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From: mycroft@hal.gnu.ai.mit.edu (Charles Hannum)
Newsgroups: comp.os.386bsd.questions
Subject: Re: gcc compile error: enquire.c
Date: 5 May 1993 05:37:56 -0400
Organization: dis
Lines: 53
Message-ID: <1s81tk$ei2@hal.gnu.ai.mit.edu>
References: <1993May4.111203.28674@ucthpx.uct.ac.za> <hastyC6IMpp.639@netcom.com>
NNTP-Posting-Host: hal.ai.mit.edu
How many times do I have to say this? Can someone please put it in a
FAQ list?
In article <hastyC6IMpp.639@netcom.com> hasty@netcom.com (Amancio Hasty
Jr) writes:
>
> The problem is inaccurate rounding (in atof()) more than the lack of
> digits.
No. The problem is entirely in printf() (really in cvt()), NOT in
atof(). I have inspected the output of atof() bit by bit, and it is
well within IEEE specification.
>> #define DBL_MAX 1.7976931348623157E+308
>
> The correct rounding of the last three digits is 159, not 157.
This is wrong. Please read the IEEE specification (again?). The
digits `157' are the `best' approximation.
> One value that works is
>
> #define DBL_MAX 1.7976931348623147E+308
> ^ was 5
This is a kluge, but it does mostly work.
> gcc prints 58 for the last two digits and gas converts the number
> correctly. Printing the number then puts 68 in the last 2 digits...
Doesn't it strike you as a little odd that printf() generates a
representation which is not even in the range of doubles? If you'd
just look at the bloody code one of these days, you'd see why.
details:
atof("1.7976931348623157e+308") returns 0x7fefffffffffffff, which is
the maximum double value and is correct. However, printf() of the
previous yields `1.7976931348623168e+308', which isn't even within the
floating point range. It is clearly printf() that is broken, and a
quick inspection of the code is enough to determine that it uses a
pessimal algorithm.
I've tested atof() with many other values, and it has never been off
by more than is allowed by IEEE 754 (though it is not optimal).
--
\ / Charles Hannum, mycroft@ai.mit.edu
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