[rrd-developers] Re: Representation of NAN and INF

[Alex van den Bogaerdt]

Matt Zimmerman wrote:
> 
> Mathematically speaking, 1.0/0.0 is undefined while 0.0/0.0 is indeterminate,
> and those do not mean the same thing.  From a programming standards
> perspective, as far as I know, IEEE 754 says that there are multiple
> permissible representations for NaN and infinity (not all bits are defined).
> So there are values for which, within rrdtool:
> 
> isnan(x) && x != DNAN
> and
> isinf(x) && x != DINF

Good to know.

> I would hope that rrdtool only uses these macros to obtain reasonable values to
> use in its internal storage format, and not for any comparisons (where the IEEE
> functions should be used).

A quick scan doesn't reveal any such comparisons.  I'll check more careful
later on.

> As long as this is true, the expressions for DNAN and DINF are a perfectly
> valid way to obtain some values which will satisfy finite/isnan/isinf in the
> expected ways:  IEEE defines that isnan(0.0/0.0) and isinf(1.0/0.0) will both
> be true.

Do you also happen to know the official way to handle isnan(1.0/0.0) ?

> I missed the original post...is there some compiler which is not handling these
> expressions correctly?

It was about porting to VB OCX or something alike.  I don't know if it
was asked because it fails in VB or not.

cheers,
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