IPC SystemV Semaphores

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

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

void semaphore_wait(int semid)
{
    struct sembuf sb = {0, -1, 0}; // P operation
    semop(semid, &sb, 1);
}

void semaphore_signal(int semid)
{
    struct sembuf sb = {0, +1, 0}; // V operation
    semop(semid, &sb, 1);
}

int main()
{
    key_t key = ftok(".", 65);
    int semid = semget(key, 1, 0666 | IPC_CREAT);

    union semun sem_union;
    sem_union.val = 1;             // Initialize semaphore to 1
    semctl(semid, 0, SETVAL, sem_union);
    pid_t pid = fork();
    if (pid == 0)
    {
        /* Child process */
        sleep(1);
        printf("Child: waiting for semaphore...\n");
        semaphore_wait(semid);
        printf("Child: entered critical section\n");
        semaphore_signal(semid);
        printf("Child: leaving critical section\n");
        exit(0);
    }
    else if (pid > 0)
    {
        /* Parent process */
        printf("Parent: entering critical section\n");
        semaphore_wait(semid);
        sleep(3);
        printf("Parent: leaving critical section\n");
        semaphore_signal(semid);
        wait(NULL);

        /* Remove semaphore */
        semctl(semid, 0, IPC_RMID);
    }

    return 0;
}