extern int connection_max_messages;/* Max down one SMTP connection */
extern BOOL config_changed; /* True if -C used */
extern FILE *config_file; /* Configuration file */
-extern uschar *config_filename; /* Configuration file name */
+extern const uschar *config_filename; /* Configuration file name */
extern gid_t config_gid; /* Additional group owner */
extern int config_lineno; /* Line number */
extern uschar *config_main_filelist; /* List of possible config files */