Re: [FFmpeg-devel] [PATCH] lavu/video_enc_params: Avoid relying on an undefined C construct

[Anton Khirnov <anton@khirnov.net>]

Quoting Martin Storsjö (2023-01-15 23:47:41)
> The construct of using offsetof on a (potentially anonymous) struct
> defined within the offsetof expression, while supported by all
> current compilers, has been declared explicitly undefined by the
> C standards committee [1].
> 
> Current Clang git main got a patch [2] which changed this construct
> into a hard error. It seems like this will be softened into a
> warning [3] soon though, as it did break a fair number of projects.
> 
> Nevertheless - in this particular case, it's trivial to fix the
> code not to rely on the construct that the standards committee has
> explicitly called out as undefined.
> 
> [1] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm
> [2] https://github.com/llvm/llvm-project/commit/e327b52766ed497e4779f4e652b9ad237dfda8e6
> [3] https://reviews.llvm.org/D133574#4053647
> 
> Signed-off-by: Martin Storsjö <martin@martin.st>
> ---
>  libavutil/video_enc_params.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/libavutil/video_enc_params.c b/libavutil/video_enc_params.c
> index 54bfed0ed9..33592dc128 100644
> --- a/libavutil/video_enc_params.c
> +++ b/libavutil/video_enc_params.c
> @@ -27,11 +27,11 @@
>  AVVideoEncParams *av_video_enc_params_alloc(enum AVVideoEncParamsType type,
>                                              unsigned int nb_blocks, size_t *out_size)
>  {
> -    const size_t blocks_offset = offsetof(
> -        struct {
> -            AVVideoEncParams   p;
> -            AVVideoBlockParams b;
> -        }, b);
> +    struct TestStruct {
> +        AVVideoEncParams   p;
> +        AVVideoBlockParams b;
> +    };
> +    const size_t blocks_offset = offsetof(struct TestStruct, b);
>      size_t size = blocks_offset;
>      AVVideoEncParams *par;
>  
> -- 
> 2.34.1

LGTM

-- 
Anton Khirnov
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