Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!seismo!sundc!sun!gorodish!guy
From: guy%gorodish@Sun.COM (Guy Harris)
Newsgroups: comp.lang.c
Subject: Re: Comparing chars to constants
Message-ID: <31174@sun.uucp>
Date: Sun, 18-Oct-87 21:14:35 EDT
Article-I.D.: sun.31174
Posted: Sun Oct 18 21:14:35 1987
Date-Received: Mon, 19-Oct-87 02:08:54 EDT
References: <4663@elroy.Jpl.Nasa.Gov>
Sender: news@sun.uucp
Lines: 46

> On a Sun, it generates the code that is commonly expected, but on a Masscomp
> is gives a warning "Comparison always false" and optimizes away the
> if statement.

Umm, on a Sun running 3.4, what it does is say

"foo.c", line 4: warning: constant 128 is out of range of char comparison 
"foo.c", line 4: warning: value coerced to -128 for bug compatibility
"foo.c", line 4: warning: do not expect this coercion in release 4.0

and then generate the code that is commonly, but incorrectly, expected.  When
it says "do not expect this coercion", it MEANS it; this WILL go away, and the
behavior WILL be the same as it is on the Masscomp compiler.  (You will get a
warning from the compiler when it does this.)

> In reading K&R I was unsure of what is to happen, under 7.6 and 7.7 it
> claims that the "usual arithmetic conversions are performed".  From 6.6
> both sides are converted into ints, 0x80 stays the same but if p is equal
> to 0x80 it may always be sign extended on some machines depending on its
> character sign convientions.  Does this imply that the above comparision
> is truly non-portable?

You bet!  It will work just fine on a 3B[2-20], since "char" is unsigned on
those machines' C implementations (which means you'd better not do something
such as:

	char c;

	if ((c = getchar()) == EOF)

since on those machines it won't do what you might expect), but it won't work
on machines on whose C implementations "char" is signed, unless they don't
correctly implement the "usual arithmetic conversions" (that's why it says "bug
compatibility"; it's providing bug-for-bug compatibility with compilers in
earlier 3.x releases).

> Both machines give the same result if it is rewritten as:
> 
> 	if (p == (char)0x80)

Which is what you should write here.  Alternatively, you can write

	if (p == '\200')
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy@sun.com