/*
 * semaphorePOS1.c++ shows the use of POSIX 
 *      named semaphore as mutex lock
 * $ g++ -Wall semaphorePOS1.c++ -lpthread                        
*/
#include <iostream>
using namespace std;
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>
#include <sys/sem.h>
#include <fcntl.h>
#include <semaphore.h>
#include <errno.h>

int main() {
    int i, cPID, status;
    sem_t *sP;

    sP = sem_open("/abcd", O_CREAT, 0777, 1);
    if(sP == SEM_FAILED && errno != EEXIST){
       cerr << errno << " Semaphore error\n";
       exit(1);
    } 
    cPID = fork();
    if(cPID == -1){
      cerr << "fork() fails\n";
      exit(1);
    } 
    if(cPID != 0) { // Parent 
       for(i=1; i<=5; ++i) {
          sem_wait(sP); // get lock
          cout << "Indian Institute of"; 
          fflush(stdout); 
          sleep(2);
          cout << " Information Technology\n"; 
          sem_post(sP); // relese lock
          sleep(1);
       }
       waitpid(cPID, &status, 0);
       sem_close(sP);
    }
    else { // Child
       for(i=1; i<=5; ++i) {
          sem_wait(sP); // get lock
          cout << "Allahabad "; 
          fflush(stdout);
          sleep(1);
          cout << "Bhubaneswar "; 
          fflush(stdout);
          sleep(1);
          cout << "Kalyani\n"; 
          sem_post(sP); // relese lock
          sleep(2);
       }
    }
    return 0 ;
}