When compiled with the appropriate option according to the release notes for your compiler, Tools.h++ is multithread safe. In other words, all Tools.h++ functions behave the same in a multithreaded environment as in a single-threaded environment. Of course, this assumes the application program takes care either to avoid sharing individual objects between threads, or to perform locking around operations on objects shared across threads. Tools.h++ does enough internal locking to maintain its own internal integrity, and uses appropriate multithread-safe systems calls.