MPI_Scatter(3)                     Open MPI                     MPI_Scatter(3)



NAME
       MPI_Scatter - Sends data from one task to all tasks in a group.


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


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


C++ Syntax
       #include <mpi.h>
       void MPI::Comm::Scatter(const void* sendbuf, int sendcount,
            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).

       sendcount Number of elements sent to each process (integer, significant
                 only at root).

       sendtype  Datatype of send buffer elements (handle, significant only at
                 root).

       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_Scatter is the inverse operation to MPI_Gather.

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

           MPI_Send(sendbuf + i * sendcount * extent(sendtype), sendcount,
                    sendtype, i, ...)

       and each process executed a receive,

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

       An alternative description is that the root sends a message with
       MPI_Send(sendbuf, sendcount * n, sendtype, ...). This message is split
       into n equal segments, the ith segment is sent to the ith process in
       the group, and each process receives this message as above.

       The send buffer is ignored for all nonroot processes.

       The type signature associated with sendcount, sendtype at the root must
       be equal to the type signature associated with recvcount, recvtype at
       all processes (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 and types should not cause any location on
       the root to be read more than once.

       Rationale: Though not needed, the last restriction is imposed so as to
       achieve symmetry with MPI_Gather, where the corresponding restriction
       (a multiple-write restriction) is necessary.

       Example: The reverse of Example 1 in the MPI_Gather manpage. Scatter
       sets of 100 ints from the root to each process in the group.

               MPI_Comm comm;
               int gsize,*sendbuf;
               int root, rbuf[100];
               ...
               MPI_Comm_size(comm, &gsize);
               sendbuf = (int *)malloc(gsize*100*sizeof(int));
               ...
               MPI_Scatter(sendbuf, 100, 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_Scatterv
       MPI_Gather
       MPI_Gatherv




1.4.1                            Jan 14, 2010                   MPI_Scatter(3)
