title: What glutswapbuffers actually did?
body: <p>Does the front buffer pointer now points to the back buffer?Or the front buffer content was updated by the back buffer's?If that is the case,whole block of the content is updated or only the changed content is updated?</p>
  
  <p>The description of glutswapbuffers on opengl.org is</p>
  
  <blockquote>
    <p>Performs a buffer swap on the layer in use for the current window. Specifically, glutSwapBuffers promotes the contents of the back buffer of the layer in use of the current window to become the contents of the front buffer. The contents of the back buffer then become undefined. </p>
  </blockquote>
  
  <p>What does the last sentence means?the back buffer now points to null?</p>