154 lines
5.7 KiB
ReStructuredText
154 lines
5.7 KiB
ReStructuredText
.. 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.
|