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	PEP: 3123
	Title: Making PyObject_HEAD conform to standard C
	Version: $Revision$
	Last-Modified: $Date$
	Author: Martin von Löwis <martin@v.loewis.de>
	Status: Final
	Type: Standards Track
	Content-Type: text/x-rst
	Created: 27-Apr-2007
	Python-Version: 3.0
	Post-History:

	Abstract
	========

	Python currently relies on undefined C behavior, with its
	usage of ``PyObject_HEAD``. This PEP proposes to change that
	into standard C.

	Rationale
	=========

	Standard C defines that an object must be accessed only through a
	pointer of its type, and that all other accesses are undefined
	behavior, with a few exceptions. In particular, the following
	code has undefined behavior::

	struct FooObject{
	PyObject_HEAD
	int data;
	};

	PyObject foo(struct FooObjectf){
	return (PyObject*)f;
	}

	int bar(){
	struct FooObject *f = malloc(sizeof(struct FooObject));
	struct PyObject *o = foo(f);
	f->ob_refcnt = 0;
	o->ob_refcnt = 1;
	return f->ob_refcnt;
	}

	The problem here is that the storage is both accessed as
	if it where struct ``PyObject``, and as struct ``FooObject``.

	Historically, compilers did not have any problems with this
	code. However, modern compilers use that clause as an
	optimization opportunity, finding that ``f->ob_refcnt`` and
	``o->ob_refcnt`` cannot possibly refer to the same memory, and
	that therefore the function should return 0, without having
	to fetch the value of ob_refcnt at all in the return
	statement. For GCC, Python now uses ``-fno-strict-aliasing``
	to work around that problem; with other compilers, it
	may just see undefined behavior. Even with GCC, using
	``-fno-strict-aliasing`` may pessimize the generated code
	unnecessarily.

	Specification
	=============

	Standard C has one specific exception to its aliasing rules precisely
	designed to support the case of Python: a value of a struct type may
	also be accessed through a pointer to the first field. E.g. if a
	struct starts with an ``int``, the ``struct *`` may also be cast to
	an ``int *``, allowing to write int values into the first field.

	For Python, ``PyObject_HEAD`` and ``PyObject_VAR_HEAD`` will be changed
	to not list all fields anymore, but list a single field of type
	``PyObject``/``PyVarObject``::

	typedef struct _object {
	_PyObject_HEAD_EXTRA
	Py_ssize_t ob_refcnt;
	struct _typeobject *ob_type;
	} PyObject;

	typedef struct {
	PyObject ob_base;
	Py_ssize_t ob_size;
	} PyVarObject;

	#define PyObject_HEAD PyObject ob_base;
	#define PyObject_VAR_HEAD PyVarObject ob_base;

	Types defined as fixed-size structure will then include PyObject
	as its first field, PyVarObject for variable-sized objects. E.g.::

	typedef struct {
	PyObject ob_base;
	PyObject start, stop, *step;
	} PySliceObject;

	typedef struct {
	PyVarObject ob_base;
	PyObject **ob_item;
	Py_ssize_t allocated;
	} PyListObject;

	The above definitions of ``PyObject_HEAD`` are normative, so extension
	authors MAY either use the macro, or put the ``ob_base`` field explicitly
	into their structs.

	As a convention, the base field SHOULD be called ob_base. However, all
	accesses to ob_refcnt and ob_type MUST cast the object pointer to
	PyObject* (unless the pointer is already known to have that type), and
	SHOULD use the respective accessor macros. To simplify access to
	ob_type, ob_refcnt, and ob_size, macros::

	#define Py_TYPE(o) (((PyObject*)(o))->ob_type)
	#define Py_REFCNT(o) (((PyObject*)(o))->ob_refcnt)
	#define Py_SIZE(o) (((PyVarObject*)(o))->ob_size)

	are added. E.g. the code blocks ::

	#define PyList_CheckExact(op) ((op)->ob_type == &PyList_Type)

	return func->ob_type->tp_name;

	needs to be changed to::

	#define PyList_CheckExact(op) (Py_TYPE(op) == &PyList_Type)

	return Py_TYPE(func)->tp_name;

	For initialization of type objects, the current sequence ::

	PyObject_HEAD_INIT(NULL)
	0, /* ob_size */

	becomes incorrect, and must be replaced with ::

	PyVarObject_HEAD_INIT(NULL, 0)

	Compatibility with Python 2.6
	=============================

	To support modules that compile with both Python 2.6 and Python 3.0,
	the ``Py_*`` macros are added to Python 2.6. The macros ``Py_INCREF``
	and ``Py_DECREF`` will be changed to cast their argument to ``PyObject *``,
	so that module authors can also explicitly declare the ``ob_base``
	field in modules designed for Python 2.6.

	Copyright
	=========

	This document has been placed in the public domain.



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