nsISeekableStream Interface

  seek

  This method moves the stream offset of the steam implementing this
  interface.

   @param whence specifies how to interpret the 'offset' parameter in
                 setting the stream offset associated with the implementing
                 stream.
     
   @param offset specifies a value, in bytes, that is used in conjunction
                 with the 'whence' parameter to set the stream offset of the 
                 implementing stream.  A negative value causes seeking in 
                 the reverse direction.

   @throws NS_BASE_STREAM_CLOSED if called on a closed stream.

void seek(in long whence, in long long offset)

  setEOF

  This method truncates the stream at the current offset.

   @throws NS_BASE_STREAM_CLOSED if called on a closed stream.

void setEOF()

  tell

  This method reports the current offset, in bytes, from the start of the 
  stream. 

   @throws NS_BASE_STREAM_CLOSED if called on a closed stream.

long long tell()