.. Sources: ``_ .. index:: single: finalization .. _topic-finalization: Finalization ============ It is sometimes necessary to perform actions when a block of memory :term:`dies `. For example, a block may represent the acquisition of an external resource such as a file handle or a network connection. When the block dies, the corresponding resource must be released. This procedure is known as :term:`finalization`. A block requiring finalization must be registered by calling :c:func:`mps_finalize`:: mps_addr_t ref = block_requiring_finalization; mps_finalize(arena, &ref); A block that been registered for finalization becomes *finalizable* as soon as the :term:`garbage collector` observes that it would otherwise be :term:`reclaimed` (that is, the only thing keeping it alive is the fact that it needs to be finalized). If a block is finalizable the MPS may choose to finalize it (by posting a finalization message: see below) at *any* future time. .. note:: This means that a block that was determined to be finalizable, but then became unconditionally :term:`live` by the creation of a new :term:`strong reference` to it, may still be finalized. :term:`Weak references (1)` do not prevent blocks from being finalized. At the point that a block is finalized, weak references will still validly refer to the block. The fact that a block is registered for finalization prevents weak references to that block from being :term:`splatted `. See :ref:`topic-weak`. The Memory Pool System finalizes a block by posting a *finalization message* to the :term:`message queue` of the :term:`arena` in which the block was allocated. .. note:: This design avoids the problems that can result from the :term:`garbage collector` calling a function in the client program to do the finalization. In such an implementation, the client program's finalization code may end up running concurrently with other code that accesses the underlying resource, and so access to the resource need to be guarded with a lock, but then an unlucky scheduling of finalization can result in deadlock. See :ref:`Boehm (2002) ` for a detailed discussion of this issue. The :term:`message type` of finalization messages is :c:func:`mps_message_type_finalization`, and the client program must enable the posting of these messages by calling :c:func:`mps_message_type_enable` before any block becomes finalizable:: mps_message_type_enable(arena, mps_message_type_finalization()); When a finalization message has been retrieved from the message queue by calling :c:func:`mps_message_get`, the finalization reference may be accessed by calling :c:func:`mps_message_finalization_ref`. The finalization message keeps the block alive until it is discarded by calling :c:func:`mps_message_discard`. .. note:: The client program may choose to keep the finalized block alive by keeping a strong reference to the finalized object after discarding the finalization message. This process is known as :term:`resurrection` and in some finalization systems requires special handling, but in the MPS this just is just the usual result of the rule that strong references keep objects alive. It is fine to re-register a block for finalization after retrieving its finalization message from the message queue. This will cause it to be finalized again should all strong references disappear again. See :ref:`topic-message` for details of the message mechanism. .. index:: single: finalization; multiple Multiple finalizations ---------------------- A block may be registered for finalization multiple times. A block that has been registered for finalization *n* times will be finalized at most *n* times. This may mean that there are multiple finalization messages on the queue at the same time, or it may not (it may be necessary for the client program to discard previous finalization messages for a block before a new finalization messages for that block are posted to the message queue). The MPS provides no guarantees either way: a client program that registers the same block multiple times must cope with either behaviour. .. index:: pair: finalization; cautions .. _topic-finalization-cautions: Cautions -------- 1. Don't rely on finalization for your program to work. Treat it as an optimization that enables the freeing of resources that the garbage collector can prove are unreachable. 2. The MPS provides no guarantees about the promptness of finalization. The MPS does not finalize a block until it determines that the block is finalizable, which may require a full garbage collection in the worst case, and such a collection may not :ref:`scheduled ` for some time. Or the block may never become finalizable because it is incorrectly determined to be reachable due to an :term:`ambiguous reference` pointing to it. Or the block may never become finalizable because it remains reachable through a reference, even if that reference might never be used. 3. Even when blocks are finalized in a reasonably timely fashion, the client needs to process the finalization messages in time to avoid the resource running out. For example, in the Scheme interpreter, finalization messages are only processed at the end of the read–eval–print loop, so a program that opens many files may run out of handles even though the associated objects are all finalizable, as shown here: .. code-block:: none