diff --git a/port.h b/port.h
index c64723a..bc9c3df 100644
--- a/port.h
+++ b/port.h
@@ -25,6 +25,8 @@
 #include "fsm.h"
 #include "transport.h"
 
+struct clock; /*forward declaration*/
+
 /** Opaque type. */
 struct port;