#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>

main()
{
  int fd, *a;

  /* allocate shared memory */
  if( (fd=open("/dev/zero", O_RDWR)) < 0 ){
    perror("open /dev/zero error");
    exit(1);
  }
  if( (a=mmap(0, sizeof(int), PROT_READ | PROT_WRITE,
              MAP_SHARED, fd, 0)) ==  -1 ){
    perror("mmap error");
    exit(1);
  }

  *a = 14;

  printf("a = %d\n", *a);

  if( (fork()) == 0){
    *a = 21;
  }

  printf("a = %d\n", *a);
}