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Xlib (also known as libX11) is an
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 wit ...
protocol client
library A library is a collection of materials, books or media that are accessible for use and not just for display purposes. A library provides physical (hard copies) or digital access (soft copies) materials, and may be a physical location or a vir ...
written in the
C programming language ''The C Programming Language'' (sometimes termed ''K&R'', after its authors' initials) is a computer programming book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally designed and implemented the language, as well as ...
. It contains
function Function or functionality may refer to: Computing * Function key, a type of key on computer keyboards * Function model, a structured representation of processes in a system * Function object or functor or functionoid, a concept of object-oriente ...
s for interacting with an X
server Server may refer to: Computing *Server (computing), a computer program or a device that provides functionality for other programs or devices, called clients Role * Waiting staff, those who work at a restaurant or a bar attending customers and su ...
. These functions allow
programmer A computer programmer, sometimes referred to as a software developer, a software engineer, a programmer or a coder, is a person who creates computer programs — often for larger computer software. A programmer is someone who writes/creates ...
s to write programs without knowing the details of the
X protocol The X Window System core protocolRobert W. Scheifler and James Gettys: ''X Window System: Core and extension protocols, X version 11, releases 6 and 6.1'', Digital Press 1996, RFC 1013Grant EdwardsAn Introduction to X11 User Interfaces/ref> is the ...
. Few applications use Xlib directly; rather, they employ other libraries that use Xlib functions to provide
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 widg ...
s: *
X Toolkit Intrinsics X Toolkit Intrinsics (also known as Xt, for X toolkit) is a library that implements an API to facilitate the development of programs with a graphical user interface (GUI) for the X Window System. It can be used in the C or C++ languages. Th ...
(Xt) * Athena widget set (Xaw) * Motif * FLTK *
GTK GTK (formerly GIMP ToolKit and GTK+) is a free and open-source cross-platform widget toolkit for creating graphical user interfaces (GUIs). It is licensed under the terms of the GNU Lesser General Public License, allowing both free and propriet ...
* Qt (X11 version) * Tk * SDL (Simple DirectMedia Layer) * SFML (Simple and Fast Multimedia Library) Xlib appeared around 1985, and is used in
GUI 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, inste ...
s for many
Unix-like A Unix-like (sometimes referred to as UN*X 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 system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...
s. A re-implementation of Xlib was introduced in 2007 using
XCB XCB (''X protocol C-language Binding'') is a library implementing the client-side of the X11 display server protocol. XCB is written in the C programming language and distributed under the MIT License. The project was started in 2001 by B ...
.


Data types

The main types of data in Xlib are the Display structure and the types of the identifiers. Informally, a display is a physical or virtual device where graphical operations are done. The Display structure of the Xlib library contains information about the display, but more importantly it contains information relative to the channel between the client and the server. For example, in a
Unix-like A Unix-like (sometimes referred to as UN*X 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 system, the Display structure contains the file handle of the
socket Socket may refer to: Mechanics * Socket wrench, a type of wrench that uses separate, removable sockets to fit different sizes of nuts and bolts * Socket head screw, a screw (or bolt) with a cylindrical head containing a socket into which the hexag ...
of this channel (this can be retrieved using the ConnectionNumber macro.) Most Xlib functions have a Display structure as an argument because they either operate on the channel or are relative to a specific channel. In particular, all Xlib functions that interact with the server need this structure for accessing the channel. Some other functions need this structure, even if they operate locally, because they operate on data relative to a specific channel. Operations of this kind include for example operations on the event queue, which is described below. Windows, colormaps, etc. are managed by the server, which means that the data about their actual implementation is all stored in the server. The client operates on these objects by using their ''identifiers''. The client cannot directly operate on an object, but can only request the server to perform the operation specifying the identifier of the object. The types Windows, Pixmap, Font, Colormap, etc. are all identifiers, which are 32-bit integers (just as in the X11 protocol itself). A client 'creates' a window by requesting that the server create a window. This is done via a call to an Xlib function that returns an identifier for the window, that is, a number. This identifier can then be used by the client for requesting other operations on the same window to the server. The identifiers are unique to the server. Most of them can be used by different applications to refer to the same objects. For example, two applications connecting with the same server use the same identifier to refer to the same window. These two applications use two different channels, and therefore have two different structures; however, when they request operations on the same identifier, these operations will be done on the same object.


