X-Sybari-Space: 00000000 00000000 00000000 00000000
From: Martin Stromberg <Martin DOT Stromberg AT epl DOT ericsson DOT se>
Message-Id: <200304061424.QAA21600@lws256.lu.erisoft.se>
Subject: Re: Support for nan(0x[0-9a-f]*) in strto*, *printf
To: djgpp-workers AT delorie DOT com
Date: Sun, 6 Apr 2003 16:24:01 +0200 (MET DST)
In-Reply-To: <3E8F48A3.2B0723E6@phekda.freeserve.co.uk> from "Richard Dawe" at Apr 05, 2003 10:20:35 PM
X-Mailer: ELM [version 2.5 PL3]
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Reply-To: djgpp-workers AT delorie DOT com
Errors-To: nobody AT delorie DOT com
X-Mailing-List: djgpp-workers AT delorie DOT com
X-Unsubscribes-To: listserv AT delorie DOT com
Precedence: bulk

HELO

Richard said:
> But I still don't think that this identity will hold: strtof(sprintf(F)) == F.

It can't (if F is a NaN) as any float is converted to a double before
being pushed on the stack by the calling routine (which results in
some hardware specific mangling).

> Consider the test program t-nan4.c:
> 
> ---Start t-nan4.c---
> #include <stdio.h>
> #include <stdlib.h>
> #include <libc/ieee.h>
> 
> int
> main (void)
> {
>   float_t  ft;
>   double_t dt;
>   float    f;
>   double   d;
> 
>   ft.mantissa = 1U;
>   ft.exponent = 0x7fU;
>   ft.sign     = 0U;
> 
>   f  = *(float *) &ft;
>   d  = f;
>   dt = *(double_t *) &d;
> 
>   printf("float(0x%x, 0x%x, 0x%x) -> double(0x%x:0x%x, 0x%x, 0x%x)\n",
>          ft.mantissa, ft.exponent, ft.sign,
>          dt.mantissah, dt.mantissal, dt.exponent, dt.sign);
> 
>   return(EXIT_SUCCESS);
> }
> ---End t-nan4.c---
> 
> This produces this output for me:
> 
> float(0x1, 0x7f, 0x0) -> double(0x0:0x20000000, 0x3ff, 0x0)
> 
> The float nan(0x1) has been converted into a double version. This would get
> formatted by sprintf as "nan(0x20000000)". But then strtof will convert it to
> an all-ones NaN, which is not the same as we started with.

Right. But we can't stop float-double promotion to *printf().

Interesting that it stayed a SNaN (if I see the bits right). (This
doesn't match my tests.)

> I'm starting to think that printf should not output nan(0x...), if
> strtox(sprintf(X)) == X identities don't all hold. But it seems wrong to have
> asymmetric support in printf & strto*.

For the float case it almost meaningless, but for the other formats
where (hopefully) the bit patterns remain untouched by the hardware
it's still useful. (Useful for somebody trying to understand where the
NaN came from supposedly.)


Right,

						MartinS