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285 lines
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ReStructuredText
285 lines
10 KiB
ReStructuredText
.. highlight:: c
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.. _supporting-cycle-detection:
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Supporting Cyclic Garbage Collection
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====================================
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Python's support for detecting and collecting garbage which involves circular
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references requires support from object types which are "containers" for other
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objects which may also be containers. Types which do not store references to
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other objects, or which only store references to atomic types (such as numbers
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or strings), do not need to provide any explicit support for garbage
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collection.
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To create a container type, the :c:member:`~PyTypeObject.tp_flags` field of the type object must
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include the :c:macro:`Py_TPFLAGS_HAVE_GC` and provide an implementation of the
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:c:member:`~PyTypeObject.tp_traverse` handler. If instances of the type are mutable, a
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:c:member:`~PyTypeObject.tp_clear` implementation must also be provided.
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:c:macro:`Py_TPFLAGS_HAVE_GC`
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Objects with a type with this flag set must conform with the rules
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documented here. For convenience these objects will be referred to as
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container objects.
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Constructors for container types must conform to two rules:
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#. The memory for the object must be allocated using :c:macro:`PyObject_GC_New`
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or :c:macro:`PyObject_GC_NewVar`.
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#. Once all the fields which may contain references to other containers are
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initialized, it must call :c:func:`PyObject_GC_Track`.
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Similarly, the deallocator for the object must conform to a similar pair of
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rules:
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#. Before fields which refer to other containers are invalidated,
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:c:func:`PyObject_GC_UnTrack` must be called.
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#. The object's memory must be deallocated using :c:func:`PyObject_GC_Del`.
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.. warning::
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If a type adds the Py_TPFLAGS_HAVE_GC, then it *must* implement at least
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a :c:member:`~PyTypeObject.tp_traverse` handler or explicitly use one
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from its subclass or subclasses.
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When calling :c:func:`PyType_Ready` or some of the APIs that indirectly
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call it like :c:func:`PyType_FromSpecWithBases` or
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:c:func:`PyType_FromSpec` the interpreter will automatically populate the
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:c:member:`~PyTypeObject.tp_flags`, :c:member:`~PyTypeObject.tp_traverse`
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and :c:member:`~PyTypeObject.tp_clear` fields if the type inherits from a
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class that implements the garbage collector protocol and the child class
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does *not* include the :c:macro:`Py_TPFLAGS_HAVE_GC` flag.
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.. c:macro:: PyObject_GC_New(TYPE, typeobj)
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Analogous to :c:macro:`PyObject_New` but for container objects with the
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:c:macro:`Py_TPFLAGS_HAVE_GC` flag set.
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.. c:macro:: PyObject_GC_NewVar(TYPE, typeobj, size)
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Analogous to :c:macro:`PyObject_NewVar` but for container objects with the
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:c:macro:`Py_TPFLAGS_HAVE_GC` flag set.
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.. c:function:: PyObject* PyUnstable_Object_GC_NewWithExtraData(PyTypeObject *type, size_t extra_size)
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Analogous to :c:macro:`PyObject_GC_New` but allocates *extra_size*
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bytes at the end of the object (at offset
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:c:member:`~PyTypeObject.tp_basicsize`).
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The allocated memory is initialized to zeros,
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except for the :c:type:`Python object header <PyObject>`.
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The extra data will be deallocated with the object, but otherwise it is
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not managed by Python.
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.. warning::
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The function is marked as unstable because the final mechanism
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for reserving extra data after an instance is not yet decided.
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For allocating a variable number of fields, prefer using
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:c:type:`PyVarObject` and :c:member:`~PyTypeObject.tp_itemsize`
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instead.
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.. versionadded:: 3.12
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.. c:macro:: PyObject_GC_Resize(TYPE, op, newsize)
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Resize an object allocated by :c:macro:`PyObject_NewVar`.
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Returns the resized object of type ``TYPE*`` (refers to any C type)
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or ``NULL`` on failure.
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*op* must be of type :c:expr:`PyVarObject *`
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and must not be tracked by the collector yet.
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*newsize* must be of type :c:type:`Py_ssize_t`.
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.. c:function:: void PyObject_GC_Track(PyObject *op)
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Adds the object *op* to the set of container objects tracked by the
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collector. The collector can run at unexpected times so objects must be
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valid while being tracked. This should be called once all the fields
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followed by the :c:member:`~PyTypeObject.tp_traverse` handler become valid, usually near the
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end of the constructor.
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.. c:function:: int PyObject_IS_GC(PyObject *obj)
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Returns non-zero if the object implements the garbage collector protocol,
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otherwise returns 0.
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The object cannot be tracked by the garbage collector if this function returns 0.
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.. c:function:: int PyObject_GC_IsTracked(PyObject *op)
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Returns 1 if the object type of *op* implements the GC protocol and *op* is being
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currently tracked by the garbage collector and 0 otherwise.
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This is analogous to the Python function :func:`gc.is_tracked`.
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.. versionadded:: 3.9
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.. c:function:: int PyObject_GC_IsFinalized(PyObject *op)
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Returns 1 if the object type of *op* implements the GC protocol and *op* has been
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already finalized by the garbage collector and 0 otherwise.
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This is analogous to the Python function :func:`gc.is_finalized`.
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.. versionadded:: 3.9
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.. c:function:: void PyObject_GC_Del(void *op)
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Releases memory allocated to an object using :c:macro:`PyObject_GC_New` or
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:c:macro:`PyObject_GC_NewVar`.
