MPI_INEIGHBOR_ALLTOALLW(3)			     Open MPI				MPI_INEIGHBOR_ALLTOALLW(3)

       MPI_Neighbor_alltoallw  <#mpi-neighbor-alltoallw>,  MPI_Ineighbor_alltoallw, MPI_Neighbor_alltoallw_init <#
       mpi-neighbor-alltoallw-init> — All processes send data of different types to, and receive data of different
       types from, all processes

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Neighbor_alltoallw(const void *sendbuf, const int sendcounts[],
	       const MPI_Aint sdispls[], const MPI_Datatype sendtypes[],
	       void *recvbuf, const int recvcounts[], const MPI_Aint rdispls[],
	       const MPI_Datatype recvtypes[], MPI_Comm comm)

	  int MPI_Ineighbor_alltoallw(const void *sendbuf, const int sendcounts[],
	       const MPI_Aint sdispls[], const MPI_Datatype sendtypes[],
	       void *recvbuf, const int recvcounts[], const MPI_Aint rdispls[],
	       const MPI_Datatype recvtypes[], MPI_Comm comm, MPI_Request *request)

	  int MPI_Neighbor_alltoallw_init(const void *sendbuf, const int sendcounts[],
	       const MPI_Aint sdispls[], const MPI_Datatype sendtypes[],
	       void *recvbuf, const int recvcounts[], const MPI_Aint rdispls[],
	       const MPI_Datatype recvtypes[], MPI_Comm comm, MPI_Info info, MPI_Request *request)

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

	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER SENDCOUNTS(*), SENDTYPES(*)
	       INTEGER RECVCOUNTS(*), RECVTYPES(*)
	       INTEGER(KIND=MPI_ADDRESS_KIND) SDISPLS(*), RDISPLS(*)
	       INTEGER COMM, IERROR

	  MPI_INEIGHBOR_ALLTOALLW(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPES,
	       RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPES, COMM, REQUEST, IERROR)

	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER SENDCOUNTS(*), SENDTYPES(*)
	       INTEGER RECVCOUNTS(*), RECVTYPES(*)
	       INTEGER(KIND=MPI_ADDRESS_KIND) SDISPLS(*), RDISPLS(*)
	       INTEGER COMM, REQUEST, IERROR

	  MPI_NEIGHBOR_ALLTOALLW_INIT(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPES,
	       RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPES, COMM, INFO, REQUEST, IERROR)

	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER SENDCOUNTS(*), SENDTYPES(*)
	       INTEGER RECVCOUNTS(*), RECVTYPES(*)
	       INTEGER(KIND=MPI_ADDRESS_KIND) SDISPLS(*), RDISPLS(*)
	       INTEGER COMM, INFO, REQUEST, IERROR

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_Neighbor_alltoallw(sendbuf, sendcounts, sdispls, sendtypes, recvbuf,
		       recvcounts, rdispls, recvtypes, comm, ierror)

	       TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
	       TYPE(*), DIMENSION(..) :: recvbuf
	       INTEGER, INTENT(IN) :: sendcounts(*), recvcounts(*)
	       INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN) :: sdispls(*), rdispls(*)
	       TYPE(MPI_Datatype), INTENT(IN) :: sendtypes(*), recvtypes(*)
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

	  MPI_Ineighbor_alltoallw(sendbuf, sendcounts, sdispls, sendtypes, recvbuf,
		       recvcounts, rdispls, recvtypes, comm, request, ierror)

	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	       INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), recvcounts(*)
	       INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN), ASYNCHRONOUS ::
	       sdispls(*), rdispls(*)
	       TYPE(MPI_Datatype), INTENT(IN), ASYNCHRONOUS :: sendtypes(*),
	       recvtypes(*)
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       TYPE(MPI_Request), INTENT(OUT) :: request
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

	  MPI_Neighbor_alltoallw_init(sendbuf, sendcounts, sdispls, sendtypes, recvbuf,
		       recvcounts, rdispls, recvtypes, comm, info, request, ierror)

	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	       INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), recvcounts(*)
	       INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN), ASYNCHRONOUS ::
	       sdispls(*), rdispls(*)
	       TYPE(MPI_Datatype), INTENT(IN), ASYNCHRONOUS :: sendtypes(*),
	       recvtypes(*)
	       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 (in bytes,  offset  from  sendbuf)  from
	 which to send data to neighbor i.

       • sendtypes: Datatype array, where entry i specifies the datatype to use when sending data to neighbor i.

       • 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 (in bytes, offset from recvbuf) to which
	 data from neighbor j should be written.

       • recvtypes:  Datatype array, where entry j specifies the datatype to use when receiving data from neighbor
	 j.

       • 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_alltoallw <#mpi-neighbor-alltoallw>	 is  a	generalized  collective	 operation  in	which  all
       processes  send	data to and receive data from all neighbors. It adds flexibility to MPI_Neighbor_alltoallv
       <#mpi-neighbor-alltoallv> by allowing the user to specify  the  datatype	 of  individual	 data  blocks  (in
       addition	 to  displacement  and element count). Its operation can be thought of in the following way, where
       each process performs 2n (n being the number of neighbors in the 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, sendtypes[i], r0, ..., comm, ...);
	      MPI_Irecv(recvbuf + rdispls[i] * extent(recvtype),
			recvcount, recvtypes[i], r0, ..., comm, ...);
	      ++i;
	      MPI_Isend(sendbuf + sdispls[i] * extent(sendtype),
			sendcount, sendtypes[i], r1, ..., comm, &req[i]);
	      MPI_Irecv(recvbuf + rdispls[i] * extent(recvtype),
			recvcount, recvtypes[i], r1, ..., comm, ...);
	      ++i;
	  }

	  MPI_Wait_all (...);

	  MPI_Comm_size(comm, &n);
	  for (i = 0, i < n; i++)
	    MPI_Send(sendbuf + sdispls[i], sendcounts[i],
		 sendtypes[i], i, ..., comm);
	  for (i = 0, i < n; i++)
	    MPI_Recv(recvbuf + rdispls[i], recvcounts[i],
		 recvtypes[i], i, ..., comm);

       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 and types of data to different processes in the communicator.

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

       The specification of counts, types, 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 bytes. Compare this to MPI_Neighbor_alltoallv <#
       mpi-neighbor-alltoallv>, where these offsets are measured in units of sendtype and recvtype, respectively.

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_alltoallv <#mpi-neighbor-alltoallv>

	  • 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_INEIGHBOR_ALLTOALLW(3)
