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[Bug libc/4943] Inconsistent rounding behaviour for sprintf and IEEE doubles
- From: "vincent+libc at vinc17 dot org" <sourceware-bugzilla at sourceware dot org>
- To: glibc-bugs at sources dot redhat dot com
- Date: 21 Sep 2007 23:43:10 -0000
- Subject: [Bug libc/4943] Inconsistent rounding behaviour for sprintf and IEEE doubles
- References: <20070820142205.4943.paul@inet.co.za>
- Reply-to: sourceware-bugzilla at sourceware dot org
------- Additional Comments From vincent+libc at vinc17 dot org 2007-09-21 23:43 -------
(In reply to comment #12)
> > Now, C99 describes the behavior of the printf family in 7.19.6.1#13. For the
> > examples given in this bug report, C99 says that the result should be
> > correctly rounded. C99 does not say which result should be returned in case
> > of a tie, but in practice, it relies on the definition given in IEEE-754.
>
> This is interesting. I wonder whether we could dig up a definition for the
> phrase "correctly rounded".
It is defined by C99 in the section "Terms and definitions". Perhaps you should
take a bit time to read the standards.
> You already know my opinion on what is meant by the
> phrase. Again, are we not assuming too much about the scope of the IEEE-754?
> From what I understand, that document is describing how to round the result of
> calculations, not how to round decimal numbers arithmetically.
IEEE-754 covers the arithmetic operations, the square root, binary format
conversions, binary FP <-> integer, *and* binary <-> decimal conversions (I've
already mentioned Section 5.6 twice, have you read it?). All of these operations
basically correspond to a math function over the real numbers, then a rounding
operation.
> As you say, there are many more numbers that are inexactly stored than those
> which are exact. It took a while for us to find the fact that our reports
> were sometimes generating the wrong numbers. It works most of the time. Our
> problem is that those reports are being used by investment houses to make
> decisions sometimes worth billions, and we just cannot have those decisions
> made against wrong numbers.
Before reporting inexistent bugs, I think you should really learn more about
standards and computer arithmetic, probably starting with:
http://www2.hursley.ibm.com/decimal/
--
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