This paper focuses on a technique to empower commercial-off-the-shelf (COTS) systems with an execution environment, and corresponding services, to support realtime and embedded applications. By leveraging COTS systems, we are able to reduce the potentially expensive maintenance and development costs of proprietary solutions. We describe a system called “Hijack” that enables user-level services to take control of features such as CPU scheduling, interrupt handling and synchronization. In contrast to other approaches that support real-time tasks within the kernel of commodity systems such as Linux, Hijack provides the basis for predictable thread execution at user-level. No changes to the kernel source code are required to support this approach. Instead, Hijack works by using a combination of kernel module support and an interposed execution environment between traditional process address spaces and the kernel. This technique enables system calls and hardware interrupts to be interc...