.. _mpi_scatter:


MPI_Scatter
===========

.. include_body

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


SYNTAX
------


C Syntax
^^^^^^^^

.. code-block:: c

   #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)

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


Fortran Syntax
^^^^^^^^^^^^^^

.. code-block:: fortran

   USE MPI
   ! or the older form: 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

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


Fortran 2008 Syntax
^^^^^^^^^^^^^^^^^^^

.. code-block:: fortran

   USE mpi_f08
   MPI_Scatter(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
   		root, comm, ierror)
   	TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
   	TYPE(*), DIMENSION(..) :: recvbuf
   	INTEGER, INTENT(IN) :: sendcount, recvcount, root
   	TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
   	TYPE(MPI_Comm), INTENT(IN) :: comm
   	INTEGER, OPTIONAL, INTENT(OUT) :: ierror

   MPI_Iscatter(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
   		root, comm, request, ierror)
   	TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
   	TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
   	INTEGER, INTENT(IN) :: sendcount, recvcount, root
   	TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
   	TYPE(MPI_Comm), INTENT(IN) :: comm
   	TYPE(MPI_Request), INTENT(OUT) :: request
   	INTEGER, OPTIONAL, INTENT(OUT) :: ierror

   MPI_Scatter_init(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
   		root, comm, info, request, ierror)
   	TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
   	TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
   	INTEGER, INTENT(IN) :: sendcount, recvcount, root
   	TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
   	TYPE(MPI_Comm), INTENT(IN) :: comm
   	TYPE(MPI_Info), INTENT(IN) :: info
   	TYPE(MPI_Request), INTENT(OUT) :: request
   	INTEGER, OPTIONAL, INTENT(OUT) :: ierror


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).
* ``info``: Info (handle, persistent only).

OUTPUT PARAMETERS
-----------------
* ``recvbuf``: Address of receive buffer (choice).
* ``request``: Request (handle, non-blocking and persistent only).
* ``ierror``: Fortran only: Error status (integer).

DESCRIPTION
-----------

:ref:`MPI_Scatter` is the inverse operation to :ref:`MPI_Gather`.

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

.. code-block:: c

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

and each process executed a receive,

.. code-block:: c

       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 :ref:`MPI_Gather`, where the corresponding restriction
(a multiple-write restriction) is necessary.

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

.. code-block:: c

           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
------

.. include:: ./ERRORS.rst

.. seealso::
   * :ref:`MPI_Scatterv`
   * :ref:`MPI_Gather`
   * :ref:`MPI_Gatherv`
