Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!bloom-beacon!LARRY.MCRCIM.MCGILL.EDU!mouse
From: mouse@LARRY.MCRCIM.MCGILL.EDU (der Mouse)
Newsgroups: comp.windows.x
Subject: XGetWindowAttributes: documentation/reality clash
Message-ID: <8904240439.AA12537@Larry.McRCIM.McGill.EDU>
Date: 24 Apr 89 04:39:29 GMT
Sender: daemon@bloom-beacon.MIT.EDU
Organization: The Internet
Lines: 52

I think XGetWindowAttributes is not behaving according to the
documentation.  (System: Sun-3s running release 3.5, X11R3 with Purdue
patches in place, and it's not any specific machine that it's failing
on.)

I'm writing my own window manager and I have occasion to want to
inquire as to whether a given window ID still refers to a window.

In the Xlib document, I find the following promising lines:

	4.1.  Obtaining Window Information

	Xlib provides functions that you can use to obtain informa-
	tion about the window tree, the window's current attributes,
	the window's current geometry, or the current pointer coor-
NOTE>	dinates.  Because they are most frequently used by window
NOTE>	managers, these functions all return a status to indicate
NOTE>	whether the window still exists.
[documentation for XQueryTree omitted]
	To obtain the current attributes of a given window, use
	XGetWindowAttributes.
[documentation for XGetWindowAttributes omitted]

Fine, think I, and write

	int window_exists(w) Window w;
	{
	 XWindowAttributes attr;
	 return(XGetWindowAttributes(disp,w,&attr)?1:0);
	}

but then I get a rude shock when my wm crashes the first time this
function is called under circumstances when I would expect it to return
false (ie, the window doesn't exist).  I see the following printed by
the wm when it dies:

	X Protocol error:  not a valid window ID
	  Major opcode of failed request:  3 (X_GetWindowAttributes)
	  Minor opcode of failed request:  0
	  Resource id in failed request:  0x1100002
	  Serial number of failed request:  1657
	  Current serial number in output stream:  1657

What am I misreading?  What *is* the proper way to tell whether a
window still exists?  (I want to avoid XSync()ing and looking for
window destruction events - which certainly ought to work!  This
function is called only with the server grabbed.)

					der Mouse

			old: mcgill-vision!mouse
			new: mouse@larry.mcrcim.mcgill.edu