Chapter 2. POSIX Standard

Table of Contents
XSI System Interfaces and Extensions


POSIX, that stands for Portable Operating System Interface, is a standard that is being jointly developed by the IEEE and The Open Group. It defines a standard operating system interface and environment, including a command interpreter (or "shell"), and common utility programs to support applications portability at the source code level. The current revision of POSIX is The Open Group Base Specifications Issue 6 and also the IEEE Std 1003.1-2001.

This standard is composed by four major components:

The IEEE Std 1003.1-2001 standard is a single common revision to IEEE Std 1003.1-1996, IEEE Std 1003.2-1992, and the Base Specifications of The Open Group Single UNIX Specification, Version 2. In order to develope the current revision several base documents has been used. The base documents that are involved in the definition of system interfaces are:

As it can be observed this standard includes support for source portability of applications with realtime requirements, but this support is maintly optional for POSIX-conforming implementations. The specific functional areas included for realtime support and their definitions[1] are basically the following:

Another optional support that can be interesting for the developemnt of embedded applications is the Threads support. This extension to POSIX defines functionality to support multiple flows of control within a process. These flows of control are called threads and they share their address space and most of the resources and attributes defined in the operating system for the owner process.

The specific functional areas included in threads support are:

Finally, the IEEE Std 1003.1-2001 standard also proposes a set of tracing facilities that can be quite useful at the development stage of a embedded real-time application. The tracing facilities defined in the standard allow a process to select a set of trace event types, to activate a trace stream of the selected trace events as they occur in the flow of execution, and to retrieve the recorded trace events. A trace event is a data object that represents an action executed by the system, and that is recorded in a trace stream. The trace events can be retrieved later from the trace stream, allowing the system behaviour analysis.

All these functionalities are not mandotory in a POSIX-conforming implementation, but defined as a set of options that may be supported by that system. In this line, the IEEE Std 1003.1-2001 standard defines several XSI extensions that groups together several of these options in so-called XSI Option Groups. The option groups that can be of interest for embedded real-time applications are described in the next section.



This definitionas are extracted from the IEEE Std 1003.1-2001 standard.