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#include <stdio.h>
#include <stdlib.h>
#include "mpi.h"
#define MAXPROC 8 /* Max number of procsses */
#define NAMELEN 80 /* Max length of machine name */
#define LENGTH 8 /* Size of matrix is LENGTH * LENGTH */
main(int argc, char* argv[]) {
int i, j, np, me, count;
const int nametag = 42; /* Tag value for sending name */
const int datatag = 43; /* Tag value for sending data */
const int root = 0; /* Root process in scatter */
MPI_Status status; /* Status object for receive */
char myname[NAMELEN]; /* Local host name string */
char hostname[MAXPROC][NAMELEN]; /* Received host names */
int x[LENGTH][LENGTH]; /* Send buffer */
int y[LENGTH]; /* Receive buffer */
int *sendcount, *displs; /* Arrays for sendcounts and displacements */
MPI_Init(&argc, &argv); /* Initialize MPI */
MPI_Comm_size(MPI_COMM_WORLD, &np); /* Get nr of processes */
MPI_Comm_rank(MPI_COMM_WORLD, &me); /* Get own identifier */
gethostname(&myname, NAMELEN); /* Get host name */
/* Check that we have one process for each row in the matrix */
if (np != LENGTH) {
if (me == 0) {
printf("You have to use %d processes\n", LENGTH);
}
MPI_Finalize();
exit(0);
}
/* Allocate memory for the sendcount and displacements arrays */
sendcount = (int *)malloc(LENGTH*sizeof(int));
displs = (int *)malloc(LENGTH*sizeof(int));
/* Initialize sendcount and displacements arrays */
for (i=0; i<LENGTH; i++) {
sendcount[i] = i+1;
displs[i] = i*LENGTH;
}
if (me == 0) { /* Process 0 does this */
/* Initialize the matrix x with values 0 .. LENGTH*LENGTH-1 */
for (i=0; i<LENGTH; i++) {
for (j=0; j<LENGTH; j++) {
x[i][j] = i*LENGTH+j;
}
}
/* Print out the matrix before it is distributed */
printf("The matrix is\n");
for (i=0; i<LENGTH; i++) {
for (j=0; j<LENGTH; j++) {
printf("%4d ", x[i][j]);
}
printf("\n");
}
printf("\nProcess %d on host %s is scatteringing the lower triangular part of the matrix x to all %d processes\n\n", me, myname, np);
/* Scatter the lower triangular part of array x to all proceses, place it in y */
MPI_Scatterv(&x, sendcount, displs, MPI_INT, &y, LENGTH, MPI_INT, root, MPI_COMM_WORLD);
/* Process zero prints out own portion of the scattered array */
for (i=0; i<sendcount[me]; i++) {
printf("%4d", y[i]);
}
printf(" ");
printf(" from process %d on host %s\n", me, myname);
/* Receive messages with hostname and the scattered data */
/* from all other processes */
for (i=1; i<np; i++) {
MPI_Recv (&hostname[i], NAMELEN, MPI_CHAR, i, nametag, MPI_COMM_WORLD, \
&status);
MPI_Recv (&y, LENGTH, MPI_INT, i, datatag, MPI_COMM_WORLD, &status);
MPI_Get_count(&status, MPI_INT, &count); /* Get nr of elements in message */
/* Print out the elements we received from this process */
for (j=0; j<count; j++) {
printf("%4d", y[j]);
}
for (j=0; j<LENGTH-count; j++) { /* Format output to look like a matrix */
printf(" ");
}
printf(" from process %d on host %s\n", i, hostname[i]);
}
printf("Ready\n");
} else { /* all other processes do this */
/* Receive the scattered matrix from process 0, place it in array y */
MPI_Scatterv(&x, sendcount, displs, MPI_INT, &y, LENGTH, MPI_INT, root, MPI_COMM_WORLD);
/* Send own name back to process 0 */
MPI_Send (&myname, NAMELEN, MPI_CHAR, 0, nametag, MPI_COMM_WORLD);
/* Send the received array back to process 0 */
MPI_Send (&y, sendcount[me], MPI_INT, 0, datatag, MPI_COMM_WORLD);
}
MPI_Finalize();
}