#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <unistd.h>

#include <sys/types.h>
#include <fcntl.h>           /* For O_* constants */
#include <sys/stat.h>        /* For mode constants */
#include <sys/mman.h>        /* For mode constants */
#include <semaphore.h>

#define SEM_NAME  "mysem"

#define FILE_MODE       (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)

int count = 0;

int main(int argc, char *argv[])
{
   int    fd, i, nloop, zero = 0;
   int    *ptr;
   int    status;
   pid_t  pid;
   sem_t  *sem;

   if ( argc != 3 ) {
      fprintf(stderr, "usage: %s <pathname> <#loops>\n", argv[0]);
      exit(-1);
   }

   nloop = atoi(argv[2]);

   fd = open(argv[1], O_RDWR | O_CREAT, FILE_MODE);
   assert(fd != -1);

   write(fd, &zero, sizeof(int)); 
   ptr = mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
   assert(ptr != MAP_FAILED);

   sem = sem_open(SEM_NAME, O_CREAT | O_EXCL, FILE_MODE, 1); 
   sem_unlink(SEM_NAME);

   setbuf(stdout, NULL);

   if ( (pid = fork()) == 0 ) {
      for ( i = 0; i < nloop; i++ ) {
         sem_wait(sem);
         printf("child: %d\n", (*ptr)++);
         sem_post(sem);
      }
      exit(0);
   }

   for ( i = 0; i < nloop; i++ ) {
      sem_wait(sem);
      printf("parent: %d\n", (*ptr)++);
      sem_post(sem);
   }

   waitpid(pid, &status, 0);

   exit(0);
}