll() again. It should
then arrange for select() or poll() to wait for the type of I/O that
gl_pending_io() reports. This is necessary because any signal handler
that calls gl_handle_signal() will frequently change the type of I/O
that gl_get_line() is waiting for.

If a signal arrives between the statements that configure the
arguments of select() or poll() and the calls to these functions, the
signal will not be seen by these functions, which will then not be
aborted. If these functions are waiting for keyboard input from the
user when the signal is received, and the signal handler arranges to
redraw the input line to accommodate a terminal resize or the
resumption of the process. This redisplay will be delayed until the
user presses the next key. Apart from puzzling the user, this clearly
is not a serious problem. However there is a way, albeit complicated,
to completely avoid this race condition. The following steps
illustrate this.

1. Block all of the signals that gl_get_line() catches, by
passing the signal set returned by gl_list_signals() to
sigprocmask(2).

2. Call gl_pending_io() and set up the arguments of select()
or poll() accordingly.

3. Call sigsetjmp(3C) with a non-zero savemask argument.

4. Initially this sigsetjmp() statement will return zero,
indicating that control is not resuming there after a
matching call to siglongjmp(3C).

5. Replace all of the handlers of the signals that
gl_get_line() is configured to catch, with a signal
handler that first records the number of the signal that
was caught, in a file-scope variable, then calls
siglongjmp() with a non-zero val argument, to return
execution to the above sigsetjmp() statement. Registering
these signal handlers can conveniently be done using the
gl_tty_signals() function.

6. Set the file-scope variable that the above signal handler
uses to record any signal that is caught to -1, so that we
can check whether a signal was caught by seeing if it
contains a valid signal number.

7. Now unblock the signals that were blocked in step 1. Any
signal that was received by the process in between step 1
and now will now be delivered, and trigger our signal
handler, as will any signal that is received until we
block these signals again.

8. Now call select() or poll().

9. When select returns, again block the signals that were
unblocked in step 7.

If a signal is arrived any time during the above steps,
our signal handler will be triggered and cause control to
return to the sigsetjmp() statement, where this time,
sigsetjmp() will return non-zero, indicating that a signal
was caught. When this happens we simply skip the above
block of statements, and continue with the following
statements, which are executed regardless of whether or
not a signal is caught. Note that when sigsetjmp()
returns, regardless of why it returned, the process signal
mask is returned to how it was when sigsetjmp() was
called. Thus the following statements are always executed
with all of our signals blocked.

10. Reinstate the signal handlers that were displaced in step
5.

11. Check whether a signal was caught, by checking the file-
scope variable that the signal handler records signal
numbers in.

12. If a signal was caught, send this signal to the
application again and unblock only this signal so that it
invokes the signal handler which was just reinstated in
step 10.

13. Unblock all of the signals that were blocked in step 7.

Signals Caught By gl_get_line()
Since the application is expected to handle signals in non-blocking
server mode, gl_get_line() does not attempt to duplicate this when it
is being called. If one of the signals that it is configured to catch
is sent to the application while gl_get_line() is being called,
gl_get_line() reinstates the caller's signal handlers, then
immediately before returning, re-sends the signal to the process to
let the application's signal handler handle it. If the process is not
terminated by this signal, gl_get_line() returns NULL, and a
following call to gl_return_status() returns the enumerated value
GLR_SIGNAL.

Aborting Line Input

Often, rather than letting it terminate the process, applications
respond to the SIGINT user-interrupt signal by aborting the current
input line. This can be accomplished in non-blocking server-I/O mode
by not calling gl_handle_signal() when this signal is caught, but by
calling instead the gl_abandon_line() function. This function
arranges that when gl_get_line() is next called, it first flushes any
pending output to the terminal, discards the current input line,
outputs a new prompt on the next line, and finally starts accepting
input of a new input line from the user.

Signal Safe Functions

Provided that certain rules are followed, the gl_normal_io(),
gl_raw_io(), gl_handle_signal(), and gl_abandon_line() functions can
be written to be safely callable from signal handlers. Other
functions in this library should not be called from signal handlers.
For this to be true, all signal handlers that call these functions
must be registered in such a way that only one instance of any one of
them can be running at one time. The way to do this is to use the
POSIX sigaction() function to register all signal handlers, and when
doing this, use the sa_mask member of the corresponding sigaction
structure to indicate that all of the signals whose handlers invoke
the above functions should be blocked when the current signal is
being handled. This prevents two signal handlers from operating on a
GetLine object at the same time.

To prevent signal handlers from accessing a GetLine object while
gl_get_line() or any of its associated public functions are operating
on it, all public functions associated with gl_get_line(), including
gl_get_line() itself, temporarily block the delivery of signals when
they are accessing GetLine objects. Beware that the only signals that
they block are the signals that gl_get_line() is currently configured
to catch, so be sure that if you call any of the above functions from
signal handlers, that the signals that these handlers are assigned to
are configured to be caught by gl_get_line(). See
gl_trap_signal(3TECLA).

Using Timeouts To Poll

If instead of using select() or poll() to wait for I/O your
application needs only to get out of gl_get_line() periodically to
briefly do something else before returning to accept input from the
user, use the gl_inactivity_timeout(3TECLA) function in non-blocking
server mode to specify that a callback function that returns
GLTO_CONTINUE should be called whenever gl_get_line() has been
waiting for I/O for more than a specified amount of time. When this
callback is triggered, gl_get_line() will return NULL and a following
call to gl_return_status() will return GLR_BLOCKED.

The gl_get_line() function will not return until the user has not
typed a key for the specified interval, so if the interval is long
and the user keeps typing, gl_get_line() might not return for a
while. There is no guarantee that it will return in the time
specified.

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