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A Tale of Two Standards

Back in the UNIX world, there were two competing standards, POSIX and X/Open. Between 1993 and 1996, X/Open was UNIX. The UNIX trademark was managed by X/Open, which defined the requirements and testing suites used to determine whether a system was allowed to be called UNIX. Then X/Open and the Open Software Foundation (OSF) merged to form The Open Group, which now controls the UNIX trademark.

The Open Software Foundation had attempted to create its own UNIX standard. OSF/1 was intended to be a standard implementation that could be extended by vendors, as competition to System V and 4.3BSD. OSF/1 lasted from 1990 until 1994, by which point only Digital was still selling an OSF/1 derivative.

The OSF that merged with X/Open to form The Open Group had already undergone one merger. In response to the creation of the OSF and the planned development of OSF/1, AT&T formed UNIX International. In 1994, these two organizations merged to counter the growing threat from Microsoft. The joint body was responsible for overseeing development of the Common Desktop Environment, which now incorporated the OSF’s Motif widget set and provided a standard GUI for UNIX systems.

Many of the members of The Open Group were involved in the development of yet another standard, the Common API Specification, which was quite popular. POSIX, being an IEEE standard, was not freely available. This fact proved a handicap for some developers; for example, Linus Torvalds didn’t have access to the POSIX specification when he began implementing Linux, and was forced to guess based on the behavior of existing systems and the descriptions in their man pages.

In 1998, the Austin Common Standards Revision Group began producing a new standard in an attempt to harmonize the existing ones. This group was run by The Open Group, which provides the group’s chairman and performs administrative functions, but includes many more members. The outcome of this process was the Single UNIX Specification (SUS), commonly known as UNIX98. The idea behind the Austin Group was that a single specification would be created and then adopted by ISO, the IEEE, and The Open Group. The same core material forms the basis of POSIX, SUSv3, and ISO/IEC 9945, although each has its own extensions.

These days, X/Open can be regarded as obsolete, leaving just POSIX and the Single UNIX Specification standing. Fortunately for implementers, these standards retain a large overlap.

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