File: gl_io_mode.in

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.\" Copyright (c) 2002, 2003, 2004, 2012 by Martin C. Shepherd
.\" 
.\" All rights reserved.
.\" 
.\" Permission is hereby granted, free of charge, to any person obtaining a
.\" copy of this software and associated documentation files (the
.\" "Software"), to deal in the Software without restriction, including
.\" without limitation the rights to use, copy, modify, merge, publish,
.\" distribute, and/or sell copies of the Software, and to permit persons
.\" to whom the Software is furnished to do so, provided that the above
.\" copyright notice(s) and this permission notice appear in all copies of
.\" the Software and that both the above copyright notice(s) and this
.\" permission notice appear in supporting documentation.
.\" 
.\" THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
.\" OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
.\" MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT
.\" OF THIRD PARTY RIGHTS. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
.\" HOLDERS INCLUDED IN THIS NOTICE BE LIABLE FOR ANY CLAIM, OR ANY SPECIAL
.\" INDIRECT OR CONSEQUENTIAL DAMAGES, OR ANY DAMAGES WHATSOEVER RESULTING
.\" FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
.\" NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION
.\" WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
.\" 
.\" Except as contained in this notice, the name of a copyright holder
.\" shall not be used in advertising or otherwise to promote the sale, use
.\" or other dealings in this Software without prior written authorization
.\" of the copyright holder.
.TH gl_io_mode @FUNC_MANEXT@
.SH NAME
 gl_io_mode, gl_raw_io, gl_normal_io, gl_tty_signals, gl_abandon_line,
 gl_handle_signal, gl_pending_io \- How to use gl_get_line() from an external event loop.
.SH SYNOPSIS
.nf
#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);

.fi

.SH DESCRIPTION

The \f3gl_get_line()\f1 function, which is documented separately in
the \f3gl_get_line(@FUNC_MANEXT@)\f1 man page, supports two different I/O modes.
These are selected by calling the \f3gl_io_mode()\f1 function.

.sp
.nf
  int gl_io_mode(GetLine *gl, GlIOMode mode);
.fi
.sp

The \f3mode\f1 argument of this function specifies the new I/O mode,
and must be one of the following.

.sp
.nf
  GL_NORMAL_MODE   -  Select the normal blocking-I/O mode.
                      In this mode \f3gl_get_line()\f1
                      doesn't return until either an error
                      occurs of the user finishes entering a
                      new line. This mode is the focus of
                      the \f3gl_get_line(@FUNC_MANEXT@)\f1 man page.

  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 \f3gl_get_line()\f1 from
                      an external I/O-driven event loop.
                      This mode is the focus of this man
                      page.
.fi
.sp

Newly created \f3GetLine\f1 objects start in normal I/O
mode, so to switch to non-blocking server mode requires an
initial call to \f3gl_io_mode()\f1.

.SH SERVER I/O MODE

In non-blocking server I/O mode, the application is required
to have an event loop which calls \f3gl_get_line()\f1
whenever the terminal file descriptor can do the type I/O
that \f3gl_get_line()\f1 is waiting for. To determine which
type of I/O \f3gl_get_line()\f1 is waiting for, the
application calls the \f3gl_pending_io()\f1 function.

.sp
.nf
  GlPendingIO gl_pending_io(GetLine *gl);
.fi
.sp

The return value of this function is one of the following two
enumerated values.

.sp
.nf
  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 keyboad.
.fi
.sp

If the application is using either the \f3select()\f1 or \f3poll()\f1
system calls to watch for I/O on a group of file descriptors, then it
should call the \f3gl_pending_io()\f1 function before each call to
these functions to see which direction of I/O it should tell them to
watch for, and configure their arguments accordingly. In the case of
the \f3select()\f1 system call, this means using the \f3FD_SET()\f1
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 \f3gl_get_line()\f1 is
either a pointer to a completed input line, or \f3NULL\f1. However,
whereas in normal I/O mode a \f3NULL\f1 return value always means that
an error occurred, in non-blocking server mode, \f3NULL\f1 is also
returned when \f3gl_get_line()\f1 can't read or write to the terminal
without blocking. Thus in non-blocking server mode, in order to
determine when a \f3NULL\f1 return value signifies that an error
occurred or not, it is necessary to call the \f3gl_return_status()\f1
function. If this function returns the enumerated value,
\f3GLR_BLOCKED\f1, as documented in the \f3gl_get_line(@FUNC_MANEXT@)\f1 man page,
this means that \f3gl_get_line()\f1 is waiting for I/O, and no error
has occurred.

