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 need a while loop because the thread that called pthread_cond_wait might wake up even when the condition you are waiting for isn't reached. This phenomenon is called "spurious wakeup".

This is not a bug, it is the way the conditional variables are implemented.

This can also be found in man pages:

Spurious wakeups from the pthread_cond_timedwait() or pthread_cond_wait() functions may occur. Since the return from pthread_cond_timedwait() or pthread_cond_wait() does not imply anything about the value of this predicate, the predicate should be re-evaluated upon such return.

Update regarding the actual code:

void* proc_add(void *name) 
{
    struct vars *my_data = (struct vars*)name;

    printf("In thread Addition and my id = %d\n",pthread_self());

    while(1) {

        pthread_mutex_lock(&mutexattr);

        while(!my_data->ipt){  // If no input get in
            pthread_cond_wait(&mutexaddr_add,&mutexattr);  // Wait till signalled
        }

        my_data->opt = my_data->a + my_data->b;
        my_data->ipt=1;
        pthread_cond_signal(&mutexaddr_opt);

        pthread_mutex_unlock(&mutexattr);

        if(my_data->end)
            pthread_exit((void *)0);
        }
    }
}