def writeOffsetTable(self, output):
        """Writes all of the object reference offsets."""
        all_positions = []
        writtenReferences = list(self.writtenReferences.items())
        writtenReferences.sort(key=lambda x: x[1])
        for obj,order in writtenReferences:
            # Porting note: Elsewhere we deliberately replace empty unicdoe strings
            # with empty binary strings, but the empty unicode string
            # goes into writtenReferences.  This isn't an issue in Py2
            # because u'' and b'' have the same hash; but it is in
            # Py3, where they don't.
            if bytes != str and obj == unicodeEmpty:
                obj = b''
            position = self.referencePositions.get(obj)
            if position is None:
                raise InvalidPlistException("Error while writing offsets table. Object not found. %s" % obj)
            output += self.binaryInt(position, self.trailer.offsetSize)
            all_positions.append(position)
        return output
Writes all of the object reference offsets.