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

       INCLUDE 'mpif.h'
       MPI_WIN_ALLOCATE_SHARED(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_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