Explain the Thread Local Storage (TLS)   

Windows NT provides unique functions for per-thread data management. Thread local storage (TLS) is a concept defined in NT so developers can manage global data independently for each thread. Generally, a mechanism is provided for setting and getting the thread local storage for each thread. Upon process or DLL initialization thread local storage must be initializes via the TlsAllocFunction. This process allocates a TLS table and returns a TLS index that is used with all other TLS functions.

Once initialized, each subsequently created thread would typically allocate memory and save the memory pointer on a per-thread basis. TlsSetValue( ) is used to associate a data pointer with the calling thread. Subsequent access to that thread data would be available via the TlsGetValue( ) function call. When all threads are done with per-thread data management, TlsFree( ) should be issued to release the TLS index.