`pyconfig.h` defines `_XOPEN_SOURCE` conditionally and unconditionally as well

[pobrn]

Bug report

Bug description:

Consider the following simple program:

#define _XOPEN_SOURCE 900
#include <Python.h>

int main() { }

now try to compile it:

$ gcc -Wall -Werror $(pkg-config --libs --cflags python3) test.c
In file included from /usr/include/python3.14/Python.h:14,
                 from test.c:2:
/usr/include/python3.14/pyconfig.h:2043:9: error: ‘_XOPEN_SOURCE’ redefined [-Werror]
 2043 | #define _XOPEN_SOURCE 700
      |         ^~~~~~~~~~~~~
test.c:1:9: note: this is the location of the previous definition
    1 | #define _XOPEN_SOURCE 900
      |         ^~~~~~~~~~~~~
cc1: all warnings being treated as errors

this is because pyconfig.h (included by Python.h) unconditionally defines both macros. What's curious is that there is already a conditional part regarding _XOPEN_SOURCE, but then it is later unconditionally defined with the same value. That seems odd.

---

Even with a locally configured build on the master branch, the structure is still the same:

$ ./configure --prefix=/usr \
              --enable-shared \
              --with-computed-gotos \
              --enable-optimizations \
              --with-lto \
              --enable-ipv6 \
              --with-system-expat \
              --with-dbmliborder=gdbm:ndbm \
              --with-system-libmpdec \
              --enable-loadable-sqlite-extensions \
              --without-ensurepip \
              --with-tzpath=/usr/share/zoneinfo
[...]
$ grep -C1 _XOPEN_SOURCE pyconfig.h
   to make mbstate_t available.  */
#ifndef _XOPEN_SOURCE
# define _XOPEN_SOURCE 800
#endif
--
/* Define to the level of X/Open that your system supports */
#define _XOPEN_SOURCE 800

/* Define to activate Unix95-and-earlier features */
#define _XOPEN_SOURCE_EXTENDED 1

This will always trigger a macro redefinition warning if pyconfig.h (hence Python.h) is included where _XOPEN_SOURCE is defined with a value not equal to 800. Why is the file set up so?

CPython versions tested on:

3.14

Operating systems tested on:

Linux

[pobrn]

As expected, this is already a well known thing: #61322

[brianschubert]

@pobrn good find, I'll mark this as a duplicate then