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# PetscDeviceFree

Free device-aware memory obtained with `PetscDeviceMalloc()` or `PetscDeviceCalloc()`

## Synopsis

    #include <petscdevice.h>
    PetscErrorCode PetscDeviceFree(PetscDeviceContext dctx, void *ptr)

Not Collective, Asynchronous, Auto-dependency aware

## Input Parameters

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

- ptr - The pointer to free, may be `NULL`

## Notes

`ptr` is set to `PETSC_NULLPTR` on successful deallocation.

`ptr` must have been allocated using `PetscDeviceMalloc()`, `PetscDeviceCalloc()` not `PetscMalloc()` or related routines

This routine falls back to using `PetscFree()` if PETSc was not configured with device support. The user should note that `PetscFree()` frees only host memory.

## DAG representation

      time ->

      -> dctx -/- |= CALL =| - dctx ->
      -> ptr -/

## 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

`PetscDeviceMalloc()`, `PetscDeviceCalloc()`

## Level

beginner

## Location

include/petscdevice.h

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