MPI_Scatter(3OpenMPI)													     MPI_Scatter(3OpenMPI)

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 restric-
       tion (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++ func-
       tions 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

Open MPI 1.2							  September 2006					     MPI_Scatter(3OpenMPI)