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-X Window System Architecture Overview HOWTO
-!!!X Window System Architecture Overview HOWTO
-!Daniel Manrique
-
-roadmr@entropia.com.mx
-
-
-__Revision History__Revision 1..12001-05-22Revised by: dmSome grammatical corrections, pointed out by Bill !StaehleRevision 1.02001-05-20Revised by: dmInitial LDP release.
-
-
-
-
-
-This document provides an overview of the X Window
-System's architecture, give a better understanding of its design,
-which components integrate with X and fit together to provide a
-working graphical environment and what choices are there regarding
-such components as window managers, toolkits and widget libraries, and
-desktop environments.
-
-
-
-
-
-----; __Table of Contents__; 1. Preface; 2. Introduction; 3. The X Window System Architecture: overview; 4. Window Managers; 5. Client Applications; 6. Widget Libraries or toolkits; 7. What we have so far; 8. Desktop environments to the rescue; 9. Specific Desktop Environments; 10. How it all fits together; 11. A day in the life of an X system; 12. Copyright and License----
-!!!1. Preface
-
-This document aims to provide an overview of the X Window System's
-architecture, hoping to give people a better understanding of why it's
-designed the way it's designed, which components integrate with X and
-fit together to provide a working graphical environment and what
-choices are there regarding those components.
-
-
-
-We explore several concepts that get mentioned a lot but might be a
-bit unclear for those without a technical background, such as widgets
-and toolkits, window managers and desktop environments. Some examples
-of how these components interact during day-to-day use of applications
-are provided.
-
-
-
-This document is, deliberately, not too technically oriented. It's
-based on the author's (empirical) knowledge of the subject, and while
-it's primarily meant as a non-technical introduction, it can certainly
-benefit from any kind of comments, further examples and explanations,
-and technical corrections. The author welcomes all questions and
-comments regarding this document and can be reached at ''roadmr@entropia.com.mx''.
-
-----
-!!!2. Introduction
-
-Back when UNIX was a new thing, around 1970, graphical user
-interfaces were only a weird thing being played with in a laboratory
-(Xerox's PARC to be precise). Nowadays, however, any operating system
-in hopes of being competitive needs to have a GUI subsystem. GUIs are
-supposed to be easier to use. This is not much of a concern under
-UNIX, which has traditionally been, to some extent, pretty
-user-hostile, preferring versatility over ease of use. However, there
-are several reasons why a GUI is desirable even on a UNIX system. For
-instance, given UNIX's multitasking nature, it's natural to have a lot
-of programs running at any given time. A GUI gives more control over
-how things are displayed on-screen, thus providing with better
-facilities for having a lot of programs on-screen at the same time.
-Also, some kinds of information are better displayed in graphical form
-(some, even, can only be displayed in graphical form; like pr0n and
-other inherently graphical data).
-
-
-
-Historically, UNIX has had a lot of improvements done by
-academic types. A good example is the BSD networking code added to it
-in the late 1970's, which was, of course, the product of work at
-the University of California at
-Berkeley. As it turns out, the X Window System (also called X, but
-never X Windows), which is the foundation for most GUI subsystems
-found in modern UNIX (unices?), Linux and the BSD's included, was also
-the result of an academic project, namely the Athena project at the Massachusetts Institute of Technology (MIT).
-
-
-
-Unix has been a multiuser, multitasking, timesharing operating
-system since its beginnings. Also, since the incorporation of
-networking technologies, it's had the ability to allow a user to
-connect remotely and perform work on the system. Previously this was
-accomplished either via dumb serial terminals, or network connections
-(the legendary telnet).
-
-
-
-When the time came to develop a GUI system that could run
-primarily under Unix, these concepts were kept in mind and
-incorporated into the design. Actually, X has a pretty complex design,
-which has often been mentioned as a disadvantage. However, because of
-its design, it's also a really versatile system, and this will become
-quite clear as we explain how all the parts comprising a GUI under
-Unix fit together.
-
-
-
-Before taking a look at X's architecture, a really brief tour of
-its history, and how it ended up on your Linux system, is in order.
