Matthias Troyer <troyer@phys.ethz.ch> wrote in news:6A1ACC3A-7E4B-41FF-8EAD-17BD127AE8D4@phys.ethz.ch: < 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.