Main function called upon script execution.

def main():
    """Main function called upon script execution."""
    # First get the wireless interface index.
    if OPTIONS['<interface>']:
        pack = struct.pack('16sI', OPTIONS['<interface>'].encode('ascii'), 0)
        sk = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
        try:
            info = struct.unpack('16sI', fcntl.ioctl(sk.fileno(), 0x8933, pack))
        except OSError:
            return error('Wireless interface {0} does not exist.'.format(OPTIONS['<interface>']))
        finally:
            sk.close()
        if_index = int(info[1])
    else:
        if_index = -1

    # Then open a socket to the kernel. Same one used for sending and receiving.
    sk = nl_socket_alloc()  # Creates an `nl_sock` instance.
    ret = genl_connect(sk)  # Create file descriptor and bind socket.
    if ret < 0:
        reason = errmsg[abs(ret)]
        return error('genl_connect() returned {0} ({1})'.format(ret, reason))

    # Now get the nl80211 driver ID. Handle errors here.
    driver_id = genl_ctrl_resolve(sk, b'nl80211')  # Find the nl80211 driver ID.
    if driver_id < 0:
        reason = errmsg[abs(driver_id)]
        return error('genl_ctrl_resolve() returned {0} ({1})'.format(driver_id, reason))

    # Setup the Generic Netlink message.
    msg = nlmsg_alloc()  # Allocate a message.
    if OPTIONS['<interface>']:
        genlmsg_put(msg, 0, 0, driver_id, 0, 0, nl80211.NL80211_CMD_GET_INTERFACE, 0)  # Tell kernel: send iface info.
        nla_put_u32(msg, nl80211.NL80211_ATTR_IFINDEX, if_index)  # This is the interface we care about.
    else:
        # Ask kernel to send info for all wireless interfaces.
        genlmsg_put(msg, 0, 0, driver_id, 0, NLM_F_DUMP, nl80211.NL80211_CMD_GET_INTERFACE, 0)

    # Add the callback function to the nl_sock.
    has_printed = list()
    nl_socket_modify_cb(sk, NL_CB_VALID, NL_CB_CUSTOM, callback, has_printed)

    # Now send the message to the kernel, and get its response, automatically calling the callback.
    ret = nl_send_auto(sk, msg)
    if ret < 0:
        reason = errmsg[abs(ret)]
        return error('nl_send_auto() returned {0} ({1})'.format(ret, reason))
    print('Sent {0} bytes to the kernel.'.format(ret))
    ret = nl_recvmsgs_default(sk)  # Blocks until the kernel replies. Usually it's instant.
    if ret < 0:
        reason = errmsg[abs(ret)]
        return error('nl_recvmsgs_default() returned {0} ({1})'.format(ret, reason))