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.. c:function:: void PyObject_GC_UnTrack(void *op)
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Remove the object *op* from the set of container objects tracked by the
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collector. Note that :c:func:`PyObject_GC_Track` can be called again on
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this object to add it back to the set of tracked objects. The deallocator
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(:c:member:`~PyTypeObject.tp_dealloc` handler) should call this for the object before any of
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the fields used by the :c:member:`~PyTypeObject.tp_traverse` handler become invalid.
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.. versionchanged:: 3.8
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The :c:func:`!_PyObject_GC_TRACK` and :c:func:`!_PyObject_GC_UNTRACK` macros
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have been removed from the public C API.
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The :c:member:`~PyTypeObject.tp_traverse` handler accepts a function parameter of this type:
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.. c:type:: int (*visitproc)(PyObject *object, void *arg)
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Type of the visitor function passed to the :c:member:`~PyTypeObject.tp_traverse` handler.
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The function should be called with an object to traverse as *object* and
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the third parameter to the :c:member:`~PyTypeObject.tp_traverse` handler as *arg*. The
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Python core uses several visitor functions to implement cyclic garbage
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detection; it's not expected that users will need to write their own
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visitor functions.
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The :c:member:`~PyTypeObject.tp_traverse` handler must have the following type:
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.. c:type:: int (*traverseproc)(PyObject *self, visitproc visit, void *arg)
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Traversal function for a container object. Implementations must call the
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*visit* function for each object directly contained by *self*, with the
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parameters to *visit* being the contained object and the *arg* value passed
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to the handler. The *visit* function must not be called with a ``NULL``
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object argument. If *visit* returns a non-zero value that value should be
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returned immediately.
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To simplify writing :c:member:`~PyTypeObject.tp_traverse` handlers, a :c:func:`Py_VISIT` macro is
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provided. In order to use this macro, the :c:member:`~PyTypeObject.tp_traverse` implementation
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must name its arguments exactly *visit* and *arg*:
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.. c:function:: void Py_VISIT(PyObject *o)
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If *o* is not ``NULL``, call the *visit* callback, with arguments *o*
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and *arg*. If *visit* returns a non-zero value, then return it.
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Using this macro, :c:member:`~PyTypeObject.tp_traverse` handlers
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look like::
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static int
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my_traverse(Noddy *self, visitproc visit, void *arg)
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{
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Py_VISIT(self->foo);
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Py_VISIT(self->bar);
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return 0;
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}
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The :c:member:`~PyTypeObject.tp_clear` handler must be of the :c:type:`inquiry` type, or ``NULL``
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if the object is immutable.
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.. c:type:: int (*inquiry)(PyObject *self)
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Drop references that may have created reference cycles. Immutable objects
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do not have to define this method since they can never directly create
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reference cycles. Note that the object must still be valid after calling
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this method (don't just call :c:func:`Py_DECREF` on a reference). The
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collector will call this method if it detects that this object is involved
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in a reference cycle.
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Controlling the Garbage Collector State
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---------------------------------------
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The C-API provides the following functions for controlling
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garbage collection runs.
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.. c:function:: Py_ssize_t PyGC_Collect(void)
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Perform a full garbage collection, if the garbage collector is enabled.
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(Note that :func:`gc.collect` runs it unconditionally.)
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Returns the number of collected + unreachable objects which cannot
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be collected.
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If the garbage collector is disabled or already collecting,
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returns ``0`` immediately.
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Errors during garbage collection are passed to :data:`sys.unraisablehook`.
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This function does not raise exceptions.
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.. c:function:: int PyGC_Enable(void)
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Enable the garbage collector: similar to :func:`gc.enable`.
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Returns the previous state, 0 for disabled and 1 for enabled.
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.. versionadded:: 3.10
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.. c:function:: int PyGC_Disable(void)
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Disable the garbage collector: similar to :func:`gc.disable`.
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Returns the previous state, 0 for disabled and 1 for enabled.
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.. versionadded:: 3.10
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.. c:function:: int PyGC_IsEnabled(void)
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Query the state of the garbage collector: similar to :func:`gc.isenabled`.
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Returns the current state, 0 for disabled and 1 for enabled.
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.. versionadded:: 3.10
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Querying Garbage Collector State
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--------------------------------
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The C-API provides the following interface for querying information about
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the garbage collector.
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.. c:function:: void PyUnstable_GC_VisitObjects(gcvisitobjects_t callback, void *arg)
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Run supplied *callback* on all live GC-capable objects. *arg* is passed through to
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all invocations of *callback*.
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.. warning::
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If new objects are (de)allocated by the callback it is undefined if they
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will be visited.
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Garbage collection is disabled during operation. Explicitly running a collection
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in the callback may lead to undefined behaviour e.g. visiting the same objects
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multiple times or not at all.
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.. versionadded:: 3.12
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.. c:type:: int (*gcvisitobjects_t)(PyObject *object, void *arg)
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Type of the visitor function to be passed to :c:func:`PyUnstable_GC_VisitObjects`.
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*arg* is the same as the *arg* passed to ``PyUnstable_GC_VisitObjects``.
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Return ``0`` to continue iteration, return ``1`` to stop iteration. Other return
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values are reserved for now so behavior on returning anything else is undefined.
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.. versionadded:: 3.12
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