#include <fcntl.h>           /* For O_* constants */
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>

#include <sys/mman.h>
#include <sys/stat.h>        /* For mode constants */
#include <sys/wait.h>

#define DEBUG 1

#define SHM_KEY "/key"
#define SHM_SZ 8192


#ifdef DEBUG
#define dprintf(format, ...) printf(format, ##__VA_ARGS__)
#else
#define dprintf(format, ...) (void) 0
#endif

#define error(format, ...) do {			\
  fprintf(stderr, format, ## __VA_ARGS__);	\
  exit(0);					\
} while(0)


int main(int argc, char* *argv) {
  int fd = shm_open(SHM_KEY, O_CREAT| O_RDWR, 0666);
  if (fd<0)
    error("shm_open failed");

  /* set the size of the shared memory segment */
  if (ftruncate(fd, SHM_SZ) < 0)
    error("ftruncate failed");

  if (fork() == 0 ) {
    void *buffer = (void *)0x20000000;
    volatile int *ptr, *ptr2;

    dprintf("[%d] I'm the child process\n", getpid());

    /* map the shared memory segment in the current address space */
    if((buffer = mmap(buffer, SHM_SZ, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0)) <0)
      error("mmap failed");

    dprintf("child process starts working\n");

    sleep(10);

    dprintf("child process stops working\n");

    dprintf("child process notifies the parent process\n");
    ptr = buffer;
    *ptr = 1;

    dprintf("child process starts waiting\n");

    ptr2 = ptr + 1;
    while (*ptr2 == 0);

    dprintf("child process stops waiting\n");

    /* unmap the memory region */
    if (munmap(buffer, SHM_SZ) < 0)
      error("munmap failed");
    /* close the shared memory segment */
    if (close(fd) < 0)
      error("close failed");

    return 0;
  } else {
    void *buffer = (void *)0x10000000;
    volatile int *ptr, *ptr2;

    dprintf("[%d] I'm the parent process\n", getpid());

    /* map the shared memory segment in the current address space */
    if((buffer = mmap(buffer, SHM_SZ, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0)) <0)
      error("mmap failed");

    dprintf("parent process starts waiting\n");

    ptr = buffer;
    while(*ptr == 0);

    dprintf("parent process stops waiting\n");

    dprintf("parent process starts working\n");

    sleep(10);

    dprintf("parent process stops working\n");

    dprintf("parent process notifies the child process\n");

    ptr2 = ptr + 1;
    *ptr2 = 1;

    /* unmap the memory region */
    if (munmap(buffer, SHM_SZ) < 0)
      error("munmap failed");
    /* close the shared memory segment */
    if (close(fd) < 0)
      error("close failed");
  }

  wait(0);

  if (shm_unlink(SHM_KEY) < 0)
    error("shm_unlink failed");

  return EXIT_SUCCESS;
}