MPI_NEIGHBOR_ALLTOALLV(3)			     Open MPI				 MPI_NEIGHBOR_ALLTOALLV(3)

MPI_Neighbor_alltoallv,	  MPI_Ineighbor_alltoallv   <#mpi-ineighbor-alltoallv>,	  MPI_Neighbor_alltoallv_init	<#
mpi-neighbor-alltoallv-init> — All processes send different amounts of data to, and receive different  amounts	of
data from, all neighbors

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Neighbor_alltoallv(const void *sendbuf, const int sendcounts[],
	       const int sdispls[], MPI_Datatype sendtype,
	       void *recvbuf, const int recvcounts[],
	       const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm)

	  int MPI_Ineighbor_alltoallv(const void *sendbuf, const int sendcounts[],
	       const int sdispls[], MPI_Datatype sendtype,
	       void *recvbuf, const int recvcounts[],
	       const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm,
	       MPI_Request *request)

	  int MPI_Neighbor_alltoallv_init(const void *sendbuf, const int sendcounts[],
	       const int sdispls[], MPI_Datatype sendtype,
	       void *recvbuf, const int recvcounts[],
	       const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm,
	       MPI_Info info, MPI_Request *request)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_NEIGHBOR_ALLTOALLV(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
	       RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, IERROR)

	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPE
	       INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPE
	       INTEGER COMM, IERROR

	  MPI_INEIGHBOR_ALLTOALLV(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
	       RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, REQUEST, IERROR)

	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPE
	       INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPE
	       INTEGER COMM, REQUEST, IERROR

	  MPI_NEIGHBOR_ALLTOALLV_INIT(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
	       RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, INFO, REQUEST, IERROR)

	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPE
	       INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPE
	       INTEGER COMM, INFO, REQUEST, IERROR

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_Neighbor_alltoallv(sendbuf, sendcounts, sdispls, sendtype, recvbuf,
		       recvcounts, rdispls, recvtype, comm, ierror)

	       TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
	       TYPE(*), DIMENSION(..) :: recvbuf
	       INTEGER, INTENT(IN) :: sendcounts(*), sdispls(*), recvcounts(*),
	       rdispls(*)
	       TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

	  MPI_Ineighbor_alltoallv(sendbuf, sendcounts, sdispls, sendtype, recvbuf,
		       recvcounts, rdispls, recvtype, comm, request, ierror)

	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	       INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), sdispls(*),
	       recvcounts(*), rdispls(*)
	       TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       TYPE(MPI_Request), INTENT(OUT) :: request
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

	  MPI_Neighbor_alltoallv_init(sendbuf, sendcounts, sdispls, sendtype, recvbuf,
		       recvcounts, rdispls, recvtype, comm, info, request, ierror)

	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	       INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), sdispls(*),
	       recvcounts(*), rdispls(*)
	       TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       TYPE(MPI_Info), INTENT(IN) :: info
	       TYPE(MPI_Request), INTENT(OUT) :: request
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS
       • sendbuf: Starting address of send buffer.

       • sendcounts: Integer array, where entry i specifies the number of elements to send to neighbor i.

       • sdispls:  Integer  array,  where  entry  i  specifies	the displacement (offset from sendbuf, in units of
	 sendtype) from which to send data to neighbor i.

       • sendtype: Datatype of send buffer elements.

       • recvcounts: Integer array, where entry j specifies the number of elements to receive from neighbor j.

       • rdispls: Integer array, where entry j specifies the  displacement  (offset  from  recvbuf,  in	 units	of
	 recvtype) to which data from neighbor j should be written.

       • recvtype: Datatype of receive buffer elements.

       • comm: Communicator over which data is to be exchanged.

       • info: Info (handle, persistent only).

OUTPUT PARAMETERS
       • recvbuf: Address of receive buffer.

       • request: Request (handle, non-blocking only).

       • ierror: Fortran only: Error status.

DESCRIPTION
       MPI_Neighbor_alltoallv  is  a  generalized  collective  operation  in  which all processes send data to and
       receive data from all neighbors. It adds flexibility to MPI_Neighbor_alltoall  <#mpi-neighbor-alltoall>	by
       allowing	 the user to specify data to send and receive vector-style (via a displacement and element count).
       The operation of this routine can be thought of as follows, where each process performs	2n  (n	being  the
       number  of  neighbors  in  to topology of communicator comm) independent point-to-point communications. The
       neighbors and buffer layout are determined by the topology of comm.

