Re: [PATCH] Fix backwards compatibility problem in libstdc++

[Paolo Carlini <pcarlini at suse dot de>]

Hi Jakub,

> It seems the
> http://gcc.gnu.org/ml/libstdc++/2004-10/msg00394.html
> changes broke libstdc++ backwards compatibility, some GCC 3.4.x compiled
> programs and/or libraries misbehave with GCC 4.0.x and later libstdc++.so.6.
> The problem is if some 3.4.x program/library has an inlined copy of
> _S_construct or _M_clone (that means it initializes _M_length and
> _M_refdata's last char, but not _M_refcount) and calls out of line
> _S_create (which in GCC 3.4.x used to clear _M_refcount, but after
> the above patch it no longer does), then _M_refcount is uninitialized.
> When running 3.4.x code against 3.4.x libstdc++ or 4.[01].x code against
> 4.[01].x libstdc++ there is no problem, in the first case _M_refcount
> is initialized in _S_create, in the latter in _S_construct/_M_clone that
> call it.
>  
Sob :(

> I'm afraid the only safe fix is now to clear _M_refcount in _S_create
> in addition to clearing it in its callers (well, it is just one int/long
> store, so it shouldn't slow things that much).
>
> Ok for trunk/4.1/4.0?
>  
Yes, certainly ok for now, we can always try to figure out something a 
tad cleaner at a later stage (the comment is very appropriate indeed).

Thanks,
Paolo.