On Windows operating systems before Windows Vista, __declspec( thread ) has some limitations. If a DLL declares any data or object as __declspec( thread ), it can cause a protection fault if dynamically loaded. After the DLL is loaded with LoadLibrary, it causes system failure whenever the code references the __declspec( thread ) data. Because the global variable space for a thread is allocated at run time, the size of this space is based on a calculation of the requirements of the application plus the requirements of all the DLLs that are statically linked. When you use LoadLibrary, you cannot extend this space to allow for the thread local variables declared with __declspec( thread ). Use the TLS APIs, such as TlsAlloc, in your DLL to allocate TLS if the DLL might be loaded with LoadLibrary.