c - 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

while(cond is false)
   pthread_cond_wait

My question is, we want to cond_wait only because condition is false. Then why should i take the pain of explicitly putting an if/while loop. I can understand that without any if/while check before cond_wait we will directly hit that and it wont return at all. Is the condition check solely for solving this purpose or does it have anyother significance. If it for solving an unnecessary condition wait, then putting a condition check and avoiding the cond_wait is similar to polling?? I am using cond_wait like this.

void* proc_add(void *name){
    struct vars *my_data = (struct vars*)name;
    printf("In thread Addition and my id = %d
",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);
    }
}

The logic is, I am asking the input thread to process the data whenever an input is available and signal the output thread to print it.

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
",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);
        }
    }
}