From: "Luke A. Kanies" <luke@...>
Date: 2003-12-17T04:53:47+09:00
Subject: Re: extremely strange segfault

On Wed, 17 Dec 2003, Daniel Kelley wrote:

> "Luke A. Kanies" <luke@madstop.com> writes:
>
> > This is just about the strangest problem I've ever had, because it will
> > appear if I comment a print statement out, but then disappear if I just
> > delete the print statement.
>
> If this is the strangest bug you ever had, you are doing pretty good.
>
> What you have come across is a "Heisenbug":
>
> http://wombat.doc.ic.ac.uk/foldoc/foldoc.cgi?heisenbug

That's what this was, though.  I would encounter the bug, so then I'd add
some print statements to try to bracket the bug and it would go away.
Okay, then I'd just comment the statements out, maybe their execution
fixed it; still no bug.  Okay, delete the statements entirely; now the bug
is back.

The Heisenberg nature of the bug did not stand up to deeper scrutiny, but
it certainly fooled my initial (usually sufficient) debugging.

Luke

-- 
"I think that's how Chicago got started.  A bunch of people in New York
said, 'Gee, I'm enjoying the crime and the poverty, but it just isn't
cold enough. Let's go west.' "
     --Richard Jeni