# MPI_Win_create

Create an MPI Window object for one-sided communication

## Synopsis

    int MPI_Win_create(void *base, MPI_Aint size, int disp_unit, MPI_Info info,
                       MPI_Comm comm, MPI_Win * win)

This is a collective call executed by all processes in the group of comm. It returns a window object that can be used by these processes to perform RMA operations. Each process specifies a window of existing memory that it exposes to RMA accesses by the processes in the group of comm. The window consists of size bytes, starting at address base. In C, base is the starting address of a memory region. In Fortran, one can pass the first element of a memory region or a whole array, which must be 'simply contiguous' (for 'simply contiguous', see also MPI 3.0, Section 17.1.12 on page 626). A process may elect to expose no memory by specifying size = 0.

## Input Parameters

base
  initial address of window (choice)
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

win
  window object returned by the call (handle)

## Notes

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.

The info argument provides optimization hints to the runtime about the expected usage pattern of the window. The following info keys are predefined.

no_locks
  If set to true, then the implementation may assume that passive target synchronization (i.e., `MPI_Win_lock`, `MPI_Win_lock_all`) will not be used on the given window. This implies that this window is not used for 3-party communication, and RMA can be implemented with no (less) asynchronous agent activity at this process.

accumulate_ordering
  Controls the ordering of accumulate operations at the target. The argument string should contain a comma-separated list of the following read/write ordering rules, where e.g. "raw" means read-after-write: "rar,raw,war,waw".

accumulate_ops
  If set to same_op, the implementation will assume that all concurrent accumulate calls to the same target address will use the same operation. If set to same_op_no_op, then the implementation will assume that all concurrent accumulate calls to the same target address will use the same operation or `MPI_NO_OP`. This can eliminate the need to protect access for certain operation types where the hardware can guarantee atomicity. The default is same_op_no_op.

## Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as `malloc` or other non-MPICH runtime routines that are themselves not interrupt-safe.

## Notes for Fortran

All MPI routines in Fortran (except for `MPI_WTIME` and `MPI_WTICK`) have an additional argument `ierr` at the end of the argument list. `ierr` is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the `call` statement.

All MPI objects (e.g., `MPI_Datatype`, `MPI_Comm`) are of type `INTEGER` in Fortran.

## Errors

All MPI routines (except `MPI_Wtime` and `MPI_Wtick`) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with `MPI_Comm_set_errhandler` (for communicators), `MPI_File_set_errhandler` (for files), and `MPI_Win_set_errhandler` (for RMA windows). The MPI-1 routine `MPI_Errhandler_set` may be used but its use is deprecated. 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; however, MPI implementations will attempt to continue whenever possible.

MPI_SUCCESS
  No error; MPI routine completed successfully.

MPI_ERR_ARG
  Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., `MPI_ERR_RANK`).

MPI_ERR_COMM
  Invalid communicator. A common error is to use a null communicator in a call (not even allowed in `MPI_Comm_rank`).

MPI_ERR_INFO
  Invalid Info

MPI_ERR_OTHER
  Other error; use `MPI_Error_string` to get more information about this error code.

MPI_ERR_SIZE

## See Also

MPI_Win_allocate MPI_Win_allocate_shared MPI_Win_create_dynamic MPI_Win_free