[FFmpeg-devel] [PATCH 6/7] avcodec: Make avcodec_decoder_subtitles2 accept a const AVPacket*

[Andreas Rheinhardt]

Anton Khirnov:
> Quoting Andreas Rheinhardt (2022-03-31 00:49:57)
>> From: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
>>
>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
>> ---
>>  doc/APIchanges            | 3 +++
>>  fftools/ffmpeg.c          | 4 ++--
>>  fftools/ffprobe.c         | 2 +-
>>  libavcodec/avcodec.h      | 3 +--
>>  libavcodec/decode.c       | 9 ++++-----
>>  libavcodec/version.h      | 2 +-
>>  tools/target_dec_fuzzer.c | 4 ++--
>>  7 files changed, 14 insertions(+), 13 deletions(-)
>>
>> diff --git a/doc/APIchanges b/doc/APIchanges
>> index 1a9f0a303e..326a3c721c 100644
>> --- a/doc/APIchanges
>> +++ b/doc/APIchanges
>> @@ -14,6 +14,9 @@ libavutil:     2021-04-27
>>  
>>  API changes, most recent first:
>>  
>> +2022-03-30 - xxxxxxxxxx - lavc 59.26.100 - avcodec.h
>> +  avcodec_decode_subtitle2() now accepts const AVPacket*.
> 
> I vaguely recall C++ having a problem with such changes. Anybody
> remembers the details? Do we care?
> 

You did something like this in 0a0f19b577d54ff2e72cc9a0fe027952db83f332
without a major bump; IIRC there are more instances likes this.

For C functions, the signature is not part of the exported symbol name
(as zhilizhao already said) and the ABIs just presume that the caller
and callee agree on the signature. Constifying this parameter does not
break this, as "pointers to qualified or unqualified versions of
compatible types shall have the same representation and alignment
requirements" (C11 6.2.5/26).

Ordinary API usage will be fine, because the conversion from AVPacket*
to const AVPacket* is safe and therefore performed automatically; it
requires no cast. API-wise there is only one thing that might cause
problems: If one does not use avcodec_decode_subtitle2 directly, but
via a function pointer like this:

#include <libavcodec/avcodec.h>

typedef int (*unconst)(AVCodecContext *, AVSubtitle *, int *, AVPacket *);

void foo(void)
{
    unconst funcptr = avcodec_decode_subtitle2;

    /* use funcptr */
}

Here the initialization of avcodec_decode_subtitle2 is not fine; for C
compilers will by default emit a warning, for C++ it is common to fail.
But avcodec_decode_subtitle2 is the only function with this signature
at all, so it makes no sense to use function pointers instead of using
this function directly. I don't think that anyone does and therefore my
answer to your last question is: I don't really care given that ordinary
usages are fine.

(Notice that the situation is different if one constified a level
further down, e.g. by changing const AVCodec *av_codec_iterate(void
**opaque) to const AVCodec *av_codec_iterate(const void **opaque) as
there is no implicit conversion in this case. So the ordinary API usage
would result in a warning or error. Might this be the reason for what
you vaguely remember?)

- Andreas