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PEP 3123 -- Making PyObject_HEAD conform to standard C

Title:Making PyObject_HEAD conform to standard C
Author:Martin von Löwis <martin at>
Type:Standards Track


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


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{
  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.


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 {
  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

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.