[FFmpeg-devel] [PATCH] restoring binary compatibility with ffmpeg 0.5

[Måns Rullgård]

Michael Niedermayer <michaelni at gmx.at> writes:

> On Sun, Jun 06, 2010 at 10:32:19PM +0100, M?ns Rullg?rd wrote:
>> Michael Niedermayer <michaelni at gmx.at> writes:
>> 
>> > On Sun, Jun 06, 2010 at 10:59:09PM +0200, Reimar D?ffinger wrote:
>> >> On Sun, Jun 06, 2010 at 10:45:18PM +0200, Michael Niedermayer wrote:
>> >> > On Sun, Jun 06, 2010 at 10:29:25PM +0200, Reimar D?ffinger wrote:
>> >> > > On Sun, Jun 06, 2010 at 09:38:07PM +0200, Reinhard Tartler wrote:
>> >> > > > In any case, find below the 'best' fix, that admittedly only works on
>> >> > > > gnu platforms. Michael, please comment if you prefer the half fix that
>> >> > > > fixes the issue on gcc/gas platforms (and doesn't regress on others) or
>> >> > > > bumping major of libavformat.
>> >> > > 
>> >> > > How sure are we this is actually correct?
>> >> > > The cases I could find documented (which only involve one library)
>> >> > > also require changes to the version script.
>> >> > > In case it was only one library, something like this:
>> >> > > Index: libavcodec.v
>> >> > > ===================================================================
>> >> > > --- libavcodec.v        (revision 23508)
>> >> > > +++ libavcodec.v        (working copy)
>> >> > > @@ -1,3 +1,7 @@
>> >> > > +LIBAVFORMAT_52 {
>> >> > > +        global: av_init_packet;
>> >> > > +};
>> >> > >  LIBAVCODEC_$MAJOR {
>> >> > > +        global: av_init_packet;
>> >> > >          global: *;
>> >> > > -};
>> >> > > +} LIBAVFORMAT_52;
>> >> > > 
>> >> > > However here we have the problem that this would break e.g. on Solaris,
>> >> > > since a symbol is not allowed to have multiple versions there.
>> >> > 
>> >> > +LIBAVFORMAT_52 {
>> >> > +        global: av_init_packet;
>> >> > +};
>> >> >  LIBAVCODEC_$MAJOR {
>> >> >          global: *;
>> >> > -};
>> >> > +} LIBAVFORMAT_52;
>> >> > 
>> >> > above does not work?
>> >> 
>> >> It does, but it does not do what I guess you want.
>> >> It will only generate av_init_packet@@LIBAVFORMAT_52,
>> >
>> > yes
>> >
>> >> so it will still break compatibility to some previous versions.
>> >
>> > do you assume this or have you tested this?
>> 
>> It changes the name of the symbol.  Anything linked against current
>> libavcodec will fail after this change.
>
> IMO the linker is suposed to find symbols in the linked versions
> gnu ldso does not, so this doesnt work for us. I still think though
> that this behavior of gnu ldso is not terribly sane

How do you expect it to figure that a differently named symbol is
actually the same thing?  If an app lists foo at LIBAVFORMAT_52 as an
undefined symbol, some library must provide that exact name.  For
dynamic symbol resolution, the version tag is effectively part of the
symbol name.

-- 
M?ns Rullg?rd
mans at mansr.com