Includes/internal/pycore_code.h uses static_assert() but does not inlcude pymacro.h

[dnicolodi]

Bug report

Bug description:

Includes/internal/pycore_code.h uses static_assert() but does not inlcude pymacro.h, should it?

AFAIU pymacro.h makes sure that static_assert() is correctly defined for all supported compilers and platforms. Not including it in Includes/internal/pycore_code.h implicitly relies on pymacro.h being included before or via transitive includes.

I've found this while investigating a Cython extension module build failure. Cython includes the private header and thus requires static_macro() to be defined.

CPython versions tested on:

3.13, CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-123747: Avoid static_assert() in internal header files #123779
• [3.13] gh-123747: Avoid static_assert() in internal header files (#123779) #123785

[vstinner]

In general, I would suggest to only include <pycore_xxx.h> headers after including <Python.h>.

[dnicolodi]

Sure, the compilation error is unrelated (Cython requires only C99 which does not have static_assert()). This report is more about a question of principle: should CPython internal headers be independent or can they rely on #include order for the definitions they use?

[vstinner]

Do you get the error if you include <Python.h> first?

[dnicolodi]

Yes. But the error is unrelated to what I am trying to say.

[vstinner]

This report is more about a question of principle: should CPython internal headers be independent or can they rely on #include order for the definitions they use?

They are not independent. <Python.h> must always be included first.

[dnicolodi]

Thanks. There is nothing wrong then.

[scoder]

It turns out that this is an issue, even after #include "Python.h":
cython/cython#6385 (comment)
This issue was found in Cython generated code, which always includes Python.h but then also needs pycore_frame.h later, which includes pycore_code.h.

Since Cython code is not commonly compiled with C11 (we currently require C99), there are platforms where the resulting code does not compile

[scoder]

Since it is really just an assertion that gets in the way here, I propose to comment it out for the final release.

[vstinner]

I don't understand how to reproduce the issue. Can somone provide a reproducer? Does the issue only affect macOS? With which C compiler and which compiler options?

[picnixz]

FTR, here's the comment concerning static_assert (it may help people that are too lazy to find it in the codebase):

// gh-91782: On FreeBSD 12, if the _POSIX_C_SOURCE and _XOPEN_SOURCE macros are
// defined, <sys/cdefs.h> disables C11 support and <assert.h> does not define
// the static_assert() macro.
// https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=255290
//
// macOS <= 10.10 doesn't define static_assert in assert.h at all despite
// having C11 compiler support.
//
// static_assert is defined in glibc from version 2.16. Compiler support for
// the C11 _Static_assert keyword is in gcc >= 4.6.
//
// MSVC makes static_assert a keyword in C11-17, contrary to the standards.
//
// In C++11 and C2x, static_assert is a keyword, redefining is undefined
// behaviour. So only define if building as C, not C++ (if __cplusplus is
// not defined), and only for C11-17.

[dnicolodi]

I don't understand how to reproduce the issue. Can somone provide a reproducer? Does the issue only affect macOS? With which C compiler and which compiler options?

I can reproduce the issue compiling the most simple Cython extension module:

def foo():
    return 42

with Cython 3.0.11 with clang from Xtools:

$ cc --version
Apple clang version 14.0.0 (clang-1400.0.29.202)

using an explicit -std=c99 compilation option.

A more synthetic reproducer is trying to compile something like:

#include "Python.h"
#include "internal/pycore_frame.h"
int main() { }

In this way:

$ cc foo.c -o foo.o -DPy_BUILD_CORE=1 $(python3.13-config --cflags) -std=c99

Omitting the -std=c99 or setting it to -std=c11 or higher the snippet compiles.

As pointed out before, the issue is that static_assert() is a C11 concept. CPython requires C11 thus it is fine. However, Cython requires only C99 and it includes internal CPython headers which assume C11 features.

For some reason that I have not investigated, static_assert() is defined when compiling with GCC on Linux or MSVC on Windows when requesting C99 compliance.

[vstinner]

Apple clang version 14.0.0 (clang-1400.0.29.202)

I cannot reproduce the issue on Linux (Fedora 40) with clang 18.1.6.

See also discussion capi-workgroup/decisions#30

[vstinner]

I wrote PR gh-123779 to avoid static_assert() in the internal header files.

[encukou]

Could Cython re-#define static_assert(EXPR, MSG) to nothing if compiling in C99 mode?

[vstinner]

Fixed by ef4b69d.

[vstinner]

Thanks for your bug report @dnicolodi!

[scoder]

Thanks @vstinner