X is a network protocol (usually carried over TCP/IP or Unix-domain sockets). X clients, usually applications, utilities, or some such, usually deal with X by using Xlib or any of a number of libraries that sit atop Xlib (motif, Xaw, gtk, etc.). They connect to an X server, which interprets X protocol requests, performing the requested drawing operations on some sort of bit-mapped device, and sending keystrokes, mouse clicks, and other events to the client as appropriate.

The X Window System reference implementation, originally developed at MIT, was owned by The Open Group when this writeup was first written. Now it belongs to the X.Org foundation. Softwarewise, it consists of Xlib, the X toolkit (Xt), a number of clients, and a number of servers specific to certain video cards (or other display mechanisms). The X Protocol specifications are also considered to be part of the X Window System. At one time, motif (which is not available under a free licence, though clones such as lesstif exist) was considered almost a de facto part of X; once XFree86 (later XOrg) acheived massive popularity, however, this was not so much the case. Many proprietary Unix vendors that ship with X still include motif, however.

One of X's greatest assets is its network-transparence. Of course, it's really only transparent over a medium-speed connection or better; X over a modem is very sluggish. Most people who do not have philosophical objections to the entire X model feel that the colour model and font handling are its weakest points.

Those who do feel that the entire design of X is fundamentally broken usually point to NeWS; 8 1/2, the Plan 9 windowing system; Berlin; and whatever Apple's latest offering might be. They may be right. Quite possibly the only reason X triumphed over NeWS was that X was much more open.

The X window system is a client-server based GUI environment. It was first developed as part of the Athena project at the Massachusetts Institute of Technology with the help of DEC. First released in 1984.

The version most commonly used nowadays is XFree86 which is based on X386.

Now that powerful processors and video cards are common, the client-server functionality of the X Window System is starting to fade. It's still implemented, but the main use now is on desktop computers.

Many people complain about X, and it is crufty, but I like it.

Sure, as a programmer I've seen the horror that is the result of Xlib, colormaps, and some really obscure and outdated things. But GDK abstracts that pretty well. And hey, if you can find any other graphics library with these many applications that's streamable over a network (I do use remote X sometimes, and it is great!), I'd like to see it.

Many people complain about no standard widget library or window manager. These people are truly clueless. That's the beauty of X! It only does graphics. It's an independent network protocol. It's a foundation. Build on it. Use whatever window manager you like. If you want something slim, use wm2. If you want a swiss army chainsaw bloat thingy that looks reel purty, use enlightenment. If you want something in between, there's WindowMaker. If you like C++, code for KDE. If you want C, code for GTK+. If you want commercial UNIX, code for Motif. If you've got a thing for Objective C, code for GNUstep. Do you really want somebody like Microsoft telling you how to manage your windows, what your widget API is like, et cetera?

As for all this talk of desktop shit, I don't care about that. My computer is used as a workstation, not a desktop. It's running fvwm2 for chrissake.

So, if you complain about X, shame on you. Most people who do are ignorant (the word "slashbot" comes to mind). The people with valid points, there are some, and I agree with them in part, but hey, use what works!

Dear Frater 219/tftv256 and X Consortium, while we innocent, ignorant end users respect your right to call your software as you want, we hope you would also sometimes consider us and our needs.

  1. X - I realize that you consider X Windows System, Version 11 to be, having been in past, present and until the day when X Windows System, Version 11 becomes obsolete, among the 26 most important software engineering projects, but your opinion is not a common consensus.

    Let me clarify: Using a single letter, especially such letter as 'X' that has been attributed to be the most mysterious character and is in general considered one of the more significant letters, implies certain importance your software may possess. Single letter always sounds more striking than a long string of letters such as this one (865 so far). Now, the supply of single-letter names is very scarce. In english, there are no more than 26 such available!

    Conclusion: Although letter 'X' to refer to this software is the most common practise today, it carries implied superiority to competition with more elaborate names, such as Berlin or Microsoft Windows.

  2. X Windows System: This name is proper: it is descriptive, telling the exact purpose of software in its name, which is of great convenience to those unfortunate ones who are not familiar with your Product, but still carries an unique identifier, 'X', with it to distinguish from other Windowing Systems. However, this name as such is the 'long form name', which generally aren't very practical in everyday speech.

    Attempt to create a short form name: 'X Windows System', hmm, well, if we choose 'X', we lose any connection to purpose of software, in addition to problems mentioned in item one. 'Windows System' loses identifier and thus is unacceptable, 'X System' is still too generic since there can be many systems. 'Windows' is taken, and 'System' is, as name, worth null. Thus, 'X Windows' would be the most logical choice. Alternative spellings might be X-windows, Xwin or a software disaster of biblical portions. And lo!, so we see why and how the common populace reached name 'x-windows'.

