Dynamic-link library: Encyclopedia II - Dynamic-link library - Background

Dynamic-link library - Background

The initial purpose for DLLs was saving both disk space and memory required for applications. Any code which many applications share could be separated into a DLL which only exists as a single disk file and a single instance in memory. Extensive use of DLLs allowed early versions of Windows to work under tight memory conditions.

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. This basic form of modularity allows for relatively compact patches and service packs for large applications, such as Microsoft Office, Microsoft Visual Studio, and even Microsoft Windows itself.

Another benefit of the modularity is the use of generic interfaces for plug-ins. A single interface may be developed which allows old as well as new modules to be integrated seamlessly at run-time into pre-existing applications, without any modification to the application itself. This concept of dynamic extensibility is taken to the extreme with ActiveX.

With this many benefits, using DLLs also has a drawback: the DLL hell, when several applications conflict on which version of a shared DLL library is to be used. Such conflicts can usually be resolved by placing the different versions of the problem DLL into the applications' folders, rather than a system-wide folder; however, this also nullifies the savings provided by using shared DLLs. Currently, Microsoft .NET is targeted as a solution to the problem of DLL hell by allowing side-by-side coexistence of different versions of a same shared library. With modern computers which have plenty of disk space and memory, it can be a reasonable approach.

Adapted from the Wikipedia article "Background", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki