computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...

, a windowing system (or window system) is software that manages separately different parts of display screens. It is a type of

graphical user interface The GUI ( "UI" by itself is still usually pronounced . or ), graphical user interface, is a form of user interface that allows users to interact with electronic devices through graphical icons and audio indicator such as primary notation, inst ...

(GUI) which implements the WIMP (

window A window is an opening in a wall, door, roof, or vehicle that allows the exchange of light and may also allow the passage of sound and sometimes air. Modern windows are usually glazed or covered in some other transparent or translucent mat ...

icon An icon () is a religious work of art, most commonly a painting, in the cultures of the Eastern Orthodox, Oriental Orthodox, and Catholic churches. They are not simply artworks; "an icon is a sacred image used in religious devotion". The most ...

s, menus, pointer) paradigm for a

user interface In the industrial design field of human–computer interaction, a user interface (UI) is the space where interactions between humans and machines occur. The goal of this interaction is to allow effective operation and control of the machine f ...

. Each currently running application is assigned a usually resizable and usually rectangular surface of the display to present its GUI to the user; these windows may overlap each other, as opposed to a tiling interface where they are not allowed to overlap. Usually a window decoration is drawn around each window. The programming of both the window decoration and of available widgets inside of the window, which are graphical elements for direct user interaction, such as sliders, buttons, etc., is eased and simplified through the use of

widget toolkit A widget toolkit, widget library, GUI toolkit, or UX library is a library or a collection of libraries containing a set of graphical control elements (called ''widgets'') used to construct the graphical user interface (GUI) of programs. Most widge ...

Technical details

The main component of any windowing system is usually called the display server, although alternative denominations such as window server or compositor are also in use. Any application that runs and presents its GUI in a window, is a client of the display server. The display server and its clients communicate with each other over a

communications protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchr ...

, which is usually called display server protocol, the display server being the mediator between the clients and the user. It receives all the input from the

kernel Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine learn ...

, that the kernel receives from all attached

input device In computing, an input device is a piece of equipment used to provide data and control signals to an information processing system, such as a computer or information appliance. Examples of input devices include keyboards, mouse, scanners, cameras ...

s, such as keyboard, pointing devices, or

touchscreen A touchscreen or touch screen is the assembly of both an input ('touch panel') and output ('display') device. The touch panel is normally layered on the top of an electronic visual display of an information processing system. The display is ofte ...

and transmits it to the correct client. The display server is also responsible for the output of the clients to the computer monitor. The output of sound is usually not managed by the display server, but the sound volume is usually handled through GUI applets and it is the display server who decides which applications are on top. A windowing system enables the computer user to work with several programs at the same time. Each program presents its GUI in its own window, which is generally a rectangular area of the screen. From a programmer's point of view, a windowing system implements graphical primitives. For example: rendering

fonts In metal typesetting, a font is a particular size, weight and style of a typeface. Each font is a matched set of type, with a piece (a " sort") for each glyph. A typeface consists of a range of such fonts that shared an overall design. In mod ...

or drawing a line on the screen. It provides an abstraction of the graphics hardware for use by higher-level elements of the graphical interface such as a window manager. A display server protocol can be network capable or even network transparent, facilitating the implementation of thin clients.

Display server

Schema of the layers of the graphical user interface

A display server or window server is a program whose primary task is to coordinate the input and output of its clients to and from the rest of the operating system, the hardware, and each other. The display server communicates with its clients over the display server protocol, a

, which can be network-transparent or simply network-capable. The display server is a key component in any

, specifically the windowing system.

Display server communications protocols

X11

One example of a display server is the

X.Org Server X.Org Server is the free and open-source implementation of the X Window System display server stewarded by the X.Org Foundation. Implementations of the client-side X Window System protocol exist in the form of ''X11 libraries'', which serve a ...

, which runs on top of the kernel (usually a

Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, an ...

-like kernel, such as

Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, w ...

or BSD). It receives user input data (e.g. from evdev on Linux) and passes it to one of its clients. The display server also receives data from its clients; it processes the data, it does the compositing and on Linux it passes the data to one of three kernel components –

DRM DRM may refer to: Government, military and politics * Defense reform movement, U.S. campaign inspired by Col. John Boyd * Democratic Republic of Madagascar, a former socialist state (1975–1992) on Madagascar * Direction du renseignement milita ...

, gem or KMS driver. The component writes the data into the

framebuffer A framebuffer (frame buffer, or sometimes framestore) is a portion of random-access memory (RAM) containing a bitmap that drives a video display. It is a memory buffer containing data representing all the pixels in a complete video frame. Mode ...

and content of the framebuffer is transmitted to the connected screen and displayed. X relies on GLX. One of the implementations of display server concept is

X Window System The X Window System (X11, or simply X) is a windowing system for bitmap displays, common on Unix-like operating systems. X provides the basic framework for a GUI environment: drawing and moving windows on the display device and interacting wi ...