  3. X Version 11: This item is really the item one with version number added. Thus, it carries all the faults mentioned in item one. In addition, it is notable that version number as a part of name may not be the most sensible thing. Usually version numbers are only mentioned when the number truly is significant. Consider the difference between sentences "I ran X on linux and it crashed" vs. "I ran X on linux 2.3.10 and it crashed". The latter clearly implies that the crash in second sentence was caused specifically by this development version of linux kernel, while first only slightly suggests that it might have been something about linux, but don't go killing penguins yet. So, using 'X version 11' as a general name for your window system constantly carries a stress on the version number with it, which is not always desirable.

    In addition to above, name 'X version 11' is too long and commonly collapses back to 'X', which turns this into item one.

  4. X Windows System, Version 11: See items 2 and 3.

  5. X11: This name seems to be the common standard name for X Windows System, Version 11 in technical contexts. Likely it is because it is sufficiently short and conveys the message without the ambiguity that plain 'X' would have. Still, it is just a hybrid of items 1 and 3, and as such, has the implied self-promotion and the hollow emphasis on version number.

Finally, a question: Why is 'X-windows' such a bad name? Are you afraid that it will get confused with Microsoft Windows? Admittedly Microsoft's choice of operating system name was among the most poor naming choices ever made, at least to the software community, but instead of yielding, fight. Do you, a proud defender of your home nation, stop calling yourself chauvinist because the treasonous feminists have given this proud term an undeserved implication of misogynia? Does a circus geek with long traditions of being laughed at by the audience stop calling himself a geek, simply because some pencil-necked kids who like computers and have pale skin have caused the term 'geek' to be overloaded with ideas of obsession with computers? No! Call yourself 'X-windows', and do it with pride, and one day they will be confusing Microsoft Windows with X-Windows, not the other way around.

The X Window system was originally designed to run exactly three programs: xterm, xclock, and xload. Originally these were meant to be started in the .xinitrc file, using manually set location and geometry settings. Any additional instances of those programs were to be started also using manually-set location and geometry via your xterm.

As an afterthought, the developers thought they'd throw in a window manager to let people move things around. They produced the Tab Window Manager, twm. It's still included with the base X distribution on any Unix machine. Try running it sometime, it's pretty sad.

The reason X sucks so much at being a GUI is because it was not actually designed to do anything but run a few minor graphical hacks that were hardly better than ASCII graphics (read: xload, xmh), and to allow people to run two terminals side by side. It doesn't support innate drag and drop. It doesn't have any OS-level human interface conventions. It doesn't even have an active desktop without the aid of programs which run cheap hacks to give it one.

This is why X-Windows is so user-hostile compared to MacOS...and even Microsoft Windows. Funny that I should have such opinions, when my own nick is a play on the naming convention of X applications...

The following can be found in the default xinitrc file (/usr/X11R6/xinit/xinitrc), dated August 22nd, 1991... (note: this is still included in current distribution of XFree86 3.x)

# start some nice programs

twm &
xclock -geometry 50x50-1+1 &
xterm -geometry 80x50+494+51 &
xterm -geometry 80x20+494-0 &
exec xterm -geometry 80x66+0+0 -name login

Try creating a new user account. Log in with that, and rm -f ~/.xinitrc. Then, startx. AND WITNESS THE SUCK.

Regarding msgs about how X11 is a network model and not a GUI: I'm not talking about the semantic details of how it works. For all practical purposes, anything running under X11 has the same technical limitations. The modularity of it is, really, what does it in. It already runs in greater penalty of the CPU and memory than it ought to; piling Gnome or KDE on top of that is just plain ludicrous. The only "user friendly" elements must be written by 3rd party developers, and this causes these basic UI functions to be nonstandard and higher-level than they are with any other GUI model. So, if it pleases pedants, I'm not putting down the protocol itself, I'm putting down the legacy that it has wrought.

The term "X Windows" as a nickname for the X Window System isn't heresy -- it's potential trademark infringement.

MIT created a network protocol for graphical application presentation, and called it "X". (It was an improvement over an earlier protocol called "W". Hence the name -- they weren't just trying to sound cool.) The software that ran X was called the "X Window System", but people commonly called it "X Windows". It caught on with Unix vendors, who shipped it on their workstations. Meanwhile, on the other side of the continent, Microsoft created a crude graphical shell for MS-DOS, and called it "Windows". Microsoft got a trademark on the name "Windows" as applied to graphical software.

