x11 app to monitor if there is a window in fullscreen mode

xx.c

/* Compile: gcc xx.c -o xx -lX11 */

#include <X11/X.h>
#include <X11/Xlib.h>
#include <X11/Xatom.h>
#include <X11/Xutil.h>
#include <X11/keysym.h>
#include <X11/Xos.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

#define err(fmt, args...)                               \
    do { fprintf(stderr, fmt, ## args); } while(0)

static Window fs_win = None;
static int fs_win_state = 0;
static int in_fs = 0;

enum { STATE_NONE, STATE_CREATED, STATE_MAPPED };

static void
fullscreen(Window window, int win_state, char *msg)
{
    int old_in_fs = in_fs;
    
    in_fs = (window != None && win_state == STATE_MAPPED);
    fs_win = window;
    fs_win_state = win_state;
    if (old_in_fs != in_fs) 
        err("fs %s, win %x, src %s\n",
            in_fs ? "yes" : "no", (unsigned int) window, msg);
}

int main()
{
    Display *display;
    Window root;
    int screen;
    XEvent ev;
    int w, h;
    
    display = XOpenDisplay(0);
    if (display == NULL) {
        err("cannot open display\n");
        exit(1);
    }
    screen = DefaultScreen(display);
    root = DefaultRootWindow(display);
    w = DisplayWidth(display, screen);
    h = DisplayHeight(display, screen);

    XSelectInput(display, root, SubstructureNotifyMask);
    for (;;) {
        XNextEvent(display, &ev);
        if (ev.type == ConfigureNotify) {
            if (ev.xconfigure.x == 0 && ev.xconfigure.y == 0
                && ev.xconfigure.width == w && ev.xconfigure.height == h)
                fullscreen(ev.xconfigure.window, STATE_MAPPED, "configure");
            else  if (ev.xconfigure.window == fs_win)
                fullscreen(None, STATE_NONE, "configure");
        } else if (ev.type == CreateNotify) {
            if (ev.xcreatewindow.x == 0 && ev.xcreatewindow.y == 0
                && ev.xcreatewindow.width == w && ev.xcreatewindow.height == h
                && ev.xcreatewindow.override_redirect)
                fullscreen(ev.xcreatewindow.window, STATE_CREATED, "create");
        } else if (ev.type == MapNotify) {
            if (ev.xmap.window == fs_win)
                fullscreen(ev.xmap.window, STATE_MAPPED, "map");
        } else if (ev.type == DestroyNotify) {
            if (ev.xdestroywindow.window == fs_win)
                fullscreen(None, STATE_NONE, "destroy");
        }
    }

}