; but see extension
) is a computer program
that interacts with a host application
(a web browser
or an email client
, for example) to provide a certain, usually very specific, function "on demand". Applications support plugins for many reasons. Some of the main reasons include: enabling third-party developers
to create capabilities to extend an application, to support features yet unforeseen, to reduce the size of an application, and to separate source code
from an application because of incompatible software licenses
Examples of applications and their plug-ins include:
- Email clients use plug-ins to decrypt and encrypt email (Pretty Good Privacy)
- Graphics software use plug-ins to support file formats and process images (Adobe Photoshop)
- Media players use plug-ins to support file formats and apply filters (foobar2000, GStreamer, Quintessential, VST, Winamp, XMMS)
- Packet sniffers use plug-ins to decode packet formats (OmniPeek)
- Remote sensing applications use plug-ins to process data from different sensor types (Opticks)
- Software development environments use plug-ins to support programming languages (Eclipse, jEdit, MonoDevelop)
- Web browsers use plug-ins to play video and presentation formats (Flash, QuickTime, Microsoft Silverlight, 3DMLW)
- Some digital mixing consoles allow plug-ins to extend features such as reverberation effects, equalization and compression.
- Some Web content management system use plug-ins to increase your functionalities (Joomla, WordPress).
The host application provides services which the plug-in can use, including a way for plug-ins to register themselves with the host application and a protocol
by which data is exchanged with plug-ins. Plug-ins are dependent on these services provided by the host application and do not usually work by themselves. Conversely, the host application is independent of the plug-ins, making it possible for plug-ins to be added and updated dynamically without changes to the host application.
Open application programming interfaces (APIs) provide a standard interface, allowing third parties to create plugins that interact with the host application. A stable API allows third-party plug-ins to function as the original version changes and to extend the lifecycle of obsolete applications. The Adobe Photoshop and After Effects plug-in APIs have become a standard and been adopted to some extent by competing applications. Other examples of such APIs include Audio Units and VST.
For example, a network switch may ship with an unoccupied but non-standard port to accommodate various optional physical layer connectors, while games and productivity applications often use plug-in architectures which allow original and third-party publishers to add functionality.
Manufacturers can use plug-ins to create vendor lock-in by limiting upgrade options to only those available from or endorsed by the original manufacturer. IBM's Micro Channel Architecture, technically superior to Industry Standard Architecture as a way to add components to IBM PCs, largely failed to gain wide support due to the difficulty in getting certification for third-party devices. The Microsoft Flight Simulator series is famous for its aircraft add-ons.
Plug-ins and Extensions
Plug-ins are slightly different from extensions
, which modify or add to existing functionality. The main difference is that plug-ins generally rely on the host application's user interface
and have a well-defined boundary to their possible set of actions. Extensions generally have fewer restrictions on their actions, and may provide their own user interfaces. They sometimes are used to decrease the size of the host application and offer optional functions. Mozilla Firefox
and related software use Add-on
as an inclusive term for a category of augmentation modules that are subdivided into plug-ins, themes, search engines and a well-developed extension
system which reduces the feature creep
that plagued the Mozilla Application Suite
Plug-ins can be traced back as far as the mid 1970s, when the EDT text editor
running on the Unisys VS/9 operating system
using the Univac 90/60
series mainframe computer
, provided the ability to run a program from the editor and to allow such program to access the editor buffer, allowing an edit session in memory to be accessed by an external program. The plug-in program could make calls to the editor to have it perform text editing services upon the buffer that the editor shared with the plug-in. This feature was used by the Waterloo
to allow interactive compilation of Fortran
programs being edited by EDT.
Perhaps the first software applications on PCs to include a plug-in function were HyperCard and QuarkXPress on the Macintosh, both released in 1987. In 1988, Silicon Beach Software included plug-in functionality in Digital Darkroom and SuperPaint, and the term plug-in was coined by Ed Bomke. Currently, plug-ins are typically implemented as shared libraries that must be installed in a place prescribed by the host application. HyperCard supported a similar facility, but it was more common for the plug-in code to be included in the HyperCard documents (called stacks) themselves. This way, the HyperCard stack became a self-contained application in its own right, which could be distributed as a single entity that could be run by the user without the need for additional installation steps.
The following plug-in frameworks are organized by programming language
and can be used by software developers to add plug-in capability to their application.