-
-
-
-X was developed by the Athena project, and released in 1984. In
-1988 an entity called the "X Consortium" took over X, and to this day
-handles its development and distribution. The X specification is
-freely available, this was a smart move as it has made X almost
-ubiquitous. This is how XFree86 came to be. XFree86 is the
-implementation of X we use on our Linux computers. XFree86 also works
-on other operating systems, like the *BSD lineage, OS/2 and maybe
-others. Also, despite its name, XFree86 is also available for other
-CPU architectures.
-
-----
-!!!3. The X Window System Architecture: overview
-
-X was designed with a client-server architecture. The
-applications themselves are the clients; they communicate with the
-server and issue requests, also receiving information from the server.
-
-
-
-The X server maintains exclusive control of the display and
-services requests from the clients. At this point, the advantages of
-using this model are pretty clear. Applications (clients) only need to
-know how to communicate with the server, and need not be concerned
-with the details of talking to the actual graphics display device. At
-the most basic level, a client tells the server stuff like "draw a
-line from here to here", or "render this string of text, using this
-font, at this position on-screen".
-
-
-
-This would be no different from just using a graphics library to
-write our application. However the X model goes a step further. It
-doesn't constrain the client being in the same computer as the server.
-The protocol used to communicate between clients and server can work
-over a network, or actually, any "inter-process communication
-mechanism that provides a reliable octet stream". Of course, the
-preferred way to do this is by using the TCP/IP protocols. As we can
-see, the X model is really powerful; the classical example of this is
-running a processor-intensive application on a Cray computer, a
-database monitor on a Solaris server, an e-mail application on a small
-BSD mail server, and a visualization program on an SGI server, and
-then displaying all those on my Linux workstation's screen.
-
-
-
-So far we've seen that the X server is the one handling the
-actual graphics display. Also, since it's the X server which runs on
-the physical, actual computer the user is working on, it's the X
-server's responsibility to perform all actual interactions with the
-user. This includes reading the mouse and keyboard. All this
-information is relayed to the client, which of course will have to
-react to it.
-
-
-
-X provides a library, aptly called Xlib, which handles all
-low-level client-server communication tasks. It sounds obvious that,
-then, the client has to invoke functions contained within Xlib to get
-work done.
-
-
-
-At this point everything seems to be working fine. We have a
-server in charge of visual output and data input, client applications,
-and a way for them to communicate between each other. In picturing a
-hypothetical interaction between a client and a server, the client
-could ask the server to assign a rectangular area on the screen. Being
-the client, I'm not concerned with where i'm being displayed on the
-screen. I just tell the server "give me an area X by Y pixels in
-size", and then call functions to perform actions like "draw a line
-from here to there", "tell me whether the user is moving the mouse in
-my screen area" and so on.
-
-----
-!!!4. Window Managers
-
-However, we never mentioned how the X server handles
-manipulation of the clients' on-screen display areas (called
-windows). It's obvious, to anyone who's ever used a GUI, that you need
-to have control over the "client windows". Typically you can move and
-arrange them; change size, maximize or minimize windows. How, then,
-does the X server handle these tasks? The answer is: it doesn't.
-
-
-
-One of X's fundamental tenets is "we provide mechanism, but not
-policy". So, while the X server provides a way (mechanism) for window
-manipulation, it doesn't actually say how this manipulation behaves
-(policy).
-
-
-
-All that mechanism/policy weird stuff basically boils down to
-this: it's another program's responsibility to manage the on-screen
-space. This program decides where to place windows, gives mechanisms
-for users to control the windows' appearance, position and size, and
-usually provides "decorations" like window titles, frames and buttons,
-that give us control over the windows themselves. This program, which
-manages windows, is called (guess!) a "window manager".
-
-
-
-"The window manager in X is just another client -- it is not
-part of the X window system, although it enjoys special privileges --
-and so there is no single window manager; instead, there are many,
-which support different ways for the user to interact with windows and
-different styles of window layout, decoration, and keyboard and
-colormap focus."
-
-
-
-The X architecture provides ways for a window manager to
-perform all those actions on the windows; but it doesn't actually
-provide a window manager.
-
-
-
-There are, of course, a lot of window managers, because since
-the window manager is an external component, it's (relatively) easy to
-write one according to your preferences, how you want windows to look,
-how you want them to behave, where do you want them to be, and so on.
