ex17-advisory-locking should not work

[b1ns4oi]

if ((pid = fork()) < 0) {
        err_sys("fork error");
    } else if (pid > 0) {           /* parent */
        for (i = 0; i < NLOOPS; i += 2) {
            sleep(1);
            printf("parent: before lock\n");
            writew_lock(fd, 0, SEEK_SET, 1);
            printf("parent: got lock\n");

            if ((counter = update((long *)area)) != i)
                err_quit("parent: expected %d, got %d", i, counter);

            un_lock(fd, 0, SEEK_SET, 1);
            printf("parent: unlock\n");
        }
    } else {                        /* child */
        for (i = 1; i < NLOOPS + 1; i += 2) {
            sleep(1);
            printf("child: before lock\n");
            writew_lock(fd, 0, SEEK_SET, 1);
            printf("child: got lock\n");

            if ((counter = update((long *)area)) != i)
                err_quit("child: expected %d, got %d", i, counter);

            un_lock(fd, 0, SEEK_SET, 1);
            printf("child: unlock\n");
        }
    }

After parent un_lock(fd, 0, SEEK_SET, 1); , parent itself can still have a change to acquire the lock again (althrough there's a sleep(1) here, but it just can't make sure child always go next).