MPS Toy Scheme Example 9960, 0> (define (repeat n f _) (if (eqv? n 0) '() (repeat (- n 1) f (f)))) repeat 10840, 0> (repeat 300 (lambda () (open-input-file "scheme.c")) 0) open-input-file: cannot open input file A less naïve interpreter might process finalization messages on a more regular schedule, or might take emergency action in the event of running out of open file handles by carrying out a full garbage collection and processing any finalization messages that are posted as a result. If you are designing a programming language then it is generally a good idea to provide the programmer with a mechanism for ensuring prompt release of scarce resources. For example, Scheme provides the ``(with-input-from-file)`` procedure which specifies that the created port has :term:`dynamic extent` (and so can be closed as soon as the procedure exits). 4. The MPS does not finalize objects in the context of :c:func:`mps_arena_destroy` or :c:func:`mps_pool_destroy`. :c:func:`mps_pool_destroy` should therefore not be invoked on pools containing objects registered for finalization. .. note:: Under normal circumstances, finalization code can assume that objects referenced by the object being finalized ("object F") have themselves not yet been finalized. (Because object F is keeping them alive.) If finalization code is run at program exit, this assumption is no longer true. It is much more difficult to write correct code if it has to run under both circumstances. This is why Java's ``System.runFinalizersOnExit`` is deprecated. See Appendix A of :ref:`Boehm (2002) ` for a discussion of this problem. 4. Not all :term:`pool classes` support finalization. In general, only pools that manage objects whose liveness is determined by garbage collection do so. See the :ref:`pool`. .. index:: single: finalization; interface Finalization interface ---------------------- .. c:function:: mps_res_t mps_finalize(mps_arena_t arena, mps_addr_t *ref_p) Register a :term:`block` for :term:`finalization`. ``arena`` is the arena in which the block lives. ``ref_p`` points to a :term:`reference` to the block to be registered for finalization. Returns :c:macro:`MPS_RES_OK` if successful, or another :term:`result code` if not. This function registers the block pointed to by ``*ref_p`` for finalization. This block must have been allocated from a :term:`pool` in ``arena``. Violations of this constraint may not be checked by the MPS, and may be unsafe, causing the MPS to crash in undefined ways. .. note:: This function receives a pointer to a reference. This is to avoid placing the restriction on the :term:`client program` that the C call stack be a :term:`root`. .. c:function:: mps_res_t mps_definalize(mps_arena_t arena, mps_addr_t *ref_p) Deregister a :term:`block` for :term:`finalization`. ``arena`` is the arena in which the block lives. ``ref_p`` points to a :term:`reference` to the block to be deregistered for finalization. Returns :c:macro:`MPS_RES_OK` if successful, or :c:macro:`MPS_RES_FAIL` if the block was not previously registered for finalization. .. note:: This function receives a pointer to a reference. This is to avoid placing the restriction on the :term:`client program` that the C call stack be a :term:`root`. .. index:: pair: finalization; message Finalization messages --------------------- .. c:function:: mps_message_type_t mps_message_type_finalization(void) Return the :term:`message type` of finalization messages. Finalization messages are used by the MPS to implement :term:`finalization`. When the MPS detects that a block that has been registered for finalization (by calling :c:func:`mps_finalize`) is finalizable, it finalizes it by posting a :term:`message` of this type. Note that there might be delays between the block becoming finalizable, the MPS detecting that, and the message being posted. In addition to the usual methods applicable to messages, finalization messages support the :c:func:`mps_message_finalization_ref` method which returns a reference to the block that was registered for finalization. .. seealso:: :ref:`topic-message`. .. c:function:: void mps_message_finalization_ref(mps_addr_t *ref_o, mps_arena_t arena, mps_message_t message) Returns the finalization reference for a finalization message. ``ref_o`` points to a location that will hold the finalization reference. ``arena`` is the :term:`arena` which posted the message. ``message`` is a message retrieved by :c:func:`mps_message_get` and not yet discarded. It must be a finalization message: see :c:func:`mps_message_type_finalization`. The reference returned by this method is a reference to the block that was originally registered for :term:`finalization` by a call to :c:func:`mps_finalize`. .. note:: The reference returned is subject to the normal constraints, such as might be imposed by a :term:`moving ` collection, if appropriate. For this reason, it is stored into the location pointed to by ``ref_o`` in order to enable the :term:`client program` to place it directly into scanned memory, without imposing the restriction that the C stack be a :term:`root`. The message itself is not affected by invoking this method. Until the client program calls :c:func:`mps_message_discard` to discard the message, it will refer to the object and prevent its reclamation. .. seealso:: :ref:`topic-message`.