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 details of the X protocol. Few applications use Xlib directly; rather, they employ other libraries that use Xlib functions to provide widget toolkits: X Toolkit Intrinsics (Xt) Athena widget set (Xaw) Motif FLTK GTK Qt (X11 version) Tk SDL (Simple DirectMedia Layer) SFML (Simple and Fast Multimedia Library) Xlib appeared around 1985, and is used in GUIs for many Unix-like operating systems. A re-implementation of Xlib was introduced in 2007 using XCB. 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 operating system, the Display structure contains the file handle of the socket 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