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(python2.1-api.info)Supporting Cyclic Garbarge Collection
Supporting Cyclic Garbarge 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 `tp_flags' field of the type object
must include the `Py_TPFLAGS_GC' and provide an implementation of the
`tp_traverse' handler. The computed value of the `tp_basicsize' field
must include `PyGC_HEAD_SIZE' as well. If instances of the type are
mutable, a `tp_clear' implementation must also be provided.
`Py_TPFLAGS_GC'
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.
`PyGC_HEAD_SIZE'
Extra memory needed for the garbage collector. Container objects
must include this in the calculation of their tp_basicsize. If the
collector is disabled at compile time then this is `0'.
Constructors for container types must conform to two rules:
1. The memory for the object must be allocated using `PyObject_New()'
or `PyObject_VarNew()'.
2. Once all the fields which may contain references to other
containers are initialized, it must call `PyObject_GC_Init()'.
`void PyObject_GC_Init(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 `tp_traverse' handler become valid,
usually near the end of the constructor.
Similarly, the deallocator for the object must conform to a similar
pair of rules:
1. Before fields which refer to other containers are invalidated,
`PyObject_GC_Fini()' must be called.
2. The object's memory must be deallocated using `PyObject_Del()'.
`void PyObject_GC_Fini(PyObject *op)'
Remove the object OP from the set of container objects tracked by
the collector. Note that `PyObject_GC_Init()' can be called again
on this object to add it back to the set of tracked objects. The
deallocator (`tp_dealloc' handler) should call this for the object
before any of the fields used by the `tp_traverse' handler become
invalid.
*Note:* Any container which may be referenced from another object
reachable by the collector must itself be tracked by the
collector, so it is generally not safe to call this function
anywhere but in the object's deallocator.
The `tp_traverse' handler accepts a function parameter of this type:
`int (*visitproc)(PyObject *object, void *arg)'
Type of the visitor function passed to the `tp_traverse' handler.
The function should be called with an object to traverse as OBJECT
and the third parameter to the `tp_traverse' handler as ARG.
The `tp_traverse' handler must have the following 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. If VISIT returns a non-zero
value then an error has occurred and that value should be returned
immediately.
The `tp_clear' handler must be of the `inquiry' type, or `NULL' if the
object is immutable.
`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 `Py_DECREF()'
on a reference). The collector will call this method if it
detects that this object is involved in a reference cycle.
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