# MPI_Win_allocate

Create and allocate an MPI Window object for one-sided communication.

## Synopsis

    int MPI_Win_allocate(MPI_Aint size, int disp_unit, MPI_Info info,
                         MPI_Comm comm, void *baseptr, MPI_Win * win)

This 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.

## 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
  base address of the window in local memory

win
  window object returned by the call (handle)

## 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 guarentee 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_shared MPI_Win_create MPI_Win_create_dynamic MPI_Win_free