On Sat, 5 May 2007, Linus Torvalds wrote:
On Sat, 5 May 2007, Esben Nielsen wrote:
I have been wondering why you use usigned for timers anyway. It is also like
that in hrtimers. Why not use signed and avoid (almost) all worries about wrap
around issues. The trick is that when all
a < b
is be replaced by
a - b < 0
the code will work on all 2-complement machines even if the (signed!) integers
a and b wrap around.
No. BOTH of the above are buggy.
The C language definition doesn't allow signed integers to wrap (ie it's
undefined behaviour), so "a-b < 0" can be rewritten by the compiler as a
simple signed "a < b".
And the unsigned (or signed) "a < b" is just broken wrt any kind of
wrap-around (whether wrapping around zero or the sign bit).
So the _only_ valid way to handle timers is to
- either not allow wrapping at all (in which case "unsigned" is better,
since it is bigger)
- or use wrapping explicitly, and use unsigned arithmetic (which is
well-defined in C) and do something like "(long)(a-b) > 0".
Notice? The signed variant is basically _never_ correct.
What is (long)(a-b) ? I have tried to look it up in the C99 standeard but
I can't find it. Maybe it is in the referred LIA-1 standeard, which I
can't find with google.
I think the best would be to use "a-b > ULONG_MAX/2" when you mean "a<b"
as that should be completely portable.
According to C99 Appendix H2.2
(http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf) an
implementation can choose to do modulo signed integers as it is
mandatory for unsigned integers. If an implementation have choosen
to do that it must be a bug to to do the "a-b < 0" -> "a<b" optimization.
I have never experienced a compiler/architecture combination _not_ doing
wrapped signed integers.
Esben
Linus
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
