[Python-3000] Draft PEP: Dropping PyObject_HEAD (original) (raw)

"Martin v. Löwis" martin at v.loewis.de
Sat Apr 28 00:44:25 CEST 2007

I propose the PEP below for Py3k.

Regards, Martin

PEP: 3122 Title: Dropping PyObject_HEAD Version: Revision:54998Revision: 54998 Revision:54998 Last-Modified: Date:2007−04−2710:31:58+0200(Fr,27Apr2007)Date: 2007-04-27 10:31:58 +0200 (Fr, 27 Apr 2007) Date:2007−04−2710:31:58+0200(Fr,27Apr2007) Author: Martin v. Löwis <martin at v.loewis.de> Status: Draft 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 cause any problems with this code. However, modern compiler 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 dropped, and PyObject gets defined to contain all fields explicitly::

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;

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;

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, a macro::

#define Py_Type(o) (((PyObject*)o)->ob_type)

is added.

Copyright

This document has been placed in the public domain.

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