--- a/unclutter.c
+++ b/unclutter.c
@@ -24,6 +24,7 @@
 #include <X11/Xutil.h>
 #include <X11/Xproto.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include "vroot.h"
 
 char *progname;