.. _mpi_win_allocate_shared:


MPI_Win_allocate_shared
=======================

.. include_body

:ref:`MPI_Win_allocate_shared` |mdash| One-sided MPI call that allocates shared
memory and returns a window object for RMA operations.


SYNTAX
------


C Syntax
^^^^^^^^

.. code-block:: c

   #include <mpi.h>

   int MPI_Win_allocate_shared (MPI_Aint size, int disp_unit, MPI_Info info,
                                MPI_Comm comm, void *baseptr, MPI_Win *win)


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

.. code-block:: fortran

   USE MPI
   ! or the older form: INCLUDE 'mpif.h'
   MPI_WIN_ALLOCATE_SHARED(SIZE, DISP_UNIT, INFO, COMM, BASEPTR, WIN, IERROR)
   	INTEGER(KIND=MPI_ADDRESS_KIND) SIZE, BASEPTR
   	INTEGER DISP_UNIT, INFO, COMM, WIN, IERROR


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

.. code-block:: fortran

   USE mpi_f08
   MPI_Win_allocate_shared(size, disp_unit, info, comm, baseptr, win, ierror)
   	USE, INTRINSIC :: ISO_C_BINDING, ONLY : C_PTR
   	INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN) :: size
   	INTEGER, INTENT(IN) :: disp_unit
   	TYPE(MPI_Info), INTENT(IN) :: info
   	TYPE(MPI_Comm), INTENT(IN) :: comm
   	TYPE(C_PTR), INTENT(OUT) :: baseptr
   	TYPE(MPI_Win), INTENT(OUT) :: win
   	INTEGER, OPTIONAL, INTENT(OUT) :: ierror


INPUT PARAMETERS
----------------
* ``size``: Size of window in bytes (nonnegative integer).
* ``disp_unit``: Local unit size for displacements, in bytes (positive integer).
* ``info``: Info argument (handle).
* ``comm``: Communicator (handle).

OUTPUT PARAMETERS
-----------------
* ``baseptr``: Initial address of window.
* ``win``: Window object returned by the call (handle).
* ``ierror``: Fortran only: Error status (integer).

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

:ref:`MPI_Win_allocate_shared` is a collective call executed by all
processes in the group of *comm*. On each process, it allocates memory
of at least *size* bytes that is shared among all processes in *comm*,
and returns a pointer to the locally allocated segment in *baseptr*
that can be used for load/store accesses on the calling process. The
locally allocated memory can be the target of load/store accesses by
remote processes; the base pointers for other processes can be queried
using the function :ref:`MPI_Win_shared_query`. The call also returns
a window object that can be used by all processes in *comm* to perform
RMA operations. The *size* argument may be different at each process
and *size* = 0 is valid. It is the user's responsibility to ensure
that the communicator *comm* represents a group of processes that can
create a shared memory segment that can be accessed by all processes
in the group. The discussions of rationales for :ref:`MPI_Alloc_mem`
and :ref:`MPI_Free_mem` in MPI-3.1 section 8.2 also apply to
:ref:`MPI_Win_allocate_shared`; in particular, see the rationale in
MPI-3.1 section 8.2 for an explanation of the type used for
*baseptr*. The allocated memory is contiguous across process ranks
unless the info key *alloc_shared_noncontig* is specified. Contiguous
across process ranks means that the first address in the memory
segment of process i is consecutive with the last address in the
memory segment of process i -
1. This may enable the user to calculate remote address offsets with
local information only.

The following info keys are supported:

alloc_shared_noncontig
   If not set to *true*, the allocation strategy is to allocate
   contiguous memory across process ranks. This may limit the
   performance on some architectures because it does not allow the
   implementation to modify the data layout (e.g., padding to reduce
   access latency).

blocking_fence
   If set to *true*, the osc/sm component will use :ref:`MPI_Barrier` for
   :ref:`MPI_Win_fence`. If set to *false* a condition variable and counter
   will be used instead. The default value is *false*. This info key is
   Open MPI specific.

For additional supported info keys see :ref:`MPI_Win_create`.


NOTES
-----

Common choices for *disp_unit* are 1 (no scaling), and (in C syntax)
*sizeof(type)*, for a window that consists of an array of elements of
type *type*. The later choice will allow one to use array indices in RMA
calls, and have those scaled correctly to byte displacements, even in a
heterogeneous environment.

Calling :ref:`MPI_Win_free` will deallocate the memory allocated by
:ref:`MPI_Win_allocate_shared`. It is thus erroneous to manually free
*baseptr*.


C NOTES
-------

While *baseptr* is a ``void *`` type, this is to allow easy use of any
pointer object for this parameter. This argument is really a ``void **``
type.


ERRORS
------

.. include:: ./ERRORS.rst

.. seealso::
   * :ref:`MPI_Alloc_mem`
   * :ref:`MPI_Free_mem`
   * :ref:`MPI_Win_allocate`
   * :ref:`MPI_Win_create`
   * :ref:`MPI_Win_shared_query`
   * :ref:`MPI_Win_free`