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(libc.info)Testing for Delivery

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Blocking to Test for Delivery of a Signal
-----------------------------------------

   Now for a simple example.  Suppose you establish a handler for
`SIGALRM' signals that sets a flag whenever a signal arrives, and your
main program checks this flag from time to time and then resets it.
You can prevent additional `SIGALRM' signals from arriving in the
meantime by wrapping the critical part of the code with calls to
`sigprocmask', like this:

     /* This variable is set by the SIGALRM signal handler. */
     volatile sig_atomic_t flag = 0;
     
     int
     main (void)
     {
       sigset_t block_alarm;
     
       ...
     
       /* Initialize the signal mask. */
       sigemptyset (&block_alarm);
       sigaddset (&block_alarm, SIGALRM);
     
       while (1)
         {
           /* Check if a signal has arrived; if so, reset the flag. */
           sigprocmask (SIG_BLOCK, &block_alarm, NULL);
           if (flag)
             {
               ACTIONS-IF-NOT-ARRIVED
               flag = 0;
             }
           sigprocmask (SIG_UNBLOCK, &block_alarm, NULL);
     
           ...
         }
     }