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.