Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!usc!snorkelwacker!spdcc!ima!haddock!karl
From: karl@haddock.ima.isc.com (Karl Heuer)
Newsgroups: comp.lang.c
Subject: Re: isascii (was: Another pitfall.  Signed chars and ctype.h)
Message-ID: <15962@haddock.ima.isc.com>
Date: 15 Feb 90 22:40:34 GMT
References: <Cuunry2@cs.psu.edu> <1990Feb12.043324.5259@sq.sq.com> <2448@ditsyda.oz>
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Organization: Interactive Systems, Cambridge, MA 02138-5302
Lines: 13

In article <2448@ditsyda.oz> evans@ditsyda.oz (Bruce Evans) writes:
>Why doesn't ANSI C guarantee isdigit() (etc.) on *all* characters?

(The above seems to mean, "both signed and unsigned characters".)

Assume a character set in which (char)-1 is printable, e.g. ISO Latin 1.  Your
proposal would require that isprint((int)(signed char)(-1)) test as true.  But
isprint(EOF) is required to test false.  Thus, this would require that EOF be
defined as a value other than -1.  This is permitted by the Standard (I'm not
entirely sure why), but it would be A Bad Thing to create conditions that
*require* it.

Karl W. Z. Heuer (karl@ima.ima.isc.com or harvard!ima!karl), The Walking Lint