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From: ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe)
Newsgroups: comp.lang.c
Subject: Re: Determining machine characteristics (was: IsUnsigned() function?)
Message-ID: <3473@goanna.cs.rmit.oz.au>
Date: 26 Jul 90 01:46:25 GMT
References: <1990Jul16.214155.5087@Neon.Stanford.EDU> <2936@mtung.ATT.COM> <17148@haddock.ima.isc.com>
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 20

In article <17148@haddock.ima.isc.com>, karl@haddock.ima.isc.com (Karl Heuer)
provides some nice macros
> 	#define is_twos_complement() ((-2 | -3) == -1)
> 	#define is_ones_complement() ((-1 & -2) == -3)
> 	#define is_sign_magnitude()  ((-1 | -2) == -3)
and says
> I'd also like to be able to do `is_big_endian()' and `is_little_endian()', but
> I think these require run-time code.

It's clear that an arithmetic expression which appears in _code_ must be
evaluated as if evaluated by the target machine, but need arithmetic
expressions in #ifs be evaluated the same way?  Is it possible for
	#if is_ones_complement()
to succeed and
	if (is_ones_complement())
to fail?

-- 
Science is all about asking the right questions.  | ok@goanna.cs.rmit.oz.au
I'm afraid you just asked one of the wrong ones.  | (quote from Playfair)