Date: Sat, 31 May 1997 12:42:33 -0800 (AKDT) From: Steve Howe <un_x@anchorage.net> To: Bruce Evans <bde@zeta.org.au> Cc: freebsd-hackers <hackers@FreeBSD.ORG> Subject: Re: Borland 16bit bcc vs cc/gcc (float) Message-ID: <Pine.BSF.3.95q.970531121927.257B-100000@aak.anchorage.net> In-Reply-To: <199705310930.TAA08679@godzilla.zeta.org.au>
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On Sat, 31 May 1997, Bruce Evans wrote:
> This code needs to use nonstandard long double math functions
> (powl() and fmodl()) to work for values > DBL_MAX. FreeBSD
> does not support these functions.
> This code also requires long double precision to actually work.
> Long double precision is not the default in FreeBSD. You can
> set it using fpsetprec().
i don't have a man page for fpsetprec() (2.2.1).
how do i find out about it? and if i set "long double"
precision, will pow/fmod, etc, work as powl/fmodl ?
> Printing of long double values with full precision is not
> supported in FreeBSD. This is why the same value is printed
> for `ld-9' as for `ld'.
are there plans to? why shouldn't it?
> >I'm not much surprised that the use of non-standard components (long
> >double) produces unexpected results. You multiply a long double with
> >a double (result of pow()), so who tells you whether the compiler does
> >it by first extending the result of pow() to long double format (thus
> >`inventing' missing precision digits), or by first truncating the long
> >double (although i wouldn't expect this)?
>
> The ANSI C standard :-). However, the standard doesn't specify that
> long double precision is strictly more precise than double precsision
> or the amount of precision of double precsision.
right. and sizeof() tells me there is a difference.
float = 4 bytes
double = 8 bytes
long double = 12 bytes
it seems a little silly to have long doubles, and
not to be able to make good use of them! :)
you can't use them with math functions ...
you can't print them ...
/*****************************************************************************/
unsigned char *ftous (unsigned char *s, long double val, unsigned char base) {
/* float to unsigned string */
/* if base>10 adjust */
unsigned char *p=s; /* to alpha numeral */
if ( val < 0 ) val = -val; /* val+=0.49999999; <- needed for Borland C */
^^^^^^^^^^^^^^^
Shouldn't be necessary. The algorithm requires `val' to have no fractional
part.
do { *p++ = itoan(fmod(val, base)); printf("- %s %Lf\n", s, val);
val/=base;
^^^^^^^^^
> Bug. The fractional part needs to be subtracted somewhere, perhaps
> using `val = floor(val / base);' here.
actually, speed is too important to call floor() each time.
i don't see why i should have to, i find precision always
rounds down (in fact, that's why i do the .49999999 thing).
i've been using the code for years, and it works fine with
64 bit string-int conversions, (under DOS ...).
} while ( val >= 1 );
*p = 0; return( strrvs(s, 0) );}
/*****************************************************************************/
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