BLOCKSIZE = 1024 (or so)
lines = 0 ; lines_wanted = 10
seek to end of file

while lines < lines_wanted
  seek back BLOCKSIZE bytes
  read block into buffer
  pos = end of buffer

  while pos > 0
    use rindex to find \n starting at pos
    if found
      lines++
      pos = index of \n
    else
      last