XGetWindowAttributes doesn't return a correct window location

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04-10-2024

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XGetWindowAttributes: Why Your Window Location Might Be Off

The Problem:

You're using the XGetWindowAttributes function in your X11 application to get information about a window, including its position. But, when you retrieve the location using the x and y fields of the XWindowAttributes structure, the values seem to be incorrect.

The Scenario:

#include <X11/Xlib.h>

int main() {
    Display *display = XOpenDisplay(NULL);
    Window window = DefaultRootWindow(display);

    XWindowAttributes attributes;
    XGetWindowAttributes(display, window, &attributes);

    printf("Window location: (%d, %d)\n", attributes.x, attributes.y);

    XCloseDisplay(display);
    return 0;
}

In this code, we expect the output to be the actual location of the root window. However, you might find that the x and y values are not accurate.

Analysis:

The root window is the top-level window in the X11 hierarchy. It represents the entire display area. While it's possible to have a root window with an offset from the screen origin, this is unusual. The confusion arises because the x and y values in the XWindowAttributes structure represent the position of the window relative to its parent.

Clarification:

• Root Window: The root window doesn't have a parent window, so its x and y values are always 0.
• Child Windows: For child windows, the x and y values will be relative to the top-left corner of their parent window.

Solution:

To get the actual screen coordinates of a window, you need to traverse the window hierarchy up to the root window and sum up the offsets of each parent window:

#include <X11/Xlib.h>

int main() {
    Display *display = XOpenDisplay(NULL);
    Window window = DefaultRootWindow(display);

    int screen_x = 0, screen_y = 0;
    Window current_window = window;

    while (current_window != root) {
        XWindowAttributes attributes;
        XGetWindowAttributes(display, current_window, &attributes);
        screen_x += attributes.x;
        screen_y += attributes.y;
        current_window = attributes.parent;
    }

    printf("Window location: (%d, %d)\n", screen_x, screen_y);

    XCloseDisplay(display);
    return 0;
}

Additional Value:

• Remember that XGetWindowAttributes provides a wealth of information about a window, including its size, border width, and other attributes.
• This issue is particularly common when dealing with child windows, as their positioning is relative to their parent.
• Always check the parent window relationship to avoid unexpected behavior when working with window positions in X11.

References:

• Xlib Programming Manual
• X Window System Architecture