Re: Specify how undefined weak symbol should be resolved in executable

[Michael Matz <matz@suse.de>]

Hi,

On Wed, 24 Feb 2016, H.J. Lu wrote:

> >> I was saying as far as ld was concerned, weak defined and non-weak 
> >> defined dynamic symbols would be treated equally at run-time.  Do you 
> >> agree with me?
> >
> > I at least don't.  The difference is that a defined weak symbol (at 
> > link edit time) might become undefined at runtime.  A defined non-weak 
> > symbol can't.  So they have to be handled differently.
> >
> 
> How?

Like right now (mostly)?  Resolution of weak symbols needs to be deferred 
to the dynamic linker, in particular they mustn't be resolved (to either 
zero or an address) at link edit time.  There's one case where weak 
symbols can be resolved early: if at link edit time the executable itself 
provides a definition of 'foo' then weak references can be directly 
resolved to that one (because at runtime of that exectuable there's no 
possibility to not have this symbol defined).

To not do that (i.e. resolve them always at link edit time) breaks various 
current uses of weak symbols.  We could do that of course, but I don't 
think that would be useful to users.


Ciao,
Michael.