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

[Tomas Härdin]

ons 2019-08-21 klockan 11:04 +0200 skrev 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.

I'm fairly sure this is only an API break. C ABI doesn't care about
constness. But also:

> @@ -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

This should still be perfectly compatible with all user code since
adding const is fine..

/Tomas