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From: decot@hpisod2.UUCP
Newsgroups: comp.lang.c
Subject: fabs(x) vs. (x) < 0 ? -(x) : (x)
Message-ID: <2550005@hpisod2.HP>
Date: Wed, 28-Jan-87 16:33:01 EST
Article-I.D.: hpisod2.2550005
Posted: Wed Jan 28 16:33:01 1987
Date-Received: Sat, 31-Jan-87 05:53:08 EST
References: <4943@mimsy.UUCP>
Lines: 21

> In article <4477@ut-ngp.UUCP> jjr@ngp.UUCP (Jeff Rodriguez) writes:
> >Why isn't fabs() implemented as a macro [ (X) < 0 ? -(X) : (X) ]?
> 
> I think it's primarily because of things like "y = fabs(sin(x))", which would
> be inefficent, and "y = fabs(*px++)", which would be wrong.
> 
> Generally, the standard library functions are not implemented as macros unless
> they evaluate each argument exactly once.  (There are exceptions, though.)
> 
> Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint

You could implement fabs() as a macro as follows:

    #define fabs(X)     ((_fabs = (X)), (_fabs < 0? -_fabs : _fabs))

if _fabs were declared as a float in the math library.

Of course, then you would have to include <math.h> to use fabs(), but you
probably already do anyway.

Dave