sigdelset() deletes signal signo from signal set set. The results are undefined if an object does not call sigemptyset() or sigfillset() before calling sigdelset(). sigdelset() returns 0 on success. sigdelset() returns -1 when not successful. It also sets errno to indicate the error. If the signo argument is not a valid signal number or is an unsupported signal number, the sigdelset() function will fail with error EINVAL.