The sigpending function stores the set of signals that are
    blocked from delivery and pending to the calling process in the
    location pointed to by the set argument.

    Call either the sigemptyset or the sigfillset function at least
    once for each object of type sigset_t prior to any other use of
    that object. If you do not initialize an object in this way and
    supply an argument to the sigpending function, the result is
    undefined.

    See also sigemptyset and sigfillset in this section.