/**
 * Simple program demonstrating shared memory in POSIX systems.
 *
 * Figure 3.16
 *
 * @author Gagne, Galvin, Silberschatz
 * Operating System Concepts  - Seventh Edition
 * Copyright John Wiley & Sons - 2005.
 * Modified by Prof. Krishna Sivalingam UMBC, Feb. 2006
 */

#include <stdio.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>

int main()
{
  /* the identifier for the shared memory segment */
  int segment_id;
  /* a pointer to the shared memory segment */
  char *shared_memory1, *shared_memory2;
  /* the size (in bytes) of the shared memory segment */ 
  const int segment_size = 64;
  pid_t pid;
  
  /** allocate  a shared memory segment */
  segment_id = shmget(IPC_PRIVATE, segment_size, S_IRUSR | S_IWUSR);

  pid = fork();  
  
  if (pid < 0)
    {				/* error occurred */
      fprintf (stderr, "Fork Failed\n");
      exit (-1);		/* Get out. */
    }
  else if (pid == 0)
    {				/* child process */
      printf ("** I am the child %d **\n", pid);

      /** attach the shared memory segment */
      shared_memory2 = (char *) shmat(segment_id, NULL, 0);
      if (shared_memory2 < 0) 
	{
	  perror("Child: Error in Attach");
	  exit(-1);
	}
      
      printf("shared memory segment %d attached at address %p\n", segment_id, shared_memory2);
      
      /** write a message to the shared memory segment   */
      sprintf(shared_memory2, "Message from child");
  
      /** now print out the string from shared memory */
      printf("Child sez: *%s*\n", shared_memory2);
      
      /** now detach the shared memory segment */ 
      if ( shmdt(shared_memory2) == -1) {
	fprintf(stderr, "Unable to detach\n");
      }
  

      exit(0);
      
    }
  else
    {				/* parent process */
      printf ("I am the parent %d\n", pid);

      wait (NULL);
      printf ("Child Complete\n"); 

      /** attach the shared memory segment */
      shared_memory1 = (char *) shmat(segment_id, NULL, 0);
      if (shared_memory1 < 0) 
	{
	  perror("Child: Error in Attach");
	  exit(-1);
	}

      printf("shared memory segment %d attached at address %p\n", segment_id, shared_memory1);
      
      printf("Parent sez: *%s*\n", shared_memory1);

      /* write a message to the shared memory segment  */
      sprintf(shared_memory1, "Message from parent"); 
  
      /** now print out the string from shared memory */
      printf("Parent sez: *%s*\n", shared_memory1);
      
      /** now detach the shared memory segment */ 
      if ( shmdt(shared_memory1) == -1) {
	fprintf(stderr, "Unable to detach\n");
      }


      /** now remove the shared memory segment */
      shmctl(segment_id, IPC_RMID, NULL); 
  
      exit (0);
    }



  return 0;
}