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# PetscDeviceMemcpy

Copy memory in a device-aware manner

## Synopsis

    #include <petscdevice.h>
    PetscErrorCode PetscDeviceMemcpy(PetscDeviceContext dctx, void *PETSC_RESTRICT dest, const void *PETSC_RESTRICT src, std::size_t n)

Not Collective, Asynchronous, Auto-dependency aware

## Input Parameters

- dctx - The `PetscDeviceContext` used to copy the memory

- dest - The pointer to copy to

- src - The pointer to copy from

- n - The amount (in bytes) to copy

## Notes

Both `dest` and `src` must have been allocated by `PetscDeviceMalloc()` or `PetscDeviceCalloc()`.

`src` and `dest` cannot overlap.

If both `src` and `dest` are on the host this routine is fully synchronous.

The user should prefer `PetscDeviceArrayCopy()` over this routine as it automatically computes the number of bytes to copy from the size of the pointer types.

## DAG representation

      time ->

      -> dctx - |= CALL =| - dctx ->
      -> dest --------------------->
      -> src ---------------------->

## 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

`PetscDeviceArrayCopy()`, `PetscDeviceMalloc()`, `PetscDeviceCalloc()`, `PetscDeviceFree()`

## Level

intermediate

## Location

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

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Index of all manual pages