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

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

[Martin Storsjö via ffmpeg-devel]

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

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[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]

> 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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