/*  Shaun Cooper
    January 23, 2005

    simple  demonstration of shared memory segments and forked processes.

    Output:  the output of the shared array updated by processes 1 and 2


*/

#include <errno.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/ipc.h>
#include <sys/shm.h>



/* value to size the array */

#define MAXNUM 10000

/* define the arrays */

/* a pointer to the shared memory segment */
int *in;




/* simple routine to start up to child processes and place values into a shared
memory segment */

main()
{
  int status1, status2;
  int i,j;
  int done;
  int beg,mid,end;
  int level;
  int share_key_in;
  int share_key_out;
  pid_t child1, child2;
  struct shmid_ds item;



  /* define shared memory */

  if ( (share_key_in=shmget(IPC_PRIVATE,MAXNUM * 4 , IPC_CREAT|0666)) < 0)
      perror("Cannot get shared memory\n");

  /* attach shared memory segment */
  if ( (in=shmat(share_key_in,(void *) 0, 0)) == (void *) -1)
     perror("cannot attach to shared memory\n");




  /* simple array initialization of the shared memory segment  */

  for (j=0; j< MAXNUM; j++)
    {   in[j]=0;
    }


  /* start up child 1 */
       child1=fork();

       if ( child1==0) {
	  /* shared memory is already attached due to the rules of fork() */

           in[1]=1;

          exit(0); /* child 1 is now out */

	}
       else {
 
        /* start up child 2 */
	  child2=fork();
	  if (child2==0) { 
	   
	      /* sahred memory for second child  already attached by rule*/
                 in[2]=2;
	              }

	  else {

	       /* parent process */
	       /* waitpid(child1,&status1, 0);
	       waitpid(child2, &status2, 0); */

             /* ONLY the parent process should destroy the shared memory segment*/

               printf("%d %d %d \n", in[0],in[1],in[2]); /* should get 0, 1, 2 */
               shmctl(share_key_in,IPC_RMID,&item);
	  }
	}




}