.. _mpi_neighbor_allgatherv:


MPI_Neighbor_allgatherv
=======================

.. include_body

:ref:`MPI_Neighbor_allgatherv`, :ref:`MPI_Ineighbor_allgatherv`,
:ref:`MPI_Neighbor_allgatherv_init` |mdash| Gathers and distributes data from and
to all neighbors. Each process may contribute a different amount of
data.

.. The following directive tells the man page generation script to
   generate multiple bindings for this file.
.. mpi-bindings: MPI_Neighbor_allgatherv, MPI_Ineighbor_allgatherv, MPI_Neighbor_allgatherv_init

.. The following file was automatically generated
.. include:: ./bindings/mpi_neighbor_allgatherv.rst

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).
* ``info Info (handle, persistent only).``:

OUTPUT PARAMETERS
-----------------
* ``recvbuf``: Address of receive buffer (choice).
* ``request``: Request (handle, non-blocking and persistent only).
* ``ierror``: Fortran only: Error status (integer).

DESCRIPTION
-----------

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

For a Cartesian topology, created with :ref:`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 :ref:`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
------

.. include:: ./ERRORS.rst

.. seealso::
   * :ref:`MPI_Neighbor_allgather`
   * :ref:`MPI_Cart_create`
   * :ref:`MPI_Graph_create`
   * :ref:`MPI_Dist_graph_create`