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NAME

       MPI_Alltoallw  -  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_Alltoallw(void *sendbuf, int *sendcounts,
            int *sdispls, MPI_Datatype *sendtypes,
            void *recvbuf, int *recvcounts,
            int *rdispls, MPI_Datatype *recvtypes, MPI_Comm comm)

Fortran Syntax

       INCLUDE 'mpif.h'
       MPI_ALLTOALLW(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPES,
            RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPES, COMM, IERROR)

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

C++ Syntax

       #include <mpi.h>
       void MPI::Comm::Alltoallw(const void* sendbuf,
            const int sendcounts[], const int sdispls[],
            const MPI::Datatype sendtypes[], void* recvbuf,
            const int recvcounts[], const int rdispls[],
            const MPI::Datatype recvtypes[])

INPUT PARAMETERS

       sendbuf     Starting address of send buffer.

       sendcounts  Integer array, where entry i specifies the number of elements to send to  rank
                   i.

       sdispls     Integer array, where entry i specifies the displacement (in bytes, offset from
                   sendbuf) from which to send data to rank i.

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

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

       rdispls     Integer array, where entry j specifies the displacement (in bytes, offset from
                   recvbuf) to which data from rank j should be written.

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

       comm        Communicator over which data is to be exchanged.

OUTPUT PARAMETERS

       recvbuf     Address of receive buffer.

       IERROR      Fortran only: Error status.

DESCRIPTION

       MPI_Alltoallw is a generalized collective operation in which all processes  send  data  to
       and  receive  data  from  all  other  processes.  It  adds flexibility to MPI_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  processes  in  communicator  comm)
       independent point-to-point communications (including communication with itself).

            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 process 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.

       WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR

       When the communicator is an inter-communicator, the gather operation occurs in two phases.
       The  data  is  gathered  from  all  the members of the first group and received by all the
       members of the second group.  Then the data is gathered from all the members of the second
       group  and  received by all the members of the first.  The operation exhibits a symmetric,
       full-duplex behavior.

       The first group defines the root process.  The root process uses MPI_ROOT as the value  of
       root.  All other processes in the first group use MPI_PROC_NULL as the value of root.  All
       processes in the second group use the rank of the root process in the first group  as  the
       value of root.

       When  the  communicator  is  an  intra-communicator,  these  groups  are the same, and the
       operation occurs in a single phase.

NOTES

       The MPI_IN_PLACE option is not available for any form of all-to-all communication.

       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_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 value of the function and
       Fortran routines in the last argument. C++ functions do not return errors. If the  default
       error  handler  is  set  to  MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception
       mechanism will be used to throw an MPI:Exception object.

       Before the error value is returned, the current MPI error handler is called.  By  default,
       this  error  handler aborts the MPI job, except for I/O function errors. The error handler
       may   be   changed   with   MPI_Comm_set_errhandler;   the   predefined   error    handler
       MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not
       guarantee that an MPI program can continue past an error.

SEE ALSO

       MPI_Alltoall
       MPI_Alltoallv