/*
** semdemo.c -- demonstrates semaphore use as a file locking mechanism
*/

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

int main(void)
{
    key_t key;
    int semid;
    struct sembuf sb = {0, -1, 0};  /* set to allocate resource */

    if ((key = ftok("semdemo.c", 'J')) == -1) {
        perror("ftok");
        exit(1);
    }

    /* grab the semaphore set created by seminit.c: */
    if ((semid = semget(key, 1, 0)) == -1) {
        perror("semget");
        exit(1);
    }

    printf("Press return to lock: ");
    getchar();
    printf("Trying to lock...\n");

    if (semop(semid, &sb, 1) == -1) {
        perror("semop");
        exit(1);
    }

    printf("Locked.\n");
    printf("Press return to unlock: ");
    getchar();

    sb.sem_op = 1; /* free resource */
    if (semop(semid, &sb, 1) == -1) {
        perror("semop");
        exit(1);
    }

    printf("Unlocked\n");

    return 0;
}