Heisenbug, noun

[jducoeur]

"A computer error that goes away every time you look at it closely, making it difficult to diagnose."

As, for example, spending the evening trying to figure out why your Internet connection has become flaky -- only to realize in the morning that the cablemodem is plugged into the light switch. D'oh...

Comments

[jducoeur]

Ouch. Yeah, that's a pretty brutal and very classic Heisenbug. While it's not quite universal, I do usually assume that a Heisenbug involves concurrency in *some* fashion: that's probably true 95% of the time.

I confess to being morbidly curious what they were doing to allow level 2 re-entrancy but not level 3 -- that smells like somebody hacked something horribly...

[goldsquare.livejournal.com]

I didn't get too into that - but from the behavior and a little hackerish use of the old DOS DEBUG commands obscure "reverse assemble" option... They just made a tiny little stack that only went 2 deep, and were sure that was enough.

Remember back in those days (and even now with embedded devices), space cost money and no one used space they didn't have to.

[jducoeur]

True. My very first summer job, working for my father, was to add a printer subsystem for the POS system we were building. We had a total of 16k of EEPROM, not expandable, and that was mostly already full. I pretty much had to invent refactoring from first principles (with some suggestions from Dad, of course), just to clear enough space to get the job done.

Fun times: it was my first experience of "they didn't tell me it was impossible when I got the assignment"...