The main classes of the messaging system are in the following file:
http://crystalspace3d.org/cel/docs/online/api/messaging_8h_source.htmlEach iCelEntity implements the iMessageChannel interface, it can be queried by iCelEntity::QueryMessageChannel(). The property classes then use these message channels to send and receive messages, they implement the iMessageSender and iMessageReceiver interfaces for that purpose.