NAME

       MPI_Alltoallv  -  All  processes  send  different amount of data to, and receive different
       amount of data from, all processes

SYNTAX

C Syntax

       #include <mpi.h>
       int MPI_Alltoallv(void *sendbuf, int *sendcounts,
            int *sdispls, MPI_Datatype sendtype,
            void *recvbuf, int *recvcounts,
            int *rdispls, MPI_Datatype recvtype, MPI_Comm comm)

Fortran Syntax

       INCLUDE 'mpif.h'

       MPI_ALLTOALLV(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
            RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, IERROR)

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

C++ Syntax

       #include <mpi.h>
       void MPI::Comm::Alltoallv(const void* sendbuf,
            const int sendcounts[], const int displs[],
            const MPI::Datatype& sendtype, void* recvbuf,
            const int recvcounts[], const int rdispls[],
            const MPI::Datatype& recvtype)

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 (offset from sendbuf,
                   in units of sendtype) from which to send data to rank i.

       sendtype    Datatype of send buffer elements.

       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 (offset from recvbuf,
                   in units of recvtype) to which data from rank j should be written.

       recvtype    Datatype of receive buffer elements.

       comm        Communicator over which data is to be exchanged.

OUTPUT PARAMETERS

       recvbuf     Address of receive buffer.

       IERROR      Fortran only: Error status.

DESCRIPTION

       MPI_Alltoallv is a generalized collective operation in which all processes  send  data  to
       and receive data from all other processes. It adds flexibility to MPI_Alltoall by allowing
       the user to specify data to send and receive vector-style (via a displacement and  element
       count).  The  operation  of  this routine can be thought of as follows, 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] * extent(sendtype),
                    sendcounts[i], sendtype, i, ..., comm);
            for (i = 0, i < n; i++)
                MPI_Recv(recvbuf + rdispls[i] * extent(recvtype),
                    recvcounts[i], recvtype, 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 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 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  units  of  sendtype  and  recvtype,
       respectively. Compare this to MPI_Alltoallw, where these offsets are measured in bytes.

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_Alltoallw