Locking semaphores in C problem sys/sem

EDIT: This peace of code is pretty fine (so take it as example of semaphores ;). Bug in my program was in another place - found by my friend.

I have problem with my functions. Sometimes two processes enter into critical section. I can't find problem in this after I spent 10 hours by debugging. On what I should aim? Is there any possibility to have bud in this piece of code?

    // lock semaphore
    static int P(int sem_id)
    {
        struct sembuf sem_b;
        sem_b.sem_num = 0;
        sem_b.sem_op = -1; /* P() */
        sem_b.sem_flg = 0;
        if (semop(sem_id, &sem_b, 1) == -1) {
              // error
                 return(0);
        }
        return(1);
    }

    // unlock semaphore
    static int V(int sem_id)
    {
        struct sembuf sem_b[1];
        sem_b.sem_num = 0;
        sem_b.sem_op = 1; /* V() */
        sem_b.sem_flg = 0;
        if (semop(sem_id, &sem_b, 1) == -1) {
              // error
            return(0);
        }
        return(1);
    }

    static int set_semval(int sem_id)  {
        // set to 1 (opened semaphore)
        if (semctl(sem_id, 0, SETVAL, 1) == -1) {
              // error
            return(0);
        }
        return(1);
    }

    static int get_val(int sem_id)
    {
        union semun sem_union;
        //sem_union.val = 0; ?
        return semctl(sem_id, 0, GETVAL, sem_union);
    }

The action loop:

// semaphores init
int mutex;
if ((mutex=semget(key+2, 1, 0666))>=0) {
    // semaphore exists
    fprintf(stderr,"semaphore exists for key %d\n", key+2);
}

if ((mutex=semget(key+2, 1, 0666 | IPC_CREAT)) == -1) { 
    exit(EXIT_FAILURE);
}
if (!set_semval(mutex)) {
    exit(EXIT_FAILURE);
}
fork()    // some times with good conditionals

// in some children
while(1) {
        P(mutex);   
        assert(get_val(mutex)==0开发者_如何学C); // always ok
        action();  // sometimes made by two processes at same time - fault
        V(mutex);
}   

Pls, feel free to edit my question.

Many thanks

in your 'action loop' what do you do if your semaphore does not exist?

currently your 3rd parameter to semget is 0666 or the PERMISSION_RW constant. You may want to use:

shmget(key, 1, PERMISSION_RW | IPC_CREAT);

this way if your semaphore does not exist it will create one.

So bug was at another place...