When \f3gl_get_line()\f1 returns \f3NULL\f1 and
\f3gl_return_status()\f1 indicates that this is due to blocked
terminal I/O, the application should call \f3gl_get_line()\f1 again
when the type of I/O reported by \f3gl_pending_io()\f1 becomes
possible. The \f3prompt\f1, \f3start_line\f1 and \f3start_pos\f1
arguments of \f3gl_get_line()\f1 will be ignored on these calls.  If
you need to change the prompt of the line that is currently being
edited, then you can call the \f3gl_replace_prompt()\f1 function
(documented in the \f3gl_get_line(@FUNC_MANEXT@) man page) between calls to
\f3gl_get_line()\f1.

.SH 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
\f3gl_get_line()\f1. If this weren't the case, the external event loop
wouldn't be able to detect individual key-presses, and the basic line
editing implemented by the terminal driver would clash with the
editing provided by \f3gl_get_line()\f1. What this means is that any
time that the terminal needs to be used for other things than entering
a new input line with \f3gl_get_line()\f1, 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
isn't done, then depending on the characteristics of the shell that
was used to invoke the program, the user may end up with a hung
terminal. To this end, the \f3gl_normal_io()\f1 function is provided
for switching the terminal back to the state that it was in when raw
mode was last established.

.sp
.nf
  int gl_normal_io(GetLine *gl);
.fi
.sp

What this function does is first flush any pending output to the
terminal, then move 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 \f3gl_raw_io()\f1
function.

.sp
.nf
  int gl_raw_io(GetLine *gl);
.fi
.sp

This 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 \f3gl_normal_io()\f1 was called, then switches back
to raw, non-blocking terminal mode ready to continue entry of the
input line when \f3gl_get_line()\f1 is next called.

Note that in non-blocking server mode, if \f3gl_get_line()\f1 is
called after a call to \f3gl_normal_io()\f1, without an intervening
call to \f3gl_raw_io()\f1, \f3gl_get_line()\f1 will call
\f3gl_raw_mode()\f1 itself, and the terminal will remain in this mode
when \f3gl_get_line()\f1 returns.

.SH 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 \f3gl_get_line()\f1, but in
non-blocking server mode, since the terminal is left in raw mode
between calls to \f3gl_get_line()\f1, 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
\f3gl_get_line()\f1, such as the \f3SIGWINCH\f1 signal, which tells it
when the terminal size has changed, the \f3gl_tty_signals()\f1
function is provided for installing signal handlers for all pertinent
signals.

.sp
.nf
  int gl_tty_signals(void (*term_handler)(int),
                     void (*susp_handler)(int),
                     void (*cont_handler)(int),
                     void (*size_handler)(int));
.fi
.sp

What this does is use \f3gl_get_line()\f1's internal list of signals
to assign specified signal handlers to groups of signals. The
arguments of this function are as follows.

.sp
.nf
  term_handler  -  This is the signal handler that is to be
                   used to trap signals that by default
                   terminate any process that receives
                   them (eg. SIGINT or SIGTERM).

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

  cont_handler  -  This is the signal handler that is to be
                   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 (eg. SIGWINCH).
.fi
.sp

These arguments can all be the same, if so desired, and you can
specify \f3SIG_IGN\f1 (ignore this signal) or \f3SIG_DFL\f1 (use the
system-provided default signal handler) instead of a function where
pertinent. In particular, it is rarely useful to trap \f3SIGCONT\f1,
so the \f3cont_handler\f1 argument will usually be \f3SIG_DFL\f1 or
\f3SIG_IGN\f1.

The \f3gl_tty_signals()\f1 function uses the POSIX \f3sigaction()\f1
function to install these signal handlers, and it is careful to use
the \f3sa_mask\f1 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
\f3sigsetjmp()\f1 buffer or a signal-received flag.

The signal handlers that are installed by this function, should call
the \f3gl_handle_signal().

.sp
.nf
  void gl_handle_signal(int signo, GetLine *gl, int ngl);
.fi
.sp

The \f3signo\f1 argument tells this function which signal it is being
asked to respond to, and the \f3gl\f1 argument should be a pointer to
the first element of an array of \f3ngl\f1 \f3GetLine\f1 objects. If
your application only has one of these objects, just pass its pointer
as the \f3gl\f1 argument and specify \f3ngl\f1 as \f31\f1.

