[FFmpeg-devel] [PATCH 1/6] avformat/format: add av_demuxer_find_by_ext

[Andreas Rheinhardt]

Anton Khirnov:
> Quoting Gyan Doshi (2020-02-01 06:15:30)
>>
>>
>> On 31-01-2020 10:41 pm, Andreas Rheinhardt wrote:
>>> Gyan Doshi:
>>>> Allows selecting demuxer by extension which are more widely recognized
>>>> by users.
>>>>
>>>> Conditional cast added since this function will usually be called after
>>>> av_find_input_format, and so matches its return type.
>>> That's not a good point. av_demuxer_find_by_ext() already always
>>> returns const AVInputFormat *, so you casting the const away when
>>> returning is pointless. Furthermore, any caller that wants to use this
>>> new function can simply use a pointer to const AVInputFormat to work
>>> with both av_find_input_format() and av_demuxer_find_by_ext(). And
>>> after all, adding const makes the code more future-proof
>>> (av_find_input_format() will return const AVInputFormat * after the
>>> next major bump).
>>
>> Ok, I don't think I should add const to the pointers at the receiving 
>> end (fftools) since they are global variables and may not be acceptable 
>> as const. So I'll cast away the const when receiving and remove the 
>> conditional cast.
> 
> Why not? Nothing can conceivably modify the content of those structs, so
> simply adding const to all their uses should work.
> 
Then avformat_open_input() would have to be modified to accept a const
AVInputFormat *, otherwise one would get compiler warnings. And then
one would have to cast the const away in avformat_open_input() (should
probably already have been made in 3aa6208d to allow users to write
future-proof-code).
But the av_opt-API seems to be based around non-const structures which
may be a problem with the av_opt_find()-calls in open_input_file() in
ffmpeg_opt.c.

- Andreas