[FFmpeg-devel] [PATCH 1/3] lavformat: Prepare to make avio_enum_protocols const correct

[Andreas Rheinhardt]

Andreas Rheinhardt:
> Using avio_enum_protocols works as follows: One initializes a pointer to
> void and gives avio_enum_protocols the address of said pointer as
> argument; the pointer will be updated to point to a member of the
> url_protocols array. Now the address of the pointer can be reused for
> another call to avio_enum_protocols.
> Said array consists of constant pointers (to constant URLProtocols),
> but the user now has a pointer to non-const to it; of course it was always
> intended that the user is not allowed to modify what the pointer points
> to and this has been enforced by hiding the real type of the underlying
> object. But it is better to use a const void ** as parameter to enforce
> this. This way avio_enum_protocols can be implemented without resorting
> to casting a const away or ignoring constness as is done currently.
> 
> Given that this amounts to an ABI and API break, this can only be done
> at the next major version bump; as usual, the break is currently hidden
> behind an appropriate #if.
> 
> It was also necessary to change the type of a pointer used in
> avio_enum_protocols. This makes the line that is not const correct move
> as long as the old function signature is used. With the new signature,
> avio_enum_protocols will be const correct.
> 
> This change will eventually force changes in their callers, e.g. to
> show_protocols in fftools/cmdutils. (This function contains all currently
> existing calls to avio_enum_protocols in FFmpeg's codebase. It hasn't
> been touched in this commit.)
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
> ---
>  libavformat/avio.h      | 4 ++++
>  libavformat/protocols.c | 6 +++++-
>  libavformat/version.h   | 3 +++
>  3 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/libavformat/avio.h b/libavformat/avio.h
> index 9141642e75..e067ea8985 100644
> --- a/libavformat/avio.h
> +++ b/libavformat/avio.h
> @@ -805,7 +805,11 @@ int avio_close_dyn_buf(AVIOContext *s, uint8_t **pbuffer);
>   *
>   * @return A static string containing the name of current protocol or NULL
>   */
> +#if FF_API_NONCONST_ENUM_PROTOCOLS
>  const char *avio_enum_protocols(void **opaque, int output);
> +#else
> +const char *avio_enum_protocols(const void **opaque, int output);
> +#endif
>  
>  /**
>   * Pause and resume playing - only meaningful if using a network streaming
> diff --git a/libavformat/protocols.c b/libavformat/protocols.c
> index ad95659795..c722f9a897 100644
> --- a/libavformat/protocols.c
> +++ b/libavformat/protocols.c
> @@ -91,9 +91,13 @@ const AVClass *ff_urlcontext_child_class_next(const AVClass *prev)
>  }
>  
>  
> +#if FF_API_NONCONST_ENUM_PROTOCOLS
>  const char *avio_enum_protocols(void **opaque, int output)
> +#else
> +const char *avio_enum_protocols(const void **opaque, int output)
> +#endif
>  {
> -    const URLProtocol **p = *opaque;
> +    const URLProtocol * const *p = *opaque;
>  
>      p = p ? p + 1 : url_protocols;
>      *opaque = p;
> diff --git a/libavformat/version.h b/libavformat/version.h
> index 9814db8633..b0b9264382 100644
> --- a/libavformat/version.h
> +++ b/libavformat/version.h
> @@ -106,6 +106,9 @@
>  #ifndef FF_API_AVIOFORMAT
>  #define FF_API_AVIOFORMAT               (LIBAVFORMAT_VERSION_MAJOR < 59)
>  #endif
> +#ifndef FF_API_NONCONST_ENUM_PROTOCOLS
> +#define FF_API_NONCONST_ENUM_PROTOCOLS  (LIBAVFORMAT_VERSION_MAJOR < 59)
> +#endif
>  
>  
>  #ifndef FF_API_R_FRAME_RATE
> I am unsure what to do next given the feedback I received: If ABI
compability is no problem, then should I simply add the const and add
an entry to doc/APIchanges? Or is a deprecation period necessary?

- Andreas