Operating Systems: Virtualization, Concurrency & Persistence/

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How to Implement Semaphores

How to Implement Semaphores

Let's see how we can make a semaphore out of a lock and a condition variable.

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Finally, let’s use our low-level synchronization primitives, locks, and condition variables, to build our own version of semaphores called … (drum roll here) … Zemaphores. This task is fairly straightforward, as you can see in the code snippet below.

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typedef struct __Zem_t {

    int value;

    pthread_cond_t cond;

    pthread_mutex_t lock;

} Zem_t;



// only one thread can call this

void Zem_init(Zem_t *s, int value) {

    s->value = value;

    Cond_init(&s->cond);

    Mutex_init(&s->lock);

}



void Zem_wait(Zem_t *s) {

    Mutex_lock(&s->lock);

    while (s->value <= 0)

        Cond_wait(&s->cond, &s->lock);

    s->value--;

    Mutex_unlock(&s->lock);

}



void Zem_post(Zem_t *s) {

    Mutex_lock(&s->lock);

    s->value++;

    Cond_signal(&s->cond);

    Mutex_unlock(&s->lock);

}

Explanation

In the code above, we use just one lock and one ...