checking for overflow in C

[Jim Shankland]

In article <10218 at smoke.BRL.MIL> gwyn at brl.arpa (Doug Gwyn) writes:
>In article <13367 at dartvax.Dartmouth.EDU> shallit at eleazar.dartmouth.edu (Jeffrey Shallit) writes:
>>... enlighten me about the officially approved way of checking overflow when
>>multiplying two integers.
>...
>You're looking for tactics when what is needed is better strategy.
>How did you get your algorithm into the state where an overflow is
>even possible?  Sounds to me like the algorithm needs to be better
>engineered.

I can think of lots of cases where the operands of an arithmetic operation
aren't known until run-time.  The only way to ensure that the operation
won't overflow is to perform some check on the operands, which is exactly
what he was inquiring about.  I don't know how else he might "better
engineer" his algorithm to avoid the problem.

I understand the reasons for C's overflow semantics (or lack of them, if
you will).  Nonetheless, while it's not a problem most of the time,
when it is a problem, it's a big one.  Operations like multiplication
end up becoming a subroutine -- often one written in assembler.

Jim Shankland
jas at ernie.berkeley.edu

"Blame it on the lies that killed us, blame it on the truth that ran us down"