Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!rochester!uhura.cc.rochester.edu!ur-valhalla!badri
From: badri@valhalla.ee.rochester.edu (Badri Lokanathan)
Newsgroups: comp.lang.c
Subject: Re: Sizes, alignments, and maxima (was: Contiguous Arrays)
Summary: Just for the sake of argument
Message-ID: <1839@valhalla.ee.rochester.edu>
Date: 24 Feb 89 18:15:08 GMT
References: <2508@ssc-vax.UUCP> <340009@hplvli.HP.COM>
Organization: UR Dept. of Electrical Engg, Rochester NY 14627
Lines: 46

In article <340009@hplvli.HP.COM>, boyne@hplvli.HP.COM (Art Boyne) writes:
> Sorry, but read Henry's response more carefully:  it is possible that the
> *computation* of x-=i will cause an abort/trap/etc - without attempting
> any *access*.  This is especially likely if x=-i would result in x pointing
> to a negative-index array location (consider a machine whose address
> registers trapped on a negative segment offset and whose compiler put
> the array into a separate segment, perhaps due to segment size limitations).  
> Henry is correct: pointer arithmetice isn't guaranteed unless you keep it
> *within* an array.
> 

On a slightly different note and out of curiosity, I tried the
following experiment on a sun 3 running OS3.4:

main()
{
	unsigned i, j, k, val;

	i = 100; j = 200; k = 300;

	val = i - j + k;
	printf("%u\n", val);

	val = (i - j) + k;
	printf("%u\n", val);

	val = i - j;
	printf("%u\n", val);
}

The output was
200
200
4294967196

Thus even though the intermediate value was rubbish (-100), it still
worked correctly. (I checked the assembler output for difference too.)
Could anybody tell me of a machine where this will not run?
Similarly with the pointer problem, while everybody has
said that a problem *might* occur, is there a machine where
failure will definitely occur?
-- 
,------------------------.    "O! the beautiful bombs falling!
| Badri Lokanathan       |     Children leap as deer to catch them.
| badri@ee.rochester.edu |     Mothers burst open like flowers.
`------------------------'     Fathers spin away into orange air."  -Dick Bakken