160 lines
4.4 KiB
Plaintext
160 lines
4.4 KiB
Plaintext
PEP: 3123
|
||
Title: Making PyObject_HEAD conform to standard C
|
||
Version: $Revision$
|
||
Last-Modified: $Date$
|
||
Author: Martin v. Löwis <martin@v.loewis.de>
|
||
Status: Accepted
|
||
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 FooObject*f){
|
||
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; variable-sized objects PyVarObject. 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 is 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 is 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.
|
||
|
||
|
||
|
||
..
|
||
Local Variables:
|
||
mode: indented-text
|
||
indent-tabs-mode: nil
|
||
sentence-end-double-space: t
|
||
fill-column: 70
|
||
coding: utf-8
|
||
End:
|