diff --git a/libavcodec/xvmc.h b/libavcodec/xvmc.h
index d9aa154f8f..da27f95675 100644
--- a/libavcodec/xvmc.h
+++ b/libavcodec/xvmc.h
@@ -40,7 +40,7 @@
 
 struct xvmc_render_state {
     //these are not changed by the decoder!
-    int  magic;
+    int  magic; ///< used as check for memory corruption by regular pixel routines
 
     short * data_blocks;
     XvMCMacroBlock * mv_blocks;