Thread Local Storage is implemented through the Win32 API layer and the compiler. For more information, see the Win32 API documentation for TlsAlloc, TlsGetValue, TlsSetValue, and TlsFree.

The Visual C++ compiler includes a keyword to make TLS operations more automatic, rather than through the API layer. This syntax is described in the next section, Compiler Implementation for TLS.

To support TLS, a new attribute, thread, has been added to the C and C++ languages and is supported by the Visual C++ compiler. This attribute is an extended storage class modifier, as described in the previous section. Use the __declspec keyword to declare a thread variable. For example, the following code declares an integer thread local variable and initializes it with a value:

__declspec( thread ) int tls_i = 1;

Concepts