After completing the unit the student knows the principles and practice of implementing real time and embedded systems, especially what comes to the use of operating system services via the operating system calls and library functions. He/she can develop systems that consist of multiple communicating processes or threads. The features of the standard IEEE Std 1003.1-2001, -2004, -2008 (POSIX.1) is followed and a special emphasis is put to the real time extensions of the standard like timers, real time signals, asynchronous i/o, semaphores, shared memory objects and so on.