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

1.6.5                                             Jun 26, 2013                                  MPI_Alltoallw(3)