The Great and Terrible implementation of MPI-2

function index


Retrieves graph topology information associated with a communicator
int MPI_Graph_get(
  MPI_Comm comm,
  int maxindex,
  int maxedges,
  int *index,
  int *edges


[in] communicator with graph structure (handle)
[in] length of vector index in the calling program (integer)
[in] length of vector edges in the calling program (integer)
[out] array of integers containing the graph structure (for details see the definition of MPI_GRAPH_CREATE)
[out] array of integers containing the graph structure


Functions MPI_GRAPHDIMS_GET and MPI_GRAPH_GET retrieve the graph-topology information that was associated with a communicator by MPI_GRAPH_CREATE.

The information provided by MPI_GRAPHDIMS_GET can be used to dimension the vectors index and edges correctly for calls to MPI_GRAPH_GET.

Thread and Interrupt Safety

This routine is both thread- and interrupt-safe. This means that this routine may safely be used by multiple threads and from within a signal handler.

Notes for Fortran

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr at the end of the argument list. ierr is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the call statement.

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.


All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

No error; MPI routine completed successfully.
Invalid topology. Either there is no topology associated with this communicator, or it is not the correct type (e.g., MPI_CART when expecting MPI_GRAPH).
Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).
Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK).

Example Code

The following sample code illustrates MPI_Graph_get.

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

int main( int argc, char *argv[] )
    int errs = 0, i, k;
    int dims[2], periods[2], wsize;
int outdims[2], outperiods[2], outcoords[2];
    int topo_type;
    int *index, *edges, *outindex, *outedges;
    MPI_Comm comm1, comm2;

    MPI_Init( &argc, &argv );
    MPI_Comm_size( MPI_COMM_WORLD, &wsize );

/* Create a cartesian topology, get its characteristics, then
        dup it and check that the new communicator has the same properties */
    dims[0] = dims[1] = 0;
    MPI_Dims_create( wsize, 2, dims );
    periods[0] = periods[1] = 0;
    MPI_Cart_create( MPI_COMM_WORLD, 2, dims, periods, 0, &comm1 );
    MPI_Comm_dup( comm1, &comm2 );
    MPI_Topo_test( comm2, &topo_type );
    if (topo_type != MPI_CART) {
        printf( "Topo type of duped cart was not cart\n" );fflush(stdout);
    else {
        MPI_Cart_get( comm2, 2, outdims, outperiods, outcoords );
for (i=0; i<2; i++) {
            if (outdims[i] != dims[i]) {
                printf( "%d = outdims[%d] != dims[%d] = %d\n", outdims[i], i, i, dims[i] );fflush(stdout);
if (outperiods[i] != periods[i]) {
                printf( "%d = outperiods[%d] != periods[%d] = %d\n", outperiods[i], i, i, periods[i] );fflush(stdout);
    MPI_Comm_free( &comm2 );
    MPI_Comm_free( &comm1 );

/* Now do the same with a graph topology */
if (wsize >= 3) {
        index = (
int*)malloc(wsize * sizeof(int) );
        edges = (
int*)malloc(wsize * 2 * sizeof(int) );
if (!index || !edges) {
            printf( "Unable to allocate %d words for index or edges\n", 3 * wsize );fflush(stdout);
            MPI_Abort( MPI_COMM_WORLD, 1 );
        index[0] = 2;
for (i=1; i<wsize; i++) {
            index[i] = 2 + index[i-1];
for (i=0; i<wsize; i++) {
            edges[k++] = (i-1+wsize) % wsize;
            edges[k++] = (i+1) % wsize;
        MPI_Graph_create( MPI_COMM_WORLD, wsize, index, edges, 0, &comm1 );
        MPI_Comm_dup( comm1, &comm2 );
        MPI_Topo_test( comm2, &topo_type );
if (topo_type != MPI_GRAPH) {
            printf( "Topo type of duped graph was not graph\n" );fflush(stdout);
else {
int nnodes, nedges;
            MPI_Graphdims_get( comm2, &nnodes, &nedges );
if (nnodes != wsize) {
                printf( "Nnodes = %d, should be %d\n", nnodes, wsize );fflush(stdout);
if (nedges != 2*wsize) {
                printf( "Nedges = %d, should be %d\n", nedges, 2*wsize );fflush(stdout);
            outindex = (
int*)malloc(wsize * sizeof(int) );
            outedges = (
int*)malloc(wsize * 2 * sizeof(int) );
if (!outindex || !outedges) {
                printf( "Unable to allocate %d words for outindex or outedges\n", 3 * wsize );fflush(stdout);
                MPI_Abort( MPI_COMM_WORLD, 1 );

            MPI_Graph_get( comm2, wsize, 2*wsize, outindex, outedges );
for (i=0; i<wsize; i++) {
if (index[i] != outindex[i]) {
                    printf( "%d = index[%d] != outindex[%d] = %d\n", index[i], i, i, outindex[i] );fflush(stdout);
for (i=0; i<2*wsize; i++) {
if (edges[i] != outedges[i]) {
                    printf( "%d = edges[%d] != outedges[%d] = %d\n", edges[i], i, i, outedges[i] );fflush(stdout);
            free( outindex );
            free( outedges );
        free( index );
        free( edges );
        MPI_Comm_free( &comm2 );
        MPI_Comm_free( &comm1 );

return 0;