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MPI_Graph_neighbors(3OpenMPI)                                                  MPI_Graph_neighbors(3OpenMPI)



NAME
       MPI_Graph_neighbors  - Returns the neighbors of a node associated with a graph topology.


SYNTAX
C Syntax
       #include <mpi.h>
       int MPI_Graph_neighbors(MPI_Comm comm, int rank, int maxneighbors,
            int *neighbors)


Fortran Syntax
       INCLUDE 'mpif.h'
       MPI_GRAPH_NEIGHBORS(COMM, RANK, MAXNEIGHBORS, NEIGHBORS, IERROR)
            INTEGER   COMM, RANK, MAXNEIGHBORS, NEIGHBORS(*), IERROR


C++ Syntax
       #include <mpi.h>
       void Graphcomm::Get_neighbors(int rank, int maxneighbors,
            int neighbors[]) const


INPUT PARAMETERS
       comm      Communicator with graph topology (handle).

       rank      Rank of process in group of comm (integer).

       maxneighbors
                 Size of array neighbors (integer).


OUTPUT PARAMETERS
       neighbors Ranks of processes that are neighbors to specified process (array of integers).

       IERROR    Fortran only: Error status (integer).


DESCRIPTION
       Example:  Suppose that comm is a communicator with a shuffle-exchange topology. The group has 2n mem-bers. members.
       bers. Each process is labeled by a(1), ..., a(n) with a(i) E{0,1}, and has three neighbors:  exchange
       (a(1), ...,  a(n) = a(1), ..., a(n-1), a(n) (a = 1 - a), shuffle (a(1), ..., a(n)) = a(2), ..., a(n),
       a(1), and unshuffle (a(1), ..., a(n)) = a(n), a(1), ..., a(n-1). The graph adjacency list  is  illus-trated illustrated
       trated below for n=3.

                      exchange       shuffle        unshuffle
           node       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)


ERRORS
       Almost all MPI routines return an error value; C routines as the value of the  function  and  Fortran
       routines  in  the  last argument. C++ functions do not return errors. If the default error handler is
       set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism will be used to  throw
       an MPI:Exception object.

       Before  the  error value is returned, the current MPI error handler is called. By default, this error
       handler aborts the MPI job, except for I/O function errors. The error handler  may  be  changed  with
       MPI_Comm_set_errhandler;  the  predefined  error handler MPI_ERRORS_RETURN may be used to cause error
       values to be returned. Note that MPI does not guarantee that an MPI  program  can  continue  past  an
       error.


SEE ALSO
       MPI_Graph_neighbors_count




Open MPI 1.2                                   September 2006                  MPI_Graph_neighbors(3OpenMPI)