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 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 illus-
       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++ func-
       tions 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)