--- a/geekcode.h
+++ b/geekcode.h
@@ -22,6 +22,7 @@
 #ifndef __INCLUDED_GEEKCODE_H__
 #define __INCLUDED_GEEKCODE_H__
 
+#include <stdlib.h> /* exit() */
 
 #define VERSION "1.7.3" /* Added in v1.2 */