In the case where we have a real-time application with a number of processes running concurrently, it would be a normal situation when some of these processes need to communicate between them. We said already that the classical UNIX system only knows pipes and signals as communication mechanisms. Interprocess communication, suitable for real-time applications is an essential part of the POSIX standard, which adds a number of mechanisms to the minimal UNIX set. In the following we will briefly describe the various IPC mechanisms and how they can be invoked. We will follow as much as possible the POSIX standard, except where the facilities are not implemented in Linux. In that case we will describe the mechanism Linux makes available.