--- a/master/mailbox.c	Thu Dec 06 18:43:56 2012 +0100
+++ b/master/mailbox.c	Fri Dec 07 19:51:55 2012 +0100
@@ -163,7 +163,7 @@
  * \return Pointer to the received data, or ERR_PTR() code.
  */
 uint8_t *ec_slave_mbox_fetch(const ec_slave_t *slave, /**< slave */
-                             ec_datagram_t *datagram, /**< datagram */
+                             const ec_datagram_t *datagram, /**< datagram */
                              uint8_t *type, /**< expected mailbox protocol */
                              size_t *size /**< size of the received data */
                              )

--- a/master/mailbox.h	Thu Dec 06 18:43:56 2012 +0100
+++ b/master/mailbox.h	Fri Dec 07 19:51:55 2012 +0100
@@ -52,7 +52,7 @@
 int      ec_slave_mbox_prepare_check(const ec_slave_t *, ec_datagram_t *);
 int      ec_slave_mbox_check(const ec_datagram_t *);
 int      ec_slave_mbox_prepare_fetch(const ec_slave_t *, ec_datagram_t *);
-uint8_t *ec_slave_mbox_fetch(const ec_slave_t *, ec_datagram_t *,
+uint8_t *ec_slave_mbox_fetch(const ec_slave_t *, const ec_datagram_t *,
                              uint8_t *, size_t *);
 
 /*****************************************************************************/