Re: __weak vs ifdef

[Linus Torvalds <torvalds@linux-foundation.org>]

On Wed, 17 Feb 2010, Grant Likely wrote:
> 
> Question.  If I use this pattern, and use the __weak attribute on core
> code functions wrapped with a #ifndef, then how does it mesh with
> EXPORT_SYMBOL*() statements?

You can't. Or at least not the traditional way, which is to put the 
EXPORT_SYMBOL next to the definition of what gets exported.

If you use __weak, you need to make sure that all users of said weak 
symbol are in another file. There are some compiler and/or binutils bugs 
with using weak symbols in the same translation unit, so the rule is that 
a weak definition hes to be somewhere else from the use - including 
EXPORT_SYMBOL.

> I also assume that at the core code site, the EXPORT_SYMBOL() must
> appear inside the #ifndef block so that a #define override doesn't
> break things.  Correct?

See above. You can't put it inside the _same_ #ifndef block. You'd have to 
put it in a different file, but yes, inside an ifndef. At which point the 
linker will just pick the right definition.

However, in general, this probably gets ugly enough that the whole __weak 
thing is not great for exported stuff. It's likely mostly useful for some 
"generic" arch functionality that is always compiled in.

		Linus