// demo shared variables and interference
#include <stdio.h>
#include <sys/types.h>
#include <sys/shm.h>
#include <errno.h>

int *x;

// int MAX = 10;
int MAX = 100000000;

C0() {
  int i;
  int tmp;
  for (i=1; i<=MAX; i++) {
    *x = *x + 1;
    // printf("x = %d\n", *x);
  }
  exit(1);
}

C1() {
  int i;
  int tmp;
  for (i=1; i<=MAX; i++) {
    *x = *x+1;
    // printf("       x = %d\n", *x);
  }
  // exit(1);
}

main() {
  int pid; 
  int shmid, shm_address;
  int NUM_SHARED_VARS = 1;

  if ((shmid = shmget(IPC_PRIVATE, sizeof(int), 0600)) == -1) {
    printf("Error in shmget");
    exit(0);
  }
  if ((int) (shm_address = shmat(shmid, 0, 0)) == -1) {
    printf("Error in shmat");
    exit(0);
  }

  x = (int *) shm_address;

  *x = 0;

  pid = fork();
  if (pid == 0) {
    C0();
  }
  else {
    C1();
    while (wait(NULL) >= 0 || errno != ECHILD) continue;
    printf("\nThe final value of x is: %d\n\n", *x);
    
    if (shmdt(shm_address) == -1) {
      printf("Error in shmdt");
      exit(0);
    }

    if (shmctl(shmid, IPC_RMID, (struct shmid_ds *)0) == -1) {
      printf("Error in shmctl");
      exit(0);
    }
    return 0;
  }
}