avcodec/progressframe: Explain how unnamed union can simplify accesses

This relies on the common initial seqence guarantee
(and on C11 support for unnamed members).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/libavcodec/progressframe.h b/libavcodec/progressframe.h
index dc841f30d2..428a461659 100644
--- a/libavcodec/progressframe.h
+++ b/libavcodec/progressframe.h
@@ -58,6 +58,18 @@
 
 struct AVCodecContext;
 
+/**
+ * The ProgressFrame structure.
+ * Hint: It is guaranteed that the AVFrame pointer is at the start
+ *       of ProgressFrame. This allows to use an unnamed
+ *       union {
+ *            struct {
+ *                AVFrame *f;
+ *            };
+ *            ProgressFrame pf;
+ *       };
+ *       to simplify accessing the embedded AVFrame.
+ */
 typedef struct ProgressFrame {
     struct AVFrame *f;
     struct ProgressInternal *progress;