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The 1973 rewrite of Unix in C made it unprecedentedly easy to port and modify. As a result, the ancestral Unix diverged into a family of operating systems early on. Unix standards originally developed to reconcile the APIs of the different branches of the family tree.
The Unix standards that evolved after 1985 were quite successful at this — so much so that they serve as valuable documentation of the API of modern Unix implementations. In fact, real-world Unixes follow published standards so closely that developers can (and frequently do) lean more on documents like the POSIX specification than on the official manual pages for the Unix variant they happen to be using.
In fact, on the newer open-source Unixes (such as Linux), it is common for operating-system features to have been engineered using published standards as the specification. We'll return to this point when we examine the RFC standards process later in this chapter.
The 4.x BSD Unixes were descended from the 1979 Version 7. After the release of 4.1BSD in 1980 the BSD line quickly developed a reputation as the cutting edge of Unix. Important additions included the vi visual editor, job control facilities for managing multiple foreground and background tasks from a single console, and improvements in signals (see Chapter═7). By far the most important addition was to be TCP/IP networking, but though Berkeley got the contract to do it in 1980, TCP/IP was not to ship in an external release for three years.
But another version, 1981's System III, became the basis of AT&T's later development. System III reworked the Version 7 terminals interface into a cleaner and more elegant form that was completely incompatible with the Berkeley enhancements. It retained the older (non-resetting) semantics of signals (again, see Chapter═7 for discussion of this point). The January 1983 release of System V Release 1 incorporated some BSD utilities (such as vi(1)).
The first attempt to bridge the gap came in February 1983 from UniForum, an influential Unix user group. Their Uniforum 1983 Draft Standard (UDS 83) described a “core Unix System” consisting of a subset of the System III kernel and libraries plus a file-locking primitive. AT&T declared support for UDS 83, but the standard was an inadequate subset of evolving practice based on 4.1BSD. The problem was exacerbated by the July 1983 release of 4.2BSD, which added many new features (including TCP/IP networking) and introduced some subtle incompatibilities with the ancestral Version 7.
The 1984 divestiture of the Bell operating companies and the beginnings of the Unix wars (see Chapter═2) significantly complicated matters. Sun Microsystems was leading the workstation industry in a BSD direction; AT&T was trying to get into the computer business and use control of Unix as a strategic weapon even as it continued to license the operating system to competitors like Sun. All the vendors were making business decisions to differentiate their versions of Unix for competitive advantage.
During the Unix wars, technical standardization became something that cooperating technical people pushed for and most product managers accepted grudgingly or actively resisted. The one large and important exception was AT&T, which declared its intention to cooperate with user groups in setting standards when it announced System V Release 2 (SVr2) in January 1984. The second revision of the UniForum Draft Standard, in 1984, both tracked and influenced the API of SVr2. Later Unix standards also tended to track System V except in areas where BSD facilities were clearly functionally superior (thus, for example, modern Unix standards describe the System V terminal controls rather than the BSD interface to the same facilities).
In 1985, AT&T released the System V Interface Definition (SVID). SVID provided a more formal description of the SVr2 API, incorporating UDS 84. Later revisions SVID2 and SVID3 tracked the interfaces of System V releases 3 and 4. SVID became the basis for the POSIX standards, which ultimately tipped most of the Berkeley/AT&T disputes over system and C library calls in AT&T's favor.
But this would not become obvious for a few years yet; meanwhile, the Unix wars raged on. For example, 1985 saw the release of two competing API standards for file system sharing over networks: Sun's Network File System (NFS) and AT&T's Remote File System (RFS). Sun's NFS prevailed because Sun was willing to share not merely specifications but open-source code with others.
The lesson of this success should have been all the more pointed because on purely logical grounds RFS was the superior model. It supported better file-locking semantics and better mapping among user identities on different systems, and generally made an effort to get the finer details of Unix file system semantics precisely right, unlike NFS. The lesson was ignored, however, even when it was repeated in 1987 by the open-source X windowing system's victory over Sun's proprietary Networked Window System (NeWS).
After 1985 the main thrust of Unix standardization passed to the Institute of Electrical and Electronic Engineers (IEEE). The IEEE's 1003 committee developed a series of standards generally known as POSIX. These went beyond describing merely systems calls and C library facilities; they specified detailed semantics of a shell and a minimum command set, and also detailed bindings for various non-C programming languages. The first release in 1990 was followed by a second edition in 1996. The International Standards Organization adopted them as ISO/IEC 9945.
Key POSIX standards include the following:
- 1003.1 (released 1990)
Library procedures. Described the C system call API, much like Version 7 except for signals and the terminal-control interface.
- 1003.2 (released 1992)
Standard shell and utilities. Shell semantics strongly resemble those of the System V Bourne shell.
- 1003.4 (released 1993)
Real-time Unix. Binary semaphores, process memory locking, memory-mapped files, shared memory, priority scheduling, real-time signals, clocks and timers, IPC message passing, synchronized I/O, asynchronous I/O, real-time files.
In the 1996 Second Edition, 1003.4 was split into 1003.1b (real-time) and 1003.1c (threads).
Despite being underspecified in a couple of key areas such as signal-handling semantics and omitting BSD sockets, the original POSIX standards became the basis of all later Unix standardization work. They are still cited as an authority, albeit indirectly through references like POSIX Programmer's Guide [Lewine]. The de facto Unix API standard is still “POSIX plus sockets”, with later standards mainly adding features and specifying conformance in unusual edge cases more closely.
