[Python-Dev] Linux Python linking with G++?

[David Abrahams]

"Martin v. Löwis" <martin at v.loewis.de> writes:

> David Abrahams wrote:
>>>Not entirely. By extending Modules/Setup 
>> 
>> 
>> You mean 
>> http://cvs.sourceforge.net/viewcvs.py/python/python/dist/src/Modules/Setup.dist?view=markup
>> ?
>
> I mean Modules/Setup. It is generated from Modules/Setup.dist
> (plus some additional information) in the build process.
>
>> I contend that either:
>> 
>>    a.  Anyone making that sort of extension with a C++ module should
>>    explicitly request --with-cxx, or
>> 
>>    b.  The python build system should automatically detect that
>>    --with-cxx is needed based on the presence of C++ extension
>>    modules.
>> 
>> Frankly I think b. would require an impractical amount of work and,
>> speaking as an author of C++ extension modules, I don't think a. is
>> much of a burden to impose.
>
> It is the burden of change. 

Well, as you say:

  What *has* changed now is that the configure test suddenly determines
  that you need to link with g++ on Linux if main was compiled with g++.
  This was not the case before, but now is (since g++ 3.2 or something).

> Contributions are welcome.

Let me first try to discover what contribution would be acceptable.
What if:

  - we add a configure test that runs after the existing test
    determines that --with-cxx is needed (but not when --with-cxx is
    explicitly specified on the command line)

  - This test runs a 'C' executable that tries to load a C++ dynamic
    library with dlopen.  

  - The test returns an error code if the the dynamic library's static
    and dynamic initializers have not been run properly

  - If the test fails we disable --with-cxx

??

I'm trying to intrude on the existing behavior as little as possible,
yet get the semantics we want for ELF/Linux in a way that stands a
good chance of generalizing to other platforms.

> However, you will find that with a), people will still pass --with-cxx,
> because they tend to "enable" all features they can find.

That's acceptable to me.  We could probably circumvent some of those
cases by improving the configure --help text.

> I personally could accept --with-cxx and ccpython.cc to be removed
> again, but I'm uncertain whether that may break distutils in some
> way.

Well, that would certainly be an easy "solution," but it would break
HP/UX, and it would break anyone that needs to statically link C++
modules on some platforms.  It's a much more drastic change than it
would be to only have the system use --with-cxx when the person
running configure asks for it explicitly.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com