semclt is always returning -1

Question

I am trying to learn use semophores. What I am trying to do is initialize a semaphore. Then set its value to 1, get that value and print it. But every time I try to do it, it shows me -1. My code is given below. Thanks in advance.

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <time.h>
#include <sys/ipc.h>
#include <sys/sem.h>

union semun {
    int val;
    struct semid_ds *buf;
    ushort *array;
    struct seminfo *__buf;
    void *__pad;
};

int main(){
    int pid, status, semid, value;
    union semun semopts;

    semid = semget(IPC_PRIVATE, 1, IPC_CREAT);

    semopts.val = 1;
    semctl(semid, 0, SETVAL, semopts);

    value = semctl(semid, 0, GETVAL, 0);
    printf("Value  = %d\n", value);

    return 0;
}

Answer 1

The code is missing to at least grant itself read/write access to the semaphore set created.

To do so modifiy the code like this:

if (-1 == (semid = semget(IPC_PRIVATE, 1, IPC_CREAT | IPC_EXCL | S_IRUSR  | S_IWUSR)))
{
  perror("semget() failed");
  exit (EXIT_FAILURE);
}