Hi, Currently Threads's Win32 TLS implementation uses a global static variable thread::current_thread_tls_key that is defined in src/win32/thread.cpp. As you may know, static variables have a tendency of not being allocated in a very specific order. Say, that you have a statically allocated class A that ends up calling thread::set_current_thread_data() (eg. uses thread::thread_specific_ptr). If compiler initializes this class before it initializes current_thread_tls_key (this did happen to me on MSVC9) things go boom: a) Compiler initializes class A, thread::set_current_thread_data() is called, thread::create_current_thread_tls_key() is called once (as expected) and ::TlsAlloc() result is stored to thread::current_thread_tls_key. b) Compiler initializes thread::current_thread_tls_key, TLS_OUT_OF_INDEXES is stored and, as you may guess, c) Any following thread::set_current_thread_data calls will fail as the proper result from ::TlsAlloc() was overwritten. Don't know why this haven't happened before, as previous versions use a static variable, too. Maybe some compilers (MSVC in this case) omit initialization code unless initial value is a non-zero (as with TLS_OUT_OF_INDEXES [0xffffffff] in this case) value. I propose that thread::current_thread_tls_key is moved to a static function that sole purpose is to return reference to that variable (and thus ensure that current_thread_tls_key is initialized before it is used for the first time). Also, move thread::current_thread_tls_init_flag to thread::set_current_thread_data() to make sure this won't happen to that, too. Attached a patch that does the above trick (did move TLS_OUT_OF_INDEXES back to a macro as otherwise the code got too cumbersome, does get undef'd unless it was already defined by Winapi). If you like it, I can submit a ticket to Trac that includes the patch. (Haven't looked what happens during destruction, but guess it's not safe to use TLS features thru Thread.) -- Pekka