Is it correct to call semget () and then semop () without calling semctl ()?

Question

I was looking at an example semaphore program in the Beej Guide for Unix IPC .

The sample program contains the following semaphore initialization code. I just post a snippet from it that is relevant to the question. To view the full code, follow the link above.

/*
** initsem() -- more-than-inspired by W. Richard Stevens' UNIX Network
** Programming 2nd edition, volume 2, lockvsem.c, page 295.
*/
int initsem(key_t key, int nsems)  /* key from ftok() */
{
    int i;
    union semun arg;
    struct semid_ds buf;
    struct sembuf sb;
    int semid;

    semid = semget(key, nsems, IPC_CREAT | IPC_EXCL | 0666);

    if (semid >= 0) { /* we got it first */
        sb.sem_op = 1; sb.sem_flg = 0;
        arg.val = 1;

        printf("press return\n"); getchar();

        for(sb.sem_num = 0; sb.sem_num < nsems; sb.sem_num++) { 
            /* do a semop() to "free" the semaphores. */
            /* this sets the sem_otime field, as needed below. */
            if (semop(semid, &sb, 1) == -1) {
                int e = errno;
                semctl(semid, 0, IPC_RMID); /* clean up */
                errno = e;
                return -1; /* error, check errno */
            }
        }

This is what I can’t understand. As soon as it semget()creates semaphores and successfully returns with a valid semaphore identifier, the semaphores themselves are uninitialized and are in an undefined state. This is confirmed by the man page semget.

Semaphore initialization

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Answer 1

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• , , . semctl() SETVAL SETALL .

Answer 2

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