On 2020-10-27 21:17 +0100, Marc Nieper-Wißkirchen wrote:
> > > > > If the member used to access the contents of a union object is not
> > > > > the same as the member last used to store a value in the object, the
> > > > > appropriate part of the object representation of the value is
> > > > > reinterpreted as an object representation in the new type as
> > > > > described in 6.2.6 (a process sometimes called "type punning").
> > > > > This might be a trap representation.
> > >
> > > "Reinterpreted as an object ..." is again implementation-defined,
> > > isn't it? Is there a way to find out how an IEEE double is mapped to
> > > an array of chars?
> >
> > Yes, it seems so.  (With ISO C, there's always more.  I'm not really
> > sure now whether their adding that footnote in the 2007 draft
> > clarified anything.)
>
> I don't think ISO C can do much about it as the binary representation
> of doubles is implementation-dependent. So, there cannot be a portable
> implementation in C.

Right, and whether the NaNs we examine have a quiet bit is also
implementation-dependent.  So the C implementation will just have to
be as-portable-as-possible.  The final say goes to the SRFI editor on
whether that, plus an FFI layer, is an acceptable sample
implementation.

Regards,

--
Wolfgang Corcoran-Mathe  <xxxxxx@sigwinch.xyz>

"Karate begins with courtesy and ends with it." --Shoshin Nagamine