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# PetscDeviceContextGetStreamHandle

Return a handle to the underlying stream of the current device context

## Synopsis

    #include <petscdevice.h>
    PetscErrorCode PetscDeviceContextGetStreamHandle(PetscDeviceContext dctx, void **handle)

## Input Parameter

- dctx - The `PetscDeviceContext` to get the stream from

## Output Parameter

- handle - A pointer to the handle to the stream

## Note

This routine is dangerous. It exists only for the most experienced users and internal PETSc development.

There is no way for PETSc’s auto-dependency system to track what the caller does with the stream.

If the user uses the stream to copy memory that was previously modified by PETSc, or launches kernels that modify memory with the stream, it is the users responsibility to inform PETSc of their actions via `PetscDeviceContextMarkIntentFromID()`. Failure to do so may introduce a race condition. This race condition may manifest in nondeterministic ways.

Alternatively, the user may synchronize the stream immediately before and after use. This is the safest option.

## Example Usage

      PetscDeviceContext dctx;
      PetscDeviceType    type;
      void               *handle;

      PetscDeviceContextGetCurrentContext(&dctx);
      PetscDeviceContextGetStreamHandle(dctx, &handle);
      PetscDeviceContextGetDeviceType(dctx, &type);

      if (type == PETSC_DEVICE_CUDA) {
        cudaStream_t stream = *(cudaStream_t *)handle;

        my_cuda_kernel<<<1, 2, 3, stream>>>();
      }

Alternatively, if type of `PetscDeviceContext` is known (for example `PETSC_DEVICE_HIP`), the user may pass in a pointer to stream handle directly:

      hipStream_t *stream;

      // note the cast to void **
      PetscDeviceContextGetStreamHandle(dctx, (void **)&stream);
      // note the dereference
      my_hip_kernel<<<1, 2, 3, *stream>>>();

## 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

`PetscDeviceContext`

## Level

developer

## Location

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

Index of all Device routines
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Index of all manual pages