NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part1/6909

Internet Message Format | 1996-08-05 | 2.0 KB

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From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.arch.arithmetic,comp.lang.c,comp.lang.c++
Subject: Re: Access carry flag from C
Date: Tue, 20 Feb 96 21:54:32 GMT
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In article <[e-mail]>
           [e-mail] "Arthur Chance" writes:

>In article <[e-mail]> Franz Korntner <[e-mail]> writes:
>> j+k will overflow when the result exceeds MAXINT
>>
>> Thus:  "if (j+k > MAXINT) overflow();" but the operation is undefined
>> if the result overflows, so the expression needs rewriting to make sure
>> this doesn't happen. The result is then "if (j>MAXINT-k) overflow();".
>
>As we must be talking about signed ints, because unsigned can't cause
>undefined behaviour by overflow, if k < 0, then MAXINT-k overflows.
>
>Basically, the C *standard* is useless on things like signed overflow
>(or word size, or what happens with right shift of -ve numbers, or
><insert your favourite "undefined behaviour" gripe here>). You have
>to look carefully at each *implementation* you use. You can usually
>find some way to do what you want, but you have to accept it's never,
>ever going to be standard C.

It certainly can be done portably although not with the efficiency that
a platform-specific solution is likely to give you.

    int j, k;

    ...

    if ((j >= 0) ? (k > INT_MAX-j) : (k < INT_MIN-j))
       ...

will test if j+k overflows. You have to perform the test before doing the
calculation since if j+k overflows the result is undefined behaviour
whatever you do afterwards.

--
-----------------------------------------
Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
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