def read(self, size=None):
    """Reads a byte string from the file-like object at the current offset.

    The function will read a byte string of the specified size or
    all of the remaining data if no size was specified.

    Args:
      size (Optional[int]): number of bytes to read, where None is all
          remaining data.

    Returns:
      bytes: data read.

    Raises:
      IOError: if the read failed.
      OSError: if the read failed.
    """
    if not self._is_open:
      raise IOError('Not opened.')

    if self._current_offset < 0:
      raise IOError(
          'Invalid current offset: {0:d} value less than zero.'.format(
              self._current_offset))

    if self._file_data is None or self._current_offset >= self._size:
      return b''

    if size is None:
      size = self._size
    if self._current_offset + size > self._size:
      size = self._size - self._current_offset

    start_offset = self._current_offset
    self._current_offset += size
    return self._file_data[start_offset:self._current_offset]