The X Window System Environment

 

What is the X Window system?

  The X Window system  provides a way of writing device independent graphical and windowing software that can be easily ported from machine to machine. X is a network-based system and supports cross-platform communication -- we can get different machines to talk to each other.

At the highest level of X there are two basic features: The window manager  and the toolkit. The window manager (wm) controls GUI aspects such as appearance of windows and interaction with the user. The toolkits are C subroutine libraries where we describe how to contsruct the GUI and how to attach the GUI to the remainder of the application. Motif is one such toolkit. Motif is built on other toolkits in the X System: Xt Intrinsics , an intermediate level toolkit, and the low level X Library (Xlib) . The relevance of these will be made apparent in due course.

X Window Principles

All forms of displaying of information in X are bit-mapped which means that every pixel on the screen is individually controllable. Therefore we can draw pictures and use text. The requirement for bit-mapped graphics means that high quality monitors are necessary. However, most computer systems provide monitors of this type.

X, like most other windowing systems, divides the screen into various parts that control input and output. Each part is called a window . A window can have many uses. A window can display graphics, receive input from a mouse, act as a standard terminal (e.g. an Xterm -- a standard text based terminal emulation window) etc..

Not all applications need to consist of a single window. We can have many windows associated with different parts of one application. Each subwindow is called a child and it usually remains under the control of it's parent window.

There is one special window, the background or root window. All other windows are children of the root .

The Window Manager

  The window manager  is responsible for manipulating windows on the screen. The window manager performs the following operations:

• Placement and movement of windows.
• Resizing of windows.
• Iconification of windows -- how the window appears when the window is minimised.
• Starting and manipulation of windows.
• Control of input to windows.

Controlling the window environment is not easy and has many facets. For instance, there may be multiple applications running simultaneously and a conflict may arise for input:

Does a keyboard input go in a window where the mouse currently points or must a window be explicitly chosen?

The window manager is also predominantly responsible for the appearance and user interaction (the look and feel)  of the interface. Since the development of X, there have been a few different window managers. The look and feel varies a lot between each window manager. The Motif window manager   (mwm) is probably the window manager you are most familiar with as this comes with the Motif system. Other window managers include Sun's Open Look window manager ( olwm)   and the Tab or Tom's window manager (twm)  .

Most major Unix vendors now supply the CDE which by default runs the desktop window manager, dtwm  . The development of the common desktop will probably result in dtwm superseding mwm. The CDE default window manager can easily be altered to run the above altenative window managers if desired. Since the CDE is built on top of motif there are many similarities between dtwm and mwm (see Section  below).

The Motif look and feel , as defined by the Motif Style Guide, is basically enforced by mwm or dtwm. Consequently, interaction between applications and these window managers are eased if the applications obey standard Motif/CDE guidelines.