[mpi_mms.c]

#include "mpi.h"
#include <stdio.h>
#include <stdlib.h>

#define NRA 62                 /* number of rows in matrix A */
#define NCA 15                 /* number of columns in matrix A */
#define NCB 7                  /* number of columns in matrix B */
#define MASTER 0               /* taskid of first task */
#define FROM_MASTER 1          /* setting a message type */
#define FROM_WORKER 2          /* setting a message type */

int main (int argc, char *argv[])
{
int     numtasks,              /* number of tasks in partition */
        taskid,                /* a task identifier */
        numworkers,            /* number of worker tasks */
        source,                /* task id of message source */
        dest,                  /* task id of message destination */
        mtype,                 /* message type */
        rows,                  /* rows of matrix A sent to each worker */
        averow, extra, offset, /* used to determine rows sent to each worker */
        i, j, k, rc;           /* misc */
double  a[NRA][NCA],           /* matrix A to be multiplied */
        b[NCA][NCB],           /* matrix B to be multiplied */
        c[NRA][NCB];           /* result matrix C */
MPI_Status status;

MPI_Init(&argc,&argv);
MPI_Comm_rank(MPI_COMM_WORLD,&taskid);
MPI_Comm_size(MPI_COMM_WORLD,&numtasks);
if (numtasks < 2 ) {
  printf("Need at least two MPI tasks. Quitting...\n");
  MPI_Abort(MPI_COMM_WORLD, rc);
  exit(1);
  }
numworkers = numtasks-1;


/**************************** master task ************************************/
   if (taskid == MASTER)
   {
      printf("mpi_mm has started with %d tasks.\n",numtasks);
      printf("Initializing arrays...\n");
      for (i=0; i<NRA; i++)
         for (j=0; j<NCA; j++)
            a[i][j]= i+j;
      for (i=0; i<NCA; i++)
         for (j=0; j<NCB; j++)
            b[i][j]= i*j;

      /* Send matrix data to the worker tasks */
      averow = NRA/numworkers;
      extra = NRA%numworkers;
      offset = 0;
      mtype = FROM_MASTER;
      for (dest=1; dest<=numworkers; dest++)
      {
         rows = (dest <= extra) ? averow+1 : averow;
         printf("Sending %d rows to task %d offset=%d\n",rows,dest,offset);
         MPI_Send(&offset, 1, MPI_INT, dest, mtype, MPI_COMM_WORLD);
         MPI_Send(&rows, 1, MPI_INT, dest, mtype, MPI_COMM_WORLD);
         MPI_Send(&a[offset][0], rows*NCA, MPI_DOUBLE, dest, mtype,
                   MPI_COMM_WORLD);
         MPI_Send(&b, NCA*NCB, MPI_DOUBLE, dest, mtype, MPI_COMM_WORLD);
         offset = offset + rows;
      }

      /* Receive results from worker tasks */
      mtype = FROM_WORKER;
      for (i=1; i<=numworkers; i++)
      {
         source = i;
         MPI_Recv(&offset, 1, MPI_INT, source, mtype, MPI_COMM_WORLD, &status);
         MPI_Recv(&rows, 1, MPI_INT, source, mtype, MPI_COMM_WORLD, &status);
         MPI_Recv(&c[offset][0], rows*NCB, MPI_DOUBLE, source, mtype,
                  MPI_COMM_WORLD, &status);
         printf("Received results from task %d\n",source);
      }

      /* Print results */
      printf("******************************************************\n");
      printf("Result Matrix:\n");
      for (i=0; i<NRA; i++)
      {
         printf("\n");
         for (j=0; j<NCB; j++)
            printf("%6.2f   ", c[i][j]);
      }
      printf("\n******************************************************\n");
      printf ("Done.\n");
   }


/**************************** worker task ************************************/
   if (taskid > MASTER)
   {
      mtype = FROM_MASTER;
      MPI_Recv(&offset, 1, MPI_INT, MASTER, mtype, MPI_COMM_WORLD, &status);
      MPI_Recv(&rows, 1, MPI_INT, MASTER, mtype, MPI_COMM_WORLD, &status);
      MPI_Recv(&a, rows*NCA, MPI_DOUBLE, MASTER, mtype, MPI_COMM_WORLD, &status);
      MPI_Recv(&b, NCA*NCB, MPI_DOUBLE, MASTER, mtype, MPI_COMM_WORLD, &status);

      for (k=0; k<NCB; k++)
         for (i=0; i<rows; i++)
         {
            c[i][k] = 0.0;
        for (j=0; j<NCA; j++)
               c[i][k] = c[i][k] + a[i][j] * b[j][k];
         }
      mtype = FROM_WORKER;
      MPI_Send(&offset, 1, MPI_INT, MASTER, mtype, MPI_COMM_WORLD);
      MPI_Send(&rows, 1, MPI_INT, MASTER, mtype, MPI_COMM_WORLD);
      MPI_Send(&c, rows*NCB, MPI_DOUBLE, MASTER, mtype, MPI_COMM_WORLD);
   }
   MPI_Finalize();
}