NLMSG_SPACE
#define NLMSG_PAYLOAD(nlh,len) ((nlh)->nlmsg_len - NLMSG_SPACE((len)))
size = NLMSG_SPACE(size0);
size = NLMSG_SPACE(sizeof(struct cn_msg) + msg->len);
#define NLMSG_PAYLOAD(nlh,len) ((nlh)->nlmsg_len - NLMSG_SPACE((len)))
.nlmsg_len = NLMSG_SPACE(sizeof(msg.rule)) +
.nlmsg_len = NLMSG_SPACE(sizeof(msg.rule)),
.nlmsg_len = NLMSG_SPACE(sizeof(msg.status)),
.nlmsg_len = NLMSG_SPACE(sizeof(msg.u.s)),