The sigwait function suspends the calling thread until at least
    one of the signals in the set argument is in the caller's set
    of pending signals. When this happens, one of those signals
    is automatically selected and removed from the set of pending
    signals. The signal number identifying that signal is then
    returned in the location referenced by sig.

    The effect is unspecified if any signals in the set argument are
    not blocked when the sigwait function is called.

    The set argument is created using the set manipulation functions
    sigemptyset, sigfillset, sigaddset, and sigdelset.

    If, while the sigwait function is waiting, a signal occurs that
    is eligible for delivery (that is, not blocked by the signal
    mask), that signal is handled asynchronously and the wait is
    interrupted.

    See also sigtimedwait and sigwaitinfo.

    See the "Error and Signal Handling" chapter of the HP C RTL
    Reference Manual for more information on signal handling.