Protocol and events

The Xlib functions that send requests to the server usually do not send these requests immediately but store them in a buffer, called the ''request buffer''. The term ''request'' in this case refers to the request from the client that is directed to the server: the request buffer can contain all kinds of requests to the server, not only those having a visible effect on the screen. The request buffer is guaranteed to be flushed (i.e., all requests done so far are sent to the server) after a call to the functions XSync or XFlush, after a call to a function that returns a value from the server (these functions block until the answer is received), and in some other conditions. Xlib stores the received events in a queue. The client application can inspect and retrieve events from the queue. While the X server sends events asynchronously, applications using the Xlib library are required to explicitly call Xlib functions for accessing the events in the queue. Some of these functions may block; in this case, they also flush the request buffer. Errors are instead received and treated asynchronously: the application can provide an error handler that will be called whenever an error message from the server is received. The content of a window is not guaranteed to be preserved if the window or one of its parts are made not visible. In this case, the application are sent an Expose event when the window of one part of it is made visible again. The application is then supposed to draw the window content again.


Functions

The functions in the Xlib library can be grouped in: # operations on the connection (XOpenDisplay, XCloseDisplay, ...); # requests to the server, including requests for operations (XCreateWindow, XCreateGC,...) and requests for information (XGetWindowProperty, ...); and # operations that are local to the client: operations on the event queue (XNextEvent, XPeekEvent, ...) and other operations on local data (XLookupKeysym, XParseGeometry, XSetRegion, XCreateImage, XSaveContext, ...)


Example

The following program creates a window with a little black square in it: /* Simple Xlib application for creating a window and drawing a box in it. gcc input.c -o output -lX11 */ #include #include #include #include int main(void) The client creates a connection with the server by calling XOpenDisplay. It then requests the creation of a window with XCreateSimpleWindow. A separate call to XMapWindow is necessary for mapping the window, that is, for making it visible on the screen. The square is drawn by calling XFillRectangle. This operation can only be performed after the window is created. However, performing it once may not be enough. Indeed, the content of the window is not always guaranteed to be preserved. For example, if the window is covered and then uncovered again, its content might require being redrawn. The program is informed that the window or a part of it has to be drawn by the reception of an Expose event. The drawing of the window content is therefore made inside the loop handling the events. Before entering this loop, the events the application is interested in are selected, in this case with XSelectInput. The event loop waits for an incoming event: if this event is a key press, the application exits; if it is an expose event, the window content is drawn. The function XNextEvent blocks and flushes the request buffer if there is no event in the queue.


Other libraries

Xlib does not provide support for buttons, menus, scrollbars, etc. Such widgets are provided by other libraries, which in turn use Xlib. There are two kinds of such libraries: * libraries built atop of the
X Toolkit Intrinsics X Toolkit Intrinsics (also known as Xt, for X toolkit) is a library that implements an API to facilitate the development of programs with a graphical user interface (GUI) for the X Window System. It can be used in the C or C++ languages. Th ...
library (Xt), which provides support for widgets but does not provide any particular widget; specific widgets are provided by
widget set Widget may refer to: * Widget (beer), a device placed in cans and bottles of beer to aid in the generation of froth * Widget (economics), a placeholder name for an unnamed, unspecified, or hypothetical manufactured good or product * Software widg ...
libraries that use Xt, such as
Xaw X Athena Widgets or Xaw is a GUI widget library for the X Window System. Developed as part of Project Athena, Xaw was written under the auspices of the MIT X Consortium as a sample widget set built on X Toolkit Intrinsics (Xt); Xt and Xaw are ...
and Motif; * libraries that provide widget sets using Xlib directly, without the Xt library, such as the X versions of
GTK GTK (formerly GIMP ToolKit and GTK+) is a free and open-source cross-platform widget toolkit for creating graphical user interfaces (GUIs). It is licensed under the terms of the GNU Lesser General Public License, allowing both free and propriet ...
, Qt, FLTK and fpGUI. Applications using any of these widget libraries typically specify the content of the window before entering the main loop and do not need to explicitly handle Expose events and redraw the window content. The
XCB XCB (''X protocol C-language Binding'') is a library implementing the client-side of the X11 display server protocol. XCB is written in the C programming language and distributed under the MIT License. The project was started in 2001 by B ...
library is an alternative to Xlib. Its two main aims are: reduction in library size and direct access to the X11 protocol. A modification of Xlib has been produced to use XCB as a low-level layer.


References


External links


X.Org Foundation's official programming documentation
including most recent version o
Xlib - C Language X Interface
in several formats.
A short tutorial on Xlib







Using Xlib for creating a screensaver module
{{Widget toolkits Application programming interfaces C (programming language) libraries X Window System Articles with example C code