[FFmpeg-devel] [PATCH] avformat/protocols: fix discarded-qualifiers compiler warning

[Andreas Rheinhardt]

Carl Eugen Hoyos:
> Am So., 14. Feb. 2021 um 18:00 Uhr schrieb Paul B Mahol <onemda at gmail.com>:
>>
>> On Sun, Feb 14, 2021 at 5:58 PM Carl Eugen Hoyos <ceffmpeg at gmail.com> wrote:
>>
>>> Am So., 14. Feb. 2021 um 17:53 Uhr schrieb Nuo Mi <nuomi2021 at gmail.com>:
>>>>
>>>> ---
>>>>  libavformat/protocols.c | 4 ++--
>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/libavformat/protocols.c b/libavformat/protocols.c
>>>> index 7df18fbb3b..0b43f66baf 100644
>>>> --- a/libavformat/protocols.c
>>>> +++ b/libavformat/protocols.c
>>>> @@ -111,10 +111,10 @@ const AVClass
>>> *ff_urlcontext_child_class_iterate(void **iter)
>>>>
>>>>  const char *avio_enum_protocols(void **opaque, int output)
>>>>  {
>>>> -    const URLProtocol **p = *opaque;
>>>> +    const URLProtocol *const *p = *opaque;
>>>>
>>>>      p = p ? p + 1 : url_protocols;
>>>> -    *opaque = p;
>>>> +    *opaque = (void*)p;
>>>
>>> I suspect that this is wrong, only avconv's protocols were const.
>>>
>>>
>> That is not valid explanation.
> 
> The warning is a regression since a merge commit from Clement (?),
> FFmpeg's protocols are not const iirc.
> 
It is irrelevant whether they are const: The above warning exists
because avio_enum_protocols returns a pointer to nonconst that points to
an entry of the const url_protocols array (i.e. it points to a const
pointer (that points to something const, but that doesn't matter)).
There are two ways to fix this: The first is by only giving the user a
pointer to const which requires changing the function signature. I once
sent such a patch, see
https://patchwork.ffmpeg.org/project/ffmpeg/patch/20190821090438.10260-2-andreas.rheinhardt@gmail.com/.
The second way requires casts. The typical way to do so is to give the
user an index in the array of protocols (by casting the pointer to
intptr_t).

- Andreas