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MPI_Win_allocate.3.rst

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MPI_Win_allocate

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

SYNTAX

C Syntax

#include <mpi.h>

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

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_WIN_ALLOCATE(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

USE mpi_f08
MPI_Win_allocate(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` is a collective call executed by all processes in the group of comm. On each process, it allocates memory of at least size bytes, returns a pointer to it, and returns a window object that can be used by all processes in comm to perform RMA operations. The returned memory consists of size bytes local to each process, starting at address baseptr and is associated with the window as if the user called :ref:`MPI_Win_create` on existing memory. The size argument may be different at each process and size = 0 is valid; however, a library might allocate and expose more memory in order to create a fast, globally symmetric allocation. The discussion of and 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`; in particular, see the rationale in MPI-3.1 section 8.2 for an explanation of the type used for baseptr.

The displacement unit argument is provided to facilitate address arithmetic in RMA operations: the target displacement argument of an RMA operation is scaled by the factor disp_unit specified by the target process, at window creation.

For 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`. 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

.. seealso::
   * :ref:`MPI_Alloc_mem`
   * :ref:`MPI_Free_mem`
   * :ref:`MPI_Win_create`
   * :ref:`MPI_Win_allocate_shared`
   * :ref:`MPI_Win_free`