//---  -lpthread
//---  -lrt   for shm_open

#include <fcntl.h>
#include <sys/mman.h> 
#include <unistd.h>
#include <pthread.h>  
#include <stdio.h>
#include <stdlib.h>


int main(void)
{
int i;
int *x;
int rt;
int shm_id;
char *addnum="myadd";
char *ptr;

pthread_mutex_t mutex;
pthread_mutexattr_t mutexattr;
pthread_mutexattr_init(&mutexattr);
pthread_mutexattr_setpshared(&mutexattr,PTHREAD_PROCESS_SHARED); //--set mutex to share in processes 

rt=fork();
if(rt==0)  //--child process
  {
   shm_id=shm_open(addnum,O_RDWR,0);
   ptr=mmap(NULL,sizeof(int),PROT_READ|PROT_WRITE,MAP_SHARED,shm_id,0);
   x=(int*)ptr;

   for(i=0;i<10;i++)
   {
     pthread_mutex_lock(&mutex);
     (*x)++;
     printf("x++:%d\n",*x);
     pthread_mutex_unlock(&mutex);
     usleep(1000);
   }
 }

else  //--parent  process
{
  shm_id=shm_open(addnum,O_RDWR|O_CREAT,0644);
  ftruncate(shm_id,sizeof(int));
  ptr=mmap(NULL,sizeof(int),PROT_READ|PROT_WRITE,MAP_SHARED,shm_id,0); //--connect to shared memory
  x=(int*)ptr;

  for(i=0;i<10;i++)
  {
    pthread_mutex_lock(&mutex);
    (*x)+=2;
    printf("x+=2:%d\n",*x);
    pthread_mutex_unlock(&mutex);
    usleep(1000);
   } 
}

  shm_unlink(addnum);
  munmap(ptr,sizeof(int));
  return 0;

}