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

NAME

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

SYNTAX

C Syntax

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

       int MPI_Iscatter(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
            void *recvbuf, int recvcount, MPI_Datatype recvtype, int root,
            MPI_Comm comm, MPI_Request *request)

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

       MPI_ISCATTER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
                 RECVTYPE, ROOT, COMM, REQUEST, IERROR)
            <type>    SENDBUF(*), RECVBUF(*)
            INTEGER   SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, ROOT
            INTEGER   COMM, REQUEST, 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).

       request   Request (handle, non-blocking only).

       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 scatter 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.