Number of threads waiting for semaphore

Question

So my problem is simple, I need to know how many threads are waiting for the semaphore(are queued). Initially I thought sem_getvalue would return a negative number showing the number of threads blocked by the semaphore but it only goes down to 0 and stays there. I also tried to declare a global variable like int waiting and incrementing it before calling sem_wait with waiting++ and decrementing it after the call but it seems like this solution is not working(by which I am baffled because I thought incrementing and decrementing won't cause race condition). I have looked at the semaphore.h but none of the functions seem to do what I want. Writing here, I came up with an imperfect solution of declaring a second semaphore and initializing it with 0 then calling sem_post on it before the actual sem_wait and a wait after the actual wait. Haven't tested whether it works but even if it does it seems a little hacky a not very nice!

Answer 1

Without getting into why you have a lot of threads waiting on a semaphore, one way to solve this is what you already have tried, having a counter. This counter has to be *atomic*, or do something like:

pthread_mutex_t wait_m = PTHREAD_MUTEX_INITIALIZER;
int wait_counter = 0;

void inc_wait_counter() 
{
    pthread_mutex_lock(&wait_m);
    wait_counter ++;
    pthread_mutex_unlock(&wait_m);

}

void dec_wait_counter() 
{
    pthread_mutex_lock(&wait_m);
    wait_counter --;
    pthread_mutex_unlock(&wait_m);

}

int get_wait_counter()
{
    /* No need to lock here for only reading wait_counter */
    return wait_counter;
}

Then just something like:

inc_wait_counter();
sem_wait(...);
dec_wait_counter();

NOTE: Using atomic integers instead, I suggest to check: How to do an atomic increment and fetch in C?