Member 6775743 wrote: The Standard Template Library (STL) that is included with Microsoft Visual C++ 6.0 is not safe for multi-threaded applications That means the library that comes with MSVC 6.0 was not to safe to use in multi-threaded applications even if the containers were not shared among threads. If you use a more recent version of MSVC (or even MSVC 6 with STLPort), that should not be a problem. Of course, you should still need to synchronize access to container objects that are shared between threads. utf8-cpp