+ * This is how to register a given function so that it gets called when a
+ * given type of message arrives.
+ *
+ * You can register several callbacks to the same kind of messages, and
+ * they will get stacked. The lastly added callback gets the message first.
+ * If it consumes the message, it should return a true value when done. If
+ * not, it should return 0, and the message will be passed to the second
+ * callback of the stack, if any.
+ *
+ * @{
+ */
+
+ /** \brief Context of callbacks (opaque structure, created by the middleware only, never by user) */
+ typedef struct s_gras_msg_cb_ctx *gras_msg_cb_ctx_t;
+
+XBT_PUBLIC void gras_msg_cb_ctx_free(gras_msg_cb_ctx_t ctx) ;
+XBT_PUBLIC gras_socket_t gras_msg_cb_ctx_from(gras_msg_cb_ctx_t ctx);
+
+ /** \brief Type of message callback functions.
+ *
+ * \param expeditor: a socket to contact who sent this message
+ * \param payload: the message itself
+ *
+ * \return true if the message was consumed by the callback,
+ * false if the message was refused by the callback (and should be
+ * passed to the next callback of the stack for this message)
+ *
+ * Once a such a function is registered to handle messages of a given
+ * type with \ref gras_cb_register(), it will be called each time such
+ * a message arrives (unless a gras_msg_wait() intercepts it on arrival).
+ *
+ * If the callback accepts the message, it should free it after use.
+ */
+ typedef int (*gras_msg_cb_t)(gras_msg_cb_ctx_t ctx,
+ void *payload);
+
+ XBT_PUBLIC void gras_cb_register (gras_msgtype_t msgtype, gras_msg_cb_t cb);
+ XBT_PUBLIC void gras_cb_unregister(gras_msgtype_t msgtype, gras_msg_cb_t cb);
+
+/** @} */
+
+/** @defgroup GRAS_msg_exchange Message exchange
+ * @ingroup GRAS_msg