# MPI_Win_lock

Begin an RMA access epoch at the target process.

## Synopsis

    int MPI_Win_lock(int lock_type, int rank, int assert, MPI_Win win)

## Input Parameters

lock_type
  Indicates whether other processes may access the target window at the same time (if `MPI_LOCK_SHARED`) or not (`MPI_LOCK_EXCLUSIVE`)
rank
  rank of locked window (nonnegative integer)
assert
  Used to optimize this call; zero may be used as a default. See notes. (integer)
win
  window object (handle)

## Notes

The name of this routine is misleading. In particular, this routine need not block, except when the target process is the calling process.

Implementations may restrict the use of RMA communication that is synchronized by lock calls to windows in memory allocated by `MPI_Alloc_mem`. Locks can be used portably only in such memory.

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

MPI_MODE_NOCHECK
  no other process holds, or will attempt to acquire a conflicting lock, while the caller holds the window lock. This is useful when mutual exclusion is achieved by other means, but the coherence operations that may be attached to the lock and unlock calls are still required.

## 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_RANK
  Invalid source or destination rank. Ranks must be between zero and the size of the communicator minus one; ranks in a receive (`MPI_Recv`, `MPI_Irecv`, `MPI_Sendrecv`, etc.) may also be `MPI_ANY_SOURCE`.

MPI_ERR_WIN
  Invalid MPI window object

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