title: Reading framebuffer at lowest level possible under X
body: <p>I want to read the framebuffer of the videocard at the lowest level possible for a security application I'm writing.<br>
  I want to be as sure as possible that what I'm reading is exactly what will be finally
  put on the bits of the hardware lighting the pixels of the screen,<br>
  and that no software layer is in the middle (or at least I want to have the lowest number possible of layers in the middle).  </p>
  
  <p>I've seen it's pretty easy to use X to grab the screen in a precise moment, but that call
  is still passing through the X server.<br>
  I would like to have something really more low level,
  even if this means messing up with some ioctl with the video card.<br>
  I've seen the existence of DRI and DRI2, but they are very very badly documented, especially
  the latter.<br>
  I can't really understand how they work. 
  Do you have any idea, reference or starting point for a good research?  </p>
  
  <p>Anything would be appreciated!</p>