Re: [FFmpeg-devel] ABI break in 5.1

[Jan Engelhardt <jengelh@inai.de>]

On Saturday 2022-07-23 15:28, Nicolas George wrote:

>Jan Engelhardt (12022-07-23):
>> 
>> This is a follow-up to a topic that was already raised earlier,
>> http://ffmpeg.org/pipermail/ffmpeg-devel/2020-July/265694.html 
>> 
>> The same has now happened to 5.0->5.1.
>
>I do not see an issue. What issue do you see?

As I have previously explained,
[ http://ffmpeg.org/pipermail/ffmpeg-devel/2020-July/265705.html ]

>"""A program may have been built with 4.3 but is combined
>with 4.2.3 at runtime, then this can happen:"""

(Or, in today's terms, a program built with 5.1 but which is
combined with 5.0 at runtime, then this can happen:

$ ./a.out 
./a.out: symbol lookup error: ./a.out: undefined symbol: avio_vprintf, 
version LIBAVFORMAT_59

Now, it was clear that

>"""Running against an older version then the build version is never
>supported
>
>A distribution should never allow this to happen."""

This is exactly what we're trying to do. ELF symbol version definitions 
are *the* tool to do this, but we keep getting screwed over by terrible 
symbol version management in ffmpeg.

libavformat.v advertises a verdef (that one being "LIBAVUTIL_59" 
currently), but you keep modifying the set of symbols, such as dropping 
avio_vprintf.

Minus that bug, libavformat.v is but used to do symbol 
visibility. For which you don't need a verdef, for starters.

diff --git a/libavutil/libavutil.v b/libavutil/libavutil.v
index fb17058df5..54dbc84e9a 100644
--- a/libavutil/libavutil.v
+++ b/libavutil/libavutil.v
@@ -1,4 +1,4 @@
-LIBAVUTIL_MAJOR {
+{
     global:
         av*;
     local:

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