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Date: Sat, 29 Nov 2003 08:31:05 -0800
From: Steve Kargl <sgk@troutmask.apl.washington.edu>
To: freebsd-standards@FreeBSD.ORG
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Subject: Re: Implementing C99's roundf(), round(), and roundl()
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On Sat, Nov 29, 2003 at 12:09:11AM -0800, David Schultz wrote:
> On Fri, Nov 28, 2003, Steve Kargl wrote:
> > Can the math functions round[fl]() be implemented in
> > terms of other math(3) functions and still conform to the 
> > C99 and POSIX standard?  For example,
> > 
> > #include <math.h>
> > 
> > float roundf(float x) {
> > 	float t;
> > 	if (x >= 0.0) {
> > 		t = ceilf(x);
> > 		if ((t - x) > 0.5) t -= 1.0;
> > 		return t;
> > 	} else {
> > 		t = ceilf(-x);
> > 		if ((t + x) > 0.5) t -= 1.0;
> > 		return -t;
> > 	}
> > }
> 
> This looks correct to me at first glance, modulo possible problems
> with overflow.  It's valuable to have simple MI implementations of
> these functions to avoid hampering efforts to port FreeBSD to new
> architectures.  Faster MD versions can always be added later.  (I
> noticed the other day that Intel has actually released an
> optimized IA64 libm, which we should consider importing.)

I don't undrestand your overflow comment.  ceil[f]() can return Inf
and nan, but in those cases round[f]() should also return Inf and nan.
The two operations, (t-x) and (t+x), should yield a value in the
range [0,1).  I'll submit a PR with a man page.

As a side comment, we need to start coding the missing C99 math(3)
functions because GCC is moving to using these in their CVS
development trees.

-- 
Steve