Re: Having fun with the following C code (UB)

[Jakub Wilk <jwilk@debian.org>]

* 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.

--
Jakub Wilk