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# PetscDeviceContextWaitForContext

Make one context wait for another context to finish

## Synopsis

    #include <petscdevice.h>
    PetscErrorCode PetscDeviceContextWaitForContext(PetscDeviceContext dctxa, PetscDeviceContext dctxb)

Not Collective

## Input Parameters

- dctxa - The `PetscDeviceContext` object that is waiting

- dctxb - The `PetscDeviceContext` object that is being waited on

## Notes

Serializes two `PetscDeviceContext`s. Serialization is performed asynchronously; the host does not wait for the serialization to actually occur.

This routine uses only the state of `dctxb` at the moment this routine was called, so any future work queued will not affect `dctxa`. It is safe to pass the same context to both arguments (in which case this routine does nothing).

## DAG representation

      time ->

      -> dctxa ---/- |= CALL =| - dctxa ->
                 /
      -> dctxb -/------------------------>

## Asynchronous API Notes

This routine is explicitly marked as exhibiting asynchronous behavior. Asynchronous behavior implies that routines launching operations on (or associated with) a `PetscDeviceContext` may return to the caller before the operation has completed.

Sequential Consistency:

Operations using the same `PetscDeviceContext` which access objects or memory regions are ordered per the language specification.

Operations using separate `PetscDeviceContext`s which access the same object or memory region are strongly write-ordered. That is, the following operations:

- `write-write`

- `write-read`

- `read-write`

are strongly ordered. Formally:

Given an operation `A-B` (e.g. `A` = `write`, `B` = `read`) on an object or memory region `M` such that `A` “happens-before” `B`, where `A` uses `PetscDeviceContext` `X` and `B` uses `PetscDeviceContext` `Y`, then `B` shall not begin before `A` completes. This implies that any side-effects resulting from `A` are also observed by `B`.

Note the omission of `read-read`; there is no implied ordering between separate `PetscDeviceContext`s for consecutive reads.

Operations using separate `PetscDeviceContext`s which access separate objects or memory regions may execute in an arbitrary order and offer no guarantee of sequential consistency.

Memory Consistency:

If this routine modifies the participating object(s) then – unless otherwise stated – the contents of any externally held references to internal data structures should be considered to be in an undefined state. A well-defined state can only be restored by re-acquiring these references through the appropriate API or by calling `PetscDeviceContextSynchronize()`.

Unless otherwise stated, exceptions to this rule are:

- References returned by the routine itself. If a routine returns a pointer, the value of the top-most pointer is guaranteed to always be valid. For example, given a routine which asynchronously allocates memory and returns a pointer to the memory, the value of said pointer is immediately valid but dereferencing the pointer may not be.

- References to structures. If a routine returns a `PetscFoo`, or array thereof then the objects themselves are always valid (though their member variables `PetscFoo->data` may not be).

## See Also

`PetscDeviceContextCreate()`, `PetscDeviceContextQueryIdle()`, `PetscDeviceContextJoin()`

## Level

beginner

## Location

src/sys/objects/device/interface/dcontext.cxx

Index of all Device routines
Table of Contents for all manual pages
Index of all manual pages