source: CIVL/examples/mpi/collective/alltoall.c

/**
 * This program illustrates MPI_Alltoall.
 * online source: http://mpi.deino.net/mpi_functions/MPI_Alltoall.html
 */
#include "mpi.h"
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>

#ifndef EXIT_SUCCESS
#define EXIT_SUCCESS 0
#define EXIT_FAILURE 1
#endif

int main( int argc, char *argv[] )
{
    int rank, size;
    int chunk = 128;
    int i;
    int *sb;
    int *rb;
    int status, gstatus;

    MPI_Init(&argc,&argv);
    MPI_Comm_rank(MPI_COMM_WORLD,&rank);
    MPI_Comm_size(MPI_COMM_WORLD,&size);
    for ( i=1 ; i < argc ; ++i ) {
        if ( argv[i][0] != '-' )
            continue;
        switch(argv[i][1]) {
            case 'm':
                chunk = atoi(argv[++i]);
                break;
            default:
                fprintf(stderr, "Unrecognized argument %s\n", argv[i]);fflush(stderr);
                MPI_Abort(MPI_COMM_WORLD,EXIT_FAILURE);
        }
    }
    sb = (int *)malloc(size*chunk*sizeof(int));
    if ( !sb ) {
        perror( "can't allocate send buffer" );fflush(stderr);
        MPI_Abort(MPI_COMM_WORLD,EXIT_FAILURE);
    }
    rb = (int *)malloc(size*chunk*sizeof(int));
    if ( !rb ) {
        perror( "can't allocate recv buffer");fflush(stderr);
        free(sb);
        MPI_Abort(MPI_COMM_WORLD, EXIT_FAILURE);
    }
    for ( i=0 ; i < size*chunk ; ++i ) {
        sb[i] = rank + 1;
        rb[i] = 0;
    }
    status = MPI_Alltoall(sb, chunk, MPI_INT, rb, chunk, MPI_INT, MPI_COMM_WORLD);
    MPI_Allreduce( &status, &gstatus, 1, MPI_INT, MPI_SUM, MPI_COMM_WORLD );
    if (rank == 0) {
        if (gstatus != 0) {
            printf("all_to_all returned %d\n",gstatus);fflush(stdout);
        }
    }
    free(sb);
    free(rb);
    MPI_Finalize();
    return(EXIT_SUCCESS);
}