C++ linking in 3.14.6? Is there a regression?

Core Development
1

setuptools.Extension used to be able on Linux with GCC automatically distinguish whether the code is a pure C or C++ and link accordingly. It seems to me that with either Python 3.14.6 something broke and I had to add patches like fix build failure with Python 3.14 by linking libstdc++ by mcepl · Pull Request #85 · SethMMorton/fastnumbers · GitHub to have C++ linking working. Is it a regression or we did screw up something in openSUSE?

2

The setup.py that you linked to uses setuptools. Why do you think that the issue is in Python and not in setuptools?

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3

Poking around in setuptools a bit it seems that their vendored distutils has a comment about this in extension.py:

def read_setup_file ...

I’m not very familiar with distutils or setuptools, but it does seem that they may be deferring this responsibility to the compiler? Either way, this is definitely a good place to start the search.

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After longer session with ChatGPT we came to this minimal situation:

$ python3.14
Python 3.14.4 (main, Apr  8 2026, 08:54:12) [GCC] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sysconfig
... 
... print(sysconfig.get_config_var("CXX"))
... print(sysconfig.get_config_var("LDCXXSHARED"))
... 
g++
g++ -shared
>>> 
$ python3.14
Python 3.14.6 (main, Jun 19 2026, 18:12:49) [GCC] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sysconfig
... 
... print(sysconfig.get_config_var("CXX"))
... print(sysconfig.get_config_var("LDCXXSHARED"))
... 
gcc
gcc -shared
>>>

I am very much willing to admit a possibility that I screwed up something in our build scripts (both of these Pythons are from our openSUSE/Tumblweed interpreters), but I really cannot see what: build parameters is something I really don’t touch unless I have to.

Any thoughts?

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5

I believe the specific change you’re running into is [3.14] gh-148294: Make configure find g++ correctly (GH-148298) by miss-islington · Pull Request #150211 · python/cpython · GitHub

The old behavior caused g++ to always be detected at build time even if it’s not present. Now, I’m guessing, your CPython build is correctly picking up that there isn’t a g++ install, and it’s falling back to gcc, breaking your case.

Can you check if the OpenSUSE tumbleweed CPython package declares a build dependency on g++? It probably should if it doesn’t. If not, then my guess is wrong!

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6

OK, it doesn’t declare any compileir dependency:

$ rpm -qR python313-devel
/bin/sh
/usr/bin/pkg-config
/usr/bin/python3
/usr/bin/python3.13
libc.so.6()(64bit)
libc.so.6(GLIBC_2.2.5)(64bit)
libpython3.13.so.1.0()(64bit)
python(abi) = 3.13
python313-base = 3.13.13
rpmlib(CompressedFileNames) <= 3.0.4-1
rpmlib(FileDigests) <= 4.6.0-1
rpmlib(PayloadFilesHavePrefix) <= 4.0-1
rpmlib(PayloadIsZstd) <= 5.4.18-1
$

I didn’t think it should be a mandatory dependency. Interesting.

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7

That’s a really fun one, a bugfix that accidently resolves some issue that changed a return value (not found vs a literal path does not exist).

I wonder if at some point something was changed somewhere to handle that path access failure that now isn’t being triggered because it’s properly handling the g++ not found error.

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Note that there’s actually a notice about this that some of us (:waving_hand:) have been ignoring without much understanding for many years:

  By default, distutils will build C++ extension modules with "$CXX".
  If this is not intended, then set CXX on the configure command line.

It does seem like AC_CHECK_TOOLS([CXX], [$CCC c++ g++ gcc CC cxx cc++ cl], [notfound]) might be a less-than-ideal check after the compiler-specific checks; we should perhaps move that up into the default case of the compiler-specific switch.

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9

That is a dependency that packaging would add as it’s not a cpython dep it’s a dep of the OP’s build process.

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10

I confirm that the configure scripts switchs to gcc if g++ is missing. I suppose that gcc is picked by this command in configure.ac:

AC_CHECK_TOOLS([CXX], [$CCC c++ g++ gcc CC cxx cc++ cl], [notfound])

If you want to keep CXX=g++ even if g++ is not installed, you can use ./configure CXX=g++ (...).

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