MPI_Graph_neighbors(3)             Open MPI             MPI_Graph_neighbors(3)



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 members. 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
       illustrated 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




1.4.1                            Jan 14, 2010           MPI_Graph_neighbors(3)
