is it really necessary for character values to be positive?

[dave at murphy.UUCP]

Summary: does EOF have to be -1?

In article <289 at haddock.UUCP>, karl at haddock.ISC.COM.UUCP (Karl Heuer) types
(in response to an earlier article that I wrote):

>Suppose I am using such a system, and one of the characters -- call it '@' --
>has a negative value.  The following program will not work:
>    main() { int c; ... c = getchar(); ... if (c == '@') ... }
>Note that getchar() returns an UNSIGNED char on success; this is to guarantee
>that none of them compare equal to EOF.  Thus, any printing character that I
>want to enclose in single quotes had better be positive, or it becomes VERY
>awkward to use.

Thanks for pointing this out, but I don't see where it should cause a major
problem.  Assuming that the character set in use doesn't take up the entire
range of int values, all that is necessary is to pick a value for EOF that
doesn't correspond to any character value.  (Newcomers: keep in mind that
the return value of getchar and getc is defined as being an int, not a
char, even though it is often treated like a char.)  This way, getchar can
return a possibly negative value, and EOF won't collide with any legit
character value.

Would defining EOF to be something other than -1 cause a problem?  I don't
think so.  K&R says, on p. 144: "The standard library defines the symbolic
constant EOF to be -1 (with a #define in the file 'stdio.h'), but tests
should be written in terms of EOF, not -1, so as to be independent of the
specific value."  I don't this is a situation like with NULL where the
actual value has a special meaning in the language definition, so I don't
see why it couldn't be changed.  People who are testing for -1 or for a
negative value instead of using EOF deserve whatever they get.

If anyone knows of any reason why the value of EOF can't be implementation-
specific, I'd like to hear about it.
---
"I used to be able to sing the blues, but now I have too much money."
-- Bruce Dickinson

Dave Cornutt, Gould Computer Systems, Ft. Lauderdale, FL
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