[Boost-users] Calling thread::join from another thread?

Given a thread A created by a thread B. Is calling the join-method of A from a thread C a well-defined operation? (With C != B && C != A) It seems to work fine, but I want to make sure that this was not just 'luck'. Assumed, the operation is not defined: Can someone imagine a way to wrap a thread such that is can be joined by an arbitrary other thread? Thanks for clarification, Wolfgang Meyer