/* shared.c - thanks to Ke Qui and Andy Barbacki for providing this program
 * Thanks to Nik Unger for removing errors/warnings.
 * for COSC 2P13, Spring 2011
 *
 * Description:
 * The program prompts the user to enter a number, n. It then creates a child process with fork.
 * The child counts to n, incrementing a shared variable, p_shared (initially 0), with each iteration.
 * The parent counts to n and decrements the value of p_shared with each iteration.
 * Given that these processes run in parallel, the result should thus always be 0.
 * However, for sufficiently large n values, this is not the case . test this empirically by 
 * compiling and running the code for various n values. 
 */

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

#define CHILD 0
#define ERROR -1

int main () 
{
	int fork_value;         /* the value returned by the fork() system call */
	int status;             /* used by the wait() system call */
	int shmid;              /* shared memory identifier returned by shmget() */
	void *shmat_value;      /* value returned by shmat() */
	int *p_shared;          /* a pointer to the shared int */
	int i;                  /* a counter */
	int n;                  /* the number times to loop */     
	struct shmid_ds buffer; /* buffer for result of shmctl */

	printf ("How many times should the program run? ");
	scanf ("%d", &n);

	//set up interprocess communication via shared variable
	shmid = shmget(IPC_PRIVATE,4,444); /* 4 bytes, rw------- */
	shmat_value = shmat(shmid, 0, 0); 
	p_shared = shmat_value;

	*p_shared = 0;                   /* initialize shared variable */

	fork_value = fork();
	if (fork_value == ERROR)
	{
		printf ("fork() failed\n");
		return 1;
	}

	if (fork_value == CHILD)	/* child process */
	{
		for (i=0; i < n; i++)
		{
			(*p_shared)++;		/* add 1 */
		}
		shmdt(shmat_value);		/* cleanup */
	}
	else                        /* parent process */
	{
		for (i=0; i < n; i++)
		{
			(*p_shared)--;      /* subtract 1 */
		}
		wait(&status);          /* wait for child to terminate */
		printf ("%d\n", *p_shared);
		
		shmdt(shmat_value);     /* cleanup */
		shmctl(shmid, IPC_RMID, &buffer);
	}
	return 0;
}