[Cfp-interest] strtodN subject sequence with minus sign

[Jim Thomas]

If no one objects, say by Friday, I'll add "(before rounding)".

-Jim

On Aug 28, 2012, at 10:11 AM, Rajan Bhakta wrote:

> Re the last point, I prefer adding the words as well for clarity. 
> 
> Regards,
> 
> Rajan Bhakta
> z/OS XL C/C++ Compiler Technical Architect
> ISO C Standards Representative for Canada
> C Compiler Development
> Contact: rbhakta at ca.ibm.com, Rajan Bhakta/Toronto/IBM
> Telephone: (905) 413-3995 
> 
> 
> From:	Jim Thomas <jaswthomas at sbcglobal.net>
> To:	"Fred J. Tydeman" <tydeman at tybor.com>
> Cc:	CFP <cfp-interest at ucbtest.org>
> Date:	08/27/2012 07:16 PM
> Subject:	Re: [Cfp-interest] strtodN subject sequence with minus sign
> Sent by:	cfp-interest-bounces at oakapple.net
> 
> 
> 
> 
> 
> On Aug 26, 2012, at 9:36 PM, Fred J. Tydeman wrote:
> 
> > On Tue, 21 Aug 2012 11:36:20 -0700 Jim Thomas wrote:
> >> 
> >>>> 
> >>>> 11.6 strtod32, strtod64, and strtod128 functions <stdlib.h>
> >>>> PDF page 31, item [4]:
> >>>> Add at the end of: 
> >>>> "If the subject sequence begins with a minus sign, the sequence is
> >>>>  interpreted as negated."
> >>>> (before rounding).
> >>> 
> >>> It says the sequence is interpreted as negated, not that the result of 
> >> rounding the unnegated sequence is negated. So no change needed
> > 
> > C11 has a footnote on negated.  That footnote is:
> > 
> > 292) It is unspecified whether a minus-signed sequence is converted 
> > to a negative number directly or by negating the value resulting from 
> > converting the corresponding unsigned sequence (see F.5); the two
> > methods may yield different results if rounding is toward positive or 
> > negative infinity. In either case, the functions honor the sign of 
> > zero if floating-point arithmetic supports signed zeros.
> > 
> > That makes it clear the "interpreted as negated" phrase has two meanings.
> 
> I believe the words give an implementation license to mis-round in this particular way. This doesn't really seem necessary since C11 without Annex F doesn't require correct rounding for strtod functions anyway. See below.
> 
> > 
> >> In the Aug teleconference, David asked would "interpreted as negative" be 
> >> better? I think it might be a little clearer. However, the current words ( 
> >> negated) match what's in C11 for the generic strtod family. I'm inclined 
> >> against making strtodN differ from strtod on this point. Does anyone think a 
> >> C11 defect report is in order?
> > 
> > C11 F.5 has:
> > 
> > Functions such as strtod that convert character sequences to floating 
> > types honor the rounding direction. Hence, if the rounding direction 
> > might be upward or downward, the implementation cannot convert a 
> > minus-signed sequence by negating the converted unsigned sequence.
> 
> This says that honoring the directed rounding modes requires negating before rounding. And we require the decimal strto functions to round correctly.
> 
> > 
> > Since strtod32/64/128 are for IEEE-754 DFP types, F.5 is in effect and
> > I believe we could differ some from strtof/d/ld.
> > 
> > I see no defect in C11.
> > 
> > I prefer adding "(before rounding)".
> 
> I still don't think it's strictly necessary, but wouldn't object to adding it.
> 
> -Jim
> 
> > 
> > ---
> > Fred J. Tydeman        Tydeman Consulting
> > tydeman at tybor.com      Testing, numerics, programming
> > +1 (775) 287-5904      Vice-chair of PL22.11 (ANSI "C")
> > Sample C99+FPCE tests: http://www.tybor.com
> > Savers sleep well, investors eat well, spenders work forever.
> > 
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