If a topology has been defined with one of the above functions, then the topology information can be looked up using inquiry functions. They all are local calls.

MPI_TOPO_TEST(comm, status)

[ IN comm] communicator (handle)

[ OUT status] topology type of communicator ` comm` (choice)

` int MPI_Topo_test(MPI_Comm comm, int *status) `

` MPI_TOPO_TEST(COMM, STATUS, IERROR) INTEGER COMM, STATUS, IERROR `

The function MPI_TOPO_TEST returns the type of topology that is assigned to a communicator.

The output value ` status` is one of the following:

[ MPI_GRAPH] graph topology

[ MPI_CART] cartesian topology

[ MPI_UNDEFINED] no topology

MPI_GRAPHDIMS_GET(comm, nnodes, nedges)

[ IN comm] communicator for group with graph structure (handle)

[ OUT nnodes] number of nodes in graph (integer) (same as number of processes in the group)

[ OUT nedges] number of edges in graph (integer)

` int MPI_Graphdims_get(MPI_Comm comm, int *nnodes, int *nedges) `

` MPI_GRAPHDIMS_GET(COMM, NNODES, NEDGES, IERROR) INTEGER COMM, NNODES, NEDGES, IERROR `

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 the following call
to MPI_GRAPH_GET.

MPI_GRAPH_GET(comm, maxindex, maxedges, index, edges)

[ IN comm] communicator with graph structure (handle)

[ IN maxindex] length of vector index in the calling program

(integer)

[ IN maxedges] length of vector edges in the calling program

(integer)

[ OUT index] array of integers containing the graph structure (for details see the definition of MPI_GRAPH_CREATE)

[ OUT edges] array of integers containing the graph structure

` int MPI_Graph_get(MPI_Comm comm, int maxindex, int maxedges, int *index, int *edges) `

` MPI_GRAPH_GET(COMM, MAXINDEX, MAXEDGES, INDEX, EDGES, IERROR) INTEGER COMM, MAXINDEX, MAXEDGES, INDEX(*), EDGES(*), IERROR `

MPI_CARTDIM_GET(comm, ndims)

[ IN comm] communicator with cartesian structure (handle)

[ OUT ndims] number of dimensions of the cartesian structure (integer)

` int MPI_Cartdim_get(MPI_Comm comm, int *ndims) `

` MPI_CARTDIM_GET(COMM, NDIMS, IERROR) INTEGER COMM, NDIMS, IERROR `

The functions MPI_CARTDIM_GET and MPI_CART_GET return the cartesian topology information that was associated with a communicator by MPI_CART_CREATE.

MPI_CART_GET(comm, maxdims, dims, periods, coords)

[ IN comm] communicator with cartesian structure (handle)

[ IN maxdims] length of vectors ` dims, periods`, and ` coords` in the calling program (integer)

[ OUT dims] number of processes for each cartesian dimension (array of integer)

[ OUT periods] periodicity ( true/ false) for each cartesian dimension (array of logical)

[ OUT coords] coordinates of calling process in cartesian structure (array of integer)

` int MPI_Cart_get(MPI_Comm comm, int maxdims, int *dims, int *periods, int *coords) `

` MPI_CART_GET(COMM, MAXDIMS, DIMS, PERIODS, COORDS, IERROR) INTEGER COMM, MAXDIMS, DIMS(*), COORDS(*), IERROR LOGICAL PERIODS(*) `

MPI_CART_RANK(comm, coords, rank)

[ IN comm] communicator with cartesian structure (handle)

[ IN coords] integer array (of size ` ndims`) specifying the cartesian coordinates of a process

[ OUT rank] rank of specified process (integer)

` int MPI_Cart_rank(MPI_Comm comm, int *coords, int *rank) `

` MPI_CART_RANK(COMM, COORDS, RANK, IERROR) INTEGER COMM, COORDS(*), RANK, IERROR `

For a process group with cartesian structure, the function MPI_CART_RANK translates the logical process coordinates to process ranks as they are used by the point-to-point routines.

For dimension ` i` with ` periods(i) = true`, if the coordinate,
` coords(i)`, is out of range, that is, ` coords(i) < 0` or

MPI_CART_COORDS(comm, rank, maxdims, coords)

[ IN comm] communicator with cartesian structure (handle)

[ IN rank] rank of a process within group of comm (integer)

[ IN maxdims] length of vector ` coord` in the calling program (integer)

[ OUT coords] integer array (of size ` ndims`) containing the cartesian coordinates of specified process (integer)

` int MPI_Cart_coords(MPI_Comm comm, int rank, int maxdims, int *coords) `

` MPI_CART_COORDS(COMM, RANK, MAXDIMS, COORDS, IERROR) INTEGER COMM, RANK, MAXDIMS, COORDS(*), IERROR `

The inverse mapping, rank-to-coordinates translation is provided by
MPI_CART_COORDS.
MPI_GRAPH_NEIGHBORS_COUNT(comm, rank, nneighbors)

[ IN comm] communicator with graph topology (handle)

[ IN rank] rank of process in group of comm (integer)

[ OUT nneighbors] number of neighbors of specified process (integer)

` int MPI_Graph_neighbors_count(MPI_Comm comm, int rank, int *nneighbors) `

` MPI_GRAPH_NEIGHBORS_COUNT(COMM, RANK, NNEIGHBORS, IERROR) INTEGER COMM, RANK, NNEIGHBORS, IERROR `

MPI_GRAPH_NEIGHBORS_COUNT and MPI_GRAPH_NEIGHBORS provide adjacency information for a general, graph topology.

MPI_GRAPH_NEIGHBORS(comm, rank, maxneighbors, neighbors)

[ IN comm] communicator with graph topology (handle)

[ IN rank] rank of process in group of comm (integer)

[ IN maxneighbors] size of array neighbors (integer)

[ OUT neighbors] ranks of processes that are neighbors to specified process (array of integer)

` int MPI_Graph_neighbors(MPI_Comm comm, int rank, int maxneighbors, int *neighbors) `

` MPI_GRAPH_NEIGHBORS(COMM, RANK, MAXNEIGHBORS, NEIGHBORS, IERROR) INTEGER COMM, RANK, MAXNEIGHBORS, NEIGHBORS(*), IERROR `

** Example**

Suppose that ` comm` is a communicator with a
shuffle-exchange topology. The group has
members.
Each process is labeled by
with
, and has three neighbors:
exchange(
(
), shuffle(
, and unshuffle(
.
The graph adjacency list is illustrated below for *n=3*.

Suppose that the communicator ` comm` has this topology associated with it.
The following code fragment cycles through the three types of neighbors
and performs an appropriate permutation for each.

C assume: each process has stored a real number A. C extract neighborhood information CALL MPI_COMM_RANK(comm, myrank, ierr) CALL MPI_GRAPH_NEIGHBORS(comm, myrank, 3, neighbors, ierr) C perform exchange permutation CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(1), 0, + neighbors(1), 0, comm, status, ierr) C perform shuffle permutation CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(2), 0, + neighbors(3), 0, comm, status, ierr) C perform unshuffle permutation CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(3), 0, + neighbors(2), 0, comm, status, ierr)

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