Mail Archives: cygwin/2007/08/29/13:13:15
On 29 August 2007 18:07, Brian Dessent wrote:
> Dmitry Karasik wrote:
>
>> #include <math.h>
>> #include <stdio.h>
>> int main( int argc, char ** argv)
>> {
>> double g = (double) 3.1415926535897900074;
>> printf("sin(%.10g)=%.10g\n", g, sin(g));
>> }
>>
>> output is :
>>
>> sin(3.141592654)=3.231089149e-15
>>
>> whereas all other sin() implementation I could find ( freebsd, linux,
>> msvc) report this:
>>
>> sin(3.141592654)=3.231085104e-015
>>
>> the difference is in 7th digit, and is significant for double precision.
>
> This appears to be due to an approximation in __kernel_sin in which if
>> x| < 2**-27 the approximation sin(x) ~ x is used.
Since when was pi less than 2^-27?
> In any case this is more of a newlib issue than a Cygwin issue as we are
> just a consumer of this code, so followups should go to the newlib list.
I'd just like to point out that I got the exact same result as cygwin
supplies on a recent linux box:
On cygwin, I get:
/tmp $ ./sin.exe
sin(3.141592654)=3.231089149e-15
... whereas on linux (x86_64), I get ...
[dk AT ucho gxlinux-ent]$ ./sin
sin(3.141592654)=3.231089149e-15
... and with mingw:
$ ./sin.exe
sin(3.141592654)=3.231085104e-015
I think it may be an artefact of FP precision and/or rounding mode, but I'd
need to do more experiments to make sure. OTOH it could be an artifact of the
printf %g specfier. Like I said, more experimentation is needed!
cheers,
DaveK
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