Provided by: libtecla1-dev_1.6.1-5_amd64 bug

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.

SYNOPSIS

       #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() function, which is documented separately in the gl_get_line(3) man page,
       supports two different  I/O  modes.   These  are  selected  by  calling  the  gl_io_mode()
       function.

         int gl_io_mode(GetLine *gl, GlIOMode mode);

       The  mode  argument  of  this  function 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()
                             doesn't return until either an error
                             occurs of the user finishes entering a
                             new line. This mode is the focus of
                             the gl_get_line(3) 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 gl_get_line() from
                             an external I/O-driven event loop.
                             This mode is the focus of this man
                             page.

       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 which
       calls gl_get_line() whenever the terminal  file  descriptor  can  do  the  type  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.

         GlPendingIO gl_pending_io(GetLine *gl);

       The return value of this function 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 keyboad.

       If the application is using either the select() or poll() system calls 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 see which direction of I/O it should tell them  to  watch  for,
       and  configure  their arguments accordingly. In the case of the select() system call, 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() can't 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, as documented in the gl_get_line(3)
       man page, this means that 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,   then   you   can   call   the
       gl_replace_prompt()  function (documented in the gl_get_line(3) man page) 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 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
       gl_get_line(). What this means is that any time that the terminal needs  to  be  used  for
       other things 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 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 gl_normal_io() function is provided for switching the
       terminal back to the state that it was in when raw mode was last established.

         int gl_normal_io(GetLine *gl);

       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 gl_raw_io() function.

         int gl_raw_io(GetLine *gl);

       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 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.

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

       What this does is use 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 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).

       These  arguments  can  all be the same, if so desired, and you can specify SIG_IGN (ignore
       this signal) or SIG_DFL (use the system-provided default  signal  handler)  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() 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() buffer or a signal-received flag.

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

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

       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 only has one of these objects, just 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.

   Terminal resize signals (SIGWINCH)
       If the signal indicates that the terminal was resized, then it arranges for the next  call
       to  gl_get_line()  to  ask  the  terminal  for  its  new  size  and  redraw the input line
       accordingly. In order that gl_get_line() be  called  as  soon  as  possible  to  do  this,
       gl_handle_signal()  also  arranges  that  the  next  call  to  gl_pending_io() will return
       GLP_WRITE. Thus if the  application  waits  for  I/O  in  select()  or  poll(),  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.

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 can't 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 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.

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.

       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 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.

       9. 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 this reports. This is
       necessary, because any signal handler  which  calls  gl_handle_signal(),  will  frequently
       change the type of I/O that gl_get_line() is waiting for.

       Unfortunately, if a signal arrives between the statements which configure the arguments of
       select() or poll() 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 accomodate 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 gl_get_line() catches,
          by passing the signal set returned by gl_list_signals() to
          sigprocmask().

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

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

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

       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 value 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.

       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 demo3.c 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 select() which encompasses steps 3 to 11. In this wrapper,  rather
       than use gl_list_signals() to figure out the signals to block, and and gl_tty_signals() to
       assign and revert signal handlers, one of its arguments  is  a  sigset_t  which  specifies
       which signals to block and assign signal handlers to. This function thus doesn't depend on
       gl_get_line() and can thus be used in other situations  where  race-condition-free  signal
       handling is required.

SIGNALS CAUGHT BY GL_GET_LINE

       Since  the  application  is  expected  to  handle  signals  in  non-blocking  server mode,
       gl_get_line() 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 gl_get_line() is
       being called, gl_get_line() 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, 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. The  way  to  do  this  in  non-
       blocking server-I/O mode is to not call gl_handle_signal() when this signal is caught, but
       instead to call the gl_abandon_line().

         void gl_abandon_line(GetLine *gl);

       This function arranges that when gl_get_line()  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.

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.

         gl_normal_io()
         gl_raw_io()
         gl_handle_signal()
         gl_abandon_line()

       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 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 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 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()).

USING TIMEOUTS TO POLL

       If instead of using select() or poll() to wait for I/O, your application just needs to get
       out of gl_get_line() 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
       gl_inactivity_timeout() function (see gl_get_line(3)), 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.

       Beware that gl_get_line() 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,  gl_get_line()  may  not
       return  for  a while. In other words there is no guarantee that it will return in the time
       specified.

THE SERVER DEMO PROGRAM

       The demo3 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  demo),  except  that
       whereas  the main demo program uses the normal blocking I/O mode, demo3 using non-blocking
       I/O and an external event loop. The source code can be found in demo3.c, and the  comments
       therein explain the various steps.

FILES

       libtecla.a      -    The tecla library
       libtecla.h      -    The tecla header file.

SEE ALSO

       libtecla(3), gl_get_line(3), tecla(7), ef_expand_file(3),
       cpl_complete_word(3), pca_lookup_file(3)

AUTHOR

       Martin Shepherd  (mcs@astro.caltech.edu)

                                                                                    gl_io_mode(3)