Xref: utzoo comp.std.c:352 comp.lang.c:12393
Path: utzoo!attcan!uunet!steinmetz!davidsen
From: davidsen@steinmetz.ge.com (William E. Davidsen Jr)
Newsgroups: comp.std.c,comp.lang.c
Subject: Invalid pointer addresses
Keywords: NULL, pointer
Message-ID: <12088@steinmetz.ge.com>
Date: 7 Sep 88 20:07:12 GMT
Reply-To: davidsen@crdos1.UUCP (bill davidsen)
Followup-To: comp.std.c
Distribution: na
Organization: GE Corp. R & D, Schenectady,NY
Lines: 25


  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.

-- 
	bill davidsen		(wedu@ge-crd.arpa)
  {uunet | philabs}!steinmetz!crdos1!davidsen
"Stupidity, like virtue, is its own reward" -me