#include <stdio.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <stdlib.h>

int powmod(int, int, int);
int powmodf(int, int,int);

int p = 103;
int g = 101;
int S_a ;
int S_b ;
int T_a ;
int T_b ;
int main ()
{
  int pid1; //Alice
  int pid2; //Bob
  int pid;  //Parent
  char *shared_memory;
  const int segment_size =4096;
  int segment_id;

  segment_id = shmget(IPC_PRIVATE, segment_size, S_IRUSR | S_IWUSR);
  shared_memory = (char *) shmat(segment_id, NULL, 0);
  printf("\n Qshrdmem: Using Shared Memory for IPC \n");
  printf("Diffie-Hellman Parameters p=%d and g=%d\n", p,g);
  printf("Parent: pid=%d \n", getpid());
  pid1 = fork(); //alice

  if (pid1 == 0 )
	{
 	printf ("Alice: pid=%d, ", getpid());
	printf ("My Parent pid=%d\n", getppid());
	S_a = rand() % 100 + 1;
	printf("\nAlice's random secret = %d", S_a);
	T_a = powmod(S_a, g , p );
	printf("Alice has a T_a of: %d ", T_a);
	sprintf(shared_memory, T_a);}




  else //create Bob
	{
	pid2=fork();
	if (pid2==0)
	{
	usleep(10000);
	printf ("Bob: pid=%d, ", getpid());
	printf ("My Parent pid=%d\n", getppid());
	S_b = rand() % 100 + 1;
        printf("\nAlice's random secret = %d", S_b);
        T_b = powmod(S_b, g , p );
        printf("Alice has a T_b of: %d ", T_b);
        sprintf(shared_memory, T_b);
	}}
  else //Parent code
	{
	usleep(10000);
	printf("Parent: pid=%d \n", getpid());
	printf("Parent view of shared memory: %s\n", shared_memory);}
//detach memory segment
  if (shmdt(shared_memory) == -1)
	{
	fprintf(stderr, "unable to detach\n");}
//remove shared memory segment
	shmct1(segment_id,IPC_RMID, NULL);

return 0;}


powmod (int a, int b, int n)
{

  int i;
  int res = a;
  for(i=1; i<b;i++)
      res = (res * a) % n;
  return res;
}
int powmodf(int a,int b, int n){

        int res=1;
        while (b)
        {
                if (b % 2) { res = (res * a) % n; }
                a = (a * a) % n;
                b /= 2;
        }
        return res;
}