Plug-in (computing)

In computing, a plug-in (or plugin, add-in, addin, add-on, or addon) is a software component that adds a specific feature to an existing computer program. When a program supports plug-ins, it enables customization.

A theme or skin is a preset package containing additional or changed graphical appearance details, achieved by the use of a graphical user interface (GUI) that can be applied to specific software and websites to suit the purpose, topic, or tastes of different users to customize the look and feel of a piece of computer software or an operating system front-end GUI (and window managers).

Purpose and examples

Applications may support plug-ins to:
* enable third-party developers to extend an application
* support easily adding new features
* reduce the size of an application by not loading unused features
* separate source code from an application because of incompatible software licenses.

Types of applications and why they use plug-ins:

* Digital audio workstations and audio editing software use audio plug-ins to generate, process or analyze sound. Ardour, Audacity, Cubase, FL Studio, Logic Pro X and Pro Tools are examples of such systems.
* Email clients use plug-ins to decrypt and encrypt email. Pretty Good Privacy is an example of such plug-ins.
* Video game console emulators often use plug-ins to modularize the separate subsystems of the devices they seek to emulate. For example, the PCSX2 emulator makes use of video, audio, optical, etc. plug-ins for those respective components of the PlayStation 2.
* Graphics software use plug-ins to support file formats and process images. A Photoshop plug-in may do this.
* Media players use plug-ins to support file formats and apply filters. foobar2000, GStreamer, Quintessential, VST, Winamp, XMMS are examples of such media players.
* Packet sniffers use plug-ins to decode packet formats. OmniPeek is an example of such packet sniffers.
* Remote sensing applications use plug-ins to process data from different sensor types; ''e.g.'', Opticks.
* Text editors and Integrated development environments use plug-ins to support programming languages or enhance the development process ''e.g.'', Visual Studio, RAD Studio, Eclipse, IntelliJ IDEA, jEdit and MonoDevelop support plug-ins. Visual Studio itself can be plugged into other applications via Visual Studio Tools for Office and Visual Studio Tools for Applications.
* Web browsers have historically used executables as plug-ins, though they are now mostly deprecated. Examples include the Adobe Flash Player, a Java virtual machine (for Java applets), QuickTime, Microsoft Silverlight and the Unity Web Player. ( Browser extensions, which are a separate type of installable module, are still widely in use.)

Mechanism

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 for the exchange of data with plug-ins. Plug-ins depend on the services provided by the host application and do not usually work by themselves. Conversely, the host application operates independently of the plug-ins, making it possible for end-users to add and update plug-ins dynamically without needing to make changes to the host application.Mozilla Firefox plugins
– Description of the difference between Mozilla Firefox plugins and extensions under the general term ''add-on''.Wordpress Plug-in API
– Description of the Wordpress Plug-in architecture.

Programmers typically implement plug-ins as shared libraries, which get dynamically loaded at run time. HyperCard supported a similar facility, but more commonly included the plug-in code in the HyperCard documents (called ''stacks'') themselves. Thus the HyperCard stack became a self-contained application in its own right, distributable as a single entity that end-users could run without the need for additional installation-steps. Programs may also implement plug-ins by loading a directory of simple script files written in a scripting language like Python or Lua.

Mozilla definition

In Mozilla Foundation definitions, the words "add-on", " extension" and "plug-in" are not synonyms. "Add-on" can refer to anything that extends the functions of a Mozilla application. Extensions comprise a subtype, albeit the most common and the most powerful one. Mozilla applications come with integrated add-on managers that, similar to package managers, install, update and manage extensions. The term, "plug-in", however, strictly refers to NPAPI-based web content renderers. Mozilla deprecated plug-ins for its products. But UXP-based applications, like web browsers Pale Moon and Basilisk, keep supporting (NPAPI) plug-ins.

History

In the mid 1970s, the EDT text editor ran on the Unisys VS/9 operating system using the UNIVAC Series 90 mainframe computer. It allowed a program to be run from the editor which can access the in-memory edit buffer.''EDT Text Editor Reference Manual'', Cinnaminson, New Jersey: