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

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_GRAPH_NEIGHBORS(COMM, RANK, MAXNEIGHBORS, NEIGHBORS, IERROR)
	INTEGER	COMM, RANK, MAXNEIGHBORS, NEIGHBORS(*), IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_Graph_neighbors(comm, rank, maxneighbors, neighbors, ierror)
	TYPE(MPI_Comm), INTENT(IN) :: comm
	INTEGER, INTENT(IN) :: rank, maxneighbors
	INTEGER, INTENT(OUT) :: neighbors(maxneighbors)
	INTEGER, OPTIONAL, INTENT(OUT) :: 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.