MPI_NEIGHBOR_ALLTOALLV_INIT(3)			     Open MPI			    MPI_NEIGHBOR_ALLTOALLV_INIT(3)

       MPI_Neighbor_alltoallv	<#mpi-neighbor-alltoallv>,   MPI_Ineighbor_alltoallv   <#mpi-ineighbor-alltoallv>,
       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  <#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_INIT(3)
