sigismember() tests whether the specified signal signo is in the specified set. If an object is not initialized by sigemptyset() or sigfillset() before being supplied to sigismember(), then the result of sigismember is undefined. sigismember() returns 1 if specified member exists in specified set sigismember() returns 0 if specified member does not exist in specified set sigismember() returns -1 and sets errno to indicate the error in case of unsuccessful completion. sigismember() sets errno to [EINVAL] incase the signo argument is not a valid signal number.