--- dclock.c
+++ dclock.c
@@ -7,6 +7,7 @@
  * manager: *Dclock.seconds: on
  */
 #include <stdio.h>
+#include <stdlib.h> /* exit() */
 #include <locale.h>
 #include <X11/Intrinsic.h>
 #include "Dclock.h"