[comp.windows.x] What's wrong with this backing-store/obscured window code? REPOST

ado@elsie.UUCP (Arthur David Olson) (11/02/89)

I'm surprised that in the attached code, the X Server is giving me
expose events with x and y values of 100 for a 100x100 window--
but only if backing store is enabled.

If you can tell me what I've done wrong, I'd appreciate hearing from you
by electronic mail.
-- 
	Arthur David Olson   ado@alw.nih.gov   ADO is a trademark of Ampex.

	Script started on Mon Oct 23 13:02:09 1989
	elsie$ cat try.c
	#include <stdio.h>
	#include <X11/Xlib.h>

	Display *	display;
	int		screen_number;
	GC		gc;
	Window		root;
	Window		first;
	Window		second;
	XEvent		xe;

	static void
	await_event(type, desc)
	char *	desc;
	{
		XSync(display, 0);
		do {
			XNextEvent(display, &xe);
			(void) printf("got event type %d\n", xe.type);
		} while (xe.type != type);
	}

	int
	main(argc, argv)
	int	argc;
	char *	argv[];
	{
		XSetWindowAttributes	xswa;
		XExposeEvent *		xeep;

		xeep = (XExposeEvent *) &xe;
		if (argc != 2)
			return 1;

		if (argv[1][0] == 'a' || argv[1][0] == 'A')
			xswa.backing_store = Always;
		else if (argv[1][0] == 'n' || argv[1][0] == 'N')
			xswa.backing_store = NotUseful;
		else if (argv[1][0] == 'w' || argv[1][0] == 'W')
			xswa.backing_store = WhenMapped;
		else	return 1;

		display = XOpenDisplay((char *) NULL);
		screen_number = DefaultScreen(display);
		gc = DefaultGC(display, screen_number);
		root = DefaultRootWindow(display);

		first = XCreateSimpleWindow(display, root,
			1, 1, 100, 100, 1, 1, 0);
		XSelectInput(display, first, ExposureMask);
		XChangeWindowAttributes(display, first, CWBackingStore, &xswa);
		XMapWindow(display, first);
		await_event(Expose, "Expose");

		second = XCreateSimpleWindow(display, first,
			26, 26, 50, 50, 1, 1, 0);
		XSelectInput(display, second, ExposureMask);
		XMapWindow(display, second);
		await_event(Expose, "Expose");

		XFillRectangle(display, first, gc, 0, 0, 100, 100);
		XSync(display, 0);

		/*
		** So much for preamble.
		*/

		XCopyArea(display, first, first, gc, 10, 20, 100, 100, 0, 0);

		for ( ; ; ) {
			await_event(GraphicsExpose, "GraphicsExpose");
			(void) printf("x\t%d\ty\t%d\twidth\t%d\theight\t%d\n",
				xeep->x, xeep->y, xeep->width, xeep->height);
		}
	}
	elsie$ cc try.c -lX11 -o try
	elsie$ try n
	got event type 12
	got event type 12
	got event type 13
	x	90	y	0	width	10	height	6
	got event type 13
	x	16	y	6	width	52	height	20
	got event type 13
	x	90	y	6	width	10	height	20
	got event type 13
	x	16	y	26	width	10	height	32
	got event type 13
	x	90	y	26	width	10	height	32
	got event type 13
	x	90	y	58	width	10	height	22
	got event type 13
	x	0	y	80	width	100	height	20
	^Celsie$ try a
	got event type 12
	got event type 12
	got event type 13
>>>>>	x	100	y	20	width	10	height	80
	got event type 13
>>>>>	x	10	y	100	width	100	height	20
	^Celsie$ exit

	script done on Mon Oct 23 13:03:06 1989

rws@EXPO.LCS.MIT.EDU (Bob Scheifler) (11/04/89)

    I'm surprised that in the attached code, the X Server is giving me
    expose events with x and y values of 100 for a 100x100 window--
    but only if backing store is enabled.

'Tis a bug, plain and simple.  Why should that surprise you? :-)