On 2014-04-10 12:42:03 +0200, Wouter Verhelst wrote: > 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.
Chapter and verse? C99, sec. 6.5.7, para. 4, quoted above, makes no such distinction. The operation is simply defined in terms of multi- plication by powers of two. If off_t is a signed type, 1 * 2 ^ (sizeof (off_t) * CHAR_BIT - 1) cannot be represented in off_t, and the behaviour is undefined. Az.
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