/*
* This is a sample usage of semaphores for parent-child coordination.
* To compile:
*
*  g++ -lpthread semaphores.cpp -o semaphores
*/

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <semaphore.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/wait.h>

    sem_t * sem_id;

static void parent(void)
{

    sem_id=sem_open("somesem", O_CREAT, 0600, 0);
    if(sem_id == SEM_FAILED) {
        perror("parent sem_open");
        return;
    }
    printf("waiting for child\n");
    if(sem_wait(sem_id) < 0) {
        perror("sem_wait");
    }
}

static void child(void)
{
    sem_id=sem_open("somesem", O_CREAT, 0600, 0);
    if(sem_id == SEM_FAILED) {
        perror("child sem_open");
        return;
    }
    printf("Posting for parent\n");
    if(sem_post(sem_id) < 0) {
        perror("sem_post");
    }
}

int main(int argc, char *argv[])
{
    pid_t pid;
    pid=fork();
    if(pid < 0) {
        perror("fork");
        exit(EXIT_FAILURE);
    }

    if(!pid) {
	sleep(1);
        child();    
    } else {
        int status;
        parent();
        wait(&status);
    }
    sem_close(sem_id);
    sem_unlink("somesem");
    return 0;
}