-Some window managers are simplistic and ugly (twm); some are flashy
-and include everything but the kitchen sink (enlightenment); and
-everything in between; fvwm, amiwm, icewm, windowmaker, afterstep,
-sawfish, kwm, and countless others. There's a window manager for every
-taste.
-
-
-
-A window manager is a "meta-client", whose most basic mission is
-to manage other clients. Most window managers provide a few additional
-facilities (and some provide a lot of them). However one piece of
-functionality that seems to be present in most window managers is a
-way to launch applications. Some of them provide a command box where
-you can type standard commands (which can then be used to launch
-client applications). Others have a nice application launching menu of
-some sort. This is not standardized, however; again, as X dictates no
-policy on how a client application should be launched, this
-functionality is to be implemented in client programs. While,
-typically, a window manager takes on this task (and each one does it
-differently), it's conceivable to have client applications whose sole
-mission is to launch other client applications; think a program
-launching pad. And of course, people have written large amounts of
-"program launching" applications.
-
-----
-!!!5. Client Applications
-
-Let's focus on the client programs for a moment. Imagine you
-wanted to write a client program from scratch, using only the
-facilities provided by X. You'd quickly find that Xlib is pretty
-spartan, and that doing things like putting buttons on screen, text,
-or nice controls (scrollbars, radio boxes) for the users, is terribly
-complicated.
-
-
-
-Luckily, someone else went to the trouble of programming these
-controls and giving them to us in a usable form; a library. These
-controls are usually known as "widgets" and of course, the library is
-a "widget library". Then I just have to call a function from this
-library with some parameters and have a button on-screen. Examples of
-widgets include menus, buttons, radio buttons, scrollbars, and
-canvases.
-
-
-
-A "canvas" is an interesting kind of widget, because it's
-basically a sub-area within the client where i can draw
-stuff. Understandably, since I shouldn't use Xlib directly, because
-that would interfere with the widget library, the library itself gives
-a way to draw arbitrary graphics within the canvas widget.
-
-
-
-Since the widget library is the one actually drawing the
-elements on-screen, as well as interpreting user's actions into input,
-the library used is largely responsible for each client's aspect and
-behavior. From a developer's point of view, a widget library also has
-a certain API (set of functions), and that might define which widget
-library i'll want to use.
-
-----
-!!!6. Widget Libraries or toolkits
-
-The original widget library, developed for the Athena Project,
-is of course the Athena widget library, also known as Athena
-Widgets. It's very basic, very ugly, and the usage is not intuitive by
-today's standards (for instance, to move a scrollbar or slider
-control, you don't drag it; instead, you click the right button to
-scroll up and the left button to scroll down). As such, it's pretty
-much not used a lot these days.
-
-
-
-Just as it happens with window managers, there are a lot of
-toolkits, with different design goals in mind. One of the earliest
-toolkits is the well-known Motif, which was part of the Open Software
-Foundation's Motif graphical environment, consisting of a window
-manager and a matching toolkit. The OSF's history is beyond the scope
-of this document. the Motif toolkit, being superior to the Athena
-widgets, became widely used in the 1980's and early 1990's.
-
-
-
-These days, Motif is not a popular toolkit choice. It's not free
-(speech), and OSF Motif costs money if you want a developer license
-(i.e. to compile your own programs with it), altough it's OK to
-distribute a binary linked against Motif. Perhaps the best-known Motif
-application, for Linux users at least, is Netscape
-Navigator/Communicator (prior to Mozilla).
-
-
-
-For a while Motif was the only decent toolkit available, and
-there's a lot of Motif software around. Of course people started
-developing alternatives, and there are plenty of toolkits, such as
-XForms, FLTK and a few others.
-
-
-
-Motif is not heard of much these days, specially in the free
-software world. The reason is that there are now better alternatives,
-in terms of licensing, performance (Motif is widely regarded as quite
-a pig) and features.
-
-
-
-One such toolkit, the widely known and used Gtk, was
-specifically created to replace Motif in the GIMP project (one
-possible meaning of Gtk is "GIMP !ToolKit, altough, with its widespread
-use, it could be interpreted as the GNU !ToolKit). Gtk is now very
-popular because it's relatively lightweight, feature-rich, extensible
-and totally free (speech). The .6 release of the GIMP included
-"Bloatif has been zorched" in the changelog. This sentence is a
-testament to Motif's bloatedness.
