nsIInputStream::available

Hi,
I've got a question about nsIInputStream::available...

It is documented as follows:
  90     /**
  91      * @return number of bytes currently available in the stream
  92      */
  93     unsigned long available();

So, my question is, what does it mean for available() to return 0?

Does it indicate that at the moment, no data is available, but at a 
later time available() may return a number > 0?

Or, does it indicate EOF?


If the former is the case, how is it possible to detect EOF on a 
blocking input stream?


-biesi