... First of all, I should say that you cannot guarantee user-mode processes to have exact control of timing because of the multi-tasking nature of Linux. ... However, for most applications using I/O ports, this does not really matter. To minimise this, you may want to nice your process to a high-priority value (see the nice(2) manual page) or use real-time scheduling (see below). ... Linux 2.x kernels have soft real time support; see the manual page for sched_setscheduler(2) for details. ...