= Source code =
{{{
/* Program:
 *   mpich_example 內建範例，計算 pi 。
 * History:
 *   2008-04-11 BETA
 *   2008-06-19 增加可重複輸入欲計算之精準度
 *   2008-06-23 加入 MPI_Barrier 以確保每個 node 在接受 n 後才執行
 * /

#include "mpi.h"
#include <stdio.h>
#include <math.h>
#include <time.h>

double f( double );
double f( double a )
{
    return (4.0 / (1.0 + a*a));
}

int main( int argc, char *argv[])
{
    int done = 0, n, myid, numprocs, i=0;
    double PI25DT = 3.141592653589793238462643;
    double mypi, pi, h, sum, x;
    double startwtime = 0.0, endwtime;
    int  namelen;
    char processor_name[MPI_MAX_PROCESSOR_NAME];
    MPI_Init(&argc,&argv);
    MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
    MPI_Comm_rank(MPI_COMM_WORLD,&myid);
    MPI_Get_processor_name(processor_name,&namelen);
    fprintf(stderr,"Process %d on %s\n",
            myid, processor_name);
    n = 0;
    while (!done)
    {
        /* 由 node 0 將使用者輸入的值送給其它的 node  */
        if (myid == 0)
        {
            printf("Enter the number of intervals: (0 quits) ");
            scanf("%d", &n);
            startwtime = MPI_Wtime();
        }

        /* 這非常重要，所有的 node 必需在此同步，才可以收到使用者輸入的 n */
        MPI_Barrier(MPI_COMM_WORLD);

        /* 將 n 送給其它的 node  */
        MPI_Bcast(&n, 1, MPI_INT, 0, MPI_COMM_WORLD);
        if (n == 0)
            done = 1;
        else
        {

            /* 此為計算 pi 的演算法 */
            h   = 1.0 / (double) n;
            sum = 0.0;
            for (i = myid + 1; i <= n; i += numprocs)
            {
                x = h * ((double)i - 0.5);
                sum += f(x);
            }
            mypi = h * sum;

            /* 將算完的結果傳給 node 0 加總 */
            MPI_Reduce(&mypi, &pi, 1, MPI_DOUBLE, MPI_SUM, 0, MPI_COMM_WORLD);
            if (myid == 0)
            {
                printf("pi is approximately %.16f, Error is %.16f\n",
                       pi, fabs(pi - PI25DT));
                endwtime = MPI_Wtime();
                printf("wall clock time = %f\n",
                       endwtime-startwtime);
            }
        }
    }
    MPI_Finalize();
    return 0;
}
}}}
= Result =
[[Image(cpi-01.png)]]