The function int pthread_cond_signal(pthread_cond_t *cond); shall unblock at least one of the threads currently blocked on the specified condition variable cond. When each thread unblocked as a result of a pthread_cond_signal() returns from its call to pthread_cond_wait() or pthread_cond_timedwait(), the thread shall own the mutex with which it called pthread_cond_wait() or pthread_cond_timedwait(). The pthread_cond_signal() functions shall have no effect if there are no threads currently blocked on cond. If successful, the pthread_cond_signal() function shall return zero; The pthread_cond_signal() function may fail if: [EINVAL] The value cond does not refer to an initialized condition variable. These functions shall not return an error code of [EINTR].