MPI_Alltoallw(3)                   Open MPI                   MPI_Alltoallw(3)



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




1.4.1                            Jan 14, 2010                 MPI_Alltoallw(3)
