/*
 Race in shared memory 
  $ g++ -Wall sharedMem4.c++              
  $ ./a.out 5000000                                    
*/
#include <iostream>
using namespace std;
#include <stdlib.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/wait.h>
#include <unistd.h>
#define SIZE 4

int main(int count, char *vect[]) {
    int shmID, *p, cPID, n, status ;
    struct shmid_ds buff;

    if(count < 2) {
       cerr << "No 2nd argument\n";
       exit(1) ;
    }
    shmID = shmget(ftok("/home/goutam", 1234), SIZE, 
                            IPC_CREAT | 0777);
    if(shmID == -1) {
       cerr << "Error in shmget" ;
       exit(1) ;
    }
    p = (int *) shmat(shmID, 0, 0777);
    p[0] = 0; // shared memory initialized to 0
    n = atoi(vect[1]);
    cPID = fork();
    if(cPID == -1){
       cerr << "fork() error\n";
       shmdt(p);
       shmctl(shmID, IPC_RMID, &buff);
       exit(1);
    }
    if(cPID > 0){ // parent
       int i;

       for(i=1; i<=n; ++i) p[0]=p[0]+1; 
       waitpid(cPID, &status, 0);      
       cout << "p[0]: " << p[0] << "\n";     
       shmdt(p);
       shmctl(shmID, IPC_RMID, &buff);
    }
    else { // child
       int i;
       for(i=1; i<=n; ++i) p[0]=p[0]-1; 
    }

    return 0 ;
}