Thread: pthread_cond_wait for binary semaphores

Originally Posted by homer_3

Are there any known algorithms for simulating pthread_cond_wait when using semaphores instead of mutexes?

A semaphore with an initial count of 1 is the same as a mutex.

Originally Posted by homer_3

Are there any known algorithms for simulating pthread_cond_wait when using semaphores instead of mutexes?

The only difference between using a semaphore and pthread_cond_wait is that signaling a semaphore will allow the next thread through even if there is not a thread currently blocking. pthread_cond_signal does nothing if no thread is waiting on a call to pthread_cond_wait.

So instead of just calling sem_post instead of pthread_cond_signal, you have to do something like this:

Code:

// blocking side
lock();
thread_blocking++;
unlock();
sem_wait();
lock();
thread_blocking--;
unlock();

// signal side
lock()
if(thread_blocking)
    sem_post();
unlock();

The calls to lock() and unlock() are just locking and unlock a simple mutex. This can also be implemented with a semaphore as brewbuck described.