Goals of X Window System:

• Provide windows on bitmapped terminals
• Protablity across different OS and maintian high performance
• Allow many mchines to cooperate within a network 
• Should not require a particular style of usre interface, "policy free."

X Protocol: Client-Sever Communication
X protocol specifies the pachaging of information between server and Xlib, even when the server and application program is running on the same machine.  

There are 4 types of packets:

• Request:
• From client to server
• A request for service or information
• Not necessarily round-trip request
• Reply:
• Replies from round-trip request
• Information but into:
• Arguments or
• Return value
• Event:
• Generated by:
• User
• Server - Side action of another event
• Window Manger
• Other clients
• Event structure
• Place in clients Event Queue that selects event type
• Error:
• Tell client that previous request invalid
• Handled by error-handling routines in Xlib

Buffering

• To save on network traffic request for information are not sent imediately to X server rather stored in buffer
• This is possible because many request do not need immediate response
  
• Buffers are flushed when client:
• Waiting on an event that is not in event queue
• Round-trip request
• Explicit flush of buffer

Resources:
So that large data structures are not transmitted over the web, many data structures are kept with the server

•  Large data structures kept on the X server:
• Windows
• Pixmaps
• Color maps
• Cursor
• Font
• Graphics Context
• Client and server can not pass reference or pointer, so pass ID number
• Reduces network traffic
• ID is return during client request for creation of data structure

Propeties: Client - client communication
So that clients can communicate artibrary data, properties are packets of information associated with a window.  Client and Window Manager communicate using properties.  

• Properties are packets of information associated with the window
• Have string names (typical upper case with underscore, eg "WM_COLORMAP") and unique numerical identifier, atom.
• Which are associated with atom identifiers - limit identifier size, for network efficient communication 
• After creating, XInternAtom(), application then use only the atom.
• Propeties are associated with a window, but multible windows can have the same propety type, consequently both atom and window ID is required in a call to the server.
• Predefined atoms (eg. XA_WM_HINTS) are contianed in Xatom.h, so clients do not need to call XInternAtom(), general beginning with XA_, for example XA_WM_HINTS
• Predefined properties have specific get and set functions
• Standard properties of each top level window are required by Window Manager
• Properties are also used for client-client communication, the server acts intermediator
  

Window Manger: A special client

• Control the resources of the display
• Decorate the top level application window by reparenting
• WM - application communication is via properties and events
• WM substructure redirection intercepts all window configuration
• Session Mangers control the start up and killing of applications

Windows:  Rectangular objects 

Windows in X windowing is a rectanglar object on the screen.  

Characteristics:

• Parent:
• Can only show in parent window
• Application top level window is child of root window
• Coordinate system:



 

• Window configuration:
• Window geometery:
• Position of upper left  coner - (x, y) origin
• Size; wdith and height
• Border
• Stack order
• Color Cfharacteristics:
• Depth - number of bits/pixel
• Visual - gray/color
• Class type:
• InputOutput - Can revieve input and display output
• InputOnly - All window can revieve input
• Attributes:
• Background - solid, tiled pixmap or transparent
• Border - solid or tiled pixmap
• Bit Gravity - how paritial windows are displayed
• Window Gravity - how child window are displayed
• Backing Store - how to save window
• Saving Under - how to save when obscured
• Events - selected events'
• Substructure Redirect Override - only for temporay windows
• Colormap - which to use
• Cursor - which to use

Window Hiearchy: window tree

• Subwindows are children of the parent
• Wiondows have stacking order among siblings

Window Visible?

1. Must be mapped with XMapWindow()
2. All ancestors must be mapped
3. Must not be obscured by siblings or ancestors siblings
4. Buffer must be flushed
5. Top level window must be Expose

Events:

• Examples
• User inputs
• Side effects
• Client - client communications
• Imply user control of application

Propagation of envents:

Events propagate up the window until the first window that has selected the event or a window that has set do_not_propagate.