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

.SS Terminal resize signals (SIGWINCH)

If the signal indicates that the terminal was resized, then it
arranges for the next call to \f3gl_get_line()\f1 to ask the terminal
for its new size and redraw the input line accordingly. In order that
\f3gl_get_line()\f1 be called as soon as possible to do this,
\f3gl_handle_signal()\f1 also arranges that the next call to
\f3gl_pending_io()\f1 will return \f3GLP_WRITE\f1. Thus if the
application waits for I/O in \f3select()\f1 or \f3poll()\f1, then the
application needs to ensure that these functions will be reliably
aborted when a signal is caught and handled by the application. More
on this below.

.SH Process termination signals.

If the signal that was caught is one of those that by default
terminates any process that receives it, then \f3gl_handle_signal()\f1
does the following steps.

1. First it blocks the delivery of all signals that can be
   blocked (ie. \f3SIGKILL\f1 and \f3SIGSTOP\f1 can't be blocked)

2. Next it calls \f3gl_normal_io()\f1 for each of the \f3ngl\f1
   \f3GetLine\f1 objects. Note that this does nothing to any of the
   \f3GetLine\f1 objects that aren't 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.

.SH 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, \f3gl_handle_signal()\f1
continues, and does the following steps.

6. It re-blocks delivery of the signal.

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

8. For any of the \f3GetLine\f1 objects that were in raw mode when
   \f3gl_handle_signal()\f1 was called, \f3gl_handle_signal()\f1 then
   calls \f3gl_raw_io()\f1, to resume entry of the input lines on
   those terminals.

9. Finally, it restores the signal process mask to how it
   was when \f3gl_handle_signal()\f1 was called.

Note that the process is suspended or terminated using the original
signal that was caught, rather than using the uncatchable
\f3SIGSTOP\f1 and \f3SIGKILL\f1 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 \f3wait()\f1 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 \f3SIGPIPE\f1 signal, the program responded by sending
itself a \f3SIGKILL\f1 signal, and the shell then printed out the
provocative statement, "Killed!".

.SH INTERRUPTING THE EVENT LOOP

If a signal is caught and handled when the application's event loop is
waiting in \f3select()\f1 or \f3poll()\f1, these functions will be
aborted with \f3errno\f1 set to \f3EINTR\f1. When this happens the
event loop should call \f3gl_pending_io()\f1, before calling
\f3select()\f1 or \f3poll()\f1 again. It should then arrange for
\f3select()\f1 or \f3poll()\f1 to wait for the type of I/O that this
reports. This is necessary, because any signal handler which calls
\f3gl_handle_signal()\f1, will frequently change the type of I/O that
\f3gl_get_line()\f1 is waiting for.

Unfortunately, if a signal arrives between the statements which
configure the arguments of \f3select()\f1 or \f3poll()\f1 and the
calls to these functions, then 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, then this
redisplay will be end up being delayed until the user hits the next
key. Apart from puzzling the user, this clearly isn't 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 \f3gl_get_line()\f1 catches,
   by passing the signal set returned by \f3gl_list_signals()\f1 to
   \f3sigprocmask()\f1.

2. Call \f3gl_pending_io()\f1 and set up the arguments of
   \f3select()\f1 or \f3poll()\f1 accordingly.

3. Call \f3sigsetjmp()\f1 with a non-zero \f3savesigs\f1 argument.

4. Initially this \f3sigsetjmp()\f1 statement will return zero,
   indicating that control isn't resuming there after a matching
   call to \f3siglongjmp()\f1.

5. Replace all of the handlers of the signals that \f3gl_get_line()\f1
   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 \f3siglongjmp()\f1 with a non-zero value argument, to
   return execution to the above \f3sigsetjmp()\f1
   statement.  Registering these signal handlers can conveniently be
   done using the \f3gl_tty_signals()\f1 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 \f3select()\f1 or \f3poll()\f1.

9. When \f3select()\f1 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
\f3sigsetjmp()\f1 statement, where this time, \f3sigsetjmp()\f1 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 \f3sigsetjmp()\f1
returns, regardless of why it returned, the process signal mask is
returned to how it was when \f3sigsetjmp()\f1 was called. Thus the
following statements are always executed with all of our signals
blocked.

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

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

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

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

Since this is complicated, note that \f3demo3.c\f1 includes a working
example of how to do this. The method used there however, is more
general than the above. What it provides is a wrapper function around
\f3select()\f1 which encompasses steps 3 to 11. In this wrapper,
rather than use \f3gl_list_signals()\f1 to figure out the signals to
block, and and \f3gl_tty_signals()\f1 to assign and revert signal
handlers, one of its arguments is a \f3sigset_t\f1 which specifies
which signals to block and assign signal handlers to. This function
thus doesn't depend on \f3gl_get_line()\f1 and can thus be used in
other situations where race-condition-free signal handling is
required.

