Why do we need a condition check before pthread_cond_wait

Question

I am trying to learn basics of pthread_cond_wait. In all the usages, I see either

if(cond is false)
   pthread_cond_wait

or

Answer 1

You must test the condition under the mutex before waiting because signals of the condition variable are not queued (condition variables are not semaphores). That is, if a thread calls pthread_cond_signal() when no threads are blocked in pthread_cond_wait() on that condition variable, then the signal does nothing.

This means that if you had one thread set the condition:

pthread_mutex_lock(&m);
cond = true;
pthread_cond_signal(&c);
pthread_mutex_unlock(&m);

and then another thread unconditionally waited:

pthread_mutex_lock(&m);
pthread_cond_wait(&c, &m);
/* cond now true */

this second thread would block forever. This is avoided by having the second thread check for the condition:

pthread_mutex_lock(&m);
if (!cond)
    pthread_cond_wait(&c, &m);
/* cond now true */

Since cond is only modified with the mutex m held, this means that the second thread waits if and only if cond is false.

The reason a while () loop is used in robust code instead of an if () is because pthread_cond_wait() does not guarantee that it will not wake up spuriously. Using a while () also means that signalling the condition variable is always perfectly safe - "extra" signals don't affect the program's correctness, which means that you can do things like move the signal outside of the locked section of code.