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

[Michael Niedermayer]

On Thu, Jun 10, 2010 at 09:52:13AM +0100, M?ns Rullg?rd wrote:
> Reinhard Tartler <siretart at tauware.de> writes:
> 
> > On Wed, Jun 09, 2010 at 19:41:33 (CEST), Michael Niedermayer wrote:
> >
> >> On Wed, Jun 09, 2010 at 03:42:13PM +0200, Reinhard Tartler wrote:
> >>> Two questions:
> >>>  a) doesn't this attribute need to be at symbol declaration and not
> >>>     definition?
> >>
> >> The way i understand it, it needs to be at the definition
> >> at the declaration it possibly means a weak refernce which is something
> >> that is 0 (instead of failing to link) if no definition is found
> >
> > I see.
> >
> > I've now tried your suggestion to (re)introduce the functions like you
> > propose:
> >
> > #if LIBAVFORMAT_VERSION_MAJOR < 53
> > [...]
> > void __attribute__((weak)) av_init_packet(AVPacket *pkt)
> > {
> >      abort();
> > }
> > #endif
> >
> > building a shared libavformat.so works without warnings/errors regarding
> > symbol collisions and results this
> >
> > $ readelf -s /tmp/ffmpeg-trunk/lib/libavformat.so | grep av_init_packet@
> >    466: 000abc00     8 FUNC    WEAK   DEFAULT   13 av_init_packet@@LIBAVFORMAT_52
> >
> > I guess this is exactly what you've expected here.
> >
> > With this setup, I've now tried both the ffplay binary from this
> > compilation and the system /usr/bin/ffplay (based on version 0.5.2). In
> > *both* cases, it seems that the binary at runtime picks up the version
> > in libavformat:
> >
> >>> readelf -s /tmp/ffmpeg-trunk/bin/ffplay | grep av_init_packet
> >     49: 00000000     0 FUNC    GLOBAL DEFAULT  UND av_init_packet at LIBAVFORMAT_52 (10)
> >
> >>> /tmp/ffmpeg-trunk/bin/ffplay /tmp/foo.avi
> > FFplay version SVN-r23556, Copyright (c) 2003-2010 the FFmpeg developers
> >   built on Jun 10 2010 08:09:17 with gcc 4.4.3
> >   configuration: --enable-avfilter --enable-libvpx --enable-shared --disable-static --enable-gpl --prefix=/tmp/ffmpeg-trunk --enable-postproc --enable-libvorbis
> >   libavutil     50.18. 0 / 50.18. 0
> >   libavcodec    52.75. 1 / 52.75. 1
> >   libavformat   52.68. 0 / 52.68. 0
> >   libavdevice   52. 2. 0 / 52. 2. 0
> >   libavfilter    1.20. 0 /  1.20. 0
> >   libswscale     0.11. 0 /  0.11. 0
> >   libpostproc   51. 2. 0 / 51. 2. 0
> > zsh: abort      /tmp/ffmpeg-trunk/bin/ffplay /srv/media/UDS-Sevilla.avi
> 
> This is what I would expect.  The weak attribute is only used when
> combining relocatable object files in to an executable or shared
> object, not during runtime dynamic linking.

until reinhard posts a readelf of libavcodec that shows a global
av_init_packet at LIBAVFORMAT_52 this conclusion is unfounded

also let me quote the code from the runtime dynamic linker
          switch (ELFW(ST_BIND) (sym->st_info))
            {
            case STB_WEAK:
              /* Weak definition.  Use this value if we don't find another.  */
              if (__builtin_expect (GLRO(dl_dynamic_weak), 0))
                {
                  if (! result->s)
                    {
                      result->s = sym;
                      result->m = (struct link_map *) map;
                    }
                  break;
                }
              /* FALLTHROUGH */
            case STB_GLOBAL:
#ifdef __mips__
              /* HACK: MIPS marks its lazy evaluation stubs with SHN_UNDEF
                 symbols, we skip them. */
              if (sym->st_shndx == SHN_UNDEF)
                break;
#endif

              /* Global definition.  Just what we need.  */
              result->s = sym;
              result->m = (struct link_map *) map;
              return 1;
            default:
              /* Local symbols are ignored.  */
              break;
            }
        }



[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I wish the Xiph folks would stop pretending they've got something they
do not.  Somehow I fear this will remain a wish. -- M?ns Rullg?rd
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