Does pthread_cond_wait(&cond_t, &mutex); unlock and then lock the mutex?

Question

I m using pthread_cond_wait(&cond_t, &mutex); in my program and I m wondering why this function needs as a second parameter a mutex variable.

Does the pthread_cond_wait() unlock the mutex at the beginning (beginning of the execution pthread_cond_wait()) and then locked when it finishes (just before leaving pthread_cond_wait())?

Answer 1

When the first thread calls pthread_cond_wait(&cond_t, &mutex); it releases the mutex and waits until condition cond_t is signaled as complete and mutex is available.

So when pthread_cond_signal is called in the other thread, it doesn't "wake up" the thread that waits yet. mutex must be unlocked first, only then there is a chance that the first thread will get a lock, which means that "upon successful return of pthread_cond_wait the mutex shall have been locked and shall be owned by the calling thread."