MPI_NEIGHBOR_ALLTOALLW(3)			     Open MPI				 MPI_NEIGHBOR_ALLTOALLW(3)

MPI_Neighbor_alltoallw,	  MPI_Ineighbor_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  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_NEIGHBOR_ALLTOALLW(3)
