This standard defines extensions to POSIX.1 {2} to support application portability at the source-code level. It is intended to be used by both application developers and system implementers. The scope of this standard is to define a portable application interface for applications with realtime constraints requiring the ability to control special devices from the application itself. The intent is to take existing realtime operating system practice and add it to the POSIX family of standards. Specifically within the scope is to define interfaces which do not preclude high-performance implementations on traditional uniprocessor realtime systems, as well as on multiprocessors. Wherever possible, the requirements of other application environments were included in the interface definition.