Dynamic-link library creation
To create a Dynamic-link library (DLL), you must create one or more source code files, and possibly a linker file for exporting the functions. If you plan to allow applications that use your DLL to use load-time dynamic linking, then you must also create an import library.
Creating source files
The source files for a DLL contain exported functions and data, internal functions and data, and an optional entry-point function for the DLL. You may use any development tools that support the creation of Windows-based DLLs.
If your DLL may be used by a multithreaded application, then you should make your DLL "thread-safe". To avoid data corruption, you must synchronize access to all of the DLL's global data. You must also ensure that you link only with libraries that are thread-safe as well. For example, Microsoft Visual C++ contains multiple versions of the C Run-time Library, one that is not thread-safe and two that are.
Exporting functions
How you specify which functions in a DLL should be exported depends on the tools that you are using for development. Some compilers allow you to export a function directly in the source code by using a modifier in the function declaration. Other times, you must specify exports in a file that you pass to the linker.
For example, using Visual C++, there are two possible ways to export DLL functions: with the __declspec(dllexport) modifier or with a module-definition (.def) file. If you use the __declspec(dllexport) modifier, then it's not necessary to use a .def file. For more info, see Exporting from a DLL.
Creating an import library
An import library (.lib) file contains information that the linker needs in order to resolve external references to exported DLL functions, so that the system can locate the specified DLL and exported DLL functions at run-time. You can create an import library for your DLL when you build your DLL.
For more info, see Building an import library and export file.
Using an import library
For example, to call the CreateWindow function, you must link your code with the import library User32.lib. The reason is that CreateWindow resides in a system DLL named User32.dll, and User32.lib is the import library used to resolve the calls in your code to exported functions in User32.dll. The linker creates a table that contains the address of each function call. Calls to functions in a DLL will be fixed up when the DLL is loaded. While the system is initializing the process, it loads User32.dll because the process depends on exported functions in that DLL, and it updates the entries in the function address table. All calls to CreateWindow invoke the function exported from User32.dll.
For more info, see Link an executable to a DLL.