Re: [boost] Ann: guid library

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

Matthias Troyer <troyer_at_[hidden]> wrote in
news:6A1ACC3A-7E4B-41FF-8EAD-17BD127AE8D4_at_[hidden]: 

< 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
> 

Hi,

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 
guid.

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.