UNIX Methods and Concepts: Putting the Genie Back in the Bottle
~ by Dr. Peter H. Salus and Warren K. Toomey, The UNIX Heritage Society
Recently, The SCO Group has asserted that IBM negligently leaked
the methods and concepts in UNIX. What The SCO Group fails to realize
is that, from day one, the methods and concepts in UNIX were out
in the open. And, as AT&T found out when UNIX was commercialized,
staunching the leakage of UNIX methods and concepts was like putting
the proverbial genie back into the bottle.
Throughout the 1970-1980 decade, AT&T made no secret of the UNIX
source code. After the system became "public," following the SOSP
paper in October 1973 (and its publication in the July 1974
Communications of the ACM), there were many requests from outside
AT&T for the new OS. The SOSP paper was itself very revealing of
the UNIX methods and concepts, and AT&T at the time (following a
consent decree) was obliged to confine its business to "telegraphy
and telephony" and not to sell software products, but to license
UNIX on a very open basis.
What the requesters received, from Ken Thompson and (later) from
Irma Biren, was a 10" tape or a disk pack with the bits of 3rd
Edition UNIX, or 4th, or 5th, or 6th. All the bits, not what we'd
call a binary version, but the source. And many recipients just
printed it out. The disk pack frequently came with a handwritten
note: "Here's your RK05 , Love Ken"; on the tape that Lou Katz
received, the note read: "Here's the tape, if it craps out, I'll
Further, from the beginning, Thompson would talk about the system
and its code: for example, at the "UNIX Users' Meeting" at Columbia
University on May 15, 1974. There were no barriers, no bars, no
hurdles. And every site had full source.
It is important to note that UNIX was never a static system, and
the userbase found it immensely useful to have the source code, so
that the system could be fixed, and enhanced to suit the users at
each individual site. Examples include the AUSAM system, developed
at UNSW in Australia, and the early BSDs. The changes made by the
users often found their way back into the main UNIX development
tree. Significant portions of AUSAM code were still visible in
System V in the late 1980s.
With a malleable OS in the hands of the users, including the source
code, the users found the urge to exchange home-grown bug fixes and
improvements to the system. Beginning in mid-1976, the UNIX Users'
Group mailed out tapes of the "software exchange." The first was
announced in the May-June "UNIX NEWS," the second followed in
November, the third in May-June 1977. AT&T's attitude forced the
users to exchange knowledge with one another. Mel Ferentz (then
publisher of "UNIX NEWS") was driven to initiate the exchange, and
Mike O'Brien (then a graduate student) to implement it.
Another avenue for the exchange of home-grown methods and concepts
was at the annual UNIX User Group (later USENIX) conferences, where
everybody bought two tapes, one full of new programs, device drivers
and system patches, and the other tape empty (over 150 attendees
at the May 1977 meeting in Urbana, IL).
After the creation of USENET in late 1979, the net.v6bugs and
net.v7bugs newsgroups were formed so that users could exchange bug
fixes on-line in the form of patches. These newsgroups were quite
active, thus "leaking" many lines of original UNIX code.
Even the UNIX developers aided and abetted the free exchange of
methods and concepts: Ken Thompson took a sabbatical at University
of California, Berkeley where he introduced UNIX and it methods and
concepts to the staff and students.
Then there is the case of the "50 bugs" tape. By the late 1970s,
AT&T had started to impose more restrictive conditions in its UNIX
licenses, stifling the exchange of UNIX code between licensees. The
licenses also did not include the ability to obtain bug fixes from
AT&T. The researchers at Bell Labs had found and fixed a significant
number of bugs in UNIX, and Ken Thompson had tried to get the patches
out, but the lawyers kept stalling him. Eventually, a tape with the
patches was "found" by Lou Katz and Reidar Bornholdt on Mountain
Avenue (the road leading to the Labs). Ken also "inadvertently"
left an image of the tape at the University of Illinois, when
visiting on his way to Berkeley, and another at Berkeley.
While the ability to exchange code was being limited, the same was
not true for the distribution of methods and concepts. The distribution
of John Lions' commentary on 6th Edition UNIX was stopped as it
contained source code. It was followed by Maurice J. Bach's book
on System V, which used pseudo-code to explain the system's internals.
Many other books followed, including those by McKusick et al,
Goodheart & Cox, and Vahalia. All of these outline the methods and
concepts in UNIX in exquisite detail.
All of this begs the question, was there ever anything in UNIX worth
protecting, and how should it have been protected? UNIX, of course,
is one of the most influential and useful operating systems in
computing history. But, was it the source code that was critical,
or the algorithms used in the system, or its methods and concepts,
or something else?
As AT&T began to productize UNIX in the late 1970s, it became
critical to protect the source code. However, AT&T dithered on how
best to do this. At this time, the ability to copyright software
was still uncertain, and the company intially chose a license plus
trade secrets approach, and did not revisit the copyright approach
until the 1980s. The result of this was the lack of copyright notices
But was the actual source code really important? Certainly, it gave
the users the ability to tailor their systems, to fix bugs, and to
extend UNIX in directions that the original designers had not chosen
to go. But the early UNIX source code didn't contain any significantly
important algorithms. The preface to the Lions' commentary indicates
that the early systems used simple algorithms (linear search etc).
What about the essential "methods and concepts" in UNIX: a hierachical
filesystem, i-nodes, multitasking, protected process address spaces,
a command-line shell etc.? None of these were new in the realm of
In fact, the useful methods and concepts in UNIX were at a much
higher level, that of the UNIX toolbox mindset: lots of well-designed
tools which perform individual actions, combined with a framework
which allows them to be connected together. And the toolbox notions
were there (according to McIlroy, Thompson and Kernighan) by 1972.
But even at this level, AT&T not only failed to protect this, but
encouraged the adoption of this mindset, e.g. with the Software
Tools book by Kernighan and Plauger (1976) and the first UNIX issue
of the Bell Systems Technical Journal in 1978.
In 1983, we had Bourne's book "The UNIX System" (with the copyright
held by "Bell Laboratories"!) -- not a good way to protect things.
Even after the advent of System V, neither AT&T nor USL attempted
to veil methods and concepts. Goodheart and Cox in The Magic Garden
Explained (1994) give full details where SVR4 is concerned, calling
it "an open systems design."
Finally, in 1995, Mike Gancarz of DEC, gave the world "The UNIX
Philosophy", showing us how "the UNIX philosophy is an approach to
developing operating systems and software that constantly looks to
the future." There is very little there that isn't in the first
three articles in the BSTJ in 1978.
In summary, what made UNIX so good, and was it protectable? The
jewel in the UNIX crown is not the source code, not the algorithms,
not the low-level methods and concepts. It is the basic design of
UNIX, its inherent philosophy and mindset, and the ability for users
to modify the system and swap changes with other users. While the
latter could be limited to some extent via copyrights and licenses,
the overall design and the inherent mindset was public from the
very beginning, and could never be protected.
To end, an addendum on the ELF magic number issue. There is an
amazingly large number of executable formats that use the magic
numbers from PDP-11 a.out files: 0407, 0410, 0413. This goes to
show that a) a magic number that has no inherent meaning is
unprotectable and b) how well the PDP-11 a.out magic numbers infected
the binaries of other platforms. Remember, they are all PDP-11
branch instructions. The concept has been with us for over 30 years.