X Windows or simply X provides a way of writing device independent graphical and windowing software that can be easily ported from machine to machine.
The X system is being adopted as a standard by nearly every computer manufacturer these days.
Before X came along every manufacturer had its own proprietary windowing system. For example PC windows, Apple Macintosh windows, Sun Windows and DEC windows.
These were all entirely different.
However, with X the programmer can now write a single application in a single language and run this program on different machines with little or no modifications.
Moreover, we can actually run programs on one machine and have the results displayed on one (or several) different machine's terminal.
The machine can be a similar computer (such as another Dec Workstation) or an entirely different machine altogether.
The possibilities are endless really.
For example in Fig. 1 below we have a ``local client'' and ``display server'' (see later for definitions) that run programs on a PC, a supercomputer and a large minicomputer all with windows being displayed on the same screen.
Fig. 1 Example of applications running across a network
This is because X is a network-based system. It has been common to have networks of the same computers for some time (cf. departments network of PC's, MAC's, Sun's and Dec's Workstations).
It is relatively easy to get machines of the same type in this network configuration to talk to each other.
Now with X we can get different machines to talk to each other.
There are a couple of versions of X windows around. You may have heard of MOTIF and OPENLOOK for example. The underlying running of these are the same. However the higher level operations of the GUI of each and the look and feel of them is different.
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 user. The toolkits are C subroutine libraries where we describe how to build windows etc.
You have been using the OPENLOOK WM on the DECSTATION already. The SUN Workstations also use OPENLOOK.
We will however be learning MOTIF toolkit. MOTIF toolkit can run under OPENLOOK WM (and OPENLOOK's toolkit (Xview) can run under MOTIF WM).
MOTIF has an appearance similar to that of Microsoft Windows on a PC.
MOTIF has become the ``standard'' version of X and indeed OPENLOOK will gradually disappear.
NOTE: MOTIF is not like microsoft windows in terms of programming. (Apart from sharing similar GUI programming principles of course.)
However the look and feel is very similar which partly explains its popularity.