-
-
-
-Another very popular toolkit these days is Qt. It was not too
-well-known until the advent of the KDE project, which utilizes Qt for
-all its GUI elements. We certainly won't get into Qt's licensing
-issues and the KDE/GNOME disjunctive. Gtk gets a lengthy mention
-because its history as a Motif replacement is interesting; Qt gets a
-brief mention because it's really popular.
-
-
-
-Finally, another alternative worth mentioning is !LessTif. The
-name is a pun on Motif, and !LessTif aims to be a free, API-compatible
-replacement for Motif. It's not clear to what extent !LessTif aims to
-be used in new development, rather than just helping those with Motif
-code use a free alternative while they (conceivably) port their apps
-to some other toolkit.
-
-----
-!!!7. What we have so far
-
-Up to this point we have an idea of how X has a client-server
-architecture, where the clients are our application programs. Under
-this client-server graphic system, we have several possible window
-managers, which manage our screen real estate; we also have our client
-applications, which are where we actually get our work done, and
-clients can be programmed using several possible different toolkits.
-
-
-
-Here's where the mess begins. Each window manager has a
-different approach to managing the clients; the behavior and
-decorations are different from one to the next. Also, as defined by
-which toolkit each client uses, they can also look and behave
-differently from each other. Since there's nothing that says authors
-have to use the same toolkit for all their applications, it's
-perfectly possible for a user to be running, say, six different
-applications, each written using a different toolkit, and they all
-look and behave differently. This creates a mess because behavior
-between the apps is not consistent. If you've ever used a program
-written with the Athena widgets, you'll notice it's not too similar to
-something written using Gtk. And you'll also remember it's a mess
-using all these apps which look and feel so different. This basically
-negates the advantage of using a GUI environment in the first place.
-
-
-
-On a more technical standpoint, using lots of different toolkits
-increases resource usage. Modern operating systems support the concept
-of dynamic shared libraries. This means that if I have two or three
-applications using Gtk, and I have a dynamic shared version of Gtk,
-then those two or three applications share the same copy of Gtk, both
-on the disk and in memory. This saves resources. On the other hand, if
-I have a Gtk application, a Qt application, something Athena-based, a
-Motif-based program such as Netscape, a program that uses FLTK and
-another using XForms, I'm now loading six different libraries in
-memory, one for each of the different toolkits. Keep in mind that all
-the toolkits provide basically the same functionality.
-
-
-
-There are other problems here. The way of launching programs
-varies from one window manager to the next. Some have a nice menu for
-launching apps; others don't, and they expect us to open a
-command-launching box, or use a certain key combination, or even open
-an xterm and launch all your apps by invoking the commands. Again,
-there's no standarization here so it becomes a mess.
-
-
-
-Finally, there are niceties we expect from a GUI environment
-which our scheme hasn't covered. Things like a configuration utility,
-or "control panel"; or a graphical file manager. Of course, these can
-be written as client apps. And, in typical free software fashion,
-there are hundreds of file managers, and hundreds of system
-configuration programs, which conceivably, further the mess of having
-to deal with a lot of disparate software components.
-
-----
-!!!8. Desktop environments to the rescue
-
-Here's where the concept of a desktop environment kicks in. The
-idea is that a desktop environment provides a set of facilities and
-guidelines aiming to standardizing all the stuff we mentioned so that
-the problems we mentioned earlier are minimized.
-
-
-
-The concept of a desktop environment is something new to people
-coming for the first time to Linux because it's something that other
-operating systems (like Windows and the Mac OS) intrinsically
-have. For example, MacOS, which is one of the earliest graphical user
-interfaces, provides a very consistent look-and-feel during the entire
-computing session. For instance, the operating system provides a lot
-of the niceties we mentioned: it provides a default file manager (the
-finder), a systemwide control panel, and single toolkit that all
-applications have to use (so they all look the same). Application
-windows are managed by the system (strictly speaking there's a window
-manager working there). Finally, there are a set of guidelines that
-tell developers how their applications should behave, recommend
-control looks and placement, and suggest behaviors according to those
-of other applications on the system. All this is done in the sake of
-consistency and ease of use.
