Split the monstrous C API manual files in smaller parts.
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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 :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;
+ }
+
+ .. versionadded:: 2.4
+
+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.