# PetscDeviceContextFork
Create a set of dependent child contexts from a parent context 
## Synopsis
```
#include <petscdevice.h> 
PetscErrorCode PetscDeviceContextFork(PetscDeviceContext dctx, PetscInt n, PetscDeviceContext **dsub)
```
Not Collective, Asynchronous


## Input Parameters

- ***dctx -*** The parent `PetscDeviceContext`
- ***n    -*** The number of children to create



## Output Parameter

- ***dsub -*** The created child context(s)



## Notes
Behaves identically to `PetscDeviceContextForkWithStreamType()` except that the prescribed
`PetscStreamType` is taken from `dctx`. In effect this routine is shorthand for:

```none
  PetscStreamType stype;

  PetscDeviceContextGetStreamType(dctx, &stype);
  PetscDeviceContextForkWithStreamType(dctx, stype, ...);
```





## 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
 `PetscDeviceContextForkWithStreamType()`, `PetscDeviceContextJoin()`,
`PetscDeviceContextSynchronize()`, `PetscDeviceContextQueryIdle()`

## Level
beginner

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/sys/objects/device/interface/dcontext.cxx.html#PetscDeviceContextFork">src/sys/objects/device/interface/dcontext.cxx</A>


---
[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/sys/objects/device/interface/dcontext.cxx)


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