	  MPI_Cart_get(comm, maxdims, dims, periods, coords);
	  for (dim = 0, i = 0 ; dim < dims ; ++dim) {
	      MPI_Cart_shift(comm, dim, 1, &r0, &r1);
	      MPI_Isend(sendbuf + sdispls[i]  * extent(sendtype),
			sendcount, sendtype, r0, ..., comm, ...);
	      MPI_Irecv(recvbuf + rdispls[i] * extent(recvtype),
			recvcount, recvtype, r0, ..., comm, ...);
	      ++i;
	      MPI_Isend(sendbuf + sdispls[i] * extent(sendtype),
			sendcount, sendtype, r1, ..., comm, &req[i]);
	      MPI_Irecv(recvbuf + rdispls[i] * extent(recvtype),
			recvcount, recvtype, r1, ..., comm, ...);
	      ++i;
	  }

       Process j sends the k-th block of its local sendbuf to neighbor k, which places the data in the j-th  block
       of its local recvbuf.

       When  a	pair  of processes exchanges data, each may pass different element count and datatype arguments so
       long as the sender specifies the same amount of data to send (in bytes) as the receiver expects to receive.

       Note that process i may send a different amount of data to process j than it receives from process j. Also,
       a process may send entirely different amounts of data to different processes in the communicator.

NEIGHBOR ORDERING
       For a distributed graph topology, created with MPI_Dist_graph_create <#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	 <#mpi-dist-graph-neighbors>  for  destinations	 and  sources, respectively. For a
       general graph topology, created with MPI_Graph_create <#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 <#
       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 <#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	<#
       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.

       The specification of counts and displacements should not cause any location to be written more than once.

       All arguments on all processes are significant. The comm argument, in particular, must  describe	 the  same
       communicator on all processes.

       The  offsets  of	 sdispls and rdispls are measured in units of sendtype and recvtype, respectively. Compare
       this to MPI_Neighbor_alltoallw <#mpi-neighbor-alltoallw>, where these offsets are measured in bytes.

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

       Before  the error value is returned, the current MPI error handler associated with the communication object
       (e.g., communicator, window, file) is called.  If no communication object is associated with the MPI  call,
       then  the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When
       MPI_COMM_SELF is not initialized (i.e.,	before	MPI_Init  <#mpi-init>/MPI_Init_thread  <#mpi-init-thread>,
       after  MPI_Finalize  <#mpi-finalize>,  or  when	using the Sessions Model exclusively) the error raises the
       initial error handler. The initial error handler can  be	 changed  by  calling  MPI_Comm_set_errhandler	<#
       mpi-comm-set-errhandler>	 on  MPI_COMM_SELF  when  using the World model, or the mpi_initial_errhandler CLI
       argument	 to  mpiexec  or  info	key   to   MPI_Comm_spawn   <#mpi-comm-spawn>/MPI_Comm_spawn_multiple	<#
       mpi-comm-spawn-multiple>.   If  no other appropriate error handler has been set, then the MPI_ERRORS_RETURN
       error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error  handler  is  called  for  all
       other MPI functions.

       Open MPI includes three predefined error handlers that can be used:

       • MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.

       • MPI_ERRORS_ABORT  An  error handler that can be invoked on a communicator, window, file, or session. When
	 called on a communicator, it acts as if MPI_Abort <#mpi-abort> was called on that communicator. If called
	 on a window or file, acts as if MPI_Abort <#mpi-abort> was called on a communicator containing the  group
	 of processes in the corresponding window or file. If called on a session, aborts only the local process.

       • MPI_ERRORS_RETURN Returns an error code to the application.

       MPI applications can also implement their own error handlers by calling:

       • MPI_Comm_create_errhandler	 <#mpi-comm-create-errhandler>	   then	    MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	<#mpi-file-create-errhandler>	  then	   MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

       See the Error Handling section of the MPI-3.1 standard for more information.

       See also:

	  • MPI_Neighbor_alltoall <#mpi-neighbor-alltoall>

	  • MPI_Neighbor_alltoallw <#mpi-neighbor-alltoallw>

	  • MPI_Cart_create <#mpi-cart-create>

	  • MPI_Graph_create <#mpi-graph-create>

	  • MPI_Dist_graph_create <#mpi-dist-graph-create>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				 MPI_NEIGHBOR_ALLTOALLV(3)