-
-
-
-This begs the question, "why didn't the X developers do things
-that way in the first place?". It makes sense; after all, it would
-have avoided all the problems we mentioned earlier. The answer is that
-in designing X, its creators chose to make it as flexible as
-possible. Going back to the policy/mechanism paradigm, the MacOS
-provides mostly policies. Mechanisms are there, but they don't
-encourage people to play with those. As a result I lose versatility;
-if I don't like the way MacOS manages my windows, or the toolkit
-doesn't provide a function I need, I'm pretty much out of luck. This
-doesn't happen under X, altough as seen before, the price of
-flexibility is greater complexity.
-
-
-
-Under Linux/Unix and X, it all comes down to agreeing on stuff
-and sticking to it. Let's take KDE for example. KDE includes a single
-window manager (kwm), which manages and controls the behavior of our
-windows. It recommends using a certain graphic toolkit (Qt), so that
-all KDE applications look the same, as far as their on-screen controls
-go. KDE further extends Qt by providing a set of environment-specific
-libraries (kdelibs) for performing common tasks like creating menus,
-"about" boxes, program toolbars, communicating between programs,
-printing, selecting files, and other things. These make the
-programmer's work easier and standardize the way these special
-features behave. KDE also provides a set of design and behavior
-guidelines to programmers, with the idea that, if everybody follows
-them, programs running under KDE will both look and behave very
-similarly. Finally, KDE provides, as part of the environment, a
-launcher panel (kpanel), a standard file manager (which is, at the
-time being, Konqueror), and a configuration utility (control panel)
-from which we can control many aspects of our computing environment,
-from settings like the desktop's background and the windows' titlebar
-color to hardware configurations.
-
-
-
-The KDE panel is an equivalent to the MS Windows taskbar. It
-provides a central point from which to launch applications, and it
-also provides for small applications, called "applets", to be
-displayed within it. This gives functionality like the small, live
-clock most users can't live without.
-
-----
-!!!9. Specific Desktop Environments
-
-We used KDE as an example, but it's by no means the earliest
-desktop environment for Unix systems. Perhaps one of the earliest is
-CDE (Common Desktop Environment), another sibling of the OSF. As per
-the CDE FAQ: "The Common Desktop Environment is a standard desktop for
-UNIX, providing services to end-users, systems administrators, and
-application developers consistently across many platforms." The key
-here is consistency. However CDE wasn't as feature-rich and easy as it
-needed to be. Along with Motif, CDE has practically disappeared from
-the free software world, having been replaced by better alternatives.
-
-
-
-Under Linux, the two most popular desktop environments are KDE
-and GNOME, but they're not the only ones. A quick internet search will
-reveal about half a dozen desktop environments: GNUStep, ROX,
-GTK+XFce, UDE, to name a few. They all provide the basic facilities we
-mentioned earlier. GNOME and KDE have had the most support, both from
-the community and the industry, so they're the most advanced ones,
-providing a large amount of services to users and applications.
-
-
-
-We mentioned KDE and the components that provide specific
-services under that environment. As a good desktop environment, GNOME
-is somewhat similar in that. The most obvious difference is that GNOME
-doesn't mandate a particular window manager (the way KDE has kwm). The
-GNOME project has always tried to be window manager-agnostic,
-acknowledging that most users get really attached to their window
-managers, and forcing them to use something that manages windows
-differently would detract from their audience. Originally GNOME
-favored the Enlightenment window manager, and currently their
-preferred window manager is Sawfish, but the GNOME control panel has
-always had a window manager selector box.
-
-
-
-Other than this, GNOME uses the Gtk toolkit, and provides a set
-of higher-level functions and facilities through the gnome-libs set of
-libraries. GNOME has its own set of programming guidelines in order to
-guarantee a consistent behavior between compliant applications; it
-provides a panel (called just "panel"), a file manager (gmc, altough
-it's probably going to be superseded by Nautilus), and a control panel
-(the gnome control center).
