Docs are incorrect re: constant initialization in the C99 standard

[jhaberman]

BPO	45306

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-09-28.00:24:44.038>
labels = ['docs']
title = 'Docs are incorrect re: constant initialization in the C99 standard'
updated_at = <Date 2021-09-28.00:24:44.038>
user = 'https://bugs.python.org/jhaberman'

bugs.python.org fields:

activity = <Date 2021-09-28.00:24:44.038>
actor = 'jhaberman'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2021-09-28.00:24:44.038>
creator = 'jhaberman'
dependencies = []
files = []
hgrepos = []
issue_num = 45306
keywords = []
message_count = 1.0
messages = ['402752']
nosy_count = 2.0
nosy_names = ['docs@python', 'jhaberman']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue45306'
versions = []

[jhaberman]

I believe the following excerpt from the docs is incorrect (https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject.tp_base):

Slot initialization is subject to the rules of initializing
globals. C99 requires the initializers to be “address
constants”. Function designators like PyType_GenericNew(),
with implicit conversion to a pointer, are valid C99 address
constants.

However, the unary ‘&’ operator applied to a non-static
variable like PyBaseObject_Type() is not required to produce
an address constant. Compilers may support this (gcc does),
MSVC does not. Both compilers are strictly standard
conforming in this particular behavior.

Consequently, tp_base should be set in the extension module’s init function.

I explained why in https://mail.python.org/archives/list/python-dev@python.org/thread/2WUFTVQA7SLEDEDYSRJ75XFIR3EUTKKO/ and on https://bugs.python.org/msg402738.

The short version: &foo is an "address constant" according to the standard whenever "foo" has static storage duration. Variables declared "extern" have static storage duration. Therefore strictly conforming implementations should accept &PyBaseObject_Type as a valid constant initializer.

I believe the text above could be replaced by something like:

MSVC does not support constant initialization of of an address
that comes from another DLL, so extensions should be set in the
extension module's init function.