diff --git a/utils/ctlgeom.h b/utils/ctlgeom.h
index d437ea4..b5e9bf4 100644
--- a/utils/ctlgeom.h
+++ b/utils/ctlgeom.h
@@ -94,6 +94,7 @@ GEOMETRIC_OBJECT object_of_point(vector3 p, vector3 *shiftby);
 vector3 shift_to_unit_cell(vector3 p);
 extern matrix3x3 square_basis(matrix3x3 lattice_basis, vector3 size);
 extern void ctl_printf(const char *fmt, ...);
+extern void (*ctl_printf_callback)(const char *s);
 
 typedef struct {
      vector3 low, high;