[FFmpeg-devel] Re: [PATCH] avcodec: Explicitly use a void cast instead of av_unused in get_bits.h (PR #20840)

[Zhao Zhili via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

> On Nov 5, 2025, at 20:17, Martin Storsjö via ffmpeg-devel <ffmpeg-devel@ffmpeg.org> wrote:
> 
> PR #20840 opened by Martin Storsjö (mstorsjo)
> URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20840
> Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20840.patch
> 
> Currently, av_unused doesn't work when building with MSVC.
> (In the future, if building in C23 mode, it could be possible
> that it would work using the [[maybe_unused]] attribute though.)
> 
> Convert this specific case of silencing an unused variable to
> a void cast; this silences almost 400 warnings in builds with
> MSVC, getting much better signal-to-noise ratio in the warnings
> there.
> 
> After this, a build with MSVC still has almost 300 warnings, but
> most of the remaining ones actually do point out inconsistencies
> that would be good to fix.
> 
> 
> From bc98f30a7365e0d3368662c3946cd5d74e6f3c51 Mon Sep 17 00:00:00 2001
> From: =?UTF-8?q?Martin=20Storsj=C3=B6?= <martin@martin.st>
> Date: Wed, 5 Nov 2025 14:10:01 +0200
> Subject: [PATCH] avcodec: Explicitly use a void cast instead of av_unused in
> get_bits.h
> 
> Currently, av_unused doesn't work when building with MSVC.
> (In the future, if building in C23 mode, it could be possible
> that it would work using the [[maybe_unused]] attribute though.)
> 
> Convert this specific case of silencing an unused variable to
> a void cast; this silences almost 400 warnings in builds with
> MSVC, getting much better signal-to-noise ratio in the warnings
> there.
> 
> After this, a build with MSVC still has almost 300 warnings, but
> most of the remaining ones actually do point out inconsistencies
> that would be good to fix.
> ---
> libavcodec/get_bits.h | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/libavcodec/get_bits.h b/libavcodec/get_bits.h
> index 85c87c65b1..c1be4f5fcf 100644
> --- a/libavcodec/get_bits.h
> +++ b/libavcodec/get_bits.h
> @@ -168,7 +168,8 @@ static inline unsigned int show_bits(GetBitContext *s, int n);
> 
> #define OPEN_READER_NOSIZE(name, gb)            \
>     unsigned int name ## _index = (gb)->index;  \
> -    av_unused unsigned int name ## _cache
> +    unsigned int name ## _cache;                \
> +    (void) name ## _cache

LGTM. (Sorry I’m blocked by anubis due to IP address so I cannot review on code.ffmpeg)

> 
> #if UNCHECKED_BITSTREAM_READER
> #define OPEN_READER(name, gb) OPEN_READER_NOSIZE(name, gb)
> -- 
> 2.49.1
> 
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