[vox-tech] totally confused about C promotion

[Peter Jay Salzman]

begin Rod Roark <rod@sunsetsystems.com> 
> It's not sqrt that is producing nonsense, but rather printf.  Printf 
> accepts any types in its arguments, and it's your responsibility to make 
> sure that the data types match up with what's in the format string.  The 
> compiler can't fix it because it's a runtime issue.
> 
> This is generally considered a weakness in the way that printf works.
 
i understand printf well enough (and printf really isn't important to
me, since i'm not really printing these values.  just calculating them).

> Regarding the math function, the compiler can cast the argument (perhaps 
> with a warning) because it knows how sqrt is declared.
 
so you're saying it's fine to pass sqrt a long double, or "it could be
fine"?

also, i think i might've mispoke before.  the info docs talk about
long double precision functions, but i'm having trouble finding them
anywhere.  perhaps they're not implemented yet.  there's something
called tdmath.h that i'm trying to look into right now.  can't see any
mention of it in the glibc docs.  looking at google right now...

pete

> On Monday 14 January 2002 15:16, Peter Jay Salzman wrote:
> > from kernighan and ritchie, page 45:
> >
> >   For example, the library routine sqrt expects a double argument, and
> >   will produce nonsense if inadvertantly handed something else.
> >
> > consider the following code1.c:
> >
> >    int main(void)
> >    {
> >    	long double var = 4.0L;
> >
> >    	printf("%Lf\n", sqrt(var));
> >    	return 0;
> >    }
> >
> > this prints nonsense (0.0000), since sqrt returns double and i'm
> > printing a long double.  next, consider this code2.c:
> >
> >    int main(void)
> >    {
> >    	long double var = 4.0L;
> >
> >    	printf("%Lf\n", (long double)sqrt(var));
> >    	return 0;
> >    }
> >
> > this works.  but i'm not really interested in just "working" because
> > this particular code is extremely important to me.  i don't want to base
> > my code on something that works by accident.
> >
> > what happens when you hand a long double to a math function which
> > expects a double?  i can't find any rules for "demotion" in K+R.
> > does code2.c work by accident or is there a concept of demoting floating
> > points to a narrower width when the function expects them to be
> > narrower?
> >
> > pete
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-- 
The mathematics [of physics] has become ever more abstract, rather than more
complicated.  The mind of God appears to be abstract but not complicated.
He also appears to like group theory.  --  Tony Zee's `Fearful Symmetry'

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