## 6.4.1. Fence

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MPI_WIN_FENCE(assert, win)
IN assert | program assertion (integer)

IN win | window object (handle)

`int MPI_Win_fence(int assert, MPI_Win win)
`

`MPI_WIN_FENCE(ASSERT, WIN, IERROR)
INTEGER ASSERT, WIN, IERROR
`

`void MPI::Win::Fence(int assert) const
`

The MPI call MPI_WIN_FENCE(assert, win) synchronizes RMA calls on win. The call is collective on the group of win. All RMA operations on win originating at a given process and started before the fence call will complete at that process before the fence call returns. They will be completed at their target before the fence call returns at the target. RMA operations on win started by a process after the fence call returns will access their target window only after MPI_WIN_FENCE has been called by the target process.

The call completes an RMA access epoch if it was preceded by another fence call and the local process issued RMA communication calls on win between these two calls. The call completes an RMA exposure epoch if it was preceded by another fence call and the local window was the target of RMA accesses between these two calls. The call starts an RMA access epoch if it is followed by another fence call and by RMA communication calls issued between these two fence calls. The call starts an exposure epoch if it is followed by another fence call and the local window is the target of RMA accesses between these two fence calls. Thus, the fence call is equivalent to calls to a subset of `post, start, complete, wait`.

A fence call usually entails a barrier synchronization: a process completes a call to MPI_WIN_FENCE only after all other processes in the group entered their matching call. However, a call to MPI_WIN_FENCE that is known not to end any epoch (in particular, a call with assert = MPI_MODE_NOPRECEDE) does not necessarily act as a barrier.

The assert argument is used to provide assertions on the context of the call that may be used for various optimizations. This is described in Section Assertions . A value of assert = 0 is always valid.

Advice to users.

Calls to MPI_WIN_FENCE should both precede and follow calls to put, get or accumulate that are synchronized with fence calls. ( End of advice to users.)

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MPI-2.0 of July 18, 1997
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