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

[Måns Rullgård]

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.

>>>  b) How do you call the 'non-weak' symbol in lavc? Or do you propose to
>>>     duplicate code? 
>>
>> I do not know in which cases the symbol can be called
>> If it cant ever then an empty function with assert(0) should do.
>
> Well, my original patch used dlopen() to force using the symbols from
> libavcodec.so.52. This should work in any case, but I guess this is not
> what we want here.

I don't see any benefit from using dlsym there.  It still needs to
perform an extra function call.

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