GUIDs are globally unique, but substrings of GUIDs aren't

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Matt Johnson's Technical Adventures — Wed, 09 Jul 2008 15:38:55 GMT

The list is a little longer today because of not posting last week. Enjoy! Microsoft Advanced Windows

re: GUIDs are globally unique, but substrings of GUIDs aren't

SuperKoko — Fri, 04 Jul 2008 01:29:23 GMT

Your reduced GUID format is a good idea, not used often enough, but it makes one more assumption... that there is only one thread or process running on each computer.

No. It just assumes that you use only one generator function (e.g. in a dynamic or static library). In that case, it's free to use shared variables and mutexes to safely manager GUIDs generation.

re: GUIDs are globally unique, but substrings of GUIDs aren't

Andy R — Thu, 03 Jul 2008 13:03:05 GMT

Your reduced GUID format is a good idea, not used often enough, but it makes one more assumption... that there is only one thread or process running on each computer.

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re: GUIDs are globally unique, but substrings of GUIDs aren't

mikeb — Mon, 30 Jun 2008 20:46:47 GMT

@SuperKoko:

>> That's still better than one GUID every 10 milliseconds with GetTickCount. <<

But you're not limited to a single UUID per 10ms tick, even if that's the hardware clock you might be stuck with. That's one of the things that section 4.2.1.2 of RFC 4122 discusses:

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A high resolution timestamp can be simulated by keeping a count of the number of UUIDs that have been generated with the same value of the system time, and using it to construct the low order bits of the timestamp. The count will range between zero and the number of 100-nanosecond intervals per system time interval.

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So you can still get a 10000 unique UUIDs per 10ms tick. But my point wasn't what timer resolution is best to use, but that by definition you can't get better resolution than 100ns resolution using Algorithm 1 to generate UUIDs. Oh, and that the clock sequence field is not intended to be used to simulate a higher resolution - it's intended to deal with clocks getting out of 'sync' (or going back in time).

re: GUIDs are globally unique, but substrings of GUIDs aren't

SuperKoko — Mon, 30 Jun 2008 18:22:52 GMT

That's nice, but it doesn't solve the theoretical problem of generating more than one UUID every 100ns (on average) - the timestamp field in the UUID is in units of 100ns.

That's still better than one GUID every 10 milliseconds with GetTickCount.

re: GUIDs are globally unique, but substrings of GUIDs aren't

pZy — Mon, 30 Jun 2008 09:23:57 GMT

German Wikipedia says for privacy reasons newer versions of Windows would not make use of the MAC address for creating GUIDs.

Otherwise it would be possible to draw conclusions about the creator.

Unfortunatley I didn't find that statement in the english wiki.

re: GUIDs are globally unique, but substrings of GUIDs aren't

mikeb — Mon, 30 Jun 2008 05:58:49 GMT

@SuperKoko:

>> On my computer, QueryPerformanceCounter has 10ns accuracy <<

That's nice, but it doesn't solve the theoretical problem of generating more than one UUID every 100ns (on average) - the timestamp field in the UUID is in units of 100ns.

re: GUIDs are globally unique, but substrings of GUIDs aren't

AndyC — Sun, 29 Jun 2008 23:46:22 GMT

@Jeff Tyrrill

"I really do wish hardware random number generators were standard order in modern processors, so we didn't have to rely on these elaborate algorithms simply to guarantee uniqueness"

If your random numbers are unique, your random number generator is broken.