Re: [gentoo-science] question about signbit

[Ertugrul Soeylemez <never@drwxr-xr-x.org>]

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Darren Dale <dd55@cornell.edu> (Sat, 22 Oct 2005 14:16:02 -0400):
> I am trying to track down the source of a bug in the SciPy development branch
> which, so far, has only been reported on a Gentoo platform (mine). It
> involves the use of the libc math function "signbit". Here is a clip from the
> manpage:
>
>        `signbit' is a generic macro which can work on all real floating-
>        point types.  It returns a non-zero value if the value of  X  has
>        its sign bit set.
>
> On my system, SciPy's signbit function reports that the sign bit is not set
> for any number, positive or negative. Could someone here help me understand
> how to test the libc signbit function? I have to admit I have no experience
> with C programming.

Hello Darren,

This doesn't have to be a bug.  I don't know how SciPy works, but many
scientific libraries provide their own number types.  In most number
encoding schemes (like floating point) the sign of a number is
determined by a single bit (the most significant bit of the whole
number, for integer types; the most significant bit of the mantissa, for
floating point types).

The scheme could be totally different for SciPy, so the bit actually
reported might be the wrong bit.  This is not a bug, but rather a
missing feature (signbit() doesn't know the type of the number).

Regards.



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