code: def nlmsg_attrlen(nlh, hdrlen):
      
      return max(nlmsg_len(nlh) - libnl.linux_private.netlink.NLMSG_ALIGN(hdrlen), 0)
docs: Length of attributes data.
  
      https://github.com/thom311/libnl/blob/libnl3_2_25/lib/msg.c#L154
  
      nlh -- Netlink message header (nlmsghdr class instance).
      hdrlen -- length of family specific header (integer).
  
      Returns:
      Integer.