Dynamic-Link Library Data

A Dynamic-Link Library (DLL) can contain global data or local data.

Variable Scope

Variables that are declared as global in a DLL source code file are treated as global variables by the compiler and linker, but each process that loads a given DLL gets its own copy of that DLL's global variables. The scope of static variables is limited to the block in which the static variables are declared. As a result, each process has its own instance of the DLL global and static variables by default.


Your development tools may allow you to override the default behavior. For example, the Visual C++ compiler supports #pragma section and the linker supports the /SECTION option. For more information, see the documentation included with your development tools.


Dynamic Memory Allocation

When a DLL allocates memory using any of the memory allocation functions (GlobalAlloc, LocalAlloc, HeapAlloc, and VirtualAlloc), the memory is allocated in the virtual address space of the calling process and is accessible only to the threads of that process.

A DLL can use file mapping to allocate memory that can be shared among processes. For a general discussion of how to use file mapping to create named shared memory, see File Mapping. For an example that uses the DllMain function to set up shared memory using file mapping, see Using Shared Memory in a Dynamic-Link Library.

Thread Local Storage

The thread local storage (TLS) functions enable a DLL to allocate an index for storing and retrieving a different value for each thread of a multithreaded process. For example, a spreadsheet application can create a new instance of the same thread each time the user opens a new spreadsheet. A DLL providing the functions for various spreadsheet operations can use TLS to save information about the current state of each spreadsheet (row, column, and so on). For a general discussion of thread local storage, see Thread Local Storage. For an example that uses the DllMain function to set up thread local storage, see Using Thread Local Storage in a Dynamic-Link Library.

Windows Server 2003 and Windows XP: The Visual C++ compiler supports a syntax that enables you to declare thread-local variables: _declspec(thread). If you use this syntax in a DLL, you will not be able to load the DLL explicitly using LoadLibrary or LoadLibraryEx on versions of Windows prior to Windows Vista. If your DLL will be loaded explicitly, you must use the thread local storage functions instead of _declspec(thread).