Chapter 19 -- DLL Basics

[Previous] [Next]

Chapter 19

Dynamic-link libraries (DLLs) have been the cornerstone of Microsoft Windows since the first version of the operating system. All the functions in the Windows API are contained in DLLs. The three most important DLLs are Kernel32.dll, which contains functions for managing memory, processes, and threads; User32.dll, which contains functions for performing user-interface tasks such as window creation and message sending; and GDI32.dll, which contains functions for drawing graphical images and displaying text.

Windows also comes with several other DLLs that offer functions for performing more specialized tasks. For example, AdvAPI32.dll contains functions for object security, registry manipulation, and event logging; ComDlg32.dll contains the common dialog boxes (such as File Open and File Save); and ComCtl32.DLL supports all of the common window controls.

In this chapter, you'll learn how to create DLLs for your own applications. Here are some reasons for using DLLs:

• They extend the features of an application. Since DLLs can be dynamically loaded into a process's address space, an application can determine at run time what actions to perform and then load the code to execute those actions on demand. For example, a DLL is useful when one company creates a product and wants to allow other companies to extend or enhance the product.
• They can be written in many programming languages. You can choose the best language for the job at hand. Perhaps your application's user interface is best programmed with Microsoft Visual Basic, but the business logic is better handled by C++. The system allows a Visual Basic program to load a C++ DLL, a Cobol DLL, a Fortran DLL, and so on.
• They simplify project management. If different groups work on different modules during the development process, the project is easier to manage. However, an application should ship with as few files as possible. I know of one company that shipped a product with one hundred DLLs—up to five DLLs per programmer. The application's initialization time was horribly slow because the system had to open one hundred disk files before the program could do anything.
• They help conserve memory. If two or more applications use the same DLL, the DLL has its pages in RAM once and the pages are shared by all of the applications. The C/C++ run-time library is a perfect example. Many applications use this library. If all these applications link to the static library, the code for functions such as sprintf, strcpy, malloc, and so on exist in memory multiple times. However, if all these applications link to the DLL C/C++ run-time library, the code for these functions is in memory only once, which means that memory is used more efficiently.
• They facilitate resource sharing. DLLs can contain resources such as dialog box templates, strings, icons, and bitmaps. Multiple applications can use DLLs to share these resources.
• They facilitate localization. Applications frequently use DLLs to localize themselves. For example, an application that contains only code and no user interface components can load the DLL containing localized user interface components.
• They help resolve platform differences. The various versions of Windows offer different functions. Frequently, developers want to call new functions if they exist on the host version. However, if your source code contains a call to a new function and your application is about to run on a version of Windows that doesn't offer that function, the operating system loader will refuse to run your process. This is true even if you never actually call the function. If you keep these new functions in a DLL, however, applications can load on an older version of Windows. Of course, you still cannot successfully call the function.
• They can serve special purposes. Windows makes certain features available only to DLLs. For example, you can install certain hooks (set using SetWindowsHookEx and SetWinEventHook) only if the hook notification function is contained in a DLL. You can extend Windows Explorer's shell by creating COM objects that must live inside a DLL. The same is true for ActiveX controls that can be loaded by a Web browser to create rich Web pages.