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 :c:member:`~PyTypeObject.tp_flags` field of the type object must 19include the :const:`Py_TPFLAGS_HAVE_GC` and provide an implementation of the 20:c:member:`~PyTypeObject.tp_traverse` handler. If instances of the type are mutable, a 21:c:member:`~PyTypeObject.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 :c:func:`PyObject_GC_New` 34 or :c:func:`PyObject_GC_NewVar`. 35 36#. Once all the fields which may contain references to other containers are 37 initialized, it must call :c:func:`PyObject_GC_Track`. 38 39 40.. c:function:: TYPE* PyObject_GC_New(TYPE, PyTypeObject *type) 41 42 Analogous to :c:func:`PyObject_New` but for container objects with the 43 :const:`Py_TPFLAGS_HAVE_GC` flag set. 44 45 46.. c:function:: TYPE* PyObject_GC_NewVar(TYPE, PyTypeObject *type, Py_ssize_t size) 47 48 Analogous to :c:func:`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 :c:type:`int` type for *size*. This might require 53 changes in your code for properly supporting 64-bit systems. 54 55 56.. c:function:: TYPE* PyObject_GC_Resize(TYPE, PyVarObject *op, Py_ssize_t newsize) 57 58 Resize an object allocated by :c:func:`PyObject_NewVar`. Returns the 59 resized object or *NULL* on failure. *op* must not be tracked by the collector yet. 60 61 .. versionchanged:: 2.5 62 This function used an :c:type:`int` type for *newsize*. This might 63 require changes in your code for properly supporting 64-bit systems. 64 65 66.. c:function:: 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 :c:member:`~PyTypeObject.tp_traverse` handler become valid, usually near the 72 end of the constructor. 73 74 75.. c:function:: void _PyObject_GC_TRACK(PyObject *op) 76 77 A macro version of :c:func:`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 :c:func:`PyObject_GC_UnTrack` must be called. 85 86#. The object's memory must be deallocated using :c:func:`PyObject_GC_Del`. 87 88 89.. c:function:: void PyObject_GC_Del(void *op) 90 91 Releases memory allocated to an object using :c:func:`PyObject_GC_New` or 92 :c:func:`PyObject_GC_NewVar`. 93 94 95.. c:function:: 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 :c:func:`PyObject_GC_Track` can be called again on 99 this object to add it back to the set of tracked objects. The deallocator 100 (:c:member:`~PyTypeObject.tp_dealloc` handler) should call this for the object before any of 101 the fields used by the :c:member:`~PyTypeObject.tp_traverse` handler become invalid. 102 103 104.. c:function:: void _PyObject_GC_UNTRACK(PyObject *op) 105 106 A macro version of :c:func:`PyObject_GC_UnTrack`. It should not be used for 107 extension modules. 108 109The :c:member:`~PyTypeObject.tp_traverse` handler accepts a function parameter of this type: 110 111 112.. c:type:: int (*visitproc)(PyObject *object, void *arg) 113 114 Type of the visitor function passed to the :c:member:`~PyTypeObject.tp_traverse` handler. 115 The function should be called with an object to traverse as *object* and 116 the third parameter to the :c:member:`~PyTypeObject.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 :c:member:`~PyTypeObject.tp_traverse` handler must have the following type: 122 123 124.. c:type:: 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 :c:member:`~PyTypeObject.tp_traverse` handlers, a :c:func:`Py_VISIT` macro is 134provided. In order to use this macro, the :c:member:`~PyTypeObject.tp_traverse` implementation 135must name its arguments exactly *visit* and *arg*: 136 137 138.. c:function:: void Py_VISIT(PyObject *o) 139 140 If *o* is not *NULL*, call the *visit* callback, with arguments *o* 141 and *arg*. If *visit* returns a non-zero value, then return it. 142 Using this macro, :c:member:`~PyTypeObject.tp_traverse` handlers 143 look like:: 144 145 static int 146 my_traverse(Noddy *self, visitproc visit, void *arg) 147 { 148 Py_VISIT(self->foo); 149 Py_VISIT(self->bar); 150 return 0; 151 } 152 153 .. versionadded:: 2.4 154 155The :c:member:`~PyTypeObject.tp_clear` handler must be of the :c:type:`inquiry` type, or *NULL* 156if the object is immutable. 157 158 159.. c:type:: int (*inquiry)(PyObject *self) 160 161 Drop references that may have created reference cycles. Immutable objects 162 do not have to define this method since they can never directly create 163 reference cycles. Note that the object must still be valid after calling 164 this method (don't just call :c:func:`Py_DECREF` on a reference). The 165 collector will call this method if it detects that this object is involved 166 in a reference cycle. 167