MPI_Scatterv(3)                    Open MPI                    MPI_Scatterv(3)



NAME
       MPI_Scatterv - Scatters a buffer in parts to all tasks in a group.


SYNTAX
C Syntax
       #include <mpi.h>
       int MPI_Scatterv(void *sendbuf, int *sendcounts, int *displs,
            MPI_Datatype sendtype, void *recvbuf, int recvcount,
            MPI_Datatype recvtype, int root, MPI_Comm comm)


Fortran Syntax
       INCLUDE 'mpif.h'
       MPI_SCATTERV(SENDBUF, SENDCOUNTS, DISPLS, SENDTYPE, RECVBUF,
                 RECVCOUNT, RECVTYPE, ROOT, COMM, IERROR)
            <type>    SENDBUF(*), RECVBUF(*)
            INTEGER   SENDCOUNTS(*), DISPLS(*), SENDTYPE
            INTEGER   RECVCOUNT, RECVTYPE, ROOT, COMM, IERROR


C++ Syntax
       #include <mpi.h>
       void MPI::Comm::Scatterv(const void* sendbuf, const int sendcounts[],
            const int displs[], const MPI::Datatype& sendtype,
            void* recvbuf, int recvcount, const MPI::Datatype&
            recvtype, int root) const


INPUT PARAMETERS
       sendbuf   Address of send buffer (choice, significant only at root).

       sendcounts
                 Integer array (of length group size) specifying the number of
                 elements to send to each processor.

       displs    Integer array (of length group size). Entry i specifies the
                 displacement (relative to sendbuf) from which to take the
                 outgoing data to process i.

       sendtype  Datatype of send buffer elements (handle).

       recvcount Number of elements in receive buffer (integer).

       recvtype  Datatype of receive buffer elements (handle).

       root      Rank of sending process (integer).

       comm      Communicator (handle).


OUTPUT PARAMETERS
       recvbuf   Address of receive buffer (choice).

       IERROR    Fortran only: Error status (integer).


DESCRIPTION
       MPI_Scatterv is the inverse operation to MPI_Gatherv.

       MPI_Scatterv extends the functionality of MPI_Scatter by allowing a
       varying count of data to be sent to each process, since sendcounts is
       now an array.  It also allows more flexibility as to where the data is
       taken from on the root, by providing the new argument, displs.

       The outcome is as if the root executed n send operations,

           MPI_Send(sendbuf + displs[i] * extent(sendtype), \
                    sendcounts[i], sendtype, i, ...)

       and each process executed a receive,

           MPI_Recv(recvbuf, recvcount, recvtype, root, ...)

       The send buffer is ignored for all nonroot processes.

       The type signature implied by sendcount[i], sendtype at the root must
       be equal to the type signature implied by recvcount, recvtype at
       process i (however, the type maps may be different). This implies that
       the amount of data sent must be equal to the amount of data received,
       pairwise between each process and the root. Distinct type maps between
       sender and receiver are still allowed.

       All arguments to the function are significant on process root, while on
       other processes, only arguments recvbuf, recvcount, recvtype, root,
       comm are significant. The arguments root and comm must have identical
       values on all processes.

       The specification of counts, types, and displacements should not cause
       any location on the root to be read more than once.

       Example 1: The reverse of Example 5 in the MPI_Gatherv manpage. We have
       a varying stride between blocks at sending (root) side, at the
       receiving side we receive 100 - i elements into the ith column of a 100
       x 150 C array at process i.

           MPI_Comm comm;
               int gsize,recvarray[100][150],*rptr;
               int root, *sendbuf, myrank, bufsize, *stride;
               MPI_Datatype rtype;
               int i, *displs, *scounts, offset;
               ...
               MPI_Comm_size( comm, &gsize);
               MPI_Comm_rank( comm, &myrank );

               stride = (int *)malloc(gsize*sizeof(int));
               ...
               /* stride[i] for i = 0 to gsize-1 is set somehow
                * sendbuf comes from elsewhere
                */
               ...
               displs = (int *)malloc(gsize*sizeof(int));
               scounts = (int *)malloc(gsize*sizeof(int));
               offset = 0;
               for (i=0; i<gsize; ++i) {
                   displs[i] = offset;
                   offset += stride[i];
                   scounts[i] = 100 - i;
               }
               /* Create datatype for the column we are receiving
                */
               MPI_Type_vector( 100-myrank, 1, 150, MPI_INT, &rtype);
               MPI_Type_commit( &rtype );
               rptr = &recvarray[0][myrank];
               MPI_Scatterv(sendbuf, scounts, displs, MPI_INT,
                            rptr, 1, rtype, root, comm);

       Example 2: The reverse of Example 1 in the MPI_Gather manpage. The root
       process scatters sets of 100 ints to the other processes, but the sets
       of 100 are stride ints apart in the sending buffer. Requires use of
       MPI_Scatterv, where stride >= 100.

           MPI_Comm comm;
               int gsize,*sendbuf;
               int root, rbuf[100], i, *displs, *scounts;

           ...

           MPI_Comm_size(comm, &gsize);
               sendbuf = (int *)malloc(gsize*stride*sizeof(int));
               ...
               displs = (int *)malloc(gsize*sizeof(int));
               scounts = (int *)malloc(gsize*sizeof(int));
               for (i=0; i<gsize; ++i) {
                   displs[i] = i*stride;
                   scounts[i] = 100;
               }
               MPI_Scatterv(sendbuf, scounts, displs, MPI_INT,
                            rbuf, 100, MPI_INT, root, comm);

USE OF IN-PLACE OPTION
       When the communicator is an intracommunicator, you can perform a gather
       operation in-place (the output buffer is used as the input buffer).
       Use the variable MPI_IN_PLACE as the value of the root process recvbuf.
       In this case, recvcount and recvtype are ignored, and the root process
       sends no data to itself.

       Note that MPI_IN_PLACE is a special kind of value; it has the same
       restrictions on its use as MPI_BOTTOM.

       Because the in-place option converts the receive buffer into a send-
       and-receive buffer, a Fortran binding that includes INTENT must mark
       these as INOUT, not OUT.


WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR
       When the communicator is an inter-communicator, the root process in the
       first group sends data to all processes in the second group.  The first
       group defines the root process.  That process uses MPI_ROOT as the
       value of its root argument.  The remaining processes use MPI_PROC_NULL
       as the value of their root argument.  All processes in the second group
       use the rank of that root process in the first group as the value of
       their root argument.   The receive buffer argument of the root process
       in the first group must be consistent with the receive buffer argument
       of the processes in the second group.


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_Gather
       MPI_Gatherv
       MPI_Scatter






1.4.1                            Jan 14, 2010                  MPI_Scatterv(3)
