Path: utzoo!attcan!uunet!lll-winken!lll-ncis!helios.ee.lbl.gov!nosc!ucsd!rutgers!njin!princeton!phoenix!rjchen
From: rjchen@phoenix.Princeton.EDU (Raymond Juimong Chen)
Newsgroups: comp.sys.ibm.pc
Subject: Re: correct code for pointer subtraction
Message-ID: <5158@phoenix.Princeton.EDU>
Date: 5 Jan 89 18:51:03 GMT
References: <607@sactoh0.UUCP>
Organization: Princeton University, NJ
Lines: 39

From article <607@sactoh0.UUCP>, by smcroft@sactoh0.UUCP (Steve M. Croft):
> In article <5137@phoenix.Princeton.EDU>, rjchen@phoenix.Princeton.EDU (Raymond Juimong Chen) writes:
>> > There is an overflow in there.
>> 
>> Exactly.  And the compiler should be smart enough to emit code to handle
>> the overflow so as to generate the "correct" answer.
>  
> At what point should the compiler not be expected to fix overflow problems?
> Whadda 'bout:
> 
> int a;
> a = (30000*30000*30000*30000*30000*30000)/(30000*30000*30000*30000*30000)
> 
> This involves more than resolving the carry bit, as was earlier suggested..

But this involves an explicit calculation which overflows an int, and
is therefore ``implementation-defined''.  The same goes for division by
zero, or generating a floating-point result which is too big.  If the
answer is too big, then the result is ``implementation-defined''.

On the other hand, the subtraction of two pointers to the same array is
PROMISED by K&R to return a "correct" answer.  Remember:  I put the
word "correct" in quotation marks to emphasize that it is guaranteed
by K&R, and therefore any compiler which returns anything different
does not observe the K&R specifications.  Whether the K&R specifications
are silly, outdated, or unimplementable is completely irrelevant.

(The thing about the carry bit was to suggest a possible method of
returning the "correct" answer for pointer subtraction.  It was certainly
not pretending to be a solution to all overflow problems!  As other
folks have mentioned, extending to a 32-bit difference also solves
the problem.)


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