Callback function called by libnl upon receiving messages from the kernel.

    Positional arguments:
    msg -- nl_msg class instance containing the data sent by the kernel.
    has_printed -- simple pseudo boolean (if list is empty) to keep track of when to print emtpy lines.

    Returns:
    An integer, value of NL_SKIP. It tells libnl to stop calling other callbacks for this message and proceed with
    processing the next kernel message.

def callback(msg, has_printed):
    """Callback function called by libnl upon receiving messages from the kernel.

    Positional arguments:
    msg -- nl_msg class instance containing the data sent by the kernel.
    has_printed -- simple pseudo boolean (if list is empty) to keep track of when to print emtpy lines.

    Returns:
    An integer, value of NL_SKIP. It tells libnl to stop calling other callbacks for this message and proceed with
    processing the next kernel message.
    """
    table = AsciiTable([['Data Type', 'Data Value']])
    # First convert `msg` into something more manageable.
    gnlh = genlmsghdr(nlmsg_data(nlmsg_hdr(msg)))

    # Partially parse the raw binary data and place them in the `tb` dictionary.
    tb = dict((i, None) for i in range(nl80211.NL80211_ATTR_MAX + 1))  # Need to populate dict with all possible keys.
    nla_parse(tb, nl80211.NL80211_ATTR_MAX, genlmsg_attrdata(gnlh, 0), genlmsg_attrlen(gnlh, 0), None)

    # Now it's time to grab the juicy data!
    if tb[nl80211.NL80211_ATTR_IFNAME]:
        table.title = nla_get_string(tb[nl80211.NL80211_ATTR_IFNAME]).decode('ascii')
    else:
        table.title = 'Unnamed Interface'

    if tb[nl80211.NL80211_ATTR_WIPHY]:
        wiphy_num = nla_get_u32(tb[nl80211.NL80211_ATTR_WIPHY])
        wiphy = ('wiphy {0}' if OPTIONS['<interface>'] else 'phy#{0}').format(wiphy_num)
        table.table_data.append(['NL80211_ATTR_WIPHY', wiphy])

    if tb[nl80211.NL80211_ATTR_MAC]:
        mac_address = ':'.join(format(x, '02x') for x in nla_data(tb[nl80211.NL80211_ATTR_MAC])[:6])
        table.table_data.append(['NL80211_ATTR_MAC', mac_address])

    if tb[nl80211.NL80211_ATTR_IFINDEX]:
        table.table_data.append(['NL80211_ATTR_IFINDEX', str(nla_get_u32(tb[nl80211.NL80211_ATTR_IFINDEX]))])

    # Print all data.
    if has_printed:
        print()
    else:
        has_printed.append(True)
    print(table.table)
    return NL_SKIP