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Jan 19, 2011


Emmanuel Garcia

I wonder how AutoCAD tracks old handles no longer used because of deleted entities. Does it leave a mock / dummy entity, keep a list or use yet another method? This is a nice informative article. Thank you for writing it.

On a similar vein, since you mentioned forensics, I wonder what the "required qualifications" are for expert testimony, as this is a highly specialized niche. I imagine some of the best and most experienced hackers were too busy to be studying Computer Science 495 in a traditional school.

Warm regards from Los Angeles,


Owen Wengerd


AutoCAD doesn't keep track of discarded handles per se; it uses the handle seed value to ensure that all new handles are larger (and therefore different) than any previously used handle.

Although it certainly doesn't hurt, an "expert" doesn't need a degree to be considered an expert. I've never testified directly as an expert witness, but I've worked as an expert consultant in several lawsuits involving DWG files -- and I only have a high school education.

Julian Hardy

"They are commonly exposed and displayed as strings, but they are really 16-digit (64-bit) hexadecimal numbers ranging from zero to more than 1.8e19. That's 18 quintillion for US readers (18 trillion for the rest of you). Or if you prefer, that's just under 1/5 of a googol."

Ummm ...

1 googol = 1.0e100 (1 x 10^100)
1/5 of a googol = 2.0e99 (2 x 10^99)

1.8e19 is less than 1 part in 10^80 of 1 googol.

For all practical intents and purposes, 1.8e19 is approximately equal to zero, when you compare it against REALLY big numbers like 1 googol!)

Steven LaKose

You may only have a high school education, but you graduated suma comlade from the school of hard knocks...

Owen Wengerd

Good catch Julian. I must have misread my slide rule.

Steve Wells

Okay, I’m three weeks late reading your article, but I also immediately noticed your misuse of Googol. So, on to entity handles.

Way back in Release 12, if you didn't have the secret password, you could still remove all the entity handles from the drawing. Specifically, in order to destroy handles, you had to:
1. Issue the command: HANDLES
2. Enter DESTROY (no keyword shortcut)
3. Enter the complete string that AutoCAD presented. One of six different strings appeared pseudorandomly. They were:

Back then, Autodesk still had a sense of humor. You might recall the brief existence in R13 of some new internal data structures. Just look in the R13c4 ReadMe information under the heading: Resurrecting Zombies. It explained how AutoCAD processed custom objects from applications that had not been loaded. Such objects would reside in the potentially visible AcDbZombieEntity class, and the non-graphical AcDbZombieObject class. If you loaded the app, Zombie Support would resurrect the zombie. The name and idea really were quite appropriate, just as with a traditional zombie; that is, a supernatural corpse reanimated to do the bidding of a powerful master. In R14 the zombie nomenclature got renamed to something a lot less fun and less apt.

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