Re: frexp man page: FLT_RADIX vs. 2

[Alejandro Colomar <alx@kernel.org>]

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Hi Matthew,

On Tue, Jan 02, 2024 at 10:50:51PM -0500, Matthew House wrote:
> On Tue, Jan 2, 2024 at 8:49 PM Alejandro Colomar <alx@kernel.org> wrote:
> > The value of that macro is defined to be 2; it can't have any other
> > value.
> >
> > ISO C defines it in
> > <http://port70.net/~nsz/c/c11/n1570.html#5.2.4.2.2p11>.
> >
> > POSIX defines it in
> > <https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/float.h.html>.
> >
> > Maybe the definition of frexp() by ISO C and POSIX should be changed to
> > define it in terms of FLT_RADIX instead of 2.
> >
> > Have a lovely day,
> > Alex
> 
> Every version of ISO C says (emphasis mine):
> 
>   The values given in the following list shall be replaced by constant
>   expressions with implementation-defined values that are *greater or
>   equal* in magnitude (absolute value) to those shown, with the same sign:
> 
>   -- radix of exponent representation, b
> 
>      FLT_RADIX                        2
> 
> And POSIX defines it similarly. So FLT_RADIX can be greater than 2 (unless
> the implementation defines __STDC_IEC_559__), it just can't be any less.
> Indeed, the subsequent EXAMPLE 1 in ISO C sets FLT_RADIX to 16.

Oops, my bad.

> 
> Thus, since frexp(3) is defined to return a power of 2, interpreting it as
> a power of FLT_RADIX is incorrect in the general case.

Hmm, then it's a bug in the manual page.  The function is curiously
defined in terms of 2 regardless of what FLT_RADIX is.  I'll fix it
tomorrow, unless anyone wants to send a patch for it before that.

Thank you both!
Alex

> 
> Thank you,
> Matthew House
> 

-- 
<https://www.alejandro-colomar.es/>
Looking for a remote C programming job at the moment.

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