SYNTAX

C Syntax

#include <mpi.h>
int MPI_Neighbor_allgather(const void *sendbuf, int  sendcount,
        MPI_Datatype sendtype, void *recvbuf, int recvcount,
         MPI_Datatype recvtype, MPI_Comm comm)
int MPI_Ineighbor_allgather(const void *sendbuf, int  sendcount,
        MPI_Datatype sendtype, void *recvbuf, int recvcount,
         MPI_Datatype recvtype, MPI_Comm comm, MPI_Request req)

Fortran Syntax

INCLUDE 'mpif.h'
MPI_NEIGHBOR_ALLGATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
                RECVTYPE, COMM, IERROR)
        <type>  SENDBUF (*), RECVBUF (*)
        INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM,
        INTEGER IERROR
MPI_INEIGHBOR_ALLGATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
                RECVTYPE, COMM, REQUEST, IERROR)
        <type>  SENDBUF (*), RECVBUF (*)
        INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM,
        INTEGER REQUEST, IERROR

INPUT PARAMETERS

sendbuf

Starting address of send buffer (choice).

sendcount

Number of elements in send buffer (integer).

sendtype

Datatype of send buffer elements (handle).

recvbuf

Starting address of recv buffer (choice).

recvcount

Number of elements received from any process (integer).

recvtype

Datatype of receive buffer elements (handle).

comm

Communicator (handle).

OUTPUT PARAMETERS

recvbuf

Address of receive buffer (choice).

request

Request (handle, non-blocking only).

IERROR

Fortran only: Error status (integer).

DESCRIPTION

MPI_Neighbor_allgather is similar to MPI_Allgather, except that only the neighboring processes receive the result, instead of all processes. The neighbors and buffer layout is determined by the topology of comm.

The type signature associated with sendcount, sendtype at a process must be equal to the type signature associated with recvcount, recvtype at any other process.

NEIGHBOR ORDERING

For a distributed graph topology, created with MPI_Dist_graph_create, the sequence of neighbors in the send and receive buffers at each process is defined as the sequence returned by MPI_Dist_graph_neighbors for destinations and sources, respectively. For a general graph topology, created with MPI_Graph_create, the order of neighbors in the send and receive buffers is defined as the sequence of neighbors as returned by MPI_Graph_neighbors. Note that general graph topologies should generally be replaced by the distributed graph topologies.

For a Cartesian topology, created with MPI_Cart_create, the sequence of neighbors in the send and receive buffers at each process is defined by order of the dimensions, first the neighbor in the negative direction and then in the positive direction with displacement 1. The numbers of sources and destinations in the communication routines are 2*ndims with ndims defined in MPI_Cart_create. If a neighbor does not exist, i.e., at the border of a Cartesian topology in the case of a non-periodic virtual grid dimension (i.e., periods[...]==false), then this neighbor is defined to be MPI_PROC_NULL.

If a neighbor in any of the functions is MPI_PROC_NULL, then the neighborhood collective communication behaves like a point-to-point communication with MPI_PROC_NULL in this direction. That is, the buffer is still part of the sequence of neighbors but it is neither communicated nor updated.

NOTES

The MPI_IN_PLACE option for sendbuf is not meaningful for this operation.