Invalid Pointers (was Re: Referencing NULL pointers)

[Gregory N. Bond]

In article <35077 at bu-cs.BU.EDU> bzs at bu-cs.BU.EDU (Barry Shein) writes:

   Obvious attacks are 0 on many (but not all) systems, a pointer into
   text space on others (eg. the address of a function), even that
   depends on certain load options being used but we'll assume that's
   under control of the author, setting some magic bit (eg. pointing into
   the wrong P space on a VAX), others?

For word-aligned machines, (e.g. 680x0), (int *)1.  In fact, in one
application involving humungous collections of 2D linked lists (linked
plains??) I used (struct foo *)0 as uninitialised pointer, and (struct
foo *)1 as initialised-to-invalid-address pointer.  Both dumped core
when dereferenced, and help track down obscure bugs.  Especialy as I
could do (struct foo *)(2*n+1) for any n, and work out which
assignment was generating the pointer that was dereferenced.  This
method could give a very large class of distinguishable invalid
pointers, (provided they weren't (char *)'s).

Not a general solution, but neat for the problem I had.

Greg.
(No comp.lang.c in Australia.  Lucky, hey?)
--
Gregory Bond, Burdett Buckeridge & Young Ltd, Melbourne, Australia
Internet: gnb at melba.bby.oz.au    non-MX: gnb%melba.bby.oz at uunet.uu.net
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