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

["Martin Storsjö" <martin@martin.st>]

On Wed, 18 Jan 2023, Anton Khirnov wrote:

> 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

Thanks, pushed now.

// Martin
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