semaphore.c

#include <pthread.h>
#include <assert.h>
#include <stdlib.h>

#include "semaphore.h"

/*	********************************************************************************
	How to implement the Dijsktra semaphore using some mutex and a queue
	******************************************************************************** */

struct sema_state {
    pthread_mutex_t mutex;
    int value;
    pthread_cond_t queue;
};

semaphore_t CreateSemaphore(int initial_value) {
    struct sema_state *s = (struct sema_state *) malloc(sizeof(struct sema_state));
    assert(s != NULL);
    int ret = pthread_mutex_init(&s->mutex, NULL);
    assert(ret == 0);
    s->value = initial_value;
    ret = pthread_cond_init(&s->queue, NULL);
    assert(ret == 0);
    return (semaphore_t) s;
}

void CloseSemaphore(semaphore_t sema) {
    struct sema_state *s = (struct sema_state *) sema;
    int ret = pthread_mutex_destroy(&s->mutex);
    assert(ret == 0);
    ret = pthread_cond_destroy(&s->queue);
    free((char *) s);
}

void P(semaphore_t sema) {
    struct sema_state *s = (struct sema_state *) sema;
    int ret = pthread_mutex_lock(&s->mutex);
    assert(ret == 0);
    s->value--;
    if (s->value < 0) {
        ret = pthread_cond_wait(&s->queue, &s->mutex);
        assert(ret == 0);
    }
    ret = pthread_mutex_unlock(&s->mutex);
    assert(ret == 0);
}

void V(semaphore_t sema) {
    struct sema_state *s = (struct sema_state *) sema;
    int ret = pthread_mutex_lock(&s->mutex);
    assert(ret == 0);
    s->value++;
    if (s->value <= 0) {
        ret = pthread_cond_signal(&s->queue);
        assert(ret == 0);
    }
    ret = pthread_mutex_unlock(&s->mutex);
    assert(ret == 0);
}