-
-----
-!!!10. How it all fits together
-
-Each user is free to choose whichever desktop environment feels
-the best. The end result is that, if you use an all-kde or all-gnome
-system, the look and feel of the environment is very consistent; and
-your applications all interact between them pretty nicely. This just
-wasn't possible when we had apps written in a hodgepodge of different
-toolkits. The range of facilities provided by modern desktop
-environments under Linux also enable some other niceties, like
-component architectures (KDE has Kparts and GNOME uses the Bonobo
-component framework), which allow you to do things like having a live
-spreadsheet or chart inside a word processing document; global
-printing facilities, similar to the printing contexts found in
-Windows; or scripting languages, which let more advanced users write
-programs to glue applications together and have them interact and
-cooperate in interesting ways.
-
-
-
-Under the Unix concept of "desktop environment", you can have
-programs from one environment running in another. I could conceivably
-use Konqueror within GNOME, or Gnumeric under KDE. They're just
-programs, after all. Of course the whole idea of a desktop environment
-is consistency, so it makes sense to stick to apps that were designed
-for your particular environment; but if you're willing to cope with an
-app that looks "out of place" and doesn't interact with the rest of
-your environment, you are completely free to do so.
-
-----
-!!!11. A day in the life of an X system
-
-This is an example of how a typical GNOME session goes, under a
-modern desktop environment in a Linux system. It's very similar to how
-things work under other environments, assuming they work on top of X.
-
-
-
-When a Linux system starts X, the X server comes up and
-initializes the graphic device, waiting for requests from
-clients. First a program called gnome-session starts, and sets up the
-working session. A session includes things such as applications I
-always open, their on-screen positions, and such. Next, the panel gets
-started. The panel appears at the bottom (usually) and it's sort of a
-dashboard for the windowing environment. It will let us launch
-programs, see which ones are running, and otherwise control the
-working environment. Next, the window manager comes up. Since we're
-using GNOME, it could be any of several different window managers, but
-in this case we'll assume we're running Sawfish. Finally, the file
-manager comes up (gmc or Nautilus). The file manager handles
-presentation of the desktop icons (the ones that appear directly on
-the desktop). At this point my GNOME environment is ready to work.
-
-
-
-So far all of the programs that have been started are clients,
-connecting to the X server. In this case the X server happens to be in
-the same computer, but as we saw before, it need not be.
-
-
-
-We'll now open an xterm to type some commands. When we click on
-the xterm icon, the panel spawns, or launches, the xterm
-application. It's another X client application, so it starts, connects
-to the X server and begins displaying its stuff. When the X server
-assigns screen space for my xterm, it lets the window manager
-(Sawfish) decorate the window with a nice titlebar, and decide where
-it will be on screen.
-
-
-
-Let's do some browsing. We click on the Netscape icon on the
-panel, and up comes a browser. Keep in mind that this browser doesn't
-use GNOME's facilities, nor does it use the Gtk toolkit. It looks a
-bit out of place here... also, it doesn't interact very nicely with
-the rest of the environment. I'll open the "File" menu. Motif is
-providing the on-screen controls, so it's the Motif library's job to
-make the appropriate calls to the underlying Xlib, draw the necessary
-on-screen elements to display the menu and let me select the "exit"
-option, closing the application.
-
-
-
-Now I open a Gnumeric spreadsheet and start doing some stuff. At
-some point I need to do some work on the xterm I had open, so I click
-on it. Sawfish sees that, and, being in charge of managing windows,
-brings the xterm to the top and gives it focus so I can work there.
-
-
-
-After that, I go back to my spreadsheet, now that I'm finished I
-want to print my document. Gnumeric is a GNOME application, so it can
-use the facilities provided by the GNOME environment. When I print,
-Gnumeric calls the gnome-print library, which actually communicates
-with the printer and produces the hard copy I need.
-
-----
-!!!12. Copyright and License
-
-Copyright (c) 2001 by Daniel Manrique
-
-
-
- Permission is granted to copy, distribute and/or modify this document
-under the terms of the ''GNU Free Documentation
-License'', Version 1.1 or any later version
-published by the Free Software Foundation with no Invariant Sections,
-no Front-Cover Texts, and no Back-Cover Texts. A copy of the license
-can be found
here.
+Describe [HowToXWindowOverviewHOWTO]
here.
