diff --git a/src/proto.h b/src/proto.h
index f04ba05..a5ca9e4 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -140,6 +140,7 @@
 int vim_snprintf_add(char *, size_t, const char *, ...) ATTRIBUTE_FORMAT_PRINTF(3, 4);
 
 int vim_snprintf(char *, size_t, const char *, ...) ATTRIBUTE_FORMAT_PRINTF(3, 4);
+size_t vim_snprintf_safelen(char *, size_t, const char *, ...) ATTRIBUTE_FORMAT_PRINTF(3, 4);
 
 int vim_vsnprintf(char *str, size_t str_m, const char *fmt, va_list ap)
 	ATTRIBUTE_FORMAT_PRINTF(3, 0);