Thanks.  I was able to blow up Smalltalk using become: as you describe,
but we aren't using become: in our code.

Any other ideas?

Thanks,

Carl

On Fri, 6 Mar 1998, Hans-Martin Mosner wrote:
> Carl E Gundel wrote:
<snip> 
> > I am trying to construct an reproducible example where an object is
> > corrupted (via some context switching or recursion perhaps), usually
> > causing a protection violation (often revealed at garbage collection).  I
> > have seen this in Smalltalk applications in the past, and suspect I am
> > seeing the same kind of problem now in our Smalltalk work.
> > 
> > I need some example code to construct a note to our developers
> > illustrating how the problem happens, what to look for, and how to avoid
<snip>
> > 
> One very easy way is to (ab)use the #become: operation.
> When you do a "self become: anotherObject" and access an inst var later
> which does not have a corresponding slot in the other object, you're
> likely
> getting into trouble. I've got bitten by this once in a database
> framework
> where objects were replaced by proxies after being committed to the
> database.
> Here's some code that has worked for me :-)
> Try to inspect the result of (3 at 4) gack for some fun.
> It does not always work on the first try, but should work sooner or
> later.
> The example is done with VisualWorks, but Squeak should show
> similar symptoms (probably when you access x instead of y).
> 
> Hans-Martin
> 
> !Point methodsFor: 'private'!
> 
> gack
> 	"Try to produce an object memory corruption via #become:"
> 	"(3 at 4) gack"
> 	self gargle.
> 	^y!
> 
> gargle
> 	| t1 t2 |
> 	t1 := Object new.
> 	t2 := String new: 4.
> 	self become: t1.
> 	^t2! !
> 
> -- 
> +--- Hans-Martin Mosner ---- Senior Smalltalk Guru :-) ---+
> | These opinions are entirely ficticious.  Any similarity |
> | to real opinions is purely coincidental and unintended. |
> +--- <hmm at heeg.de> ------ URL:http://www.heeg.de/~hmm/ ---+
> 

------------------------------------------------------------------
 Carl Gundel  carlg at world.std.com  Shoptalk Systems  508-872-5315
 author of Liberty BASIC, a 1996 PC Magazine Awards Finalist!
 http://world.std.com/~carlg/basic.html
------------------------------------------------------------------