When someone has a trademark on a name, nobody else is allowed to sell or distribute a similar product with a similar name. That's trademark law for you -- and "Windows" and "X Windows" are rather similar names, especially to a lawyer. The consortium that had come to maintain X got worried, and stuck a warning in the documentation saying that "X Windows" was not what their product was called.

According to the X manpage, the X Consortium requests that the following names be used for the current release of their software:

(Actually, the X Consortium is not in any position to request anything, seeing as it no longer exists.)

The X Window System is the foundation of the graphical environments on Unix and Unix-like operating systems, such as Linux. Since Unix predates the graphical user interface and widespread availability of computer graphics, it has no built-in facility for graphics at the lowest levels of the system. However, as the de facto standard for graphical applications under Unix-like systems, X has been near-ubiquitous on such systems for the last 10-15 years. Unlike most graphics layers, the protocol between applications and the system is network-transparent, allowing programs from multiple machines to appear on a single display without requiring external support.

X History

The X Window System was originally developed at MIT as part of their pioneering computing access program, Project Athena. X originated as an adaptation of Stanford University's W network window system to a much more efficient network protocol, completed by Bob Scheifler in May 1984. Unlike other graphics systems of the time, such as the Macintosh's QuickDraw, X was designed to be both hardware-independent and vendor-independent, since Project Athena intended to connect all systems at MIT regardless of their origin.

The initial implementation of X was quite limited, and over the next year it was extended in a number of backwards-incompatible ways. The result was X Version 9 or X9, which was the first version released under the permissive 'MIT License' in September 1985. This license would have an important effect on the future of the X Window System, allowing anyone to use and modify the code for any purpose whatsoever, provided that the original author's copyright notice is preserved and the recipient understands that the original author provides no warranty of any kind. These permissions have been broadly abused in the intervening time, but also ensured the widespread availability of X throughout the Unix world.

It soon became apparent that, despite the best efforts of X's designers, the X9 and X10 protocols were still quite hardware-dependent. A comprehensive redesign was begun under the joint auspices of MIT and Digital Equipment Corporation, which resulted in the release of X Version 11 in September 1987. Since then, there have been no backwards-incompatible changes in the core X protocol, an impressive record of compatibility.

X11 continued to be developed by a consortium centred at MIT for several years, with many important improvements made to the core X codebase. These changes did not modify the core X11 protocol itself but improved its efficiency, programming interface, and included tools. Ports to a variety of systems, including DEC's non-Unix OS VMS, were made, some of which were contributed back to the MIT X distribution, while some of them were retained as proprietary software by their authors, as the MIT license permits. During the late 1980s and early 1990s X was one theatre of the so-called 'Unix wars' with various vendors adding proprietary extensions to their own versions in an attempt to achieve differentiation from their competitors.

In 1991, X11 Release 5 was released, containing many improvements including a port to the x86-based 'PC' architecture called X386. With the emergence of free, open source Unix variants and lookalikes for the 32-bit x86 platform, this port grew in importance and soon had its own community of maintainers. These maintainers eventually broke with the original authors after they began to make their new versions proprietary, forming the XFree86 project. Over the course of the 1990s, this project would move to the forefront of X development.

The MIT X Consortium shut down at the end of 1996, with its series of successors becoming increasingly ineffectual due to the influence of differentiation-craving proprietary Unix vendors. The leverage of XFree86 as the most active developer of open source X prevented the reference implementation from switching to a new restrictive license for X11R6.4, but the reference implementation continued to rot under neglect.

XFree86 succeeded in modernizing the architecture of X's core with XFree86 4.0, but afterwards XFree86 development began to stagnate. Development was controlled quite tightly by a Core Team with commit access to the repository, and several members of this team were reluctant to give up control. While the burgeoning Linux desktop was making ever heavier demands of X, the XFree86 developers were slow to change in response.

This situation came to a head in early 2004, when the lead developer of XFree86, David Dawes, unilaterally changed the XFree86 license. The new license was widely vilified as being incompatible with the most common free software license, the GNU GPL. As a result a group of developers, led by longtime X developers Keith Packard and Jim Gettys, created a fork from the last MIT-licensed XFree86 release under the auspices of freedesktop.org. A new X consortium, the X.Org Foundation, was created by the old, moribund X.org organization and freedesktop.org, and the XFree86 fork, generally called Xorg, was imported as the new reference X implementation. A resurgence in X development followed, with the resulting changes making a noticeable difference in the modern Linux desktop.

