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

[Anton Khirnov]

Quoting Andreas Rheinhardt (2020-02-18 16:01:00)
> 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).

It's already been modified to be const on the next bump, so the warning
would only be temporary. And we are planning to do a bump pretty soon
IIUC.

And I'm not sure if changing a function parameter from pointer to
pointer-to-const is actually an API break. It's merely an extra
constraint on what a function is allowed to do.

And besides, those structs are actually constant. They must not be
modified from the outside. So compiler warnings would be entirely
appropriate to remind us about invalid code.

-- 
Anton Khirnov