On Thu, Apr 10, 2014 at 12:29:50PM +0200, Jakub Wilk wrote: > * Wouter Verhelst <wou...@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 -- To UNSUBSCRIBE, email to debian-devel-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org Archive: https://lists.debian.org/20140410104203.gd18...@grep.be