Under the stewardship of the new X.org, the reference X distribution quickly replaced XFree86 in most Linux distributions and BSD flavours. Though their first release, Xorg 6.7.0, was very little different to the poorly-licensed XFree86 4.4, the writing was on the wall and most distributions were running Xorg by early 2005. Rarely has a software fork replaced its progenitor so quickly, but virtually all X developers doing new work were founders of the fork or quickly joined. This included Keith Packard, architect of many of XFree86's innovative features, whose vocal public clashes with the XFree86 core development team were key in generating the will to fork.

The first Xorg release with a full release cycle, 6.8.0 followed in September 2004, which included a number of important additions in different degrees of stability. Most prominent but most experimental was the XComposite extension, which allowed much greater control over the display of windows. While this initially was used to implement true transparent windows, it permits a variety of uses including screen magnification for accessibility and the presence of ordinary X windows within a fully-3D interface such as OpenCroquet.

The much-delayed release of Xorg 7.0 brought with it many important improvements, but none as prominent as the division of the core X distribution into discrete modules with their own release cycles, which succeeded in accelerating the introduction of X improvements into distributions and also in lowering the barrier to entry for new X developers. Since 7.0 there have not been as many sweeping changes but a slow, ceaseless introduction of new features, bugfixes, and cleanups, similar to the Linux kernel or GNOME. X has finally joined the rest of the modern free software ecosystem.

X Architecture

The X distribution primarily consists of a display management program called the X server, a protocol that other programs, called X clients can use to display themselves on the X server's screen, and a library for writing X clients without detailed network programming, Xlib. A complete X install also includes a variety of basic clients and utilities for managing the general X environment. These parts were traditionally bundled together into a single software package, but, as of Xorg version 7, the X distribution is divided into a number of individual packages with their own release schedule.

The original X was amazingly primitive by modern standards. Lacking a facility for managing windows or launching programs, early X sessions consisted of a preselected list of programs started and positioned in the .xinitrc configuration file. Any additional programs run from a terminal emulator also required manual specification of position and size on the command line. The X developers' response to this was typical of the X development process; rather than adding window management and automatic window borders to the X server, they added a special X application, the window manager, whose sole purpose is to handle the placement, movement, and decoration of windows.

The introduction of window management highlighted an issue that has come back to haunt X many times over the years; many operations that are fundamental 'server-side' parts of other display layers require cooperation by X clients to implement. The Inter-Client Communication Conventions Manual (ICCCM) was promulgated in an attempt to standardize this cooperation, but it is a complex standard that is difficult to implement and contains a number of now-archaic restrictions. This confusion was responsible for at least a decade of poor X usability before high-level free software toolkits that abstracted away the costs of the ICCCM became ubiquitous in the early 2000s.

Another facility that X does not provide is a set of standard graphical interface elements. The X protocol is extremely low-level, dealing mainly with maintaining square drawing canvasses and routing input events to applications. An additional library above Xlib called a widget toolkit is required to produce buttons, scrollbars, menus, and text boxes, in another instance of the general X convention 'mechanism, not policy'. X distributions do contain an extremely primitive widget library called the Athena widgets or Xaw, but unfortunately Xaw widgets are more primitive than even the original Macintosh widgets, and have a number of conventions quite unlike any other widget set, especially in its idiosyncratic scroll bars.

As a result, a variety of widget toolkits have appeared over X's long lifetime. Early on, the OpenLook and Motif toolkits were available, though both were reasonably proprietary. Motif gained prominence as the basis for the first integrated desktop environment for X, the Common Desktop Environment or CDE. However, as the 1990s wore on, the heavy Windows 3.1-like Motif controls fell out of fashion, and began to dwindle when a group of students at Berkeley, frustrated with the Motif licensing terms, began developing their more modern, free replacement, GTK+. With the GIMP image editor driving GTK+ development and the Norwegian startup Trolltech releasing the X11 version of their cross-platform Qt GUI toolkit to the community, Motif's downfall became inevitable. It is a testament to the power of X's architecture that the system was not stranded with an antiquated 1980s look-and-feel and programming interface but could move ahead maintaining both backwards and forwards compatibility.

An important part of the core X protocol is a mechanism for adding new capabilities to the X protocol, generally referred to as extensions. The availability of this feature has allowed the interaction between the X server and its clients to be drastically altered without breaking compatibility. A number of important features of the modern toolkits depend on extensions, rather than the core protocol, such as the SHAPE extension for non-rectangular windows and widgets, and the MIT-SHM extension for the transfer of images through shared memory rather than the command stream.

The modularity of the X protocol also allows for many of the client-side libraries to be compatibly changed without modifying the server. Most X applications communicate with the server through Xlib, a library that attempts to smooth some of the complexities of the full X protocol by handling certain things itself. The assumptions that were made during Xlib's development in the mid-1980s have not aged well, and many of the details that it 'takes care of' have become a hindrance to modern toolkits as many of these details have become more relevant over the years. In particular, the single command queue used by Xlib limits the use of multi-threading in applications. Furthermore, the ancient Xlib code is known to have errors that are almost impossible to find in its ageing codebase.

