+SG_BEGIN_DECL()
+
+/** @addtogroup GRAS_msg
+ * @brief Defining messages and callbacks, and exchanging messages
+ *
+ * There is two way to receive messages in GRAS. The first one is to
+ * register a given function as callback to a given type of messages (see
+ * \ref gras_cb_register and associated section). But you can also
+ * explicitely wait for a given message with the \ref gras_msg_wait
+ * function.
+ *
+ * Usually, both ways are not intended to be mixed of a given type of
+ * messages. But if you do so, it shouldn't trigger any issue. If the
+ * message arrives when gras_msg_wait is blocked, then it will be routed to
+ * it. If it arrives when before or after \ref gras_msg_wait, it will be
+ * passed to the callback.
+ *
+ * For an example of use, please refer to \ref GRAS_ex_ping. The archive
+ * contains much more examples, but their are not properly integrated into
+ * this documentation yet.
+ */
+/** @defgroup GRAS_msg_decl Message declaration and retrival
+ * @ingroup GRAS_msg
+ *
+ * GRAS messages can only accept one type of payload. See \ref GRAS_dd for
+ * more information on how to describe data in GRAS.
+ *
+ * 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;
+
+XBT_PUBLIC(void) gras_msgtype_declare(const char *name,
+ gras_datadesc_type_t payload);
+XBT_PUBLIC(void) gras_msgtype_declare_v(const char *name,
+ short int version,
+ gras_datadesc_type_t payload);
+
+XBT_PUBLIC(gras_msgtype_t) gras_msgtype_by_name(const char *name);
+XBT_PUBLIC(gras_msgtype_t) gras_msgtype_by_name_or_null(const char *name);
+XBT_PUBLIC(gras_msgtype_t) gras_msgtype_by_namev(const char *name,
+ short int version);
+XBT_PUBLIC(gras_msgtype_t) gras_msgtype_by_id(int id);
+
+XBT_PUBLIC(void) gras_msgtype_dumpall(void);
+
+
+/** @} */
+/** @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, 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);