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From: ron@topaz.rutgers.edu (Ron Natalie)
Newsgroups: comp.lang.c
Subject: Re: Comparing chars to constants
Message-ID: <15706@topaz.rutgers.edu>
Date: Mon, 19-Oct-87 15:34:27 EDT
Article-I.D.: topaz.15706
Posted: Mon Oct 19 15:34:27 1987
Date-Received: Tue, 20-Oct-87 06:50:04 EDT
References: <4663@elroy.Jpl.Nasa.Gov>
Organization: Rutgers Univ., New Brunswick, N.J.
Lines: 13

Correct.  The character comparison "c == 0x80" is non portable since
chars may or may not be unsigned innately.  On machines where char's
are signed, 0x80 would likely become -0x80 (e.g.  on 32 bit ints,
0xFFFFFF80) when converted to int (usual arithmatic conversions).  The
compiler in question is being smart in noting that no conversion of a
char to an int has values outside the range -0x80 to +0x7F.

Using the (char) cast on the constant causes the conversion of char
to int to occur on the constant yielding the sign extension.  This
doesn't happen otherwise because 0x80 is already of type integer so
no conversion is called for.

-Ron