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

[Martin Storsjö]

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 at 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