UNIX 屏障（barrier）的一個可能的簡單實現（使用條件變數）

typedef struct {
	pthread_cond_t cond;
	pthread_mutex_t mutex;
	unsigned int count;
}mpthread_barrier_t;

int mpthread_barrier_init(mpthread_barrier_t *restrict barrier, unsigned int count) {
	if (barrier != NULL) {
		pthread_mutex_init(&barrier->mutex, NULL);
		pthread_cond_init(&barrier->cond, NULL);
		barrier->count = count;
	}
	return 0;
}

int mpthread_barrier_wait(mpthread_barrier_t *restrict barrier) {
	pthread_mutex_lock(&barrier->mutex);
	if (barrier->count <= 1) {
		if(barrier->count == 1)
			barrier->count--;
		pthread_mutex_unlock(&barrier->mutex);
		pthread_cond_broadcast(&barrier->cond);
		return 0;
	}
	barrier->count--;
	while (barrier->count != 0)
		pthread_cond_wait(&barrier->cond, &barrier->mutex);
	pthread_mutex_unlock(&barrier->mutex);
	return 0;
}

 

示例執行：（使用50個執行緒置一初始值為0的含50個整形元素的陣列為全1）

int arr[TNUM] = {0};
mpthread_barrier_t b;

void *thr_fn(void *arg) {
	int idx = (int)((long)arg);
//	sleep(1);
	arr[idx] = 1;
	mpthread_barrier_wait(&b);	
}

int main() {
	mpthread_barrier_init(&b, TNUM + 1);
	for (long i = 0; i < TNUM; i++) {
		pthread_t tid;
		int err = pthread_create(&tid, NULL, thr_fn, (void*)(i));
		if (err != 0)
			err_exit(err, "can't create thread");
	}
	
	mpthread_barrier_wait(&b);		/*  等待直到barrier->count 為0，表明所有50個執行緒已完成工作 */
	for(int i = 0; i < TNUM; i++)
		printf("%d ", arr[i]);
	putchar(10);
	return 0;
}
 

如有錯誤，肯請指正，不慎感激！