# 17.2.266. MPI_Neighbor_alltoallw

MPI_Neighbor_alltoallw, MPI_Ineighbor_alltoallw, MPI_Neighbor_alltoallw_init — All processes send data of different types to, and receive data of different types from, all processes

## 17.2.266.1. SYNTAX

### 17.2.266.1.1. 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)

### 17.2.266.1.2. 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

### 17.2.266.1.3. 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

## 17.2.266.2. 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).

## 17.2.266.3. OUTPUT PARAMETERS

- `recvbuf`: Address of receive buffer.

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

- `ierror`: Fortran only: Error status.

## 17.2.266.4. 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 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.

## 17.2.266.5. 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, where these offsets are measured in units of sendtype and recvtype, respectively.

## 17.2.266.6. 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_thread, after 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 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_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 was called on that communicator. If called on a window or file, acts as if 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 then MPI_Comm_set_errhandler

- MPI_File_create_errhandler then MPI_File_set_errhandler

- MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

- MPI_Win_create_errhandler then MPI_Win_set_errhandler

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

See the MPI man page for a full list of MPI error codes.

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

See also

- MPI_Neighbor_alltoall

- MPI_Neighbor_alltoallv

- MPI_Cart_create

- MPI_Graph_create

- MPI_Dist_graph_create