//
// SEMAPHORE: PSEUDO-CODE
//
sem_init(sem_t *s, int initvalue) {
    s->value = initvalue;
    s->lock = phtread_mutex_init();
    s->cond = phtread_cond_init();
}

sem_wait(sem_t *s) { 
    pthread_mutex_lock(&s->lock);
    while (s->value <= 0) 
        phtread_cond_wait(&s->cond, &s->lock) //put_self_to_sleep(); // put self to sleep
    s->value--;
    pthread_mutex_unlock(&s->lock);
}

sem_post(sem_t *s) { 
    s->value++; 
    wake_one_waiting_thread(); // if there is one
}

//
// IMPORTANT: each is done atomically
// (i.e., body of post() and wait() happen all at once)
//