Re: Dodgy functions (finitel, strold)

[Achim Gratz <Stromeko@nexgo.de>]

Doug Henderson writes:
> I modified your program to display the actual hex value of the a, b,
> and c variables. The b and c variables have different bit patterns. It
> appears that the %a format conversion is (correctly) detecting ±inf
> and NaN according to IEEE 754, and ignoring the value of all other
> bits in the variables.

I wasn't concerned about the bit patterns, but the fact that +-Inf was
recognized as valid input, but then converted to NaN.

> It appears that strtold and the implicit conversion from double to
> long double are setting some of the bits which are not used to
> represent NaN or ±Inf to different values.
>
> It appears that some of the different functions that get used to
> detect finiteness and validity are sensitive to the setting of other
> bits in the values, or are expecting particular values for these bits.

That would be a bug.  The standard defines all the valid bit patterns
for each case, so if in doubt you could exhaustively test them.

> The standard supports two representations of NaN: a signalling NaN and
> a non-signalling NaN. From what I could see, the C language does not
> distinguish between the two NaN representations, but I did not look at
> the standards docs.

You can mostly ignore that distinction unless the runtime exposes it to
you.  In a nutshell, the difference is between silently producing and
then using the NaN in further computation or raising an exception.  In
the latter case, the runtime would need to give you a way to catch and
handle the exception.


Regards,
Achim.
-- 
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+

Factory and User Sound Singles for Waldorf Blofeld:
http://Synth.Stromeko.net/Downloads.html#WaldorfSounds