What does the standard say about array access wraparound?

From: David Mathog (mathog_at_caltech.edu)
Date: 05/27/04 

Date: Thu, 27 May 2004 08:02:56 -0700

If this:

int i,sum;
int *array;
  for(sum=0, i=0; i<len; i++){
    sum += array[i];
  }

is converted to this (never mind why for the moment):

int i,sum;
int *array;
int *arrl;
  arl=&array[-len];
  for(sum=0,i=len; i<2*len; i++){
    sum += arrl[i];
  }

it should give the same result. But there are some funny
things that can happen. For instance, if &array is 1000 and
len is 100000. In that case arrl will hold an address
(1000-100000) which presumably wraps around since the
pointer should be an unsigned int (whatever size int is).
The address it points to will be MAX_POINTER - 100000 + 1000.
When the second form loop loop begins i=len (100000) so
arrl[100000] will wrap back around and point to the same
place as array[0].

Or will it?

It seems possible that this sort of array access "off the top of
memory" could trigger a fault.

What does the C standard say about this (if anything)?

Thanks,

David Mathog
mathog@caltech.edu
Manager, Sequence Analysis Facility, Biology Division, Caltech

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