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

[Måns Rullgård]

Reinhard Tartler <siretart at tauware.de> writes:

> Hi,
>
> I'm sorry for not noticing earlier, but during upgrade tests, I noticed
> that there are some functions that went missing in libavformat.so.52
> since ffmpeg 0.5. In total, they are:
>
> void ff_av_destruct_packet_nofree(AVPacket *pkt);
> void ff_av_destruct_packet(AVPacket *pkt);
> int ff_av_new_packet(AVPacket *pkt, int size);
> int ff_av_dup_packet(AVPacket *pkt);
> void ff_av_free_packet(AVPacket *pkt);
> void ff_av_init_packet(AVPacket *pkt);
>
> They all were moved quite some time ago from libavformat to libavcodec.
> The API has not been broken, as avcodec is a dependency for avformat.
> Still, this is a serious problem if we want to retain binary
> compatibility. As a testcase, I'm using the ubuntu ffplay binary (ffmpeg
> 0.5) with a LD_LIBRARY_PATH, in which ffmpeg 0.6 libraries have been
> installed to playback a VP8 file in a WEBM container. Using this setup,
> ffplay aborts with:
>
> ffplay: relocation error: ffplay: symbol av_init_packet, version LIBAVFORMAT_52 not defined in file libavformat.so.52 with link time reference
>
> The message is exactly right; the mentioned symbols have been removed
> from libavformat.so.52.
>
> The safest fix in this situation would be of course to bump major.

I think that is the proper thing to do.  We already have over 30
changes queued up for the next lavf major bump.  Piling on hacks will
merely delay the inevitable and has no long-term benefit.

> I'd really like to have this for ffmpeg 0.6.

That is ultimately for the release maintainers to decide, but please
see comments below.

> Is the attached patch acceptable for trunk?

No.

> +#if HAVE_SYMBOL_VERSIONING && LIBAVFORMAT_VERSION_MAJOR < 53
> +/* compatibility trampolines for packet functions */
>
> +void ff_av_destruct_packet_nofree(AVPacket *pkt);
> +void ff_av_destruct_packet_nofree(AVPacket *pkt)
> +{
> +    void (*fn)(AVPacket *pkt) = dlsym(avformat_handle, "av_destruct_packet_nofree");
> +    if (!fn)
> +        av_log(NULL, AV_LOG_FATAL, "Failed to locate symbol: av_destruct_nofree\n");
> +    (*fn)(pkt);

Aside, you don't need all those stars and parens.  A simple fn(pkt) is
equivalent.

> +}

Why not simply call the function directly?

> +__asm__(".symver ff_av_destruct_packet_nofree,av_destruct_packet_nofree at LIBAVFORMAT_52");

This will break with non-gcc compilers and/or non-gnu assemblers, both
of which we support.

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