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2013.12.11                                                 Navigation
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The Birth of Standard Error                                  dds blog
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Earlier today Stephen Johnson, in a mailing list run         comments
by the The Unix Heritage Society, described the birth           << The
of the standard error concept: the idea that a           Frictionless
program's error output is sent on a channel different     Development
from that of its normal output. Over the past forty       Environment
years, all major operating systems and language             Scorecard
libraries have embraced this concept.                       >> Bespoke
                                                      Infrastructures
C/A/T phototypesetter The story starts with the             Tagged as
Graphic Systems' C/A/T phototypesetter used by the               Unix
Bell Labs in the 1970s. This, washing-machine sized,          History
equipment would typeset documents by flashing a          Debug Like a
strobe to expose character glyphs that were                    Master
pre-installed on a rotating drum. The drum supported    Book cover of
four fonts (Times Roman, italic, bold, and Symbol); a       Effective
magnifying glass was used to change the font size.          Debugging
The text input normally came from a paper tape, and      Compute with
the output was rendered onto film, which then                   Style
required a cumbersome, dirty, and smelly development    Book cover of
process. Brian Walden relates that "the chemical      The Elements of
baths were nasty smelling and it gummed up the        Computing Style
rollers. You'd needed to regularly take the developer   Wikipedialize
roller and gear guts into the janitor's closet and      Add Wikipedia
scrub it with a toothbrush in the slop sink under               links
running water."

As one might expect, Bell Labs didn't use the paper
tape input. According to Doug McIlroy, "as soon as
the machine arrived, Joe Ossanna [the author of
troff] bypassed the tape reader so the C/A/T could be
driven directly from the PDP-11. The manufacturer was
astonished." The connection was one-way only, so the
corresponding device had write-only permissions, and
there was no way to obtain status feedback from the
phototypesetter. Doug McIlroy also remembers that,
amusingly, "the first technical paper that came off
the C/A/T drew a query from the journal editor, who'd
never seen a phototypeset manuscript before: had it
been published elsewhere?"

This ardous but cutting-edge phototypesetting process
set the stage for the invention of the standard error
concept. Stephen Johnson describes the incident as
follows.

    "One of the most amusing and unexpected
    consequences of phototypesetting was the Unix
    standard error file (!). After phototypesetting,
    you had to take a long wide strip of paper and
    feed it carefully into a smelly, icky machine
    which eventually (several minutes later) spat out
    the paper with the printing visible."

    "One afternoon several of us had the same
    experience -- typesetting something, feeding the
    paper through the developer, only to find a
    single, beautifully typeset line: "cannot open
    file foobar" The grumbles were loud enough and in
    the presence of the right people, and a couple of
    days later the standard error file was born..."

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