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Floyd-Hoare Verification Harmful??
Date: Thu, 19 May 88 10:35:23 EDT
From: Mitchell Wand <wand%corwin.ccs.northeastern.edu@RELAY.CS.NET>
One ought not to say things like:
"F(G(C)) := D ought to ensure that F(G(C)) = D afterwards."
too blithely. Consider the array assignment:
A[A] := 2
in a two element array A, where initially A=A=1 . This sort of
thing had program verifiers confused for a good while in the early
Date: Fri, 20 May 88 12:54 EDT
From: Howard Shrobe <email@example.com>
I wrote a paper that was distributed by hand to friends in the late '70s
called "Floyd-Hoare Verifiers Considered Harmful" that pointed this ought.
It was somewhat tongue in cheek but was based on catching Vaughn Pratt
making exactly this kind of mistake. I just moved my office and found
copies of the paper. Sussman would remember it well.
That seems like an awfully rash conclusion to draw from this kind of bug. It
would be more reasonable to say something like "Unrestricted Pointer
Manipulation Considered Harmful" or "Hiding Nasty Pointer Manipulation in
Innocuous-Looking Array Manipulation Considered Harmful" or... :-)
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