Dynamic-Link Library Creation

To create a 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, you must also create an import library.

Creating Source Files

The source files contain exported functions, internal functions, and an optional entry-point function for the DLL. You may use any development tools that support the creation of Win32-based DLLs.

If your DLL may be used by a multithreaded application, you should make your DLL "thread-safe" by linking only with libraries that have support for multiple threads. Also, be sure to synchronize access to your global data.

Exporting Functions

How you specify exported functions 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 Microsoft Visual C++, there are two possible ways to export DLL functions: with _declspec modifier or with a .DEF file. If you use the _declspec modifier, it is not necessary to use a .DEF file.

For more information about exporting functions, see the documentation included with your development tools.

Creating an Import Library

The import library (.LIB) file contains information the linker needs to resolve external references to exported DLL functions, so the system can locate the specified DLL and exported DLL functions at run time. 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. The file USER32.LIB is the import library used to resolve the call to CreateWindow in your code.

For information about creating import libraries, see the documentation included with your development tools.