Messages in this thread | | | Date | Mon, 29 Jul 2013 15:30:35 +1000 | From | caf@guarana ... | Subject | Re: [PATCH jiffies] Avoid undefined behavior from signed overflow |
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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.
...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.
- Kevin
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