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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.
+
+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
+
+   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 :cfunc:`PyObject_GC_New` or
+   :cfunc:`PyObject_GC_VarNew`.
+
+#. Once all the fields which may contain references to other containers are
+   initialized, it must call :cfunc:`PyObject_GC_Track`.
+
+
+.. cfunction:: TYPE* PyObject_GC_New(TYPE, PyTypeObject *type)
+
+   Analogous to :cfunc:`PyObject_New` but for container objects with the
+   :const:`Py_TPFLAGS_HAVE_GC` flag set.
+
+
+.. cfunction:: TYPE* PyObject_GC_NewVar(TYPE, PyTypeObject *type, Py_ssize_t size)
+
+   Analogous to :cfunc:`PyObject_NewVar` but for container objects with the
+   :const:`Py_TPFLAGS_HAVE_GC` flag set.
+
+
+.. cfunction:: PyVarObject * PyObject_GC_Resize(PyVarObject *op, Py_ssize_t)
+
+   Resize an object allocated by :cfunc:`PyObject_NewVar`.  Returns the resized
+   object or *NULL* on failure.
+
+
+.. cfunction:: 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.
+
+
+.. cfunction:: void _PyObject_GC_TRACK(PyObject *op)
+
+   A macro version of :cfunc:`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,
+   :cfunc:`PyObject_GC_UnTrack` must be called.
+
+#. The object's memory must be deallocated using :cfunc:`PyObject_GC_Del`.
+
+
+.. cfunction:: void PyObject_GC_Del(void *op)
+
+   Releases memory allocated to an object using :cfunc:`PyObject_GC_New` or
+   :cfunc:`PyObject_GC_NewVar`.
+
+
+.. cfunction:: void PyObject_GC_UnTrack(void *op)
+
+   Remove the object *op* from the set of container objects tracked by the
+   collector.  Note that :cfunc:`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.
+
+
+.. cfunction:: void _PyObject_GC_UNTRACK(PyObject *op)
+
+   A macro version of :cfunc:`PyObject_GC_UnTrack`.  It should not be used for
+   extension modules.
+
+The :attr:`tp_traverse` handler accepts a function parameter of this type:
+
+
+.. ctype:: 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:
+
+
+.. ctype:: 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 :cfunc:`Py_VISIT` macro is
+provided.  In order to use this macro, the :attr:`tp_traverse` implementation
+must name its arguments exactly *visit* and *arg*:
+
+
+.. cfunction:: 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;
+      }
+
+The :attr:`tp_clear` handler must be of the :ctype:`inquiry` type, or *NULL* if
+the object is immutable.
+
+
+.. ctype:: 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 :cfunc:`Py_DECREF` on a reference).  The collector will call
+   this method if it detects that this object is involved in a reference cycle.