When is a condition variable needed, isn't a mutex enough?

Question

I\'m sure mutex isn\'t enough that\'s the reason the concept of condition variables exist; but it beats me and I\'m not able to convince myself with a concrete scenario when

Answer 1

Mutex is for exclusive access of shared resources, while conditional variable is about waiting for a condition to be true. People may think they can implement conditional variable without the support of kernel. A common solution one might come up with is the "flag + mutex" is like:

lock(mutex)

while (!flag) {
    sleep(100);
}

unlock(mutex)

do_something_on_flag_set();

but it will never work, because you never release the mutex during the waiting, no one else can set the flag in a thread-safe way. This is why we need conditional variable, when you're waiting on a condition variable, the associated mutex is not hold by your thread until it's signaled.