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
coords(i) 
0 
| 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 2n members.
Each process is labeled by a1 , ..., an with 
| 2|c| node | exchange | shuffle | unshuffle | |
| neighbors(1) | neighbors(2) | neighbors(3) | ||
| 0 | (000) | 1 | 0 | 0 |
| 1 | (001) | 0 | 2 | 4 |
| 2 | (010) | 3 | 4 | 1 |
| 3 | (011) | 2 | 6 | 5 |
| 4 | (100) | 5 | 1 | 2 |
| 5 | (101) | 4 | 3 | 6 |
| 6 | (110) | 7 | 5 | 3 |
| 7 | (111) | 6 | 7 | 7 |
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)