# MPI_Graph_neighbors

Returns the neighbors of a node associated with a graph topologyint MPI_Graph_neighbors(MPI_Commcomm,intrank,intmaxneighbors,int*neighbors);

#### Parameters

*comm*- [in] communicator with graph topology (handle)
*rank*- [in] rank of process in group of comm (integer)
*maxneighbors*- [in] size of array neighbors (integer)
*neighbors*- [out] ranks of processes that are neighbors to specified process (array of integer)

#### Remarks

MPI_GRAPH_NEIGHBORS provides adjacency information for a general, graph topology.

Example

Suppose that `comm` is a communicator with a shuffle-exchange
topology. The group has *2 ^{n}* members. Each process is
labeled by with , and has
three neighbors: exchange( (
), shuffle( , and unshuffle(
. The graph adjacency list is illustrated below for

*n=3*.

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)

#### Thread and Interrupt Safety

This routine is both thread- and interrupt-safe. This means that this routine may safely be used by multiple threads and from within a signal handler.

#### Notes for Fortran

All MPI routines in Fortran (except for`MPI_WTIME`and

`MPI_WTICK`) have an additional argument

`ierr`at the end of the argument list.

`ierr`is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the

`call`statement.

All MPI objects (e.g., `MPI_Datatype`, `MPI_Comm`) are of type `INTEGER
`in Fortran.

#### Errors

All MPI routines (except `MPI_Wtime` and `MPI_Wtick`) return an error value;
C routines as the value of the function and Fortran routines in the last
argument. Before the value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job. The error handler
may be changed with `MPI_Comm_set_errhandler` (for communicators),
`MPI_File_set_errhandler` (for files), and `MPI_Win_set_errhandler` (for
RMA windows). The MPI-1 routine `MPI_Errhandler_set` may be used but
its use is deprecated. The predefined error handler
`MPI_ERRORS_RETURN` may be used to cause error values to be returned.
Note that MPI does *not* guarentee that an MPI program can continue past
an error; however, MPI implementations will attempt to continue whenever
possible.

*MPI_SUCCESS*- No error; MPI routine completed successfully.

*MPI_ERR_TOPOLOGY*- Invalid topology. Either there is no topology
associated with this communicator, or it is not the correct type (e.g.,
`MPI_CART`when expecting`MPI_GRAPH`).

*MPI_ERR_COMM*- Invalid communicator. A common error is to use a null
communicator in a call (not even allowed in
`MPI_Comm_rank`).

*MPI_ERR_ARG*- Invalid argument. Some argument is invalid and is not
identified by a specific error class (e.g.,
`MPI_ERR_RANK`).

*MPI_ERR_RANK*- Invalid source or destination rank. Ranks must be between
zero and the size of the communicator minus one; ranks in a receive
(
`MPI_Recv`,`MPI_Irecv`,`MPI_Sendrecv`, etc.) may also be`MPI_ANY_SOURCE`.

#### Example Code

The following sample code illustrates MPI_Graph_neighbors.

Insert code here.

## DOWNLOAD

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