> -----Original Message-----
> From: Joerg Schilling [mailto:joerg.schill...@fokus.fraunhofer.de]
> Sent: Thursday, July 19, 2018 4:53 PM
> To: vincent-o...@vinc17.net; austin-group-l@opengroup.org
> Subject: Re: About issue 0001108 and abs(INT_MIN)
>
> Vincent Lefevre <vincent-o...@vinc17.net> wrote:
>
> > The problem is not just the warning. If t is signed,
> >
> > ((t)(~((t)0) << (sizeof (t)*CHAR_BIT - 1)))
> >
> > will yield undefined behavior due to overflow. This means that
> > compilers may generate code that shows a behavior different from what
> A compiler that creates other than the expected behavior would need to create
> intentionally buggy code.
>
> The question was to create working code that neither creates a warning with
> newer
> nor with older compilers. Do you have such code?
I don't think such code (to detect whether an arbitrary type is signed or
unsigned) exists.
As Vincent correctly wrote, signed arithmetic is allowed to trap;
my understanding is that this was so C could support IBM360-derived
architectures
(non-trapping signed arithmetic is a recent addition to z-Series).
POSIX acknowledges the inability to programmatically discover the by stating,
for each typedef
it specifies (e.g. in sys/types.h), whether the type is signed or unsigned.
Where it does not, e.g., time_t
computation is significantly complicated, e.g., difftime() must be used in
portable code.
((t) 1 << sizeof (t) * CHAR_BIT - 1) is an expression that evaluates to a
t-sized word with the most
significant bit set.
Regards
Konrad
