diff --git a/src/string_func.h b/src/string_func.h
--- a/src/string_func.h
+++ b/src/string_func.h
@@ -39,6 +39,7 @@ int CDECL seprintf(char *str, const char
 int CDECL vseprintf(char *str, const char *last, const char *format, va_list ap) WARN_FORMAT(3, 0) NOACCESS(2);
 
 char *CDECL str_fmt(const char *str, ...) WARN_FORMAT(1, 2);
+char *str_vfmt(const char *str, va_list ap);
 
 std::string FormatArrayAsHex(span<const byte> data);