Bug in isatty (all systems!)

[Steven Bellovin]

> For those keepers of the true faith who savaged my coding practices,
> this is exactly my point.  I designed a large software system around
> the assumption that errno was always valid and not set as a side-effect.
> It took me 6 months (admittedly, I wasn't looking hard) to figure out
> why I was always getting "not a typewriter" (because my open routine 
> was setting a struct bit to indicate tty/non-tty for every file).
> 
> My reason for posting this was to call attention to an error in the
> documentation.  All the manuals I've seen are in error, because this
> is a known, established, important "SPECIAL CONSIDERATION".

My 4.2bsd manual describes this in the INTRO(2) section, right before the
list of errno values.  Just where would you prefer that it be mentioned?
RTFM!