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