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A window manager so I understand what the hell x-server is doing.
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A window manager so I understand what the hell x-server is doing.

X-Server in a really big nutshell...

A list of important concepts of X-Server is that it is built to be as low-level as possible. The X-Server communicates in a server-client model. A window manager is just a client communicating with the X-Server.

This image shows the x-server's relationship to input devices and window manager.

Stolen from Chuan Ji's Blog

Window Heirarchy (Hierarchy of Rectangles)


  1. The root window container: This is the main window, the area in which everything created related to it can be dispalyed
  2. A top level window: Windows created inside the root container. They can have child windows. In this case, 4 and 5 are child windows of 2. Child window 5 looks like it is being cropped out of window 2. A window is clipped to its parents boundarys. No actions can be completed or requested on the area that is cropped.
  3. Another top level window, in this case also displaying as cropped. Window 3 is a child of the root window and therefore will still only display within the boundarys of its parent 1.

Substructure Redirection

Basically, how x-server mainly functions is an application will send a request to x-server, and thats the end. The window manager has to get these requests though. Substructure Redirection is what does this. The window manager will need to know that a new top level window has been created and needs to create all the decorations (minimize, maximize and close buttons). A request can be made by an application, x-server will process it then send it to the window manager which can do whatever it wants with the request.


When you seen all the minimize buttons, maximize buttons and frames around a window, this is created by the Window Manager. Reparenting is probably the most obvious functionalliy and visual way to be distinct. Sadly, the one impossible thing is the decorations can not be matched to imformation/colours inside the application window. That data cannot be queried...

[ Xlib Programming Manual (16.3) ]

A window manager can decorate windows on the screen with titlebars and 
place little boxes on the titlebar with which the window can be moved 
or resized.This is only one possibility, modeled on the user interface on the Macintosh.

To do this, the window manager creates a child of the root somewhat larger
than the top−level window of the application.Then it calls XReparentWindow(), 
specifying the top−level window of the  application as win and the new parent
as parent. win and all its descendants will then be descendants of parent.

In the area where the new parent is visible around the top−level window of 
the application, the window manager can putanything it wants.  This could 
include text, graphics, and small windows which perform certain functions
when a button is clicked in them.

Compositing Window Manager

A compositing window manager provides programs with an "off-screen buffer". This allows for compositing of the image so it can be send to display memory. The word compositing in this case refers to the compilation of different visual elements from multiple sources to form a single image. The screen is double buffered, so no flickering!


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