EWL C++ Library is multi-thread safe provided that the operating system supports thread-safe system calls. Library has locks at appropriate places in the code for thread safety. The locks are implemented as a mutex class -- the implementation of which may differ from platform to platform.
This ensures that the library is MT-Safe internally. For example, if a buffer is shared between two string class objects (via an internal refcount), then only one string object will be able to modify the shared buffer at a given time.
Thus the library will work in the presence of multiple threads in the same way as in single thread provided the user does not share objects between threads or locks between accesses to objects that are shared.