NAME

       MPI_Win_allocate  -  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

       INCLUDE 'mpif.h'
       MPI_WIN_ALLOCATE(IZE, DISP_UNIT, INFO, COMM, BASEPTR, WIN, IERROR)
            INTEGER(KIND=MPI_ADDRESS_KIND) SIZE, BASEPTR
            INTEGER DISP_UNIT, INFO, COMM, WIN, 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

       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
       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 MPI_Alloc_mem and MPI_Free_mem  in  MPI-3.1  §
       8.2  also apply to MPI_Win_allocate; in particular, see the rationale in MPI-3.1 § 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 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.

ERRORS

       Almost all MPI routines return an error value; C routines as  the  value  of  the  function  and  Fortran
       routines in the last argument.

       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.

SEE ALSO

       MPI_Alloc_mem MPI_Free_mem MPI_Win_create MPI_Win_allocate_shared