#### 25.7.4 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);

          ...
        }
    }