Система X Window: Протоколы и Архитектура

In computing, the X Window System (commonly: X11, or X) is a network transparent windowing system for bitmap displays. This article details the protocols and technical structure of X11.

X uses a client–server model. An X server program runs on a computer with a graphical display and communicates with various client programs. The X server acts as a go between for the user and the client programs, accepting requests on TCP port 6000 plus the display number for graphical output (windows) from the client programs and displaying them to the user (display), and receiving user input (keyboard, mouse) and transmitting it to the client programs. In X, the server runs on the user's computer, while the clients may run on remote machines. This terminology reverses the common notion of client–server systems, where the client normally runs on the user's local computer and the server runs on the remote computer. The X Window terminology takes the perspective that the X Window program is at the centre of all activity, i. e. the X Window program accepts and responds to requests from applications, and from the user's mouse and keyboard input. Therefore, applications (on remote computers) are viewed as clients of the X Window server program. The communication protocol between server and client runs network transparently: the client and server may run on the same machine or on different ones, possibly with different architectures and operating systems. A client and server can communicate securely over the Internet by tunneling the connection over an encrypted connection.

What other graphical user interfaces usually call a window is a top level window in the X Window System. The term window is also used for windows that lie within another window, that is, the subwindows of a parent window. Graphical elements such as buttons, menus, icons, etc. are all realized using windows. A window can only be created as a subwindow of a parent window. This causes the windows to be arranged hierarchically in a tree. The X server automatically creates the root of the tree, called the root window. The top level windows are exactly the direct subwindows of the root window. Visibly, the root window is as large as the screen, and lies behind all other windows.

The X server stores all data about windows, fonts, etc. The client knows identifiers of these objects – integers it can use as names for them when interacting with the server. For example, if a client wishes a window to be created, it requests the server to create one and (in case of success) gets in return an identifier the server associated with the newly created window. The identifier can be later used by the client to request, for example, a string to be drawn in the window. Identifiers are unique to the server, not only to the client; for example, no two windows have the same identifier, even if created by two different clients. A client can access any object given its identifier, even if another client created the object.

Every window has a predefined set of attributes and a set of properties, all stored in the X server and accessible to the clients via appropriate requests. Attributes are data about the window, such as its size, position, background color, etc. Properties are pieces of data that are attached to a window. Unlike attributes, properties have no meaning at the level of the X Window core protocol. A client can store arbitrary data in a property of a window. A property is characterized by a name, a type, and a value. Properties resemble variables in imperative programming languages, in that the application can create a new property with a given name and of a given type and store a value in it. Properties are associated to windows: two properties with the same name can exist on two different windows while having different types and values. Properties are mostly used for inter client communication. For example, the property named WM NAME stores the name for the window; window managers typically read this property and display the name of the window at the top of it. The xprop program can display the properties of a window. In particular, xprop root shows the properties of the root window, which include the X resources (parameters of programs).

Events are packets sent by the server to the client to communicate that something has happened which may interest the client. A client can request the server to send an event to another client; this is used for communication between clients. For example, when a client requests the text that is currently selected, an event is sent to the client that is currently handling the window that holds the selection. The content of a window may be "destroyed" in some conditions (for example, if the window is covered). Whenever an area of destroyed content is made visible, the server generates an Expose event to notify the client that a part of the window has to be drawn. Other events can serve to notify clients of keyboard or mouse input, of the creation of new windows, etc. Some kinds of events are always sent to a client, but most kinds of event are sent only if the client previously stated an interest in them, because clients may only be interested in some kind of events. For example, a client may be interested in keyboard related event but not in mouse related events.

The way the X Window System handles colors can sometimes confuse users, and historically several different modes have been supported. Most modern applications use full color (24 bit color, 8 bits for each of red, green and blue), but old or specialist applications may require a different color mode. Many commercial specialist applications use PseudoColor. The X11 protocol actually uses a single 32 bit unsigned integer called a pixelvalue for representing a single color in most graphic operations. When transferring the intensity of primary colors, a 16 bit integer is used for each color component. The following representations of colors exist; not all of them may be supported on a specific device. DirectColor: A pixel value is decomposed into separate red, green, and blue subfields. Each subfield indexes a separate colormap. Entries in all colormaps can be changed. TrueColor: Same as DirectColor, except that the colormap entries are predefined by the hardware and cannot be changed. Typically, each of the red, green, and blue colormaps provides a (near) linear ramp of intensity. GrayScale: A pixel value indexes a single colormap that contains monochrome intensities. Colormap entries can be changed. StaticGray: Same as GrayScale, except that the colormap entries are predefined by the hardware and cannot be changed. PseudoColor (Chunky): A pixel value indexes a single colormap that contains color intensities. Colormap entries can be changed. StaticColor: Same as PseudoColor, except that the colormap entries are predefined by the hardware and cannot be changed.

