Problem with CheckerString::read_next_line

Hi,
I have a problem with the aspell utility failing to find all the errors in a 
text file. The source of the problem is that it stopped reading in the following 
function when next == cur_line_.
I spent some time understanding how lines are read, but I can't understand why 
the test for next==cur_line_ is there. Could someone please enlighten me?

bool CheckerString::read_next_line()
{
  if (feof(in_)) return false;
  Line * next = end_;
  inc(next);
  if (next == cur_line_) return false; //This is where we stop reading the file.
  int s = get_line(in_, *end_);
  if (s == 0) return false;
  end_ = next;
  if (out_ && end_->real.size() > 0)
    fwrite(end_->real.data(), end_->real.size(), 1, out_);
  end_->clear();
  return true;
}