# 17.2.428. MPI_Win_allocate_shared

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

## 17.2.428.1. SYNTAX

### 17.2.428.1.1. 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)

### 17.2.428.1.2. 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

### 17.2.428.1.3. 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

## 17.2.428.2. 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).

## 17.2.428.3. OUTPUT PARAMETERS

- `baseptr`: Initial address of window.

- `win`: Window object returned by the call (handle).

- `ierror`: Fortran only: Error status (integer).

## 17.2.428.4. 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 section 8.2 also apply to MPI_Win_allocate_shared; in particular, see the rationale in MPI-3.1 section 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.

## 17.2.428.5. 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 MPI_Win_free will deallocate the memory allocated by MPI_Win_allocate_shared. It is thus erroneous to manually free baseptr.

## 17.2.428.6. 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.

## 17.2.428.7. ERRORS

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

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

- `MPI_ERRORS_ARE_FATAL` Causes the program to abort all connected MPI processes.

- `MPI_ERRORS_ABORT` An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process.

- `MPI_ERRORS_RETURN` Returns an error code to the application.

MPI applications can also implement their own error handlers by calling:

- MPI_Comm_create_errhandler then MPI_Comm_set_errhandler

- MPI_File_create_errhandler then MPI_File_set_errhandler

- MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

- MPI_Win_create_errhandler then MPI_Win_set_errhandler

Note that MPI does not guarantee that an MPI program can continue past an error.

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.

See also

- MPI_Alloc_mem

- MPI_Free_mem

- MPI_Win_allocate

- MPI_Win_create

- MPI_Win_shared_query

- MPI_Win_free