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