Glossary of Terms - What Is DLL File (Dynamic Link Library) [MiniTool Wiki]

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DLL, short for Dynamic Link Library, is a type of file containing code and data that can be used by multiple programs at the same time. In Windows, many application files are not complete and executable and they are split into relatively independent dynamic link libraries, or DLL files. When executing a specific procedure, the corresponding DLL file will be called. An application can use multiple DLL files, and a DLL file may be also used by different applications.

DLL files contain functions of various programs. When a program needs to call a function, it will load the DLL firstly, then get the address of the function and finally start a call. By using DLL files, the program does not need to load all of the code at the beginning of the running but do that when needing a certain function.

In Windows, there is a DLL file called Comdlg32.dll that can perform common dialog box related functions. And each application can use the functionality included in this DLL to call an Open dialog box. This facilitates code reuse and efficient memory usage.

In the operating system, there are many types of files that are implemented as DLL, including ActiveX Controls (.ocx) files (eg: calendar control), Control Panel (.cpl) files (eg: an item that is located in Control Panel) and Device driver (.drv) files (eg: printer driver).

DLL Advantages

Uses fewer resources

A DLL can reduce the duplication of code loaded on the hard drive and in physical memory when multiple programs use the same library of functions. This has a great influence on the performance of not only the program running in the foreground but also other applications running on Windows operating system.

Promotes modular architecture

A DLL contributes to the development of modular programs. This can help users to develop applications that require multiple language versions or a modular architecture. For example, the accounting program has many modules that can be dynamically loaded when running.

Simplify Deployment and Installation

When needing to update or fix a function within a DLL file, the deployment and installation of the DLL do not require the reestablishment of a link between the program and the DLL. In addition, if multiple programs use the same DLL, these programs will benefit from the update or fix.

Linking Methods

When loading a DLL in a program, users can use two linking methods to call the exported DLL functions. And they are load-time dynamic linking and run-time dynamic linking.

In load-time dynamic linking, a program can make explicit calls to exported DLL functions like local functions. To use this linking method, provide a header (.h) file and an import library (.lib) file when compiling and linking the application. Then, the linker will provide the system with the information required to load the DLL and resolve the exported DLL function locations at load time.

In run-time dynamic linking, a program can call either the LoadLibraryEx function or the LoadLibrary function so as to load the DLL at runtime. After successfully loading the DLL, users use the GetProcAddress function to get the address of the exported DLL function that they want to call. When using this linking, no import library files are required.

DLL Troubleshooting

It is likely that DLL issues happen. Here, several tools are available to help users to troubleshoot DLL problems, for example, Dependency Walker and DLL Universal Problem Solver. Additionally, users can choose to fix DLL files by a Windows OS features, a backup or downloading specific files.

Tip: If users get an error messages saying DLL file is missing or not found, what should be done? This post - Missing DLL Files in Windows 10/8/7? Useful Ways Are Here is helpful.

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