176 lines
6.6 KiB
ReStructuredText
176 lines
6.6 KiB
ReStructuredText
.. highlight:: 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.
|
|
|
|
To create a container type, the :c:member:`~PyTypeObject.tp_flags` field of the type object must
|
|
include the :const:`Py_TPFLAGS_HAVE_GC` and provide an implementation of the
|
|
:c:member:`~PyTypeObject.tp_traverse` handler. If instances of the type are mutable, a
|
|
:c:member:`~PyTypeObject.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 :c:func:`PyObject_GC_New`
|
|
or :c:func:`PyObject_GC_NewVar`.
|
|
|
|
#. Once all the fields which may contain references to other containers are
|
|
initialized, it must call :c:func:`PyObject_GC_Track`.
|
|
|
|
|
|
.. c:function:: TYPE* PyObject_GC_New(TYPE, PyTypeObject *type)
|
|
|
|
Analogous to :c:func:`PyObject_New` but for container objects with the
|
|
:const:`Py_TPFLAGS_HAVE_GC` flag set.
|
|
|
|
|
|
.. c:function:: TYPE* PyObject_GC_NewVar(TYPE, PyTypeObject *type, Py_ssize_t size)
|
|
|
|
Analogous to :c:func:`PyObject_NewVar` but for container objects with the
|
|
:const:`Py_TPFLAGS_HAVE_GC` flag set.
|
|
|
|
|
|
.. c:function:: TYPE* PyObject_GC_Resize(TYPE, PyVarObject *op, Py_ssize_t newsize)
|
|
|
|
Resize an object allocated by :c:func:`PyObject_NewVar`. Returns the
|
|
resized object or ``NULL`` on failure. *op* must not be tracked by the collector yet.
|
|
|
|
|
|
.. c:function:: 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 :c:member:`~PyTypeObject.tp_traverse` handler become valid, usually near the
|
|
end of the constructor.
|
|
|
|
|
|
.. c:function:: int PyObject_IS_GC(PyObject *obj)
|
|
|
|
Returns non-zero if the object implements the garbage collector protocol,
|
|
otherwise returns 0.
|
|
|
|
The object cannot be tracked by the garbage collector if this function returns 0.
|
|
|
|
|
|
.. c:function:: int PyObject_GC_IsTracked(PyObject *op)
|
|
|
|
Returns 1 if the object type of *op* implements the GC protocol and *op* is being
|
|
currently tracked by the garbage collector and 0 otherwise.
|
|
|
|
This is analogous to the Python function :func:`gc.is_tracked`.
|
|
|
|
.. versionadded:: 3.9
|
|
|
|
|
|
.. c:function:: int PyObject_GC_IsFinalized(PyObject *op)
|
|
|
|
Returns 1 if the object type of *op* implements the GC protocol and *op* has been
|
|
already finalized by the garbage collector and 0 otherwise.
|
|
|
|
This is analogous to the Python function :func:`gc.is_finalized`.
|
|
|
|
.. versionadded:: 3.9
|
|
|
|
Similarly, the deallocator for the object must conform to a similar pair of
|
|
rules:
|
|
|
|
#. Before fields which refer to other containers are invalidated,
|
|
:c:func:`PyObject_GC_UnTrack` must be called.
|
|
|
|
#. The object's memory must be deallocated using :c:func:`PyObject_GC_Del`.
|
|
|
|
|
|
.. c:function:: void PyObject_GC_Del(void *op)
|
|
|
|
Releases memory allocated to an object using :c:func:`PyObject_GC_New` or
|
|
:c:func:`PyObject_GC_NewVar`.
|
|
|
|
|
|
.. c:function:: void PyObject_GC_UnTrack(void *op)
|
|
|
|
Remove the object *op* from the set of container objects tracked by the
|
|
collector. Note that :c:func:`PyObject_GC_Track` can be called again on
|
|
this object to add it back to the set of tracked objects. The deallocator
|
|
(:c:member:`~PyTypeObject.tp_dealloc` handler) should call this for the object before any of
|
|
the fields used by the :c:member:`~PyTypeObject.tp_traverse` handler become invalid.
|
|
|
|
|
|
.. versionchanged:: 3.8
|
|
|
|
The :c:func:`_PyObject_GC_TRACK` and :c:func:`_PyObject_GC_UNTRACK` macros
|
|
have been removed from the public C API.
|
|
|
|
The :c:member:`~PyTypeObject.tp_traverse` handler accepts a function parameter of this type:
|
|
|
|
|
|
.. c:type:: int (*visitproc)(PyObject *object, void *arg)
|
|
|
|
Type of the visitor function passed to the :c:member:`~PyTypeObject.tp_traverse` handler.
|
|
The function should be called with an object to traverse as *object* and
|
|
the third parameter to the :c:member:`~PyTypeObject.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 :c:member:`~PyTypeObject.tp_traverse` handler must have the following type:
|
|
|
|
|
|
.. c:type:: 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 :c:member:`~PyTypeObject.tp_traverse` handlers, a :c:func:`Py_VISIT` macro is
|
|
provided. In order to use this macro, the :c:member:`~PyTypeObject.tp_traverse` implementation
|
|
must name its arguments exactly *visit* and *arg*:
|
|
|
|
|
|
.. c:function:: void Py_VISIT(PyObject *o)
|
|
|
|
If *o* is not ``NULL``, call the *visit* callback, with arguments *o*
|
|
and *arg*. If *visit* returns a non-zero value, then return it.
|
|
Using this macro, :c:member:`~PyTypeObject.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;
|
|
}
|
|
|
|
The :c:member:`~PyTypeObject.tp_clear` handler must be of the :c:type:`inquiry` type, or ``NULL``
|
|
if the object is immutable.
|
|
|
|
|
|
.. c:type:: 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 :c:func:`Py_DECREF` on a reference). The
|
|
collector will call this method if it detects that this object is involved
|
|
in a reference cycle.
|