MPI_Graph_neighbors  Returns the neighbors of a node associated with a graph topology.
#include <mpi.h>
int MPI_Graph_neighbors(MPI_Comm comm, int rank, int maxneighbors,
int neighbors[]) 
USE MPI
! or the older form: INCLUDE ’mpif.h’
MPI_GRAPH_NEIGHBORS(COMM, RANK, MAXNEIGHBORS, NEIGHBORS, IERROR)
INTEGER 
COMM, RANK, MAXNEIGHBORS, NEIGHBORS(*), IERROR 
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 
#include <mpi.h>
void Graphcomm::Get_neighbors(int rank, int maxneighbors,
int neighbors[]) const 
comm 
Communicator with graph topology (handle). 

rank 
Rank of process in group of comm (integer). 
maxneighbors
Size of array neighbors (integer).
neighbors 
Ranks of processes that are neighbors to specified process (array of integers). 

IERROR 
Fortran only: Error status (integer). 
Example: Suppose that comm is a communicator with a shuffleexchange 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(n1), 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(n1). The graph adjacency list is illustrated below for n=3.
exchange shuffle unshuffle
node
neighbors(3) 

1 

0 
0 

0 

2 
4 

3 

4 
1 

2 

6 
5 

5 

1 
2 

4 

3 
6 

7 

5 
3 

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)
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.
MPI_Graph_neighbors_count