xlib.rb

#!/usr/bin/env ruby
require 'rubygems'
require 'ffi'
module X11
  extend FFI::Library
  ffi_lib 'X11'
  attach_function :open_display, :XOpenDisplay, [:pointer], :pointer
  attach_function :query_tree, :XQueryTree,
      [:pointer, :int, :pointer, :pointer, :pointer, :pointer], :int
  attach_function :get_window_attributes, :XGetWindowAttributes,
      [:pointer, :int, :pointer], :pointer
  attach_function :create_gc, :XCreateGC, [:pointer, :int, :int, :pointer], :pointer
  attach_function :set_foreground, :XSetForeground, [:pointer, :pointer, :ulong], :int
  attach_function :fill_rectangle, :XFillRectangle,
      [:pointer, :int, :pointer, :int, :int, :uint, :uint], :int
  attach_function :sync, :XSync, [:pointer, :int], :int
  attach_function :free_gc, :XFreeGC, [:pointer, :pointer], :int
end

class XWindowAttributes < FFI::Struct
  layout :x, :int, :y, :int,
      :width, :int, :height, :int,
      :border_width, :int,
      :depth, :int,
      :visual, :pointer,
      :root, :int,
      :class, :int,
      :bit_gravity, :int,
      :win_gravity, :int,
      :backing_store, :int,
      :backing_planes, :ulong,
      :backing_pixel, :ulong,
      :save_under, :int,
      :colormap, :ulong,
      :map_installed, :int,
      :map_state, :int,
      :all_event_masks, :long,
      :your_event_mask, :long,
      :do_not_propagate_mask, :long,
      :override_redirect, :int,
      :screen, :pointer
end

display = X11.open_display(nil)
window = ENV["WINDOWID"].to_i
attr = XWindowAttributes.new
X11.get_window_attributes(display, window, attr.to_ptr)
width = attr[:width]
height = attr[:height]
p [width, height]

root = FFI::MemoryPointer.new(:int32, 1)
parent = FFI::MemoryPointer.new(:int32, 1)
children = FFI::MemoryPointer.new(:pointer, 1)
nchildren = FFI::MemoryPointer.new(:int32, 1)

loop do
  p_window = window
  X11.query_tree(display, window, root, parent, children, nchildren)
  n = nchildren.get_int32(0)
p n
  n.times do |i|
      child = children.get_pointer(0).get_int32(i)
      X11.get_window_attributes(display, child, attr.to_ptr)
p [attr[:width], attr[:height]]
      if attr[:width] > width * 0.7 && attr[:height] > height * 0.7
p :change
          width = attr[:width]
          height = attr[:height]
          window = child
      end
  end
  break if p_window == window
end

offset_x = 2
n = nchildren.get_int32(0)
n.times do |i|
  child = children.get_pointer(0).get_int32(i)
  X11.get_window_attributes(display, child, attr.to_ptr)
  if attr[:x] <= 0 && attr[:width] < 30 && attr[:height] > height * 0.7 # scrollbar of xterm/kterm ?
    offset_x += attr[:x] + attr[:width] + attr[:border_width] * 2
    break
  end
end
offset_y = 2
p [:offset, offset_x, offset_y]

screen = attr[:screen]
red = 0x00ff0000
gc = X11.create_gc(display, window, 0, nil)
X11.set_foreground(display, gc, red)
X11.fill_rectangle(display, window, gc, 0, 0, 100, 100)
X11.sync(display, 0)
X11.free_gc(display, gc)