[FFmpeg-devel] [PR] swscale/swscale: force SwsDither and SwsAlphaBlend to 32 bits (PR #21594)

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

PR #21594 opened by Zhao Zhili (quink)
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21594
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21594.patch

The 'sws_dither' and 'alphablend' options access 'SwsDither' and
'SwsAlphaBlend' enum fields as integers. This is unsafe when the
code is compiled with -fshort-enums, as the enum size might be
smaller than an int.

Since the 'dither' and 'alpha_blend' struct members are part of the
public API, their types cannot be easily changed.

To ensure safe integer access and maintain ABI compatibility across
different compiler settings, a MAX_ENUM value is added to force the
enums to a 32-bit underlying type.


>From aa03934bcaf51b3d4e6ff5da1abf64c4cd14a885 Mon Sep 17 00:00:00 2001
From: Zhao Zhili <zhilizhao@tencent.com>
Date: Mon, 26 Jan 2026 23:50:50 +0800
Subject: [PATCH] swscale/swscale: force SwsDither and SwsAlphaBlend to 32 bits

The 'sws_dither' and 'alphablend' options access 'SwsDither' and
'SwsAlphaBlend' enum fields as integers. This is unsafe when the
code is compiled with -fshort-enums, as the enum size might be
smaller than an int.

Since the 'dither' and 'alpha_blend' struct members are part of the
public API, their types cannot be easily changed.

To ensure safe integer access and maintain ABI compatibility across
different compiler settings, a MAX_ENUM value is added to force the
enums to a 32-bit underlying type.
---
 libswscale/swscale.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/libswscale/swscale.h b/libswscale/swscale.h
index 345e5458ce..495d500f14 100644
--- a/libswscale/swscale.h
+++ b/libswscale/swscale.h
@@ -82,6 +82,7 @@ typedef enum SwsDither {
     SWS_DITHER_A_DITHER, /* arithmetic addition */
     SWS_DITHER_X_DITHER, /* arithmetic xor */
     SWS_DITHER_NB,       /* not part of the ABI */
+    SWS_DITHER_MAX_ENUM = 0x7FFFFFFF, /* force size to 32 bits, not a valid dither type */
 } SwsDither;
 
 typedef enum SwsAlphaBlend {
@@ -89,6 +90,7 @@ typedef enum SwsAlphaBlend {
     SWS_ALPHA_BLEND_UNIFORM,
     SWS_ALPHA_BLEND_CHECKERBOARD,
     SWS_ALPHA_BLEND_NB,  /* not part of the ABI */
+    SWS_ALPHA_BLEND_MAX_ENUM = 0x7FFFFFFF, /* force size to 32 bits, not a valid blend mode */
 } SwsAlphaBlend;
 
 typedef enum SwsFlags {
-- 
2.52.0

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