An XwindowServer is the program which mediates XwindowProtocol requests from X client applications and translates them into graphics on the local display hardware.  This separation of PolicyAndMechanism allows for wide hardware support and X's unique ability to run the server and clients on separate machines connected over a network.

XwindowServer''''''s exist for all platforms, from high-end Unix workstations down to HandHeld''''''s like the iPaq, and are the standard means for providing a graphical user interface for Unix systems.  The standard reference implementation used to be XFree86, but this group disbanded after a licensing dispute.  The latest reference implementation comes from X.Org (http://www.x.org/).  New development is supported by http://freedesktop.org/.  MacOsx ships with an Apple designed X server.

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''Isn't the server doing what the client usually does?''

A server receives requests from clients and performs some action(s) in response to those requests.  An XwindowServer receives drawing requests from clients and performs some drawing action(s) in response to those requests.

'''''To make it painfully clear...'''''
The '''XwindowServer''' controls one or more displays each having one or more screens that display information in response to requests from the
'''X Client,''' the computer running the program.  When you specify something like "export DISPLAY=foo:0.0", you are specifying "hostname:displaynumber.screennumber". This kind of flexibility is often misunderstood by people who's only exposure has been to PC's and other small single user machines.

''This confuses people because you normally think that the big computer in the back room is the server and the little box on your desk is the client.  X-Windows is still client-server, in the traditional sense; it's just that the boxes have the opposite roles than what you may be accustomed to.''

It's only confusing to people who don't understand what client/server actually means.
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CategoryXwindow