* receiver wakes up, it usually invokes msg_get() to pick
* the next message from the port.
*
+ * \note
+ * When you put a message into a port, such message becomes
+ * unavailable until you retrieve it using msg_get(), eg.
+ * you must not delete it or put it into another port.
+ *
* Message ports can hold any number of pending messages,
* and receivers usually process them in FIFO order.
* Other scheduling policies are possible, but not implemented