FScreenGetScrRect function

[klebertarcisio]

Hi everyone,

I would like to understand a little more about the FScreenGetScrRect function:

fvwm3/libs/FScreen.c

Lines 636 to 656 in 48dd509

	Bool FScreenGetScrRect(fscreen_scr_arg *arg, fscreen_scr_t screen,
	int *x, int *y, int *w, int *h)
	{
	struct monitor *m = FindScreen(arg, screen);
	if (m == NULL) {
	fvwm_debug(__func__, "%s: m is NULL\n", __func__);
	return (True);
	}
	if (x)
	*x = m->si->x;
	if (y)
	*y = m->si->y;
	if (w)
	*w = m->si->w;
	if (h)
	*h = m->si->h;
	return !((monitor_get_count() > 1) &&
	(strcmp(m->si->name, GLOBAL_SCREEN_NAME) == 0));
	}

This function may not change the values of x, y, w and h. That way, I always have to pass x, y, w and h initialized, right? If I don't pass these initialized values, I can't guarantee that the function initialized them unless I check the function's return, but that return is hardly used in the code.

See this example:

fvwm3/fvwm/icons.c

Lines 2041 to 2058 in 48dd509

	{
	/* if first time */
	int sx;
	int sy;
	int sw;
	int sh;
	/* Right now, the global box is hard-coded, fills the primary
	* screen, uses an 80x80 grid, and fills top-bottom,
	* left-right */
	FScreenGetScrRect(NULL, FSCREEN_PRIMARY, &sx, &sy, &sw, &sh);
	global_icon_box_ptr = fxcalloc(1, sizeof(icon_boxes));
	global_icon_box_ptr->IconBox[0] = sx;
	global_icon_box_ptr->IconBox[1] = sy;
	global_icon_box_ptr->IconBox[2] = sx + sw;
	global_icon_box_ptr->IconBox[3] = sy + sh;
	global_icon_box_ptr->IconGrid[0] = 80;
	global_icon_box_ptr->IconGrid[1] = 80;
	global_icon_box_ptr->IconFlags = ICONFILLHRZ;

Is there any possibility that global_icon_box_ptr-> IconBox [0] will receive an uninitialized value?

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[ThomasAdam]

Hi @klebertarcisio,

Yes, there's a small risk, but that's not the point here, because:

• Most compilers will set the value to 0 if undeclared in this case;
• The context for these callers is such that it's very unlikely we're going to fail finding a screen to set the values in the first place.

[danespen]

Thomas Adam <[email protected]> writes:

Hi @klebertarcisio, Yes, there's a small risk, but that's not the point here, because:

You're the expert, but as I remember, the built in default iconbox that covers the root can't be removed. Not sure what you are doing with multiple monitors that can be turned on and off, but the basic idea of the iconboxes was to have a grid to fall back to.

…

-- Dan Espen

[ThomasAdam]

Hi @danespen,

Yes -- that's still the case here; per screen iconboxes can be definedm and come and go depending on whether the monitor is attached or not. The PRIMARY monitor is now always set regardless.

[klebertarcisio]

In order to contribute to this discussion, I will post a few more places that the FScreenGetScrRect function is used in a similar way.

FScreen.c

fvwm3/libs/FScreen.c

Lines 661 to 684 in 48dd509

	void FScreenTranslateCoordinates(
	fscreen_scr_arg *arg_src, fscreen_scr_t screen_src,
	fscreen_scr_arg *arg_dest, fscreen_scr_t screen_dest,
	int *x, int *y)
	{
	int x_src;
	int y_src;
	int x_dest;
	int y_dest;
	FScreenGetScrRect(arg_src, screen_src, &x_src, &y_src, NULL, NULL);
	FScreenGetScrRect(arg_dest, screen_dest, &x_dest, &y_dest, NULL, NULL);
	if (x)
	{
	*x = *x + x_src - x_dest;
	}
	if (y)
	{
	*y = *y + y_src - y_dest;
	}
	return;
	}

FvwmIdent.c

fvwm3/modules/FvwmIdent/FvwmIdent.c

Lines 787 to 813 in 48dd509

	{
	int sx;
	int sy;
	int sw;
	int sh;
	Window JunkW;
	int JunkC;
	unsigned int JunkM;
	fscreen_scr_arg fscr;
	if (!FQueryPointer(
	dpy, Root, &JunkW, &JunkW, &x, &y, &JunkC, &JunkC,
	&JunkM))
	{
	/* pointer is on a different screen */
	x = 0;
	y = 0;
	}
	fscr.xypos.x = x;
	fscr.xypos.y = y;
	FScreenGetScrRect(&fscr, FSCREEN_XYPOS, &sx, &sy, &sw, &sh);
	if (y + height + 100 > sy + sh)
	{
	y = sy + sh - height - 10;
	mysizehints.win_gravity = SouthWestGravity;
	}