sem_t sem;

void* thread(void* arg) {
    sem_wait(&mutex);
    printf("Entering, thread id %ld\n", pthread_self());

    sleep(4);

    printf("Exiting, thread id %ld\n", pthread_self());
    sem_post(&mutex);
}

int main() {
    sem_init(&mutex, 0, 2);
    pthread_t t1, t2, t3, t4;
    pthread_create(&t1, NULL, thread, NULL);
    sleep(1);
    pthread_create(&t2, NULL, thread, NULL);
    sleep(1);
    pthread_create(&t3, NULL, thread, NULL);
    sleep(1);
    pthread_create(&t4, NULL, thread, NULL);
    pthread_join(t1, NULL);
    pthread_join(t2, NULL);
    pthread_join(t3, NULL);
    pthread_join(t4, NULL);
    sem_destroy(&mutex);
}