160 lines
6.1 KiB
ReStructuredText
160 lines
6.1 KiB
ReStructuredText
.. highlightlang:: c
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.. _supporting-cycle-detection:
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Supporting Cyclic Garbage Collection
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====================================
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Python's support for detecting and collecting garbage which involves circular
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references requires support from object types which are "containers" for other
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objects which may also be containers. Types which do not store references to
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other objects, or which only store references to atomic types (such as numbers
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or strings), do not need to provide any explicit support for garbage
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collection.
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To create a container type, the :c:member:`~PyTypeObject.tp_flags` field of the type object must
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include the :const:`Py_TPFLAGS_HAVE_GC` and provide an implementation of the
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:c:member:`~PyTypeObject.tp_traverse` handler. If instances of the type are mutable, a
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:c:member:`~PyTypeObject.tp_clear` implementation must also be provided.
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.. data:: Py_TPFLAGS_HAVE_GC
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:noindex:
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Objects with a type with this flag set must conform with the rules
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documented here. For convenience these objects will be referred to as
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container objects.
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Constructors for container types must conform to two rules:
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#. The memory for the object must be allocated using :c:func:`PyObject_GC_New`
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or :c:func:`PyObject_GC_NewVar`.
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#. Once all the fields which may contain references to other containers are
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initialized, it must call :c:func:`PyObject_GC_Track`.
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.. c:function:: TYPE* PyObject_GC_New(TYPE, PyTypeObject *type)
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Analogous to :c:func:`PyObject_New` but for container objects with the
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:const:`Py_TPFLAGS_HAVE_GC` flag set.
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.. c:function:: TYPE* PyObject_GC_NewVar(TYPE, PyTypeObject *type, Py_ssize_t size)
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Analogous to :c:func:`PyObject_NewVar` but for container objects with the
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:const:`Py_TPFLAGS_HAVE_GC` flag set.
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.. c:function:: TYPE* PyObject_GC_Resize(TYPE, PyVarObject *op, Py_ssize_t newsize)
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Resize an object allocated by :c:func:`PyObject_NewVar`. Returns the
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resized object or ``NULL`` on failure. *op* must not be tracked by the collector yet.
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.. c:function:: void PyObject_GC_Track(PyObject *op)
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Adds the object *op* to the set of container objects tracked by the
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collector. The collector can run at unexpected times so objects must be
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valid while being tracked. This should be called once all the fields
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followed by the :c:member:`~PyTypeObject.tp_traverse` handler become valid, usually near the
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end of the constructor.
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.. c:function:: void _PyObject_GC_TRACK(PyObject *op)
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A macro version of :c:func:`PyObject_GC_Track`. It should not be used for
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extension modules.
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.. deprecated:: 3.6
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This macro is removed from Python 3.8.
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Similarly, the deallocator for the object must conform to a similar pair of
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rules:
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#. Before fields which refer to other containers are invalidated,
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:c:func:`PyObject_GC_UnTrack` must be called.
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#. The object's memory must be deallocated using :c:func:`PyObject_GC_Del`.
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.. c:function:: void PyObject_GC_Del(void *op)
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Releases memory allocated to an object using :c:func:`PyObject_GC_New` or
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:c:func:`PyObject_GC_NewVar`.
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.. c:function:: void PyObject_GC_UnTrack(void *op)
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Remove the object *op* from the set of container objects tracked by the
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collector. Note that :c:func:`PyObject_GC_Track` can be called again on
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this object to add it back to the set of tracked objects. The deallocator
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(:c:member:`~PyTypeObject.tp_dealloc` handler) should call this for the object before any of
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the fields used by the :c:member:`~PyTypeObject.tp_traverse` handler become invalid.
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.. c:function:: void _PyObject_GC_UNTRACK(PyObject *op)
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A macro version of :c:func:`PyObject_GC_UnTrack`. It should not be used for
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extension modules.
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.. deprecated:: 3.6
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This macro is removed from Python 3.8.
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The :c:member:`~PyTypeObject.tp_traverse` handler accepts a function parameter of this type:
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.. c:type:: int (*visitproc)(PyObject *object, void *arg)
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Type of the visitor function passed to the :c:member:`~PyTypeObject.tp_traverse` handler.
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The function should be called with an object to traverse as *object* and
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the third parameter to the :c:member:`~PyTypeObject.tp_traverse` handler as *arg*. The
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Python core uses several visitor functions to implement cyclic garbage
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detection; it's not expected that users will need to write their own
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visitor functions.
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The :c:member:`~PyTypeObject.tp_traverse` handler must have the following type:
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.. c:type:: int (*traverseproc)(PyObject *self, visitproc visit, void *arg)
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Traversal function for a container object. Implementations must call the
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*visit* function for each object directly contained by *self*, with the
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parameters to *visit* being the contained object and the *arg* value passed
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to the handler. The *visit* function must not be called with a ``NULL``
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object argument. If *visit* returns a non-zero value that value should be
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returned immediately.
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To simplify writing :c:member:`~PyTypeObject.tp_traverse` handlers, a :c:func:`Py_VISIT` macro is
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provided. In order to use this macro, the :c:member:`~PyTypeObject.tp_traverse` implementation
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must name its arguments exactly *visit* and *arg*:
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.. c:function:: void Py_VISIT(PyObject *o)
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If *o* is not ``NULL``, call the *visit* callback, with arguments *o*
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and *arg*. If *visit* returns a non-zero value, then return it.
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Using this macro, :c:member:`~PyTypeObject.tp_traverse` handlers
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look like::
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static int
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my_traverse(Noddy *self, visitproc visit, void *arg)
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{
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Py_VISIT(self->foo);
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Py_VISIT(self->bar);
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return 0;
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}
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The :c:member:`~PyTypeObject.tp_clear` handler must be of the :c:type:`inquiry` type, or ``NULL``
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if the object is immutable.
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.. c:type:: int (*inquiry)(PyObject *self)
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Drop references that may have created reference cycles. Immutable objects
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do not have to define this method since they can never directly create
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reference cycles. Note that the object must still be valid after calling
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this method (don't just call :c:func:`Py_DECREF` on a reference). The
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collector will call this method if it detects that this object is involved
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in a reference cycle.
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