/* alltoall.c
 * This program demonstrate the basic usage of MPI_Alltoall
 * MUST USE 4 processors !
 *
 */

#include <stdio.h>
#include "mpi.h"
#define N 4
int main(int argc, char *argv[])
{
     int rank, nproc, i;
     int a[N], b[N];
     FILE *fp; char fn[80];
     MPI_Init(&argc, &argv);
     MPI_Comm_rank(MPI_COMM_WORLD, &rank);
     MPI_Comm_size(MPI_COMM_WORLD, &nproc);
     if (nproc != 4) MPI_Abort(MPI_COMM_WORLD,1);
     sprintf(fn,"data-a2a-%d",rank); 
     fp = fopen(fn,"w");
     for (i=0; i<N; i++) a[i]=i+rank*N+1; 
     for (i=0; i<N; i++) fprintf(fp,"a[%d]=%d ",i,a[i]);
     fprintf(fp,"\n");
     MPI_Alltoall(a, N/nproc, MPI_INT, 
                  b, N/nproc, MPI_INT, MPI_COMM_WORLD);
     for (i=0; i<N; i++) fprintf(fp,"b[%d]=%d ",i,b[i]);
     fprintf(fp,"\n");
     fclose(fp);
     MPI_Finalize();
     return 0;
}