diff --git a/include/ucore/mbuf.h b/include/ucore/mbuf.h
index 256354a..1b92b81 100644
--- a/include/ucore/mbuf.h
+++ b/include/ucore/mbuf.h
@@ -30,6 +30,7 @@ struct MBuf {
     /**Arbitary pointer user can associate with the mbuf*/
     void *cookie;
 
+    /*UC_MBUF_FLAGS. The upper 16 bits can be used by the application */
     uint32_t flags;
     /** Total no of bytes from bufstart*/
     uint32_t allocated;