--- a/src/uc2/formats/sk1/sk1objs/curvelow.c
+++ b/src/uc2/formats/sk1/sk1objs/curvelow.c
@@ -24,6 +24,7 @@
 
 
 #include "math.h"
+#include <stdlib.h> /* abs() */
 // #include "X11/Xlib.h"
 #include "curvelow.h"