/*
 * semSysV1.c++ shows the use of System V 
 *               semaphore as mutex lock
 * $ g++ -Wall semSysV1.c++                         
*/
#include <iostream>
using namespace std;
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/wait.h>
#define FLAGS (0644)

static int P(int semID) {
     struct sembuf buff ;

     buff.sem_num = 0 ; // On the 0th element
     buff.sem_op  = -1 ;
     buff.sem_flg = 0 ;
     if(semop(semID, &buff, 1) == -1) {
       cerr << "semop P operation error\n" ;
       return -1;
     }
     return 0 ;
}

static int V(int semID) {
     struct sembuf buff ;

     buff.sem_num = 0 ;  // On the 0th element
     buff.sem_op  = 1 ;
     buff.sem_flg = 0 ;
     if(semop(semID, &buff, 1) == -1) {
        cerr << "semop V operation error\n" ;
        return -1 ;
     }
     return 0 ;
}

int main() {
    int semID, chID ;

    if((semID = semget(IPC_PRIVATE, 1,  IPC_CREAT | FLAGS)) == -1) {
        cerr << "semget() fails\n" ;
        exit(1) ;
    }
    semctl(semID, 0, SETVAL, 1);
    chID = fork();
    if(chID == -1){
      cerr << "fork() fails\n";
      exit(1);
    }
    if(chID > 0) { // Parent 
      int status ;
                
      for(int i=0; i<=5; ++i) {
          P(semID) ;
          cout << "Indian Institute of";
          fflush(stdout);
          sleep(2);
          cout << " Information Technology\n";
          V(semID) ;
          sleep(1) ;
      }
      waitpid(chID, &status, 0) ;
      semctl(semID, 0, IPC_RMID) ;
    }
    else { // Child
      for(int i=0; i<=5; ++i) {
          P(semID) ;
          cout << "Allahabad ";
          fflush(stdout);
          sleep(1);
          cout << "Bhubaneswar ";
          fflush(stdout);
          sleep(1);
          cout << "Kalyani\n";
          V(semID) ;
          sleep(2) ;
      }
    }
    return 0 ;
}