Re: Documentation of abs(3), div(3) etc.

On Wed, 07 Feb 2007, Martijn van Buul wrote:
> * Christian Biere:
> > the manpages for abs(3) and its variants define behavior for the
> > "most negative integer" whereas the standard explicity states that
> > the behavior is undefined if the result cannot be represented.

I'd like out man page to both document what we do, and document what
parts of that are non-standard.  At present, out abs(3) seems to
accurately document what we do, but not that part of it is undefined by
the standard.

> > This is a lie anyway because the code looks like
> >
> > 	return a < 0 ? -a : a;
> >
> > whereas it obviously means
> >
> > 	return a < 0 ? -(unsigned)a : a;
> 
> This is plain nonsense, on multiple grounds. First of all, you're casting
> a signed int (known to be negative) to an unsiged int, which is pretty
> broken to begin with, secondly, you're trying to negate the resulting
> unsigned number, which isn't any better.

The suggested replacement code is correct.  Unsigned arithmetic in C is
defined in terms of modular arithmetic in mathematics.

The original code would invoke undefined behaviour if it appeared in
user-written code.  (The mathematical result of -a might be outside the
range representable by a signed int, which gives undefined bahaviour.)
Since the code in queation is part of the implementation, I think we
don't need to worry about that.

--apb (Alan Barrett)