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

[David Abrahams]

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

> David Abrahams wrote:
>> Apparently Python on some linux distros is being linked by g++ rather
>> than gcc, resulting in the C++ runtime library being linked into
>> Python; this has bad consequences for compatibility between C++
>> extension modules and Pythons that have been built with different
>> versions of GCC.  Is this behavior intentional?
>
> It's as Skip says. According to the C++ standard, a "C++ program" is
> one where all translation units are written in C++. While most platforms
> offer interoperability between C and C++ in the sense that C libraries
> can be linked into C++ programs, interoperability in the other direction
> is not always supported, i.e. C programs may not be able to use C++
> libraries. This is the specific case you are talking about: Python is
> written in C, yet the extension might be written in C++.

The C++ standard doesn't cover interoperability with 'C', or dynamic
linking (we on the C++ committee are working to fix that, but for now
that is the case) and it doesn't cover dynamic loading without
linking, which is what happens when Python loads an extension written
in C++.  You can't appeal to the standard for the rules about what
needs to be done.  All this stuff is entirely
implementation-dependent.

> Now, on some of these implementations, things can be fixed by writing
> main() as a C++ translation unit, and compiling it with the C++
> compiler. Then, Python itself is a C library to this C++ program, and
> the extension modules are C++ libraries. Then everything works fine.

C++ extension modules work fine even when main() is a 'C' program on
Linux/GCC.  The only reason that main would have to be a C++ program
on this platform and compiler is if there were C++ translation units
_linked_ into it (not loaded as in with dlopen).  Since whoever writes
the Makefile for Python also controls what's being linked into it,
there's no reason to speculate and make Python a C++ program based on
what might be linked in.

> To support this, main() must be a C++ program. Python compiles main
> using the C++ compiler if configure thinks this is necessary.
>
> Doing so is necessary for example on systems using the G++ collect2
> mechanism for static initializers: compiling main with g++ leads
> to a call to __main(), which is then created by collect2.

Extension modules that get loaded with dlopen don't get their static
initializers run by __main() anyway.

> configure thinks that using CXX for linking is necessary if compiling
> a program using CXX and linking it using CC fails.

That might be the right thing to do for some programs, but AFAICT
that's the wrong logic to use for Python.

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