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From: david@elroy.Jpl.Nasa.Gov (David Robinson)
Newsgroups: comp.lang.c
Subject: Comparing chars to constants
Message-ID: <4663@elroy.Jpl.Nasa.Gov>
Date: Sun, 18-Oct-87 02:09:10 EDT
Article-I.D.: elroy.4663
Posted: Sun Oct 18 02:09:10 1987
Date-Received: Sun, 18-Oct-87 23:49:48 EDT
Organization: Image Analysis Systems Grp, JPL
Lines: 31

I have run into a difference between the SunOS 3.4 C compiler
and the Masscomp C compiler for the following segment
of code:

foo(p)
	char p;
{
	if (p == 0x80)
		return (1);
	return (0);
}


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.  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? Both machines
give the same result if it is rewritten as:

	if (p == (char)0x80)


-- 
	David Robinson		elroy!david@csvax.caltech.edu     ARPA
				david@elroy.jpl.nasa.gov (new)
				seismo!cit-vax!elroy!david UUCP
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