diff --git a/libvo/video_out.h b/libvo/video_out.h
index c6bbcb9303..394a10d7dc 100644
--- a/libvo/video_out.h
+++ b/libvo/video_out.h
@@ -90,6 +90,7 @@ typedef struct vo_functions_s
 
 } vo_functions_t;
 
+char *vo_format_name(int format);
 // NULL terminated array of all drivers
 extern vo_functions_t* video_out_drivers[];