CODEWITHSUNDEEP
csynchronizationposixsemaphore
Manlio
Manlio

Reputation: 10865

POSIX semaphores: synchronization problem

I have a problem with synchronization using posix semaphores. Here's the code I wrote

sem_t *sem1, *sem2;

void* test1() {
    int rv;

    while(1) {
        rv = sem_wait(sem1);
        printf("sem_wait(&sem1) = %d\n",rv);
        printf("test1\n");

        rv = sem_post(sem2);
        printf("sem_post(&sem2) = %d\n\n",rv);
    }
    return NULL;
}

void* test2() {

    while(1) {

        rv = sem_wait(sem2);
        printf("sem_wait(&sem2) = %d\n",rv);
        printf("test2\n");

        rv = sem_post(sem1);
        printf("sem_post(&sem1) = %d\n\n",rv);
    }
    return NULL;
}

int main (int argc, const char * argv[]) {

pthread_t t1,t2;

sem1 = sem_open("sem1", O_CREAT | O_EXCL, 0644, 1);

if (sem1 == SEM_FAILED){
    perror("sem1_init");
    sem_close(sem1);
    return;
}   

sem2 = sem_open("sem2", O_CREAT | O_EXCL, 0644, 0);
if (sem2 == SEM_FAILED){
    perror("sem2_init");
    sem_close(sem1);
    sem_close(sem2);
    return;
}

pthread_create(&t1, NULL, &test1, NULL);
pthread_create(&t2, NULL, &test2, NULL);

pthread_join(t1, NULL);
pthread_join(t2, NULL);

return 0;
}

What I expected is that, since I initialized sem1 to 1 and sem2 to 0, test1 would be the first function to run, and then they will alternate till the end of time.
Instead it's not working, I mean in the log I read many times "test1", many times "test2", for a while alternated and then again without any order. Can someone tell me where is my error?

ps. don't know if it can be useful but I'm running MacOSX 10.6.7

EDIT: I updated my code, removing all calls to sem_init and sem_getvalue (both not present under MacOS), using sem_open to init the semaphores and it seems to work.
I found, however, a strange problem, probably because of my miscomprehension of sem_open call: every time I restart the program if I reuse the same name for semaphores I get the error File exists. How can I force to reuse the same identifier?

Besides. according to man pages, sem_wait should return 0 if successful, -1 if not. What does it mean if I receive 1 (this happen always in test2 )?

Upvotes: 3

Views: 2504

Answers (1)

Summit Guy
Summit Guy

Reputation: 305

If what you want is for the two functions to alternate, you should just use a single semaphore.

When one of your threads gets scheduled, for example t1, it locks sem1 and then keeps it and runs in a while loop until it gets interrupted. Because it's not doing a sem_post(&sem1), it doesn't give control to the other thread.

Update: Looking at this again, this code should work. I think the sem_init not being implemented on OSX is probably your issue.

Update: To answer your question about sem_wait returning 1, it looks like the real error value is returned in errno. If the return value != 0, then check there. My shot in the dark guess would be EINTR is what you are seeing.

Upvotes: 1

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