# MPI_Graph_neighbors(3) man page (version 1.7.1)

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## Name

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

## 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.