[C++-sig] [Python-Dev] GCC version compatibility

[Christoph Ludwig]

On Thu, Jul 07, 2005 at 06:27:46PM -0400, David Abrahams wrote:
> "Martin v. Löwis" <martin at v.loewis.de> writes:
> 
> > David Abrahams wrote:
> >> I'm wondering if there has been a well-known recent change either in Python
> >> or GCC that would account for these new reports.  Any relevant
> >> information would be appreciated.
[...]
> > Python is linked with g++ if configure thinks this is necessary
> 
> Right.  The question is, when should configure "think it's necessary?"

Just to add to the confusion... I encountered the case that configure decided
to use gcc for linking but it should have used g++. (It is Python 
PR #1189330 <http://tinyurl.com/dlheb>. This was on a x86 Linux system with
g++ 3.4.2.)

Background: The description of --with-cxx in the README of the Python 2.4.1
source distribution made me think that I need to configure my Python
installation with --with-configure=/opt/gcc/gcc-3.4.2/bin/g++ if I plan to use
C++ extensions built with this compiler. (That was possibly a misunderstanding
on my part, but Python should build with this option anyway.) 

configure set `LINKCC=$(PURIFY) $(CC)'. The result was that make failed when
linking the python executable due to an unresolved reference to
__gxx_personality_v0. I had to replace CC by CXX in the definition of LINKCC
to finish the build of Python.

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

I gather from posts on the Boost mailing lists that you can import
Boost.Python extensions even if Python was configured --without-cxx. (On
ELF based Linux/x86, at least.) That leaves me wondering
 * when is --with-cxx really necessary?
 * what happens if I import extensions built with different g++ versions? Will
   there be a conflict between the different versions of libstdc++ those
   extensions depend on?

Regards

Christoph

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