Messages in this thread | | | Date | Mon, 29 Jul 2013 06:54:11 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH jiffies] Avoid undefined behavior from signed overflow |
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On Mon, Jul 29, 2013 at 03:30:35PM +1000, caf@guarana.org wrote: > Quoting "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>: > > >According to the C standard 3.4.3p3, overflow of a signed integer results > >in undefined behavior. This commit therefore changes the definitions > >of time_after() and time_after_eq() to avoid this undefined behavior. > >The trick is that the subtraction is done using unsigned arithmetic, > >which according to 6.2.5p9 cannot overflow because it is defined as > >modulo arithmetic. This has the added (though admittedly quite small) > >benefit of shortening two lines of code by four characters each. > > > >Note that the C standard considers the cast from signed to > >unsigned to be implementation-defined, see 6.3.1.3p3. However, on a > >two-complement system, an implementation that defines anything other > >than a reinterpretation of the bits is free come to me, and I will be > >happy to act as a witness for its being committed to an insane asylum. > >(Although I have nothing against saturating arithmetic or signals in > >some cases, these things really should not be the default.) > > Don't worry, the case from signed to unsigned is actually well-defined - > the relevant part is 6.3.1.3p2 (in C99): > > >Otherwise, if the new type is unsigned, the value is converted by > >repeatedly adding or subtracting one more than the maximum value that > >can be represented in the new type until the value is in the range of > >the new type.
Yep, but we are going in the other direction, from unsigned to signed.
> ...which ends up just being reinterpretation of the bits on a two's > complement system, as you'd hope (after sign-extension to the width of > the target unsigned type, that is). This actually means if you were > mad enough to implement C on a sign-magnitude system, you'd be forced to > do a non-trivial conversion in this case.
Fortunately, I never used signed-magnitude systems. And even when I used ones-complement systems back in my misguided youth, I didn't write C programs for them. ;-)
Thanx, Paul
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