#include<stdio.h>
#include<stdlib.h>
#include <mpi.h>
#include <math.h>
double f(double a){
    return(4.0 / (1.0 + a*a));
}
int main(int argc, char *argv[])
{
    int n, rank, numprocs, i,rank_parts,rank_start;
    double PI25DT = 3.141592653589793238462643;
    double mypi, pi, h, sum, x;
    MPI_Win nwin, piwin;

    MPI_Init(&argc,&argv);
    MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
    MPI_Comm_rank(MPI_COMM_WORLD,&rank);
    MPI_Win_create(&n, sizeof(int), 1, MPI_INFO_NULL, MPI_COMM_WORLD, &nwin);
    MPI_Win_create(&pi, sizeof(double), 1, MPI_INFO_NULL, MPI_COMM_WORLD, &piwin);  
    while (1) {
        if (rank == 0) {
            printf("Enter the number of intervals: (0 quits)\n");
            scanf("%d",&n);
            pi = 0.0;            
        }
        MPI_Win_fence(0, nwin);
        if (rank != 0)
            MPI_Get(&n, 1, MPI_INT, 0, 0, 1, MPI_INT, nwin);
        MPI_Win_fence(0, nwin);
        if (n == 0)
            break;
        else {
            rank_parts=n/numprocs;
            rank_start=rank*rank_parts;
            h   = 1.0 / (double) n;
            sum = 0.0;
            for (i = 0; i <= rank_parts; i++) {
                x = h * ((double)(rank_start+i));
                sum += f(x);
            }
            mypi = h * sum;
            MPI_Win_fence( 0, piwin);
            MPI_Accumulate(&mypi, 1, MPI_DOUBLE, 0, 0, 1, MPI_DOUBLE,
                           MPI_SUM, piwin);
            MPI_Win_fence(0, piwin);
            if (rank == 0)
                printf("pi is approximately %.16f, Error is %.16f\n",
                       pi, fabs(pi - PI25DT));
        }
    }
    MPI_Win_free(&nwin);
    MPI_Win_free(&piwin);
    MPI_Finalize();
    return 0;
}



Runs