Re: Having fun with the following C code (UB)
On Thu, Apr 10, 2014 at 12:29:50PM +0200, Jakub Wilk wrote:
> * Wouter Verhelst <wouter@debian.org>, 2014-04-10, 12:03:
> >I've had to figure out the size of off_t in nbd-server, and have been
> >doing it without relying on overflow, for years now. It took quite a few
> >iterations to get it right, but the current definition has looked like
> >this since 2006:
> >
> >#define OFFT_MAX ~((off_t)1<<(sizeof(off_t)*8-1))
> >
> >i.e., left-shift 1 by enough bits so that the most significant bit is set,
>
> I believe that this code triggers undefined behavior. My C99 draft reads:
>
> The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are
> filled with zeros. […] If E1 has a signed type and nonnegative value, and E1
> × 2^(E2) is representable in the result type, then that is the resulting
> value; otherwise, the behavior is undefined.
Yes; the standard does this to allow for machine architectures which do
not use two's complement to store negative values. I did mention that
assumption in my previous mail.
If the architecture uses two's complement, however, then the code is
correct.
--
It is easy to love a country that is famous for chocolate and beer
-- Barack Obama, speaking in Brussels, Belgium, 2014-03-26
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