# MPI_Win_start

Start an RMA access epoch for MPI

## Synopsis

    int MPI_Win_start(MPI_Group group, int assert, MPI_Win win)

## Input Parameters

group
  group of target processes (handle)
assert
  program assertion (integer)
win
  window object (handle)

## Notes

The `assert` argument is used to indicate special conditions for the fence that an implementation may use to optimize the `MPI_Win_start` operation. The value zero is always correct. Other assertion values may be or'ed together. Assertions that are valid for `MPI_Win_start` are:

MPI_MODE_NOCHECK
  the matching calls to `MPI_WIN_POST` have already completed on all target processes when the call to `MPI_WIN_START` is made. The nocheck option can be specified in a start call if and only if it is specified in each matching post call. This is similar to the optimization of ready-send that may save a handshake when the handshake is implicit in the code. (However, ready-send is matched by a regular receive, whereas both start and post must specify the nocheck option.)

## 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_GROUP
  Null or invalid group passed to function.

MPI_ERR_WIN
  Invalid MPI window object

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