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From: Andy (atompkins_at_[hidden])
Date: 2006-11-13 11:15:29

Matthias Troyer <troyer_at_[hidden]> wrote in

< snip >

> Hi Andy,
> It seems from looking at the code that create_v4 will create
> identically the same guid if two computers (or nodes in a parallel
> computer) create a guid during the same second? This would be very
> common in any parallel program. This is why the paper you cite
> suggests using the MAC address of the computer for the lowest 6
> octets.
> Matthias


First thank you for your comments.

Version 4 guids are just random values. There really is no 'node' part
of the guid with this version. What you are talking about is a version 1
guid. They do have a node and the document does suggest using the MAC
address. I took out the code that creates version 1 guids because I
don't know of a portable way to get the MAC address. You are correct in
that the version 1 code was not following the doc to create a time-based

The create_v4 private() function is called by create() which (when
compiled with Boost.Thread) is protected by a mutex so only one thread
can create a guid at a time.

It is still possible for 2 calls to create_v4() to create the same guid
if the 2 calls were in different processes and the internal random number
generator was seeded with the same value. I am always looking for better
ways to seed the random number generator.

I plan to add a name-based create function (version 5) in the future.
This can be used to ensure that two computers (or nodes in a parallel
computer) will never create the same guid by ensuring that each computer
(or node) uses a different 'name space identifier'

Again, thank you for the comments.

Andy Tompkins.

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