Rules and Limitations for TLS

The following guidelines must be observed when declaring statically bound thread local objects and variables:

• The thread attribute can be applied only to data declarations and definitions. It cannot be used on function declarations or definitions. For example, the following code generates a compiler error:

  #define Thread  __declspec( thread )
  Thread void func();     // This will generate an error.
  

• The thread modifier might be specified only on data items with static extent. This includes global data objects (both static and extern), local static objects, and static data members of C++ classes. Automatic data objects cannot be declared with the thread attribute. The following code generates compiler errors:

  #define Thread  __declspec( thread )
  void func1()
  {
      Thread int tls_i;            // This will generate an error.
  }
  
  int func2( Thread int tls_i )    // This will generate an error.
  {
      return tls_i;
  }
  

• The declarations and the definition of a thread local object must all specify the thread attribute. For example, the following code generates an error:

  #define Thread  __declspec( thread )
  extern int tls_i;        // This will generate an error, since the
  int Thread tls_i;        // declaration and definition differ.
  

• The thread attribute cannot be used as a type modifier. For example, the following code generates a compiler error:

  char __declspec( thread ) *ch;        // Error
  

• C++ classes cannot use the thread attribute. However, C++ class objects can be instantiated with the thread attribute. For example, the following code generates a compiler error:

  #define Thread  __declspec( thread )
  class Thread C       // Error: classes cannot be declared Thread.
  {
  // Code
  };
  C CObject;
  

  Because the declaration of C++ objects that use the thread attribute is permitted, the following two examples are semantically equivalent:

  #define Thread  __declspec( thread )
  Thread class B
  {
  // Code
  } BObject;               // OK--BObject is declared thread local.
  
  class B
  {
  // Code
  };
  Thread B BObject;        // OK--BObject is declared thread local.
  

• The address of a thread local object is not considered constant, and any expression involving such an address is not considered a constant expression. In standard C, the effect of this is to forbid the use of the address of a thread local variable as an initializer for an object or pointer. For example, the following code is flagged as an error by the C compiler:

  #define Thread  __declspec( thread )
  Thread int tls_i;
  int *p = &tls_i;       //This will generate an error in C.
  

  This restriction does not apply in C++. Because C++ allows for dynamic initialization of all objects, you can initialize an object by using an expression that uses the address of a thread local variable. This is accomplished just like the construction of thread local objects. For example, the code shown earlier does not generate an error when it is compiled as a C++ source file. Note that the address of a thread local variable is valid only as long as the thread in which the address was taken still exists.

• Standard C allows for the initialization of an object or variable with an expression involving a reference to itself, but only for objects of nonstatic extent. Although C++ generally allows for such dynamic initialization of objects with an expression involving a reference to itself, this kind of initialization is not permitted with thread local objects. For example:

  #define Thread  __declspec( thread )
  Thread int tls_i = tls_i;                // Error in C and C++ 
  int j = j;                               // OK in C++, error in C
  Thread int tls_i = sizeof( tls_i )       // Legal in C and C++
  

  Note that a sizeof expression that includes the object being initialized does not represent a reference to itself and is enabled in both C and C++.

  C++ does not allow such dynamic initialization of thread data because of possible future enhancements to the thread local storage facility.

• On Windows operating systems before Windows Vista, __declspec( thread ) has some limitations. If a DLL declares any nonlocal 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 nonlocal __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.

See Also

Concepts

Thread Local Storage (TLS)

Change History