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The Single UNIX Specification Version 3 - Overview

Overview Paper
XBD Contents
XSH Contents
XCU Contents
XRAT Contents
XCURSES Contents
The Austin Group
ISO/IEC 9945
IEEE Std 1003.1
The Authorized Guide Book
Interface Tables (pdf)
Migration Guide (pdf)

The Single UNIX Specification, Version 3

Work commenced in late 1998 on the Single UNIX Specification, Version 3. The core of the Single UNIX Specification, Version 3, collectively known as the Base Specifications, was developed, and is maintained, by a joint working group of members of the IEEE Portable Applications Standards Committee, members of The Open Group, and members of ISO/IEC Joint Technical Committee 1. This joint working group is known as the Austin Group.1

The Austin Group arose out of discussions amongst the parties which started in early 1998, leading to an initial meeting and formation of the group in September 1998. This represented a seachange in attitude regarding development of two related specifications by three development organizations, which to date had been developed separately, often with the same standards developers involved.

The purpose of the Austin Group has been to revise, combine, and update the following standards: ISO/IEC 9945-1, ISO/IEC 9945-2, IEEE Std 1003.1, IEEE Std 1003.2, and the Base Specifications of The Open Group Single UNIX Specification.

After two initial meetings, an agreement was signed in July 1999 between The Open Group and the Institute of Electrical and Electronics Engineers (IEEE), Inc. to formalize the project, with the first draft of the revised specifications being made available at the same time. Under this agreement, The Open Group and IEEE agreed to share joint copyright of the resulting work. The Open Group has provided the chair and secretariat for the Austin Group.

This unique development has combined both the industry-led efforts and the formal standardization activities into a single initiative, and included a wide spectrum of participants including commercial, academia, government, and the open source communities.

The approach to specification development was one of ``write once, adopt everywhere'', with the resulting set of specifications being approved as IEEE Std 1003.1-2001 (POSIX), ISO/IEC 9945:2002 (parts 1 to 4), and The Open Group Base Specifications, Issue 6. This set of specifications forms the core of the Single UNIX Specification, Version 3.

The Base Specifications, Issue 6 consist of the following Technical Standards:

  • Base Definitions, Issue 6 (XBD)

  • Shell and Utilities, Issue 6 (XCU)

  • System Interfaces, Issue 6 (XSH)

  • Rationale (Informative)

The revision of the Base Specifications has tried to minimize the number of changes required to implementations which conform to the earlier versions of the approved standards to bring them into conformance with the current standard. Specifically, the scope of this work excluded doing any ``new'' work, but rather collecting into a single document what had been spread across a number of documents, and presenting it in what had been proven in practice to be a more effective way. Some changes to prior conforming implementations were unavoidable, primarily as a consequence of resolving conflicts found in prior revisions, or which became apparent when bringing the various pieces together.

However, since the revision now references the 1999 version of the ISO C standard, there are a number of unavoidable changes that have been made which will affect applications portability.

In addition to the Base Specifications, the Single UNIX Specification, Version 3 includes the X/Open Curses, Issue 4, Version 2 specification. Updates to X/Open Curses, Issue 4, Version 2 have been limited to production of a Corrigendum to allow it to exist in a Base Specifications, Issue 6 environment.


The Austin Group is named after the location of the inaugural meeting held at the IBM facility in Austin, Texas in September 1998.


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