Why do pthreads’ condition variable functions require a mutex?

Question

I’m reading up on `pthread.h`; the condition variable related functions (like `pthread_cond_wait(3)`) require a mutex as an argument. Why? As far as I can tell, I’m going to be creating a mutex *just* to use as that argument? What is that mutex supposed to do?

Answer 1

Pthread condition variables are used to send signals from one thread to another. Sending any signal requires a channel. The mutex variable works as the channel here.

When one thread (A) is doing some computation and another thread (B) is waiting for the result of that computation, the thread A needs to let the waiting thread B know when it has done with the computation.

So, thread B will listen to a channel (a mutex variable) for the signal to arrive and thread A will send the signal on the same channel (the same mutex variable) when it's done.