I recently proofread a manual which stated that even calculating the
value of an invalid address could cause a memory fault. I read this to
mean that code like:
        int D_temp[30],                 /* 30 elements */
            *tempr = D_temp-100;        /* int tempr[100..129] */

would cause a problem even if the pointer were not dereferenced. I would
assume that calculating a value would not EVER cause a dereference (and
thereby a fault), no matter how invalid the address.

  An example in point are the machines which will fault if you try to
dereference a NULL pointer. Simply having the value NULL doesn't cause a
problem. Is there a portion of dpANS which states that this is/isn't
allowed, or that it's implementation dependent? Seems like "common
practice" to me, and portable, as long as you don't use the pointer
other than as a starting point back to the defined array.

BTW: the example above is how you convert a Pascal program with a range
to C. Access is rangechecked by a macro package.

--

  {uunet | philabs}!steinmetz!crdos1!davidsen
"Stupidity, like virtue, is its own reward" -me