-extern int rda_interpret(redirect_block *, int, uschar *, uschar *,
- uschar *, uschar *, uschar *, ugid_block *, address_item **,
- uschar **, error_block **, int *, uschar *);
+extern int rda_interpret(redirect_block *, int, const uschar *, const uschar *,
+ const uschar *, const uschar *, const uschar *, const ugid_block *, address_item **,
+ uschar **, error_block **, int *, const uschar *);