Why `pshared == 0` can still shared between processes in `sem_init(sem, 0, 0)`?

Question

void shmSemphoreDemo() {
    printf("shmSemphoreDemo:\n");

    int shmfd = shmget(IPC_PRIVATE, sizeof(sem_t), IPC_CREAT | 0600);
    sem_t *sem = (sem_t *)shmat(shmfd, NULL, 0);
    sem_init(sem, 0, 0);

    printf("parent start\n");

    int pid = fork();
    if (pid == 0) {
        printf("child sleep start\n");
        sleep(2);
        printf("child sleep end\n");
        sem_post(sem);
        exit(0);
    }

    sem_wait(sem);

    shmdt(sem);
    shmctl(shmfd, IPC_RMID, NULL);

    printf("parent end\n");
}

The second parameter pshared in sem_init(sem, 0, 0) indicates the semaphore shared between threads or processes. But in the C code above, no matter I set it as 1 or 0, the code can always run as expected:

shmSemphoreDemo:
parent start
child sleep start
# slept 2 seconds here
child sleep end
parent end

Why?

If you misuse a function like this, you get undefined behavior. It may or may not work correctly. — interjay, Aug 31 '23 at 12:45
@interjay Could please you tell me what is the correct usage of this function? — Lane, Aug 31 '23 at 12:59
You said it yourself: use 0 if the semaphore is shared between threads in the same process, or 1 if it's shared between processes. — interjay, Aug 31 '23 at 13:06
@interjay You mean if I use 0 when between processes, it's undefined behavior? — Lane, Aug 31 '23 at 13:07
Yes. You're violating the conditions for using the function. — interjay, Aug 31 '23 at 13:13
... and the behavior being undefined is not a guarantee of *failing* behavior under any particular circumstances. — John Bollinger, Aug 31 '23 at 13:57

Why `pshared == 0` can still shared between processes in `sem_init(sem, 0, 0)`?

0 Answers0