Re: How to check a Float for NaN

[Jerry <lanceboyle@qwest.net>]

On May 1, 12:52 pm, Keith Thompson <ks...@mib.org> wrote:
> Adam Beneschan <a...@irvine.com> writes:
> > On Apr 30, 4:23 pm, I wrote:
> >> On Apr 30, 1:33 pm, Jerry <lancebo...@qwest.net> wrote:
> >> > Thanks for the insight. From my point of view, not having access to
> >> > the all the IEEE-754 features is a nuisance. I'm not sure what I'm
> >> > going to do--probably try in import the C function isnan.
>
> >> Be careful with that, too.  On my (Pentium Linux) system, the man page
> >> for "isnan" says it takes a "double", which I think means a 64-bit
> >> float.
>
> > Ha, ha, ha, the man page lied.  Apparently (on my system) isnan is a C
> > macro (in <math.h>), which will call one of three routines depending
> > on the size of the float.  But since isnan is a macro, doing an Import
> > pragma on it is likely going to fail since "isnan" is not the name of
> > an actual routine in the library.  That's on my OS, though; who knows
> > how it works on yours.
>
> > Anyway, good luck and have fun getting this to work.
>
> The C99 C standard (which is not widely implemented, at least not
> completely) specifies that isnan() is a macro that can take an
> argument of any floating-point type (float, double, long double).  It
> might typically be implemented by invoking one of three functions
> depending on the size of the argument, but the standard doesn't
> specify any particular method; it could be pure magic.  The earlier
> C90 standard doesn't provide any such floating-point classification
> macros or functions.  Some C90 implementations might provide a
> function, rather than macro, called "isnan"; perhaps that's what your
> system documents.
>
> You can't interface to a C macro from Ada (as far as I know), but you
> can easily write a wrapper function.  Assuming your C implementation
> provides the isnan() macro as specified by C99, you can do this:
>
>     #include <math.h>
>
>     int float_isnan       (float x)       { return isnan(x); }
>     int double_isnan      (double x)      { return isnan(x); }
>     int long_double_isnan (long double x) { return isnan(x); }
>
> and then provide Ada interfaces to those C functions.
>
> It's a bit of a roundabout way to do it (providing three distinct
> function wrappers for a single macro that's probably a wrapper for
> three distinct underlying functions), but it should work.
>
> --
> Keith Thompson (The_Other_Keith) <ks...@mib.org>
> Nokia
> "We must do something.  This is something.  Therefore, we must do this."
>     -- Antony Jay and Jonathan Lynn, "Yes Minister"

That looks doable.

As an aside, I remember the early Pascal compilers on Macintosh
(Apple's own and the much-missed THINK Pascal nee MacPascal) dealt
with all of the 754 spec (as far as I know, which isn't very far) and
was very well documented in a book called Apple Numerics Manual with
unspecified authorship but with a forward by Prof. Kahan of UC
Berkeley. MacPascal did 96-bit floating point when it ran without
hardware acceleration and fell back to 80 bits with a FPU.

Jerry