A dynamic link library (DLL) is a collection of small programs, which can be called upon when needed by the executable program (EXE) that is running. It is Microsoft's implementation of the library concept in the Microsoft Windows and OS/2 operating systems. These libraries usually have the file extension DLL, OCX or DRV. The idea is to have a central library where everyone can obtain the commonly used functions, as they are needed. This cuts down on code, speeds things up, is more efficient, etc. They are called dynamic links because they are put to use only when a program calls on them and they are run in the program’s own memory space. The DLL lets the executable communicate with a specific device such as a printer or may contain source code as well. Some programs may contain many different modules, and each module of the program is contained and distributed in DLLs. The use of DLLs helps promote modularization of code, code reuse, efficient memory usage, and reduced disk space. Therefore, the operating system and the programs load faster, run faster, and take less disk space on the computer. As with EXEs, DLLs can contain code, data, and resources, in any combination.
In a conventional non-shared, static library, sections of code are simply added to the calling program when its executable is built at the linking phase; if two programs use the same routine, the code has to be included in both. With dynamic linking, shared code is placed into a single, separate file. The programs that call this file are connected to it at run time, with the operating system. The notion of building up the operating system from a collection of dynamically loaded libraries is a core concept of Windows that persists even today. DLLs provide the standard benefits of shared libraries, such as modularity. Modularity allows changes to be made to code and data in a single self-contained DLL shared by several applications without any change to the applications themselves.
The advantage of DLL files is that, because they do not get loaded into random access memory (RAM) together with the main program, space is saved in RAM. When and if a DLL file is called, then it is loaded. For example, you are creating a picture through an image editor, the printer DLL file does not need to be loaded into RAM. If you decide to print the image, then the printer DLL file is loaded and a call is made to print.
A DLL is a file that cannot run on its own, it can only run from inside an executable file. To load a DLL file, an executable needs to declare the DLL function. A DLL may have many different functions in it. Then when needed the call is made with the required parameters.
A DLL file is indicated by the extension DLL in its name. Microsoft gives several definitions of a DLL : "A Dynamic Link Library (DLL) is a file of code containing functions that can be called from other executable code (either an application or another DLL). Programmers use DLLs to provide code that they can reuse and to parcel out distinct jobs. Unlike an executable (EXE) file, a DLL cannot be directly run. DLLs must be called from other code that is already executing."
Another way of putting it is that DLL files are like modules that can be plugged into different programs whenever a program needs the particular features that come with the DLL. The original concept behind DLL files was to simplify things. It was recognized that there were many functions common to a lot of software. More than one program can use the functions of a particular DLL at the same time.
A computer today may contain a thousand or more different DLL files. Also, Microsoft seems to tinker endlessly with DLL files, giving rise to many different versions of a file with the same name, not all compatible. Microsoft maintains a database with information about various DLLs to help with version conflicts.
There are several very important DLLs that contain a large number of the basic Windows functions. Since they figure so importantly in the workings of Windows, it is worth noting their names. In order to use a DLL, it has to be registered by having appropriate references entered in the Registry. It sometimes happens that a Registry reference gets corrupted and the functions of the DLL cannot be used anymore. The DLL can be re-registered.
While DLLs are the core of the Windows architecture, they have a number of drawbacks, collectively called "DLL hell". DLL hell often shows up in a Windows alert pop-up that reports something similar to "A Required DLL File, Z.DLL, was not found" or "The procedure entry point Y couldn't be located in X.DLL" when users try to run an application, or during startup. This can also manifest itself more quietly as applications just not working properly. There are a number of problems commonly encountered with DLLs – especially after numerous applications have been installed and uninstalled on a system. The difficulties include conflicts between DLL versions, difficulty in obtaining required DLLs, and having many unnecessary DLL copies. When a program uses a DLL, an issue that is called dependency may cause the program not to run. When a program uses a DLL, a dependency is created. If another program overwrites and breaks this dependency, the original program may not successfully run.
