+ * If you absolutely want use a message able to convey several datatypes,
+ * you can always say that it conveys a generic reference (see
+ * \ref gras_datadesc_ref_generic).
+ *
+ * In order to ease the upgrade of GRAS applications, it is possible to \e
+ * version the messages, ie to add a version number to the message (by
+ * default, the version is set to 0). Any messages of the wrong version will
+ * be ignored by the applications not providing any specific callback for
+ * them.
+ *
+ * This mechanism (stolen from the dynamic loader one) should ensure you to
+ * change the semantic of a given message while still understanding the old
+ * one.
+ */
+/** @{ */
+/** \brief Opaque type */
+typedef struct s_gras_msgtype *gras_msgtype_t;
+
+ void gras_msgtype_declare (const char *name,
+ gras_datadesc_type_t payload);
+ void gras_msgtype_declare_v(const char *name,
+ short int version,
+ gras_datadesc_type_t payload);
+
+ gras_msgtype_t gras_msgtype_by_name (const char *name);
+ gras_msgtype_t gras_msgtype_by_namev(const char *name, short int version);
+ gras_msgtype_t gras_msgtype_by_id(int id);
+
+/** @} */
+/** @defgroup GRAS_msg_cb Callback declaration and use
+ * @ingroup GRAS_msg
+ *
+ *
+ * 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) */
+ typedef struct s_gras_msg_cb_ctx *gras_msg_cb_ctx_t;
+
+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);
+
+ void gras_cb_register (gras_msgtype_t msgtype, gras_msg_cb_t cb);
+ void gras_cb_unregister(gras_msgtype_t msgtype, gras_msg_cb_t cb);
+
+/** @} */
+
+/** @defgroup GRAS_msg_exchange Message exchange
+ * @ingroup GRAS_msg
+ *
+ */
+/** @{ */
+
+
+ void gras_msg_send(gras_socket_t sock,
+ gras_msgtype_t msgtype,
+ void *payload);
+ void gras_msg_wait(double timeout,
+ gras_msgtype_t msgt_want,
+ gras_socket_t *expeditor,
+ void *payload);
+ void gras_msg_handle(double timeOut);
+
+/** @} */
+
+/** @defgroup GRAS_msg_rpc RPC specific functions
+ * @ingroup GRAS_msg
+ *
+ * Remote Procedure Call (RPC) are a classical mecanism to request a service
+ * from a remote host. Using this set of functions, you let GRAS doing most of
+ * the work of sending the request, wait for an answer, make sure it is the
+ * right answer from the right host and so on. Any exception raised on the
+ * server is also passed over the network to the client.
+ *
+ * Callbacks are attached to RPC incomming messages the regular way using
+ * \ref gras_cb_register.
+ *
+ * For an example of use, check the examples/gras/rpc directory of the distribution.
+ */
+/** @{ */
+
+/* declaration */
+void gras_msgtype_declare_rpc(const char *name,
+ gras_datadesc_type_t payload_request,
+ gras_datadesc_type_t payload_answer);
+
+void gras_msgtype_declare_rpc_v(const char *name,
+ short int version,
+ gras_datadesc_type_t payload_request,
+ gras_datadesc_type_t payload_answer);
+
+/* client side */
+void gras_msg_rpccall(gras_socket_t server,
+ double timeOut,
+ gras_msgtype_t msgtype,
+ void *request, void *answer);
+
+/* server side */
+void gras_msg_rpcreturn(double timeOut, gras_msg_cb_ctx_t ctx,void *answer);
+
+
+/** @} */
+
+/** @defgroup GRAS_msg_exchangeadv Message exchange (advanced interface)
+ * @ingroup GRAS_msg