The sigsuspend function replaces the signal mask of the process
    with the set of signals pointed to by the signal_mask argument.
    Then it suspends execution of the process until delivery of
    a signal whose action is either to execute a signal catching
    function or to terminate the process. You cannot block the
    SIGKILL or SIGSTOP signals with the sigsuspend function. If a
    program attempts to block either of these signals, sigsuspend
    gives no indication of the error.

    If delivery of a signal causes the process to terminate,
    sigsuspend does not return. If delivery of a signal causes a
    signal catching function to execute, sigsuspend returns after the
    signal catching function returns, with the signal mask restored
    to the set that existed prior to the call to sigsuspend.

    The sigsuspend function sets the signal mask and waits for
    an unblocked signal as one atomic operation. This means that
    signals cannot occur between the operations of setting the mask
    and waiting for a signal. If a program invokes sigprocmask SIG_
    SETMASK and sigsuspend separately, a signal that occurs between
    these functions is often not noticed by sigsuspend.

    In normal usage, a signal is blocked by using the sigprocmask
    function at the beginning of a critical section. The process then
    determines whether there is work for it to do. If there is no
    work, the process waits for work by calling sigsuspend with the
    mask previously returned by sigprocmask.

    If a signal is intercepted by the calling process and control
    is returned from the signal handler, the calling process resumes
    execution after sigsuspend, which always returns a value of -1
    and sets errno to EINTR.

    See also sigpause and sigprocmask.