, in particular its actually used version –

and

Xlib Xlib (also known as libX11) is an X Window System protocol client library written in the C programming language. It contains functions for interacting with an X server. These functions allow programmers to write programs without knowing the ...

and XCB client libraries. The X.Org Server is a display server, but in its current implementation it relies on a second program, the compositing window manager, to do the compositing. Examples are Mutter or

KWin KWin is a window manager for the X Window System and a Wayland compositor. It is released as a part of KDE Plasma 5, for which it is the default window manager. KWin can also be used on its own or with other desktop environments. KWin can be c ...

. Notable examples of display servers implementing the X11 display server protocol are

XFree86 XFree86 is an implementation of the X Window System. It was originally written for Unix-like operating systems on IBM PC compatibles and was available for many other operating systems and platforms. It is free and open source software under the X ...

, XQuartz and Cygwin/X, while client libraries implementing the X11 display server protocol are

and XCB.

Wayland

Display servers that implement the Wayland display server protocol are called Wayland compositors. Like any display server, a Wayland compositor is responsible for handling input and output for its clients and – in contrast to X11 – additionally for the

compositing Compositing is the process or technique of combining visual elements from separate sources into single images, often to create the illusion that all those elements are parts of the same scene. Live action, Live-action shooting for compositing is ...

. Examples are

Weston Weston may refer to: Places Australia * Weston, Australian Capital Territory, a suburb of Canberra * Weston, New South Wales * Weston Creek, a residential district of Canberra * Weston Park, Canberra, a park Canada * Weston, Nova Scotia * ...

, Mutter,

or Enlightenment. Wayland compositors communicate with Wayland clients over the

Wayland display server protocol Wayland may refer to: Computers * Wayland (display server protocol), a graphical display system for Unix-like computers Fiction * Jace Wayland, a character in the ''Mortal Instruments'' book series * Wayland (''Star Wars''), a planet in the '' ...

. This protocol defines that clients can directly write data into the framebuffer using the EGL rendering API. The display server still gets to decide which window is on top and thus visible to the user and also still is responsible for passing data regarding to input devices from evdev to its clients. Wayland is used to a certain degree in some Linux desktop distributions, such as Fedora. It is also well suited for mobile computing and has been adopted, for example, by the smartphone- and tablet-focused projects Tizen,

Sailfish OS Sailfish OS is a Linux-based operating system based on free software, and open source projects such as Mer as well as including a closed source UI. The project is being developed by the Finnish company Jolla. The OS first shipped with the ...

and

AsteroidOS AsteroidOS is an open source operating system designed for smartwatches. It is available as a firmware replacement for some Android Wear devices. The motto for the AsteroidOS project is "Hack your wrist." Wareable.com reviewed version 1.0 and gav ...

. An implementation of Wayland is available under the

MIT License The MIT License is a permissive free software license originating at the Massachusetts Institute of Technology (MIT) in the late 1980s. As a permissive license, it puts only very limited restriction on reuse and has, therefore, high license comp ...

, the libwayland-client and libwayland-server libraries. There is an ongoing effort to add Wayland support to

ChromeOS ChromeOS, sometimes stylized as chromeOS and formerly styled as Chrome OS, is a Linux-based operating system designed by Google. It is derived from the open-source ChromiumOS and uses the Google Chrome web browser as its principal user interfa ...

Mir

The Mir display server comes with its own Mir display server protocol which is different from those used by X11 and Wayland. Mir additionally supports the X11 protocol. It was developed by

Canonical The adjective canonical is applied in many contexts to mean "according to the canon" the standard, rule or primary source that is accepted as authoritative for the body of knowledge or literature in that context. In mathematics, "canonical examp ...

and was intended to be the display server of choice for

Ubuntu Ubuntu ( ) is a Linux distribution based on Debian and composed mostly of free and open-source software. Ubuntu is officially released in three editions: '' Desktop'', ''Server'', and ''Core'' for Internet of things devices and robots. All ...

. As of 2017, it has been replaced with the Wayland display server for desktop editions of Ubuntu. There are implementations of the Mir display server, the libmir-server and the libmir-client libraries available under the GPLv3.

SurfaceFlinger

Google Google LLC () is an American Multinational corporation, multinational technology company focusing on Search Engine, search engine technology, online advertising, cloud computing, software, computer software, quantum computing, e-commerce, ar ...

developed a display server called SurfaceFlinger for Android (another Linux kernel-based operating system primarily for mobile devices): Everything in Android is rendered to a "surface"; "surfaces" are produced by applications and placed into a queue that is managed by SurfaceFlinger. Yet another Android-specific solution is "Gralloc". Gralloc handles device memory i.e. it does allocation, arbitration, it handles synchronization via Android/Linux fence file descriptors. Gralloc competes with other solutions like e.g. Mesa's Generic Buffer Management (GBM) or Nvidia's EGLStreams. The Gralloc hardware abstraction layer (HAL) is used to allocate the buffers that underlie "surfaces". For compositing in Android, Surfaces are sent to SurfaceFlinger, which uses OpenGL ES to do the compositing. Hardware Composer HAL (HWC) was introduced in Android 3.0 and has evolved steadily over the years. Its primary purpose is to determine the most efficient way to composite buffers with the available hardware. As a HAL, its implementation is device-specific and usually done by the display hardware OEM.