Most client programs communicate with the server via the Xlib client library. Beside Xlib, the XCB library operates more closely to X protocol. In particular, most clients use libraries such as Xaw, Motif, GTK+, or Qt which in turn use Xlib for interacting with the server. Qt switched from Xlib to XCB with the 5.0 release, but client programs were almost entirely unaffected by this change.

The X Window core protocol provides mechanisms for communication between clients: window properties and events, in particular the client to client message events. However, it does not specify any protocol for such interactions. Instead, a separate set of inter client communication conventions governs these protocols. The Inter Client Communication Conventions Manual specifies the protocol for the exchange of data via selections and the interaction of applications with the window manager. Some have considered this specification difficult and confusing; consistency of application look and feel and communication is typically addressed by programming to a given desktop environment. The Inter Client Exchange protocol (ICE) specifies a framework for building protocols for interaction between clients, so that programmers can build a specific protocol on top of it. In particular, the X Session Management protocol (XSMP) is a protocol based on ICE that governs the interaction between applications with the session manager, which is the program that takes care of storing the status of the desktop at the end of an interactive session and recovering it when another session with the same user is started again. The freedesktop specifications include newer conventions, including the drag and drop convention Xdnd (used for transferring data by selecting it and dragging in another window) and the embedded application convention Xembed (which details how an application can run in a subwindow of another application).

The mechanisms of selections, cut buffers, and drag and drop in the X Window System allow a user to transfer data from one window to another. Selections and cut buffer are used (typically) when a user selects text or some other data in a window and pastes in a different window. Drag and drop is used when a user selects something in a window, then clicks on the selection and drags it into another window. Since two different applications may handle the two windows, data transfer requires different clients connected with the same X server to interact. The X Window core protocol includes some types of requests and events that are specific to selection exchange, but the transfer is mainly done using the general client to client event sending and window properties, which are not specific to selection transfer. Users can transfer data of different types between clients: it is usually text, but can also be a pixmap, a number, a list of objects, etc. Selections and drag and drop are active mechanisms: after the user selects data in a window, the client handling the window must actively support a protocol for transferring that data to the application requesting it. Cut buffers, by contrast, provide a passive mechanism: when the user selects some text, its content is transferred to a cut buffer, where it remains even if the application handling the window terminates and the window is destroyed.

Roughly, the state of a session is the “state of the desktop” at a given time: a set of windows with their current content. More precisely, it is the set of applications managing these windows and the information that allow these applications to restore the condition of their managed windows if required. A program known as the X session manager saves and restores the state of sessions. Most recognizably, using a session manager permits a user to log out from an interactive session but to find exactly the same windows in the same state when logging in again. For this to work, the session manager program stores the names of the running applications at logout and starts them again at login. In order for the state of the applications to be restored as well (which is needed to restore the content of windows), the applications must be able to save their state of execution upon request from the session manager and load it back when they start again. The X Window System includes a default session manager called xsm. Developers have written other session managers for specific desktop systems. Major examples include , , and for KDE, Xfce, and GNOME respectively.

The program known as the X display manager shows the graphical login prompt in the X Window System. More generally, a display manager runs one or more X servers on the local computer or accepts incoming connections from X servers running on remote computers. The local servers are started by the display manager, which then connects to them to present the user the login screen. The remote servers are started independently from the display manager and connect to it. In this situation, the display manager works like a graphical telnet server: an X server can connect to the display manager, which starts a session; the applications which utilize this session run on the same computer of the display manager but have input and output on the computer where the X server runs (which may be the computer in front of the user or a remote one). The X Window System ships with XDM as the basic supplied display manager. Other display managers include GDM (GNOME), KDM/SDDM (KDE), WDM (using the WINGs widget set used in Window Maker) and entrance (using the architecture used in Enlightenment v.17).

Early widget toolkits for X included Xaw (the Athena Widget Set, 1983), OLIT (OPEN LOOK Intrinsics Toolkit, 1988), XView (1988), Motif (1980s) and Tk. OLIT and XView function as the base toolkits for Sun's legacy OpenWindows desktop environment. Motif provides the base toolkit for the Common Desktop Environment (CDE), the desktop environment used on commercial Unix systems such as Solaris, AIX and HP UX. (Solaris 10 includes both CDE and GNOME, with the latter the preferred desktop environment .) Toolkits developed more recently include Qt (1991 , used by KDE), GTK+ (1997 , used by GNOME), wxWidgets (1992 ), FLTK (1998 ), FOX (1997 ) and fpGUI (2005 current).

Extension Description and notes Low Bandwidth X (LBX) Replaced by VNC tunneled over a secure shell connection, proved faster than LBX. PEX "PHIGS Extension to X"; support for PHIGS 3D scene graph API. GLX with OpenGL is frequently used instead. XImage Extension MIT SHM is used instead. XEvIE X Event Interception Extension. Designed for users who need to intercept all keyboard and mouse events. Removed in X. Org 7.5. XFree86 Misc Superseded by input properties. Removed in X. Org 7.5.