def unpack(s):
    """Unpack a MXImageRecord to string.

    Parameters
    ----------
    s : str
        String buffer from ``MXRecordIO.read``.

    Returns
    -------
    header : IRHeader
        Header of the image record.
    s : str
        Unpacked string.

    Examples
    --------
    >>> record = mx.recordio.MXRecordIO('test.rec', 'r')
    >>> item = record.read()
    >>> header, s = mx.recordio.unpack(item)
    >>> header
    HEADER(flag=0, label=14.0, id=20129312, id2=0)
    """
    header = IRHeader(*struct.unpack(_IR_FORMAT, s[:_IR_SIZE]))
    s = s[_IR_SIZE:]
    if header.flag > 0:
        header = header._replace(label=np.frombuffer(s, np.float32, header.flag))
        s = s[header.flag*4:]
    return header, s