C中共享内存的信号量无法初始化

Question

The command semctl always returns -1 (returns "Fail2" in console). What am I doing wrong?

union semun{
    int val;
    struct semid_ds *buf;
    unsigned short int *array;
    struct seminfo *__buf;
} forsem;
forsem.val = 0;


int sem;
if((sem= semget(key, 1, 0666 | IPC_CREAT) == -1)) {
    fprintf(stderr, "Fail1");
}

if (semctl(sem, 0, SETVAL, forsem) == -1) {
    fprintf(stderr, "Fail2");
}

Errno writes Invalid argument

Answer 1

You make a simple mistake: in

if((sem= semget(key, 1, 0666 | IPC_CREAT) == -1)) { fprintf(stderr, "Fail1"); }

You should write

if((sem= semget(key, 1, 0666 | IPC_CREAT)) == -1) { fprintf(stderr, "Fail1"); }

Notice the brackets?

By the way, the error is EIDRM because sem is 0 in your code, not Invalid Argument .

Answer 2

Thanks for your answer! It does not matter. The only thing it's not giving error is for semctl with the second argument 0 (number of the semaphore in the semaphore set), if I put 1 or 30 there, it returns -1.