NAME

       MPI_Win_allocate_shared - One-sided MPI call that allocates shared memory and returns a window object for
       RMA operations.

SYNTAX

C Syntax

       #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

       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

       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

       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  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 MPI_Alloc_mem and MPI_Free_mem in MPI-3.1 § 8.2 also apply to MPI_Win_allocate_shared; in particular,
       see the rationale in MPI-3.1 § 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 MPI_Barrier for 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 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_allocate MPI_Win_create MPI_Win_shared_query