.SH SIGNALS CAUGHT BY GL_GET_LINE

Since the application is expected to handle signals in non-blocking
server mode, \f3gl_get_line()\f1 doesn't 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 \f3gl_get_line()\f1 is being
called, \f3gl_get_line()\f1 reinstates the caller's signal handlers,
then just before returning, re-sends the signal to the process to let
the application's signal handler handle it. If the process isn't
terminated by this signal, \f3gl_get_line()\f1 returns \f3NULL\f1, and
a following call to \f3gl_return_status()\f1 returns the enumerated
value \f3GLR_SIGNAL\f1.

.SH 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. The way to do this in non-blocking server-I/O mode is to
not call \f3gl_handle_signal()\f1 when this signal is caught, but
instead to call the \f3gl_abandon_line()\f1.

.sp
.nf
  void gl_abandon_line(GetLine *gl);
.fi
.sp

This function arranges that when \f3gl_get_line()\f1 is next called,
it first flushes any pending output to the terminal, then discardes
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.

.SH SIGNAL SAFE FUNCTIONS

Provided that certain rules are followed, the following functions can
have been written to be safely callable from signal handlers. Other
functions in this library should not be called from signal handlers.

.sp
.nf
  gl_normal_io()
  gl_raw_io()
  gl_handle_signal()
  gl_abandon_line()
.fi
.sp

In order 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 \f3sigaction()\f1 function to register all signal
handlers, and when doing this, use the \f3sa_mask\f1 member of the
corresponding sigaction structure, to indicate that all of the signals
who's handlers invoke the above functions, should be blocked when the
current signal is being handled. This prevents two signal handlers
from operating on a \f3GetLine\f1 object at the same time.

To prevent signal handlers from accessing a \f3GetLine\f1 object while
\f3gl_get_line()\f1 or any of its associated public functions are
operating on it, all public functions associated with
\f3gl_get_line()\f1, including \f3gl_get_line()\f1 itself, temporarily
block the delivery of signals when they are accessing \f3GetLine\f1
objects. Beware that the only signals that they block are the signals
that \f3gl_get_line()\f1 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 \f3gl_get_line()\f1 (see \f3gl_trap_signal()\f1).

.SH USING TIMEOUTS TO POLL

If instead of using \f3select()\f1 or \f3poll()\f1 to wait for I/O,
your application just needs to get out of \f3gl_get_line()\f1
periodically to briefly do something else before returning to accept
input from the user, this can be done in non-blocking server mode by
using the \f3gl_inactivity_timeout()\f1 function (see
\f3gl_get_line(@FUNC_MANEXT@)\f1), to specify that a callback function that
returns \f3GLTO_CONTINUE\f1 should be called whenever
\f3gl_get_line()\f1 has been waiting for I/O for more than a specified
amount of time.

When this callback is triggered, \f3gl_get_line()\f1 will return
\f3NULL\f1, and a following call to \f3gl_return_status()\f1 will
return \f3GLR_BLOCKED\f1.

Beware that \f3gl_get_line()\f1 won't return until the user
hasn't typed a key for the specified interval, so if the
interval is long, and the user keeps typing,
\f3gl_get_line()\f1 may not return for a while. In other
words there is no guarantee that it will return in the time
specified.

.SH THE SERVER DEMO PROGRAM

The \f3demo3\f1 program that is distributed with the library, provides
a working example of how to use non-blocking server I/O mode in a real
program. As far as the user is concerned, this program operates
identically to the main demo program (called \f3demo\f1), except that
whereas the main demo program uses the normal blocking I/O mode,
\f3demo3\f1 using non-blocking I/O and an external event loop. The
source code can be found in \f3demo3.c\f1, and the comments therein
explain the various steps.

.SH FILES
.nf
libtecla.a      -    The tecla library
libtecla.h      -    The tecla header file.
.fi

.SH SEE ALSO

.nf
libtecla(@LIBR_MANEXT@), gl_get_line(@FUNC_MANEXT@), tecla(@MISC_MANEXT@), ef_expand_file(@FUNC_MANEXT@),
cpl_complete_word(@FUNC_MANEXT@), pca_lookup_file(@FUNC_MANEXT@)
.fi

.SH AUTHOR
Martin Shepherd  (mcs@astro.caltech.edu)