XSI System Interfaces and Extensions

The X/Open System Interface is the core application programming interface for systems conforming to the Single UNIX Specification. This is a superset of the mandatory requirements for conformance to IEEE Std 1003.1-2001.

A system that wants to be a XSI-conforming implementation shall meet the criteria for POSIX conformance and support all functions and headers defined in IEEE Std 1003.1-2001 as part of the XSI extension. Additionally, it shall support the following options defined in the standard: File Synchronization, Memory Mapped Files, Memory Protection, Threads, Thread Process-Shared Synchronization, Thread Stack Address Attribute and Thread Stack Address Size.

Despite of these options that are mandatory to be a XSI-conforming system, the system may also support one or more of the following XSI Option Groups: Encryption, Realtime, Advanced Realtime, Realtime Threads, Advanced Realtime Threads, Tracing, XSI STREAMS and Legacy. All the realtime option groups jointly with the tracing option group are clearly of great interest for developing embedded real-time applications. The options each option groups requires are detailed next:


The options of IEEE Std 1003.1-2001 that are grouped together in this option group are: Asynchronous, Synchronized and Prioritized Input and Output, File Synchronization, Memory Mapped Files, Shared Memory Objects, Process and Range Memory Locking, Memory Protection, Semaphores, Timers, Realtime Signals Extension, Message Passing and Process Scheduling.

Advanced Realtime

The options of the standard that are grouped together in the Advanced Realtime option group are: Advisory Information, Clock Selection, Process CPU-Time Clocks, Monotonic Clock, Timeouts, Typed Memory Objects, Spawn and Process Sporadic Server.

Realtime Threads

This option group includes the following options: Thread Priority Inheritance and Protection, and Thread Execution Scheduling.

Advanced Realtime Threads

The Advanced Realtime Threads option group requires the following POSIX options: Thread CPU-Time Clocks, Thread Sporadic Server, Spin Locks and Barriers.


This option group includes the following tracing facility options: Trace, Trace Event Filter, Trace Inherit and Trace Log.