Re: printf and long double

[Chrysostomos Nanakos <nanakos@wired-net.gr>]

  On 4/10/2010 4:33 πμ, Glynn Clements wrote:
> Nanakos Chrysostomos wrote:
>
>> I have the above C code snippet. I am trying to get a resonable
>> result in a fixed form with printf for a long double value but with
>> no luck. What am I doing wrong?
>> 	long double c = powl(10.0L,30.0L);
>>
>> 	printf("%llf %lle\n",c,c);
> On x86, long double is 80 bits, which is roughly 24 decimal digits.


I wish I could get 24 decimal digits precision with the current long 
double (80-bit). I can't get more than 15 digits even in a simple division.
I think that the extended-precision can't give more than 19 decimal 
digits of precision (log(10,2^64)=19.2). The quadruple precision can 
give at most 34 decimal digits (log(10,2^113)=34).

Please consider the example below:
--------------------------------------------------
long double a = 1.0L/7.0L;
printf("%Lf\n",a);
--------------------------------------------------

Can't get more than 15 digits precision....-:(
Is this a compiler problem or libc's problem? In x86_64 machines the 
long double values supposed to give at least 106-bit precision even when 
implemented in software. Is gcc compliant with the quadruple notation?




> The -m128bit-long-double flag only changes the alignment, not the
> accuracy of the calculations.
>
> The only reason why you're getting different results for %f and %e is
> that the default precision of 6 refers to 6 decimal places for %f but
> to 6 significant digits for %e.
>

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