Question

pthread_cond_broadcast is used to wake up several threads waiting on a condition variable. But, at the same time it is also said that we should place a mutex before the condition variable to avoid race conditions.

So, if a mutex is there, and one thread already holds it and thus waits on the variable, how can any other thread hold the same mutex (to enter to the waiting part)?

Answer 1

When a thread calls pthread_cond_wait() it needs to hold the associated mutex. The API will release the mutex while it blocks the thread. Once the API decides the thread needs to be released, it will acquire the mutex before returning from the API.

Holding the mutex is required because checking the condition then calling pthread_cond_wait() isn’t an atomic operation. The mutex (and the proper use of the mutex) prevents the thread from missing the condition becoming true in the short period between checking it and calling the wait.

But the short answer to the specific question (how can another thread obtain the mutex) is that while the thread is blocked in pthread_cond_wait(), the mutex is not held.