From: John Gabriele <jmg3000@...>
Date: 2006-09-14T17:09:05+09:00
Subject: Re: building extension modules, and linking

On 9/14/06, Vincent Fourmond <vincent.fourmond@9online.fr> wrote:
>
>   Hello !

Hi Vincent. Thanks for the reply.

>
> > But when we're building an extension module, how to you do tell GCC
> > (at link-edit time) that you want your code to link to (at runtime)
> > what's already loaded and running -- that is, to link to the ruby
> > interpreter -- rather than to some shared lib somewhere?
>
>   Well, this is highly platform-dependant. If I understand you right,
> you're asking how the system knows where to find the rb_* functions
> called from within your C code, even if you didn't link with the
> appropriate library ? The answer is relatively simple. The Makefile
> produced by extconf.rb contain linker-specific flags to say : "don't
> bother to look for missing symbols at link-time, do this at run-time".

Ohhhhhhhhhhhhhhhhhhhh.

Ok. Hm. Well then. Maybe that's the point of that "-R" link-editor arg
(mentioned in that other post I just made a few minutes ago before
seeing this one). The ld docs on -R/-rpath don't specifically say what
you so clearly express above, but they might *imply* that if read
under the right intensity lights, wearing those cheap bi-colored
3D-movie glasses, while howling under a full-moon... :)

> What happens next is when your extension is loaded, the dynamic linker
> see there are missing symbols, and it looks for them in the current
> namespace. From within a ruby interpreter, the symbols are already here
> and everything goes fine. But if for some reasons it doesn't find
> symbols, your program will crash with an undefined symbol problem.

Got it.

>   Just as an example try, with a ruby extension
>
> int main()
> {
>   void * a;
>   void (* func)();
>   a = dlopen("ruby_extension.so");
>   func = dlsym(a, "Init_extension"); /* might already crash here, make
> sure to use the right Init_ function */
>   func(); /* will crash here if not before */
> }
>
>   If you want to inspect this in more details, I advise you to have a
> look at nm and especially the output of nm -D on a shared object (small,
> if possible).
>
>   Cheers !
>
>         Vince

Thanks again Vince! :)

---John