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22. Signals: Advanced Features > Waiting for a Signal Using a Mask: sigsuspend(...

Waiting for a Signal Using a Mask: sigsuspend()

Before we explain what sigsuspend() does, we first describe a situation where we need to use it. Consider the following scenario that is sometimes encountered when programming with signals:

  1. We temporarily block a signal so that the handler for the signal doesn’t interrupt the execution of some critical section of code.

  2. We unblock the signal, and then suspend execution until the signal is delivered.

In order to do this, we might try using code such as that shown in Example 22-4.

Example 22-4. Incorrectly unblocking and waiting for a signal
sigset_t prevMask, intMask;
    struct sigaction sa;

    sigaddset(&intMask, SIGINT);

    sa.sa_flags = 0;
    sa.sa_handler = handler;

    if (sigaction(SIGINT, &sa, NULL) == -1)

    /* Block SIGINT prior to executing critical section. (At this
       point we assume that SIGINT is not already blocked.) */

    if (sigprocmask(SIG_BLOCK, &intMask, &prevMask) == -1)
        errExit("sigprocmask - SIG_BLOCK");

    /* Critical section: do some work here that must not be
       interrupted by the SIGINT handler */

    /* End of critical section - restore old mask to unblock SIGINT */

    if (sigprocmask(SIG_SETMASK, &prevMask, NULL) == -1)


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