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