DLL Basics:

A Windows program is an executable file that generally creates one or more windows and uses a message loop to receive user input. Dynamic-link libraries are generally not directly executable, and they generally do not receive messages. They are separate files containing functions that can be called by programs and other DLLs to perform certain jobs. A dynamic-link library is brought into action only when another module calls one of the functions in the library.

The term "dynamic linking" refers to the process that Windows uses to link a function call in one module to the actual function in the library module. "Static linking" occurs during program development when you link various object (.OBJ) modules, run-time library (.LIB) files, and usually a compiled resource (.RES) file to create a Windows .EXE file. Dynamic linking instead occurs at run time.

KERNEL32.DLL, USER32.DLL, and GDI32.DLL; the various driver files such as KEYBOARD.DRV, SYSTEM.DRV, and MOUSE.DRV; and the video and printer drivers are all dynamic-link libraries. These are libraries that all Windows programs can use.

Some dynamic-link libraries (such as font files) are termed "resource-only." They contain only data (usually in the form of resources) and no code. Thus, one purpose of dynamic-link libraries is to provide functions and resources that can be used by many different programs. In a conventional operating system, only the operating system itself contains routines that other programs can call on to do a job. In Windows, the process of one module calling a function in another module is generalized. In effect, by writing a dynamic-link library, you are writing an extension to Windows. Or you can think of DLLs, including those that make up Windows, as extensions to your program.

Although a dynamic-link library module can have any extension (such as .EXE or .FON), the standard extension is .DLL. Only dynamic-link libraries with the extension .DLL will be loaded automatically by Windows. If the file has another extension, the program must explicitly load the module by using the LoadLibrary or LoadLibraryEx function.

You'll generally find that dynamic libraries make most sense in the context of a large application. For instance, suppose you write a large accounting package for Windows that consists of several different programs. You'll probably find that these programs use many common routines. You could put these common routines in a normal object library (with the extension .LIB) and add them to each of the program modules during static linking with LINK. But this approach is wasteful, because each of the programs in this package contains identical code for the common routines. Moreover, if you change one of the routines in this library, you'll have to relink all the programs that use the changed routine. If, however, you put these common routines in a dynamic-link library called, for instance, ACCOUNT.DLL, you've solved both problems. Only the library module need contain the routines required by all the programs, thus requiring less disk space for the files and less memory space when running two or more of the applications simultaneously, and you can make changes to the library module without relinking any of the individual programs.

Dynamic-link libraries can themselves be viable products. For instance, suppose you write a collection of three-dimensional drawing routines and put them in a DLL called GDI3.DLL. If you then interest other software developers in using your library, you can license it to be included with their graphics programs.

Library Meaning:

Part of the confusion surrounding dynamic-link libraries results from the appearance of the word "library" in several different contexts. Besides dynamic-link libraries, we'll also be talking about "object libraries" and "import libraries."

An object library is a file with the extension .LIB containing code that is added to your program's .EXE file in the process called static linking when you run the linker. For example, in Microsoft Visual C++, the normal C run-time object library that you link with your program is called LIBC.LIB.

An import library is a special form of an object library file. Like object libraries, import libraries have the extension .LIB and are used by the linker to resolve function calls in your source code. However, import libraries contain no code. Instead, they provide the linker with information necessary to set up relocation tables within the .EXE file for dynamic linking. The KERNEL32.LIB, USER32.LIB, and GDI32.LIB files included with the Microsoft compiler are import libraries for Windows functions. If you call the Rectangle function in a program, GDI32.LIB tells LINK that this function is in the GDI32.DLL dynamic-link library. This information goes into the .EXE file so that Windows can perform dynamic linking with the GDI32.DLL dynamic-link library when your program is executed.

Object libraries and import libraries are used only during program development. Dynamic-link libraries are used during run time. A dynamic library must be present on the disk when a program is run that uses the library. When Windows needs to load a DLL module before running a program that requires it, the library file must be stored in the directory containing the .EXE program, the current directory, the Windows system directory, the Windows directory, or a directory accessible through the PATH string in the MS-DOS environment.