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/Doc/c-api/gcsupport.rst

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  1.. highlightlang:: c
  2
  3.. _supporting-cycle-detection:
  4
  5Supporting Cyclic Garbage Collection
  6====================================
  7
  8Python's support for detecting and collecting garbage which involves circular
  9references requires support from object types which are "containers" for other
 10objects which may also be containers.  Types which do not store references to
 11other objects, or which only store references to atomic types (such as numbers
 12or strings), do not need to provide any explicit support for garbage
 13collection.
 14
 15.. An example showing the use of these interfaces can be found in "Supporting the
 16.. Cycle Collector (XXX not found: ../ext/example-cycle-support.html)".
 17
 18To create a container type, the :attr:`tp_flags` field of the type object must
 19include the :const:`Py_TPFLAGS_HAVE_GC` and provide an implementation of the
 20:attr:`tp_traverse` handler.  If instances of the type are mutable, a
 21:attr:`tp_clear` implementation must also be provided.
 22
 23
 24.. data:: Py_TPFLAGS_HAVE_GC
 25   :noindex:
 26
 27   Objects with a type with this flag set must conform with the rules
 28   documented here.  For convenience these objects will be referred to as
 29   container objects.
 30
 31Constructors for container types must conform to two rules:
 32
 33#. The memory for the object must be allocated using :cfunc:`PyObject_GC_New`
 34   or :cfunc:`PyObject_GC_VarNew`.
 35
 36#. Once all the fields which may contain references to other containers are
 37   initialized, it must call :cfunc:`PyObject_GC_Track`.
 38
 39
 40.. cfunction:: TYPE* PyObject_GC_New(TYPE, PyTypeObject *type)
 41
 42   Analogous to :cfunc:`PyObject_New` but for container objects with the
 43   :const:`Py_TPFLAGS_HAVE_GC` flag set.
 44
 45
 46.. cfunction:: TYPE* PyObject_GC_NewVar(TYPE, PyTypeObject *type, Py_ssize_t size)
 47
 48   Analogous to :cfunc:`PyObject_NewVar` but for container objects with the
 49   :const:`Py_TPFLAGS_HAVE_GC` flag set.
 50
 51   .. versionchanged:: 2.5
 52      This function used an :ctype:`int` type for *size*. This might require
 53      changes in your code for properly supporting 64-bit systems.
 54
 55
 56.. cfunction:: TYPE* PyObject_GC_Resize(TYPE, PyVarObject *op, Py_ssize_t newsize)
 57
 58   Resize an object allocated by :cfunc:`PyObject_NewVar`.  Returns the
 59   resized object or *NULL* on failure.
 60
 61   .. versionchanged:: 2.5
 62      This function used an :ctype:`int` type for *newsize*. This might
 63      require changes in your code for properly supporting 64-bit systems.
 64
 65
 66.. cfunction:: void PyObject_GC_Track(PyObject *op)
 67
 68   Adds the object *op* to the set of container objects tracked by the
 69   collector.  The collector can run at unexpected times so objects must be
 70   valid while being tracked.  This should be called once all the fields
 71   followed by the :attr:`tp_traverse` handler become valid, usually near the
 72   end of the constructor.
 73
 74
 75.. cfunction:: void _PyObject_GC_TRACK(PyObject *op)
 76
 77   A macro version of :cfunc:`PyObject_GC_Track`.  It should not be used for
 78   extension modules.
 79
 80Similarly, the deallocator for the object must conform to a similar pair of
 81rules:
 82
 83#. Before fields which refer to other containers are invalidated,
 84   :cfunc:`PyObject_GC_UnTrack` must be called.
 85
 86#. The object's memory must be deallocated using :cfunc:`PyObject_GC_Del`.
 87
 88
 89.. cfunction:: void PyObject_GC_Del(void *op)
 90
 91   Releases memory allocated to an object using :cfunc:`PyObject_GC_New` or
 92   :cfunc:`PyObject_GC_NewVar`.
 93
 94
 95.. cfunction:: void PyObject_GC_UnTrack(void *op)
 96
 97   Remove the object *op* from the set of container objects tracked by the
 98   collector.  Note that :cfunc:`PyObject_GC_Track` can be called again on
 99   this object to add it back to the set of tracked objects.  The deallocator
100   (:attr:`tp_dealloc` handler) should call this for the object before any of
101   the fields used by the :attr:`tp_traverse` handler become invalid.
102
103
104.. cfunction:: void _PyObject_GC_UNTRACK(PyObject *op)
105
106   A macro version of :cfunc:`PyObject_GC_UnTrack`.  It should not be used for
107   extension modules.
108
109The :attr:`tp_traverse` handler accepts a function parameter of this type:
110
111
112.. ctype:: int (*visitproc)(PyObject *object, void *arg)
113
114   Type of the visitor function passed to the :attr:`tp_traverse` handler.
115   The function should be called with an object to traverse as *object* and
116   the third parameter to the :attr:`tp_traverse` handler as *arg*.  The
117   Python core uses several visitor functions to implement cyclic garbage
118   detection; it's not expected that users will need to write their own
119   visitor functions.
120
121The :attr:`tp_traverse` handler must have the following type:
122
123
124.. ctype:: int (*traverseproc)(PyObject *self, visitproc visit, void *arg)
125
126   Traversal function for a container object.  Implementations must call the
127   *visit* function for each object directly contained by *self*, with the
128   parameters to *visit* being the contained object and the *arg* value passed
129   to the handler.  The *visit* function must not be called with a *NULL*
130   object argument.  If *visit* returns a non-zero value that value should be
131   returned immediately.
132
133To simplify writing :attr:`tp_traverse` handlers, a :cfunc:`Py_VISIT` macro is
134provided.  In order to use this macro, the :attr:`tp_traverse` implementation
135must name its arguments exactly *visit* and *arg*:
136
137
138.. cfunction:: void Py_VISIT(PyObject *o)
139
140   Call the *visit* callback, with arguments *o* and *arg*. If *visit* returns
141   a non-zero value, then return it.  Using this macro, :attr:`tp_traverse`
142   handlers look like::
143
144      static int
145      my_traverse(Noddy *self, visitproc visit, void *arg)
146      {
147          Py_VISIT(self->foo);
148          Py_VISIT(self->bar);
149          return 0;
150      }
151
152   .. versionadded:: 2.4
153
154The :attr:`tp_clear` handler must be of the :ctype:`inquiry` type, or *NULL*
155if the object is immutable.
156
157
158.. ctype:: int (*inquiry)(PyObject *self)
159
160   Drop references that may have created reference cycles.  Immutable objects
161   do not have to define this method since they can never directly create
162   reference cycles.  Note that the object must still be valid after calling
163   this method (don't just call :cfunc:`Py_DECREF` on a reference).  The
164   collector will call this method if it detects that this object is involved
165   in a reference cycle.