symbian-qemu-0.9.1-12/python-2.6.1/Doc/c-api/gcsupport.rst
author martin.trojer@nokia.com
Fri, 31 Jul 2009 15:01:17 +0100
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.. highlightlang:: 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 collection.
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.. An example showing the use of these interfaces can be found in "Supporting the
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.. Cycle Collector (XXX not found: ../ext/example-cycle-support.html)".
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To create a container type, the :attr:`tp_flags` field of the type object must
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include the :const:`Py_TPFLAGS_HAVE_GC` and provide an implementation of the
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:attr:`tp_traverse` handler.  If instances of the type are mutable, a
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:attr:`tp_clear` implementation must also be provided.
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.. data:: Py_TPFLAGS_HAVE_GC
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   :noindex:
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   Objects with a type with this flag set must conform with the rules documented
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   here.  For convenience these objects will be referred to as 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 :cfunc:`PyObject_GC_New` or
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   :cfunc:`PyObject_GC_VarNew`.
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#. Once all the fields which may contain references to other containers are
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   initialized, it must call :cfunc:`PyObject_GC_Track`.
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.. cfunction:: TYPE* PyObject_GC_New(TYPE, PyTypeObject *type)
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   Analogous to :cfunc:`PyObject_New` but for container objects with the
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   :const:`Py_TPFLAGS_HAVE_GC` flag set.
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.. cfunction:: TYPE* PyObject_GC_NewVar(TYPE, PyTypeObject *type, Py_ssize_t size)
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   Analogous to :cfunc:`PyObject_NewVar` but for container objects with the
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   :const:`Py_TPFLAGS_HAVE_GC` flag set.
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.. cfunction:: PyVarObject * PyObject_GC_Resize(PyVarObject *op, Py_ssize_t)
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   Resize an object allocated by :cfunc:`PyObject_NewVar`.  Returns the resized
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   object or *NULL* on failure.
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.. cfunction:: void PyObject_GC_Track(PyObject *op)
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   Adds the object *op* to the set of container objects tracked by the collector.
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   The collector can run at unexpected times so objects must be valid while being
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   tracked.  This should be called once all the fields followed by the
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   :attr:`tp_traverse` handler become valid, usually near the end of the
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   constructor.
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.. cfunction:: void _PyObject_GC_TRACK(PyObject *op)
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   A macro version of :cfunc:`PyObject_GC_Track`.  It should not be used for
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   extension modules.
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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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   :cfunc:`PyObject_GC_UnTrack` must be called.
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#. The object's memory must be deallocated using :cfunc:`PyObject_GC_Del`.
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.. cfunction:: void PyObject_GC_Del(void *op)
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   Releases memory allocated to an object using :cfunc:`PyObject_GC_New` or
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   :cfunc:`PyObject_GC_NewVar`.
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.. cfunction:: 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 :cfunc:`PyObject_GC_Track` can be called again on this
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   object to add it back to the set of tracked objects.  The deallocator
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   (:attr:`tp_dealloc` handler) should call this for the object before any of the
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   fields used by the :attr:`tp_traverse` handler become invalid.
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.. cfunction:: void _PyObject_GC_UNTRACK(PyObject *op)
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   A macro version of :cfunc:`PyObject_GC_UnTrack`.  It should not be used for
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   extension modules.
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The :attr:`tp_traverse` handler accepts a function parameter of this type:
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.. ctype:: int (*visitproc)(PyObject *object, void *arg)
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   Type of the visitor function passed to the :attr:`tp_traverse` handler.  The
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   function should be called with an object to traverse as *object* and the third
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   parameter to the :attr:`tp_traverse` handler as *arg*.  The Python core uses
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   several visitor functions to implement cyclic garbage detection; it's not
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   expected that users will need to write their own visitor functions.
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The :attr:`tp_traverse` handler must have the following type:
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.. ctype:: 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 to
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   the handler.  The *visit* function must not be called with a *NULL* object
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   argument.  If *visit* returns a non-zero value that value should be returned
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   immediately.
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To simplify writing :attr:`tp_traverse` handlers, a :cfunc:`Py_VISIT` macro is
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provided.  In order to use this macro, the :attr:`tp_traverse` implementation
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must name its arguments exactly *visit* and *arg*:
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.. cfunction:: void Py_VISIT(PyObject *o)
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   Call the *visit* callback, with arguments *o* and *arg*. If *visit* returns a
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   non-zero value, then return it.  Using this macro, :attr:`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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   .. versionadded:: 2.4
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The :attr:`tp_clear` handler must be of the :ctype:`inquiry` type, or *NULL* if
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the object is immutable.
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.. ctype:: int (*inquiry)(PyObject *self)
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   Drop references that may have created reference cycles.  Immutable objects do
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   not have to define this method since they can never directly create reference
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   cycles.  Note that the object must still be valid after calling this method
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   (don't just call :cfunc:`Py_DECREF` on a reference).  The collector will call
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   this method if it detects that this object is involved in a reference cycle.