int MPI_Graph_neighbors(
  MPI_Comm comm,
  int rank,
  int maxneighbors,
  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 2ⁿ 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 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.