1.. highlight:: 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. *op* must not be tracked by the collector yet. 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 64Similarly, the deallocator for the object must conform to a similar pair of 65rules: 66 67#. Before fields which refer to other containers are invalidated, 68 :c:func:`PyObject_GC_UnTrack` must be called. 69 70#. The object's memory must be deallocated using :c:func:`PyObject_GC_Del`. 71 72 73.. c:function:: void PyObject_GC_Del(void *op) 74 75 Releases memory allocated to an object using :c:func:`PyObject_GC_New` or 76 :c:func:`PyObject_GC_NewVar`. 77 78 79.. c:function:: void PyObject_GC_UnTrack(void *op) 80 81 Remove the object *op* from the set of container objects tracked by the 82 collector. Note that :c:func:`PyObject_GC_Track` can be called again on 83 this object to add it back to the set of tracked objects. The deallocator 84 (:c:member:`~PyTypeObject.tp_dealloc` handler) should call this for the object before any of 85 the fields used by the :c:member:`~PyTypeObject.tp_traverse` handler become invalid. 86 87 88.. versionchanged:: 3.8 89 90 The :c:func:`_PyObject_GC_TRACK` and :c:func:`_PyObject_GC_UNTRACK` macros 91 have been removed from the public C API. 92 93The :c:member:`~PyTypeObject.tp_traverse` handler accepts a function parameter of this type: 94 95 96.. c:type:: int (*visitproc)(PyObject *object, void *arg) 97 98 Type of the visitor function passed to the :c:member:`~PyTypeObject.tp_traverse` handler. 99 The function should be called with an object to traverse as *object* and 100 the third parameter to the :c:member:`~PyTypeObject.tp_traverse` handler as *arg*. The 101 Python core uses several visitor functions to implement cyclic garbage 102 detection; it's not expected that users will need to write their own 103 visitor functions. 104 105The :c:member:`~PyTypeObject.tp_traverse` handler must have the following type: 106 107 108.. c:type:: int (*traverseproc)(PyObject *self, visitproc visit, void *arg) 109 110 Traversal function for a container object. Implementations must call the 111 *visit* function for each object directly contained by *self*, with the 112 parameters to *visit* being the contained object and the *arg* value passed 113 to the handler. The *visit* function must not be called with a ``NULL`` 114 object argument. If *visit* returns a non-zero value that value should be 115 returned immediately. 116 117To simplify writing :c:member:`~PyTypeObject.tp_traverse` handlers, a :c:func:`Py_VISIT` macro is 118provided. In order to use this macro, the :c:member:`~PyTypeObject.tp_traverse` implementation 119must name its arguments exactly *visit* and *arg*: 120 121 122.. c:function:: void Py_VISIT(PyObject *o) 123 124 If *o* is not ``NULL``, call the *visit* callback, with arguments *o* 125 and *arg*. If *visit* returns a non-zero value, then return it. 126 Using this macro, :c:member:`~PyTypeObject.tp_traverse` handlers 127 look like:: 128 129 static int 130 my_traverse(Noddy *self, visitproc visit, void *arg) 131 { 132 Py_VISIT(self->foo); 133 Py_VISIT(self->bar); 134 return 0; 135 } 136 137The :c:member:`~PyTypeObject.tp_clear` handler must be of the :c:type:`inquiry` type, or ``NULL`` 138if the object is immutable. 139 140 141.. c:type:: int (*inquiry)(PyObject *self) 142 143 Drop references that may have created reference cycles. Immutable objects 144 do not have to define this method since they can never directly create 145 reference cycles. Note that the object must still be valid after calling 146 this method (don't just call :c:func:`Py_DECREF` on a reference). The 147 collector will call this method if it detects that this object is involved 148 in a reference cycle. 149