sigaddset() adds signal signo to signal set set. If an application sends an object of type sigset_t to sigaddset() without first calling sigemptyset() or sigfillset(), the results are undefined. sigaddset() returns 0 on success. sigaddset() return -1 if not successful. It also sets errno to indicate the error. If the value of the signo argument is an invalid or unsupported signal number, sigaddset() will return EINVAL error status.