#### 24.7.4 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);

               ...
             }
         }