[FFmpeg-devel] [PATCH 2.0/3] libavformat/protocols.c: avio_enum_protocols(): Add more const-correctness

[Andreas Rheinhardt]

Michael Witten:
> This commit adds 'const' qualifiers to the parameters.
> 
> ---
>  libavformat/avio.h      | 2 +-
>  libavformat/protocols.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/libavformat/avio.h b/libavformat/avio.h
> index 0b35409787..3b92cf742a 100644
> --- a/libavformat/avio.h
> +++ b/libavformat/avio.h
> @@ -786,7 +786,7 @@ int avio_close_dyn_buf(AVIOContext *s, uint8_t **pbuffer);
>   *
>   * @return A static string containing the name of current protocol or NULL
>   */
> -const char *avio_enum_protocols(void **opaque, int output);
> +const char *avio_enum_protocols(void **const opaque, const int output);
>  
>  /**
>   * Get AVClass by names of available protocols.
> diff --git a/libavformat/protocols.c b/libavformat/protocols.c
> index e0b3405ab8..e671c5ab6a 100644
> --- a/libavformat/protocols.c
> +++ b/libavformat/protocols.c
> @@ -91,7 +91,7 @@ const AVClass *ff_urlcontext_child_class_iterate(void **iter)
>      return ret;
>  }
>  
> -const char *avio_enum_protocols(void **opaque, int output)
> +const char *avio_enum_protocols(void **const opaque, const int output)
>  {
>      const URLProtocol *const *p = *opaque;
>  
> 
This thing makes nothing more const-correct at all; C uses call be
value, so we only deal with our own copy of opaque which we may modify
as we please.
The reason for the const-incorrectness lies in the fact that this
function makes *opaque point to something const (namely an entry in a
const list of pointers (each pointing to a const protocol, but that is
irrelevant)), so that the user now has a pointer whose pointed to-type
is not const-qualified, despite the actual target being const.
See here:
https://patchwork.ffmpeg.org/project/ffmpeg/patch/20190821090438.10260-2-andreas.rheinhardt@gmail.com/
for how to fix the const-correctness if one wants to keep the general
design of the functions. See libavformat/allformats.c,
libavcodec/allcodecs.c, libavfilter/allfilters.c for the other approach
(that involves storing an index in the pointer).

- Andreas