I should probably elaborate on my problem a little bit more. I wanted to create a function that makes a given call atomic, but doesn't enforce that the function be called only once. I thought I could do so by using a once_flag allocated on the stack immediately before invoking call_once, so the flag would always be reinitialized to BOOST_ONCE_INIT and call_once could just offer me it's atomic guarantees, but apparently that won't work. I guess I'll just have to find some place to put my own mutex where it can be constructed safely.
I forgot to mention: 1) I had the same issue in Boost.Regex and wrote a statically initialised mutex as a solution (see boost/regex/pending/static_mutex.hpp), it might make a useful addition to the threading lib, however, I really should rewrite the Win32 implementation to use a mutex so you don't get a priority inversion and busy waiting (see http://www.netrino.com/Publications/Glossary/PriorityInversion.html). 2) It's possible to do what you want with call_once, but it's one of those things you might think is impossible until someone shows you how, and then it's embarrassingly easy, see http://lists.boost.org/Archives/boost/2005/07/90756.php for an explanation. HTH, John.