| Platform SDK: DLLs, Processes, and Threads |
Dynamic-link libraries (DLL) are modules that contain functions and data. A DLL is loaded at run time by its calling modules (.exe or .dll). When a DLL is loaded, it is mapped into the address space of the calling process.
DLLs can define two kinds of functions: exported and internal. The exported functions can be called by other modules. Internal functions can only be called from within the DLL where they are defined. Although DLLs can export data, its data is usually only used by its functions.
DLLs provide a way to modularize applications so that functionality can be updated and reused more easily. They also help reduce memory overhead when several applications use the same functionality at the same time, because although each application gets its own copy of the data, they can share the code.
The Microsoft® Win32® application programming interface (API) is implemented as a set of dynamic-link libraries, so any process that uses the Win32 API uses dynamic linking.