Question

In the man page it appears that even if you initialise a semaphore to a value of one:

sem_init(&mySem, 0, 1);

It could still be incremented to a value greater than 1 with multiple calls to

sem_post(&mySem);

But in this code example the comment seems to think differently:

sem_init(&mutex, 0, 1);      /* initialize mutex to 1 - binary semaphore */

Is it possible to initialise a strictly binary semaphore in C?

Note: The reason for doing this instead of using a mutex in this case is the sem_post and sem_wait may be called by different threads.

Answer 1

If you want a strictly binary semaphore on Linux, I suggest building one out of mutexes and condition variables.

struct binary_semaphore {
    pthread_mutex_t mutex;
    pthread_cond_t cvar;
    bool v;
};

void mysem_post(struct binary_semaphore *p)
{
    pthread_mutex_lock(&p->mutex);
    if (p->v)
        abort(); // error
    p->v = true;
    pthread_cond_signal(&p->cvar);
    pthread_mutex_unlock(&p->mutex);
}

void mysem_wait(struct binary_semaphore *p)
{
    pthread_mutex_lock(&p->mutex);
    while (!p->v)
        pthread_cond_wait(&p->cvar, &p->mutex);
    p->v = false;
    pthread_mutex_unlock(&p->mutex);
}