IPC SystemV Semaphores

#include <stdio.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/shm.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>

/* semctl union */
union semun
{
    int val;
    struct semid_ds *buf;
    unsigned short *array;
};

void sem_wait(int semid)
{
    struct sembuf sb = {0, -1, 0};
    semop(semid, &sb, 1);
}

void sem_signal(int semid)
{
    struct sembuf sb = {0, +1, 0};
    semop(semid, &sb, 1);
}

int main()
{
    key_t key = ftok(".", 75);

    /* Shared memory */
    int shmid = shmget(key, sizeof(int), 0666 | IPC_CREAT);
    int *counter = (int *)shmat(shmid, NULL, 0);
    *counter = 0;

    /* Semaphore */
    int semid = semget(key, 1, 0666 | IPC_CREAT);
    union semun arg;
    arg.val = 1;   // mutex
    semctl(semid, 0, SETVAL, arg);
    pid_t pid = fork();
    if (pid == 0)
    {
        /* Child */
        for (int i = 0; i < 5; i++)
        {
            sem_wait(semid);
            (*counter)++;
            printf("Child incremented counter to %d\n", *counter);
            sem_signal(semid);
            sleep(1);
        }
        exit(0);
    }
    else if (pid > 0)
    {
        /* Parent */
        for (int i = 0; i < 5; i++)
        {
            sem_wait(semid);
            (*counter)++;
            printf("Parent incremented counter to %d\n", *counter);
            sem_signal(semid);
            sleep(1);
        }

        wait(NULL);

        /* Cleanup */
        shmdt(counter);
        shmctl(shmid, IPC_RMID, NULL);
        semctl(semid, 0, IPC_RMID);
    }

    return 0;
}