When Xlib was written, many X applications were built directly upon it, necessitating an API that was somewhat 'user-friendly'. In the last decade, very few programs have been written on bare Xlib, using a higher-level toolkit such as GTK+ or Qt instead. As such, the authors of the new XCB (X C Bindings) library have reasoned that a small, simple library for interfacing directly to the X protocol would be useful, as its added difficulties would only be a hindrance to relatively few developers. XCB has now reached full stability, with the Xlib in Xorg 7.2 and later having been re-written on top of XCB to maintain compatibility. The toolkits have not yet been rewritten to use XCB instead of Xlib but most believe that it is only a matter of time.

X Today

Under the stewardship of the X.Org Foundation and freedesktop.org, the standard X distribution has broken out of its development rut and has become one of the more active areas for improvement of the free software desktop. With the KDE and GNOME desktop environments building modern user interfaces on X and their underlying Qt and GTK+ toolkits allowing applications to be built with similar ease as under other major platforms, X has emerged as an important competitor for the graphical interfaces of other major operating systems.

An important part of modern X is its efficiency in the most common case of a server and client running on the same machine. Lacking a network separating the two programs, they can communicate through an efficient inter-process communication method such as Unix domain sockets, and use shared-memory image transfer to speed up the most message-intensive operations. When these methods are used, X is similar in weight to competing display systems, which have themselves adopted a window server/client architecture similar to X.

One of the first tasks undertaken by X.Org was a full modularization of the core X distribution. Following this separation, the X server, client libraries, graphics drivers, and utilities could be updated independently, with semi-yearly comprehensive 'katamari' releases maintaining a common baseline. The separation of drivers from the main X server code base has enabled rapid development of many drivers, especially those for Intel and ATI graphics hardware.

The most important modern addition to X's display model is the Xrender extension. Xrender adds Porter-Duff composition as a basic display operation, and although this sounds somewhat obscure its effects are widespread and impressive. The most common use of Xrender is to display smooth, anti-aliased graphics and text, which the Xrender architecture is designed to accelerate. The advent of anti-aliased text coincided with the wholesale replacement of X's antiquated, user-hostile font system, finally ending most Linux users' font headaches.

The composition capabilities of Xrender can be used to more spectacular effect in combination with the Xcomposite extension. Xcomposite allows a particular client, called the compositing manager, to intercept window contents before they are drawn to the screen, and then to combine the windows in whatever way it sees fit to produce the final screen contents. This opens the door for advanced desktop effects similar to those used by Mac OS X and Windows Vista, with or without GPU acceleration. The Compiz window manager was built from the ground up to include a compositing manager, and is used in a number of modern distributions including Ubuntu and openSUSE to provide translucent windows, drop shadows, and Expose-like window switching, among other things.

Composition managers are not only for fancy 'bling' effects, though. Accessibility applications benefit greatly from the ability to modify the appearance of a window before it is shown, as magnification or increased contrast of windows can help visually impaired users (or even users on poor displays) use the computer's GUI. Composition can also help with the use of virtual desktops, as the desktop can be made to 'slide' across, providing a visual cue to the user. While this is most useful to the novice user, power users may also find benefits from the composition manager. Compiz can display virtual desktops on the faces of a 3D polygonal prism, called the 'desktop cube' for the common case of four desktops, which can be freely rotated to visualize and choose a desktop.

The new Plasma desktop in the KDE 4 desktop uses composition as the basis of its attempt to redefine the basic GUI elements for the better. While it is still beta software and currently implements a relatively conventional, though very pretty, desktop, Plasma aims to be a test-bed for new styles of human-computer interaction. Surely, the promise of pervasive desktop composition has yet to be fully realized by the more creative programmers in the free software world.


The X Window System has come a long way from its cradle at MIT to become the keystone of the widespread free software desktop. Through all of this it has maintained an astounding amount of compatibility; a client from the late 1980s will still be able to display on today's newest servers, even without a recompile (though they would be unlikely to work on the same machine). While the primitive nature of its drawing model limited it to the most powerful computers when it was first released, X11's overhead is now comparable to or even less than competing display layers from Apple and Microsoft. It remains under active development and will continue to be optimized and extended for many years to come.

This writeup is copyright 2008 D.G. Roberge and is released under the Creative Commons Attribution-NonCommercial-ShareAlike licence. Details can be found at http://creativecommons.org/licenses/by-nc-sa/3.0/ .

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