Re: Weak symbols and inline

[Marc Glisse <marc dot glisse at inria dot fr>]

On Fri, 28 Feb 2014, Paul Smith wrote:

> Can I move out of the clouds of standards-compliance and into the mud of
> implementations? :-).  Speaking specifically about GCC/binutils on
> GNU/Linux, although at some point I'll need to address Windows MSVC and
> MacOSX Clang/LLVM as well (not here, although I welcome pointers).
>
> I'm creating a shared library from a large existing C++ codebase.  The
> interface to the shared library is well-designed using abstract classes
> and factories, and correct handling of memory.
>
> In my shared library I want to use an alternate memory manager, and so I
> want to override global operator new/delete.
>
> However, I don't want my memory manager to "leak out" into the
> executable using the shared library; I want the executable (also written
> in C++) to be able to define its own memory management (or get the
> system new/delete by default) without impacting my library and vice
> versa.
>
>
> I build all my code with -fvisibility=hidden, then mark the factory
> functions as __attribute__((visibility("default"))).  Of course I didn't
> mark the global operator new/delete as "default".
>
> However, even though I didn't mark them that way, running nm on the .so
> shows them as public symbols ("T").  I'm assuming that these functions
> are handled specially so that the visibility=hidden default doesn't
> apply to them?  I get a compiler error if I try to force them with
> __attribute__.
>
> So then I tried to use inline versions of global new/delete.  If I
> further add __attribute__((force_inline)) then it APPEARS to do what I'd
> like: none of my objects are exporting global new/delete.

Header <new> contains:
#pragma GCC visibility push(default)
  __attribute__((__externally_visible__));

It seems hard to counter those effects.

> Of course this is a bit hackish, and more concerning it means that if I
> forget to include the header with the inline definitions into any of my
> translation units then I silently get the wrong ones.
>
>
> Is there a better supported, more "approved" way to handle this
> requirement?

I haven't looked at it closely, but maybe asking the linker directly 
(instead of telling gcc to tell the linker), for instance through a map 
file, could help?

The simplest is probably to use asm("other_name_for_new") on a declaration 
of new, so it is still visible but with a different name...

> Would the standard-conforming answer be modifying all classes to declare
> their own new/delete instead of relying on global new/delete, maybe by
> introducing a common base class for everything?  That's just not
> feasible for me at the moment: there are thousands of classes,
> templates, etc. etc.  I need something less invasive.

--
Marc Glisse