Python 3.12.2 headers contain non-C90-compliant code

[jayaddison]

Bug report

Bug description:

When compiling a program that links against the Python libraries (and uses their headerfiles) with the -Werror=declaration-after-statement compiler flag enabled, the following error occurs:

/usr/include/python3.12/object.h:233:5: error: ISO C90 forbids mixed declarations and code [-Werror=declaration-after-statement]
  233 |     PyVarObject *var_ob = _PyVarObject_CAST(ob);

This is fairly similar to previous issue #92781.

Although the C90 style may be archaic and perhaps not worth supporting at some point, maintaining support could be helpful for some downstream platforms (I encountered this in Debian bug #1057442).

Minimal test cases to show that the problem did not occur with Python 3.11.8 and that it does occur on Python 3.12.2 and to help verify any fixes.

#include "/usr/include/python3.11/Python.h"
#include "/usr/include/python3.11/object.h"

int main() {}

#include "/usr/include/python3.12/Python.h"
#include "/usr/include/python3.12/object.h"

int main() {}

Compiled in either case using:

$ gcc bugreport.c -Werror=declaration-after-statement

or equivalently, using llvm v17:

$ clang-17 bugreport.c -Werror=declaration-after-statement

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Linked PRs

• gh-116869: Make C API compatible with ISO C90 #116950
• gh-116869: Add test_cext test: build a C extension #116954
• gh-116869: Fix redefinition of the _PyOptimizerObject type #116963
• gh-116869: Enable test_cext and test_cppext on Free Threading build #116973
• gh-116869: Build test_cext with -Werror=declaration-after-statement #116990
• gh-116869: Test the limited C API in test_cext #116995
• gh-116869: Fix test_cext on RHEL7 #117010
• [3.12] gh-116869: Make C API compatible with ISO C90 (GH-116950) #117011
• gh-116869: Fix test_cext for Free Threading #117043
• gh-116869: Enable -Werror in test_cext for Free Threading #117106

[hugovk]

The build requirements for 3.12 include:

• A C11 compiler. Optional C11 features are not required.

This is since 3.11:

Changed in version 3.11: C11 compiler, IEEE 754 and NaN support are now required. On Windows, Visual Studio 2017 or later is required.

https://docs.python.org/3/using/configure.html

[vstinner]

I wrote PR gh-116950 to fix the issue.

While C11 is needed to build Python itself, the C API should be usable with ISO C90.

[colesbury]

While C11 is needed to build Python itself, the C API should be usable with ISO C90.

Where do we document that? PEP 7 says "The public C API should be compatible with C++.", not ISO C90. ISO C90 doesn't even support the inline keyword.

[vstinner]

Where do we document that?

I don't think that it's documented. It's just convenient since a few projects still use -Werror=declaration-after-statement. IMO these projects added this option when Visual Studio didn't support C99, it does now, but these projects were never updated.

In 2024, I don't see the point to support ISO C90. But the way, Python doesn't: it uses C99 "// comment" which fails with a build error with -std=c90.

But if the limited C API only exposes opaque functions, in fact, it's cheap to support -Werror=declaration-after-statement.

If tomorrow, we have a better reason to break that unwritten rule, we can revisit it.