The next player on the scene was X/Open (later renamed the Open Group), a consortium of Unix vendors formed in 1984. Their X/Open Portability Guides (XPGs) initially developed in parallel with the POSIX drafts, then after 1990 the XPGs incorporated and extended POSIX. Unlike POSIX, which attempted to capture a safe subset of all Unixes, the XPGs were oriented more toward common practice at the leading edge; even XPG1 in 1985, spanning SVr2 and 4.2BSD, included sockets.
XPG2 in 1987 added a terminal-handling API that was essentially System V curses(3). XPG3 in 1990 merged in the X11 API. XPG4 in 1992 mandated full compliance with the 1989 ANSI C standard. XPG2, 3, and 4 were heavily concerned with support of internationalization and described an elaborate API for handling codesets and message catalogs.
In reading about Unix standards you might come across references to “Spec 1170” (from 1993), “Unix 95” (from 1995) and “Unix 98” (from 1998). These were certification marks based on the X/Open standards; they are now of historical interest only. But the work done on XPG4 turned into Spec 1170, which turned into the first version of the Single Unix Specification (SUS).
In 1993 seventy-five systems and software vendors including every major Unix company put a final end to the Unix wars when they declared backing for X/Open to develop a common definition of Unix. As part of the arrangement, X/Open acquired the rights to the Unix trademark. The merged standard became Single Unix Standard version 1. It was followed in 1997 by a version 2. In 1999 X/Open absorbed the POSIX activity.
In 2001, X/Open (now The Open Group) issued the Single Unix Standard version═3. All the threads of Unix API standardization were finally gathered into one bundle. This reflected facts on the ground; the different varieties of Unix had re-converged on a common API. And, at least among old-timers who remembered the turbulence of the 1980s, there was much rejoicing.
There was, unfortunately, an awkward detail — the old-school Unix vendors who had backed the effort were under severe pressure from the new school of open-source Unixes, and were in some cases in the process of abandoning (in favor of Linux) the proprietary Unixes for which they had gone to so much effort to secure conformance.
The conformance testing needed to verify Single Unix Specification conformance is an expensive proposition. It would need to be done on a per-distribution basis, but is well out of the reach of most distributors of open-source operating systems. In any case, Linux changes so fast that any given release of a distribution would probably be obsolete by the time it could get certified.
Standards like the Single Unix Specification have not entirely lost their relevance. They're still valuable guides for Unix implementers. But how The Open Group and other institutions of the old-school Unix standardization process will adapt to the rapid tempo of open-source releases (and to the low- or zero-budget operation of open-source development groups!) remains to be seen.
In the mid-1990s, the open-source community began standardization efforts of its own. These efforts built on the source-code-level compatibility secured by POSIX and its descendants. Linux, in particular, had been written from scratch in a way that depended on the availability of Unix API standards like POSIX.
In 1998 Oracle ported its market-leading database product to Linux, in a move that was rightly seen as a major breakthrough in Linux's mainstream acceptance. The engineer in charge of the port provided a definitive demonstration that API standards had done their job when he was asked by a reporter what technical challenges Oracle had had to surmount. The engineer's reply was “We typed ‘make’.”
The problem for the new-school Unixes, therefore, was not API compatibility at the source-code level. Everybody took for granted the ability to move source code between different Linux, BSD, and proprietary-Unix distributions without more than a trivial amount of porting labor. The new problem was not source compatibility but binary compatibility. For the ground under Unix had shifted in a subtle way as a consequence of the triumph of commodity PC hardware.
In the old days, each Unix had run on what was effectively its own hardware platform. There was enough variety in processor instruction sets and machine architectures that applications had to be ported at source level to move at all. On the other hand, there were a relatively few major Unix releases, each with relatively long service lifetimes. Application vendors like Oracle could afford the cost of building and shipping separate binary distributions for each of three or four hardware/software combinations, because they could amortize the low cost of source-code porting over large customer populations and a long enough product life cycle.
But then the minicomputer and workstation vendors were swamped by inexpensive 386-based supermicros, and open-source Unixes changed the rules. Vendors found they no longer had a stable platform to ship their binaries to.
The superficial problem, at first, was the large number of Unix distributors — but as the Linux distribution market consolidated, it became clear that the real issue was the rate of change over time. APIs were stable, but the expected locations of system administrative files, utility programs, and things like the prefix of the paths to user mailbox names and system log files kept changing.
The first standards effort to develop within the new-school Linux and BSD community itself (beginning in 1993) was the Filesystem Hierarchy Standard (FHS). This was incorporated into the Linux Standards Base (LSB), which also standardized an expected set of service libraries and helper applications. Both standards became activities of the Free Standards Group, which by 2001 developed a role similar to X/Open's position amidst the old-school Unix vendors.
 The original 1986 trial-use standard was called IEEE-IX. The name ‘POSIX’ was suggested by Richard Stallman. The introduction to POSIX.1 says: “It is expected to be pronounced pahz-icks as in positive, not poh-six, or other variations. The pronounciation has been published in an attempt to promulgate a standardized way of referring to a standard operating system interface”.
 One Linux distributor, Lasermoon in Great Britain, did achieve POSIX.1 FIPS 151-2 certification — and went out of business, because potential customers didn't care.