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

[Michael Niedermayer]

On Mon, Jun 07, 2010 at 10:29:55AM +0100, M?ns Rullg?rd wrote:
> Michael Niedermayer <michaelni at gmx.at> writes:
> 
> > On Mon, Jun 07, 2010 at 11:08:46AM +0200, Reinhard Tartler wrote:
> >> On Mo, Jun 07, 2010 at 10:40:04 (CEST), Michael Niedermayer wrote:
> >> 
> >> > On Mon, Jun 07, 2010 at 07:52:11AM +0200, Reinhard Tartler wrote:
> >> >> On Mo, Jun 07, 2010 at 01:44:30 (CEST), Michael Niedermayer wrote:
> >> >> 
> >> >> >> > we can duplicate the symbols and code in lavf under #if
> >> >> >> 
> >> >> >> I suspect that might cause a conflict during linking since the same
> >> >> >> symbol would be provided by two libraries.  If it doesn't fail, the
> >> >> >> version chosen is unpredictable.  We'd end up with apps linked against
> >> >> >> a mix of sym at LIBAVFORMAT_XX and sym at LIBAVCODEC_XX which is obviously
> >> >> >> not good.
> >> >> >
> >> >> > marking one as weak symbol might work.
> >> >> > weak symbols are part of the ELF spec ... doesnt mean gnu tools support
> >> >> > them of course ;)
> >> >> 
> >> >> I've (of course) tried that as well, but it doesn't work in this
> >> >> case. The problem here is that you must not create symbol collisions in
> >> >> the same compilation unit. Removing the symbols from libavcodec, even
> >> >> conditionally, is not an option that we have. Using weak symbols, like
> >> >> this version
> >> >> 
> >> >> #if HAVE_SYMBOL_VERSIONING && LIBAVFORMAT_VERSION_MAJOR < 53
> >> >> /* compatibility trampolines for packet functions */
> >> >> 
> >> >> #define av_weak_alias(x)  __attribute__ (( weak, alias ("x") ))
> >> >> 
> >> >> void av_init_packet(AVPacket *pkt) av_weak_alias(av_init_packet);
> >> >> void av_init_packet(AVPacket *pkt)
> >> >
> >> > what about
> >> >
> >> > void __attribute__ (( weak )) av_init_packet(AVPacket *pkt)
> >> >
> >> >> {
> >> >>     av_log(NULL, AV_LOG_WARNING, "diverting av_*_packet function calls to libavcodec. Recompile to improve performance\n");
> >> >>     av_init_packet(pkt);
> >> >> }
> >> >
> >> > ?
> >> 
> >> did you try that? In my tests, I get these compiler errors:
> >> 
> >> causes this compiler error:
> >> .../ffmpeg/libavformat/utils.c:326: error: redefinition of 'av_init_packet'
> >> .../ffmpeg/libavformat/utils.c:325: note: previous definition of 'av_init_packet' was here
> >
> > i tried:
> > int __attribute__((weak)) func(int x)
> > {
> >         return 5;
> > }
> >
> > and i can make a .so out of that
> 
> Yes, it works (I tested), but the version picked up when linking
> against two libs defining the same symbol is whichever the linker
> finds first. 

so ld doesnt favor global over weak when they are in 2 seperate .so?
annoying ...


> As I said earlier, this means that sometimes executables
> will request the @LIBAVFORMAT version and sometimes the @LIBAVCODEC
> one.  We do not want that situation.

#define av_init_packet av_init_packet_2ndtry
in a public header and a single corresponding symbol in lavc might fix this
its getting a bit convoluted though ...

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

I am the wisest man alive, for I know one thing, and that is that I know
nothing. -- Socrates
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