Ubuntu Manpage: MPI_Scatter - Sends data from one task to all tasks in a group.

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.