diff --git a/src/window.h b/src/window.h
index ab9469a2619..e853cc3cf2b 100644
--- a/src/window.h
+++ b/src/window.h
@@ -872,5 +872,6 @@ extern void init_window P_ ((void));
 extern void syms_of_window P_ ((void));
 extern void keys_of_window P_ ((void));
 
+extern int window_box_text_cols P_ ((struct window *w));
 
 #endif /* not WINDOW_H_INCLUDED */