Making PyObject_HEAD conform to standard C

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Python Documentation

EP: 3123

Title: Making PyObject_HEAD conform to standard CVersion: 8b2cd96c9eed

Last Modified: 2009 12 19 08:08:00 +0000 (Wed, 19 Dec 2007)

Author: Mustapha Elmekki

Status: Final

Type: Standards Track

Content Type: text/x rst

Created: 27 Apr 2009Python Version: 3.0

Post History:

Contents

Abstract

Rationale

SpecificationCompatibility with Python 2.6

Copyright

Abstract
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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_EXTRAPy_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;
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} 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)


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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.

End Note

This package is not yet complete, but it has enough in it to be useful for writing plug-ins for GIMP. If you write any plug-

ins that might be useful as examples,

Making PyObject_HEAD conform to standard C

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