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.. highlightlang:: c .. _supporting-cycle-detection: Supporting Cyclic Garbage Collection ==================================== Python's support for detecting and collecting garbage which involves circular references requires support from object types which are "containers" for other objects which may also be containers. Types which do not store references to other objects, or which only store references to atomic types (such as numbers or strings), do not need to provide any explicit support for garbage collection. .. An example showing the use of these interfaces can be found in "Supporting the .. Cycle Collector (XXX not found: ../ext/example-cycle-support.html)". To create a container type, the :attr:`tp_flags` field of the type object must include the :const:`Py_TPFLAGS_HAVE_GC` and provide an implementation of the :attr:`tp_traverse` handler. If instances of the type are mutable, a :attr:`tp_clear` implementation must also be provided. .. data:: Py_TPFLAGS_HAVE_GC :noindex: Objects with a type with this flag set must conform with the rules documented here. For convenience these objects will be referred to as container objects. Constructors for container types must conform to two rules: #. The memory for the object must be allocated using :c:func:`PyObject_GC_New` or :c:func:`PyObject_GC_NewVar`. #. Once all the fields which may contain references to other containers are initialized, it must call :c:func:`PyObject_GC_Track`. .. c:function:: TYPE* PyObject_GC_New(TYPE, PyTypeObject *type) Analogous to :c:func:`PyObject_New` but for container objects with the :const:`Py_TPFLAGS_HAVE_GC` flag set. .. c:function:: TYPE* PyObject_GC_NewVar(TYPE, PyTypeObject *type, Py_ssize_t size) Analogous to :c:func:`PyObject_NewVar` but for container objects with the :const:`Py_TPFLAGS_HAVE_GC` flag set. .. versionchanged:: 2.5 This function used an :c:type:`int` type for *size*. This might require changes in your code for properly supporting 64-bit systems. .. c:function:: TYPE* PyObject_GC_Resize(TYPE, PyVarObject *op, Py_ssize_t newsize) Resize an object allocated by :c:func:`PyObject_NewVar`. Returns the resized object or *NULL* on failure. .. versionchanged:: 2.5 This function used an :c:type:`int` type for *newsize*. This might require changes in your code for properly supporting 64-bit systems. .. c:function:: void PyObject_GC_Track(PyObject *op) Adds the object *op* to the set of container objects tracked by the collector. The collector can run at unexpected times so objects must be valid while being tracked. This should be called once all the fields followed by the :attr:`tp_traverse` handler become valid, usually near the end of the constructor. .. c:function:: void _PyObject_GC_TRACK(PyObject *op) A macro version of :c:func:`PyObject_GC_Track`. It should not be used for extension modules. Similarly, the deallocator for the object must conform to a similar pair of rules: #. Before fields which refer to other containers are invalidated, :c:func:`PyObject_GC_UnTrack` must be called. #. The object's memory must be deallocated using :c:func:`PyObject_GC_Del`. .. c:function:: void PyObject_GC_Del(void *op) Releases memory allocated to an object using :c:func:`PyObject_GC_New` or :c:func:`PyObject_GC_NewVar`. .. c:function:: void PyObject_GC_UnTrack(void *op) Remove the object *op* from the set of container objects tracked by the collector. Note that :c:func:`PyObject_GC_Track` can be called again on this object to add it back to the set of tracked objects. The deallocator (:attr:`tp_dealloc` handler) should call this for the object before any of the fields used by the :attr:`tp_traverse` handler become invalid. .. c:function:: void _PyObject_GC_UNTRACK(PyObject *op) A macro version of :c:func:`PyObject_GC_UnTrack`. It should not be used for extension modules. The :attr:`tp_traverse` handler accepts a function parameter of this type: .. c:type:: int (*visitproc)(PyObject *object, void *arg) Type of the visitor function passed to the :attr:`tp_traverse` handler. The function should be called with an object to traverse as *object* and the third parameter to the :attr:`tp_traverse` handler as *arg*. The Python core uses several visitor functions to implement cyclic garbage detection; it's not expected that users will need to write their own visitor functions. The :attr:`tp_traverse` handler must have the following type: .. c:type:: int (*traverseproc)(PyObject *self, visitproc visit, void *arg) Traversal function for a container object. Implementations must call the *visit* function for each object directly contained by *self*, with the parameters to *visit* being the contained object and the *arg* value passed to the handler. The *visit* function must not be called with a *NULL* object argument. If *visit* returns a non-zero value that value should be returned immediately. To simplify writing :attr:`tp_traverse` handlers, a :c:func:`Py_VISIT` macro is provided. In order to use this macro, the :attr:`tp_traverse` implementation must name its arguments exactly *visit* and *arg*: .. c:function:: void Py_VISIT(PyObject *o) Call the *visit* callback, with arguments *o* and *arg*. If *visit* returns a non-zero value, then return it. Using this macro, :attr:`tp_traverse` handlers look like:: static int my_traverse(Noddy *self, visitproc visit, void *arg) { Py_VISIT(self->foo); Py_VISIT(self->bar); return 0; } .. versionadded:: 2.4 The :attr:`tp_clear` handler must be of the :c:type:`inquiry` type, or *NULL* if the object is immutable. .. c:type:: int (*inquiry)(PyObject *self) Drop references that may have created reference cycles. Immutable objects do not have to define this method since they can never directly create reference cycles. Note that the object must still be valid after calling this method (don't just call :c:func:`Py_DECREF` on a reference). The collector will call this method if it detects that this object is involved in a reference cycle.