MPI_Neighbor_allgatherv(3)

NAME
MPI_Neighbor_allgatherv, MPI_Ineighbor_allgatherv - Gathers and distributes data from and to all neighbors. Each process may contribute a different amount of data.

SYNTAX

C Syntax
#include <mpi.h>
int MPI_Neighbor_allgatherv(const void *sendbuf, int sendcount,
	MPI_Datatype sendtype, void *recvbuf, const int recvcounts[],
	const int displs[], MPI_Datatype recvtype, MPI_Comm comm)

int MPI_Ineighbor_allgatherv(const void *sendbuf, int sendcount,
	MPI_Datatype sendtype, void *recvbuf, const int recvcounts[],
	const int displs[], MPI_Datatype recvtype, MPI_Comm comm,
        MPI_Request *request)

INPUT PARAMETERS

sendbuf
    Starting address of send buffer (choice).

sendcount
    Number of elements in send buffer (integer).

sendtype
    Datatype of send buffer elements (handle).

recvcount
    Integer array (of length group size) containing the number of elements that are received from each neighbor.

displs
    Integer array (of length group size). Entry i specifies the displacement (relative to recvbuf) at which to place the incoming data from neighbor i.

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_allgatherv is similar to MPI_Neighbor_allgather in that all processes gather data from all neighbors, except that each process can send a different amount of data. The block of data sent from the jth neighbor is received by every neighbor and placed in the jth block of the buffer. The neighbors and buffer layout is determined by the topology of comm. recvbuf.

The type signature associated with sendcount, sendtype, at process j must be equal to the type signature associated with the corresponding entry in recvcounts on neighboring processes.

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.

ERRORS
Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument.

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_Neighbor_allgather MPI_Cart_create MPI_Graph_create MPI_Dist_graph_create