[Python-Dev] GCC version compatibility

["Martin v. Löwis"]

David Abrahams wrote:
>>When I looked into this problem I saw that configure in fact builds a test
>>executable that included an object file compiled with g++. If the link step
>>with gcc succeeds then LINKCC is set as above, otherwise CXX is
>>used. Obviously, on my system this test was successful so configure decided
>>to link with gcc. However, minimal changes to the source of the test program
>>caused the link step to fail. It was not obvious to me at all why the latter
>>source code should cause a dependency on the C++ runtime if the original
>>code does not. My conclusion was that this test is fragile and should be
>>skipped. 
> 
> 
> Sounds like it.  I have never understood what the test was really
> checking for since the moment it was first described to me, FWIW.

I'll describe it once more: *If* a program is compiled with the C++
compiler, is it *then* possible to still link it with the C compiler?
This is the question this test tries to answer.

>>If Python is built with --with-cxx then it should be linked with CXX
>>as well.
> 
> 
> U betcha.

Wrong. The test was introduced in response to complaints that Python
unnecessarily links with libstdc++ on some Linux systems. On these
Linux systems, it was well possible to build main() with a C++ compiler,
and still link the entire thing with gcc. Since main() doesn't use
any libstdc++ functionality, and since collect2/__main isn't used,
one would indeed expect that linking with CXX is not necessary.

>>(On ELF based Linux/x86, at least.) That leaves me wondering
>>
>> * when is --with-cxx really necessary?
> 
> 
> I think it's plausible that if you set sys.dlopenflags

This has no relationship at all. --with-cxx is much older than
sys.dlopenflags. It is used on systems where main() must be a
C++ program for C++ extension modules to work (e.g. some Linux
systems).

Regards,
Martin