Sergio Saez UPVLC ssaez@disca.upv.es Alejandro Lucero UPVLC alucero@gap.upv.es 2003 OCERA Consortium RTLinux PSignals Real-Time, POSIX, communication, IPC UNIX systems offers several possibilities for interprocess communication: signals, pipes and FIFO queues, shared memory, sockets, etc. In RTLinux, the most flexible one is shared memory, but the programmer has to use alternative synchronisation mechanism to build a safe communication mechanism between process or threads. On the other hand, signals and pipes lack certain flexibility to establish communication channels between process. In order to cover some of these weaknesses, POSIX standard proposes a message passing facility that offers: 1) Protected and synchronised access to the message queue. Access to data stored in the message queue is properly protected against concurrent operations. 2) Prioritised messages. Processes can build several flows over the same queue, and it is ensured that the receiver will pick up the oldest message from the most urgent flow. 3) Asynchronous and temporised operation. Processes have not to wait for operation to be finish, i.e., they can send a message without having to wait for someone to read that message. They also can wait an specified amount of time or nothing at all, if the message queue is full or empty. 4) Asynchronous notification of message arrivals. A receiver process can configure the message queue to be notified on message arrivals. So such a process can be working on something else until the expected message arrives.