Path: utzoo!attcan!uunet!ncrlnk!ncrcae!hubcap!gatech!bloom-beacon!EXPO.LCS.MIT.EDU!jim
From: jim@EXPO.LCS.MIT.EDU (Jim Fulton)
Newsgroups: comp.windows.x
Subject: Re: X protocol packets
Message-ID: <8812161545.AA00730@EXPO.LCS.MIT.EDU>
Date: 16 Dec 88 15:45:12 GMT
References: <106@systech.uucp>
Sender: daemon@bloom-beacon.MIT.EDU
Organization: X Consortium, MIT Laboratory for Computer Science
Lines: 74


> Can anyone provide me an explanation of the data within the X protocol
> message itself?  Like where is the 'k' encoded??
> I just do not understand the format of the data within these X message.

Look in your handy Protocol document (sources are provided doc/Protocol/ or
in _X_Window_System:_C_Library_and_Protocol_Reference_ by Scheifler, Gettys,
and Newman) under Section 11 (named Events).  In particular, you want to read
the first section describing KeyPress events (in "The Book", this is on pages
443-444).  The particular bits are described in Appendix F: Protocol Encoding
and are as follows


	1	2			event code
	1	KEYCODE			detail
	2	CARD16			sequence number
	4	TIMESTAMP		time
	4	WINDOW			root
	4	WINDOW			event
	4	WINDOW			child
	2	INT16			root-x
	2	INT16			root-y
	2	INT16			event-x
	2	INT16			event-y
	2	SETofKEYBUTMASK		state


If you are using the C lanuage interface Xlib, you'll only need to deal with
XKeyEvent structures (from <X11/Xlib.h>):

typedef struct {
	int type;		/* of event */
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window window;	        /* "event" window it is reported relative to */
	Window root;	        /* root window that the event occured on */
	Window subwindow;	/* child window */
	Time time;		/* milliseconds */
	int x, y;		/* pointer x, y coordinates in event window */
	int x_root, y_root;	/* coordinates relative to root */
	unsigned int state;	/* key or button mask */
	unsigned int keycode;	/* detail */
	Bool same_screen;	/* same screen flag */
} XKeyEvent;
typedef XKeyEvent XKeyPressedEvent;



> Like where is the 'k' encoded??

The detail field in the protocol request contains a "keycode" which is a
server-dependent number representing the physical key that was pressed just
like make/break codes under other operating systems.  The state field contains
a mask of bits indicating which keyboard modifiers where held down when this
key was pressed. 

Clients are free to use the keycode and state mask directly, although they will
usually translate it into a "keysym" which represents the symbol etched on the
key.  This is usually handled by a library routine (XKeycodeToKeysym in Xlib)
as it involves fetching the keyboard mapping (which is a table containing a
row for each keycode and a column for each modifier) and then looking up the
keysym that corresponds to the <keycode,statemask> pair.

Now, this keysym is still just a number (although keysyms, unlike keycodes, are
specified by the X Protocol).  Applications can then use the keysyms or
translate them into "strings" for various character sets using XLookupString
and XmuLookup*.  However, they should keep in mind that these routines are
designed to be used with specific character sets (e.g. ISO Latin-1 for
XLookupString) and may not return values for arbitrary combinations of
modifiers (esp. function control codes).

Jim Fulton
MIT X Consortium