Simple proof that GUID is not unique [closed]

Answer 1

This will run for a lot more than hours. Assuming it loops at 1 GHz (which it won't - it will be a lot slower than that), it will run for 10790283070806014188970 years. Which is about 83 billion times longer than the age of the universe.

Assuming Moores law holds, it would be a lot quicker to not run this program, wait several hundred years and run it on a computer that is billions of times faster. In fact, any program that takes longer to run than it takes CPU speeds to double (about 18 months) will complete sooner if you wait until the CPU speeds have increased and buy a new CPU before running it (unless you write it so that it can be suspended and resumed on new hardware).

Answer 2

[Update:] As the comments below point out, newer MS GUIDs are V4 and do not use the MAC address as part of the GUID generation (I haven't seen any indication of a V5 implementation from MS though, so if anyone has a link confirming that let me know). WIth V4 though, time is still a factor though, and the odds against duplication of GUIDs remains so small as to be irrelevant for any practical usage. You certainly would not be likely to ever generate a duplicate GUID from just a single system test such as the OP was trying to do.

Most of these answers are missing one vital point about Microsoft's GUID implementation. The first part of the GUID is based on a timestamp and another part is based on the MAC address of the network card (or a random number if no NIC is installed).

If I understand this correctly, it means that the only reliable way to duplicate a GUID would be to run simultainous GUID generations on multiple machines where the MAC addresses were the same AND where the clocks on both systems were at the same exact time when the generation occured (the timestamp is based on milliseconds if I understand it correctly).... even then there are a lot of other bits in the number that are random, so the odds are still vanishingly small.

For all practical purposes the GUIDs are universally unique.

There is a pretty good description of the MS GUID over at "The Old New Thing" blog

Answer 3

For me.. the time it takes for a single core to generate a UUIDv1 guarantees it will be unique. Even in a multi core situation if the UUID generator only allows one UUID to be generated at a time for your specific resource (keep in mind that multiple resources can totally utilize the same UUIDs however unlikely since the resource inherently part of the address) then you will have more than enough UUIDs to last you until the timestamp burns out. At which point I really doubt you would care.

Answer 4

Since part of Guid generation is based on the current machine's time, my theory to get a duplicate Guid is:

1. Perform a clean installation of Windows
2. Create a startup script that resets the time to 2010-01-01 12:00:00 just as Windows boots up.
3. Just after the startup script, it triggers your application to generate a Guid.
4. Clone this Windows installation, so that you rule out any subtle differences that may occur in subsequent boot-ups.
5. Re-image the hard drive with this image and boot-up the machine a few times.

Answer 5

The program, albeit its errors, shows proof that a GUID is not unique. Those that try to prove the contrary are missing the point. This statement just proves the weak implementation of some of the GUID variations.

A GUID is not necessary unique by definition, it is highly unique by definition. You just refined the meaning of highly. Depending on the version, the implementator (MS or others), use of VM's, etc your definition of highly changes. (see link in earlier post)

You can shorten your 128 bit table to prove your point. The best solution is to use a hash formula to shorten your table with duplicates, and then use the full value once the hash collides and based on that re-generate a GUID. If running from different locations, you would be storing your hash/full key pairs in a central location.

Ps: If the goal is just to generate x number of different values, create a hash table of this width and just check on the hash value.

Answer 6

If you're worried about uniqueness you can always purchase new GUIDs so you can throw away your old ones. I'll put some up on eBay if you'd like.