GNU Info

(libc.info)Handler Returns

---

Next: Termination in Handler Up: Defining Handlers

---

Enter node , (file) or (file)node

Signal Handlers that Return
---------------------------

   Handlers which return normally are usually used for signals such as
`SIGALRM' and the I/O and interprocess communication signals.  But a
handler for `SIGINT' might also return normally after setting a flag
that tells the program to exit at a convenient time.

   It is not safe to return normally from the handler for a program
error signal, because the behavior of the program when the handler
function returns is not defined after a program error.  Note: Program
Error Signals.

   Handlers that return normally must modify some global variable in
order to have any effect.  Typically, the variable is one that is
examined periodically by the program during normal operation.  Its data
type should be `sig_atomic_t' for reasons described in Note: Atomic
Data Access.

   Here is a simple example of such a program.  It executes the body of
the loop until it has noticed that a `SIGALRM' signal has arrived.
This technique is useful because it allows the iteration in progress
when the signal arrives to complete before the loop exits.

     #include <signal.h>
     #include <stdio.h>
     #include <stdlib.h>
     
     /* This flag controls termination of the main loop. */
     volatile sig_atomic_t keep_going = 1;
     
     /* The signal handler just clears the flag and re-enables itself. */
     void
     catch_alarm (int sig)
     {
       keep_going = 0;
       signal (sig, catch_alarm);
     }
     
     void
     do_stuff (void)
     {
       puts ("Doing stuff while waiting for alarm....");
     }
     
     int
     main (void)
     {
       /* Establish a handler for SIGALRM signals. */
       signal (SIGALRM, catch_alarm);
     
       /* Set an alarm to go off in a little while. */
       alarm (2);
     
       /* Check the flag once in a while to see when to quit. */
       while (keep_going)
         do_stuff ();
     
       return EXIT_SUCCESS;
     }