#include <X11/Xlib.h>
#include <X11/Xmu/WinUtil.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main(int argc, char **argv)
{
  Display *display;
  Window reference;
  Window root;

  if (argc > 2) {
    char *name;
    name = strrchr(argv[0], '/');
    name = name ? name+1 : argv[0];
    fprintf(stderr, "usage: %s [window-id]\n", name);
    exit(1);
  }

  if ((display = XOpenDisplay(NULL)) == NULL) {
    fprintf(stderr, "cannot open display\n");
    exit(1);
  }

  if (argc == 2) {
    int x, y;
    unsigned int width, height, border, depth;
    reference = strtol(argv[1], NULL, 10);
    XGetGeometry(
      display, reference,
      &root, &x, &y, &width, &height, &border, &depth);
  } else {
    root = DefaultRootWindow(display);
  }

  printf("%ld\n", root);

  XCloseDisplay(display);
  exit(0);
}