MPI_Graph_neighbors(3) man page (version 3.1.6)

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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

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

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Table of Contents

• Name
• Syntax
• C Syntax
• Fortran Syntax
• Fortran 2008 Syntax
• Input Parameters
• Output Parameters
• Description
• Errors
• See Also

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Page last modified: 20-Mar-2020
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