pthread_cond_wait and mutex requirement

Question

Why it is required to lock a mutex before calling pthread_cond_wait?

Also, is it required to take a lock (on the same mutex) before calling pthrea

Answer 1

Condition variables are for synchronising on a condition that you are expecting to change. Locking ensures that:

• The change can be reliably observed on the waiting threads
• The item under change isn't somehow changed by another thread at the same time one of the now-woken-up threads is observing it

A condition system that doesn't use mutexes would be much more brittle.