The sigisemptyset() function shall check for empty signal set referenced by set.
Applications shall call sigemptyset() or sigfillset() at least once for each object of type sigset_t to initialize it. If an uninitialized or NULL object is passed to sigisemptyset(), the results are undefined.
The sigisemptyset() function shall return a positive non-zero value if the signal set referenced by set is empty, or zero if this set is empty. There are no defined error returns.