Quartz Compositor

For Apple's

macOS macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and lapt ...

family of operating systems,

Quartz Compositor Quartz Compositor is the display server (and at the same time the compositing window manager) in macOS. It is responsible for presenting and maintaining rasterized, rendered graphics from the rest of the Core Graphics framework and other ren ...

fulfils the tasks of a display server and of a window manager in the windowing system.

Desktop Window Manager

For Microsoft Windows, from

Windows Vista Windows Vista is a major release of the Windows NT operating system developed by Microsoft. It was the direct successor to Windows XP, which was released five years before, at the time being the longest time span between successive releases of ...

onward,

Desktop Window Manager Desktop Window Manager (DWM, previously Desktop Compositing Engine or DCE) is the compositing window manager in Microsoft Windows since Windows Vista that enables the use of hardware acceleration to render the graphical user interface of Windows ...

enables the use of hardware acceleration to render the graphical user interface. It was originally created to enable portions of the new "Windows Aero" user experience, which allowed for effects such as transparency, 3D window switching and more. It is also included with Windows Server 2008, but requires the "Desktop Experience" feature and compatible graphics drivers to be installed. From Windows 8 onwards DWM can't be disabled and is software rendered if no suitable graphics card is installed.

List of windowing systems

For
Unix-like A Unix-like (sometimes referred to as UNX or nix) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-li ...
operating systems

8½ ' (Italian title: , ) is a 1963 surrealist comedy-drama film directed and co-written (with Tullio Pinelli, Ennio Flaiano and Brunello Rondi) by Italian filmmaker Federico Fellini. The metafictional narrative centers on Guido Anselmi, played by M ...

and rio for Plan 9 * FramebufferUI in-kernel windowing system * ''HP Windows/9000'' (on early versions of HP-UX) * Sapphire for the

PERQ The PERQ, also referred to as the Three Rivers PERQ or ICL PERQ, was a pioneering workstation computer produced in the late 1970s through the early 1980s. In June 1979, the company took its very first order from the UK's Rutherford Appleton La ...

* ManaGeR (MGR) * Metisse *

Mir ''Mir'' (russian: Мир, ; ) was a space station that operated in low Earth orbit from 1986 to 2001, operated by the Soviet Union and later by Russia. ''Mir'' was the first modular space station and was assembled in orbit from 1986 to&n ...

NeWS News is information about current events. This may be provided through many different media: word of mouth, printing, postal systems, broadcasting, electronic communication, or through the testimony of observers and witnesses to events. N ...

OpenWindows OpenWindows is a discontinued desktop environment for Sun Microsystems workstations which combined SunView, NeWS, and X Window System protocols. OpenWindows was included in later releases of the SunOS 4 and Solaris operating systems, until its r ...

* NeXT DPS * Orbital (

Redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

) * Qt Extended *

(

Mac OS X macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and la ...

) * SunView *

Twin Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...

(Text WINdows) * W Window System * Wayland *

* XFast * Xynth

For Windows NT-family operating systems

(DWM) in Microsoft Windows ( Vista and later) * ReactOS Explorer * Classic Shell *

Pokki Pokki is a free digital distribution platform and Windows Shell extension by SweetLabs, Inc. that alters the start menu to a look and feel like the second generation start menu used before Windows 8. The extension also adds additional functional ...

RetroUI RetroUI is a utility for Windows 8 (applies to the Pro edition) and Windows Server 2012 (the Server edition) by Thinix that modifies Windows Shell, especially the Start menu and taskbar. RetroUI places a Start menu and start button on the Windows ...

* Talisman Desktop

Web windowing systems

* Dojo * ExtJS *

TIBCO TIBCO Software Inc. is an American business intelligence software company founded in 1997 in Palo Alto, California. It has headquarters in Palo Alto, California, and offices in North America, Europe, Asia, the Middle East, Africa and South A ...

General Interface * Web Window Manager

Other

* DM * GEM *

Intuition Intuition is the ability to acquire knowledge without recourse to conscious reasoning. Different fields use the word "intuition" in very different ways, including but not limited to: direct access to unconscious knowledge; unconscious cognition; ...

Microwindows In computing, Nano-X is a windowing system which is full featured enough to be used on a Personal Computer, PC, an embedded system or a personal digital assistant, PDA. It is an Open Source project aimed at bringing the features of modern graphic ...

* MiniGUI * OOHG *

Visi On VisiCorp Visi On was a short-lived but influential graphical user interface-based operating environment program for IBM compatible personal computers running MS-DOS. Although Visi On was never popular, as it had steep minimum system requirement ...

* VWS (VAX Workstation Software) Some systems such as Microsoft Windows ( XP, 9x and earlier), the classic Mac OS (version 9 and earlier), and

Palm OS Palm OS (also known as Garnet OS) was a mobile operating system initially developed by Palm, Inc., for personal digital assistants (PDAs) in 1996. Palm OS was designed for ease of use with a touchscreen-based graphical user interface. It is pro ...

, contain a windowing system which is integrated with the OS.

References

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