user client

获取netlink 的family id

发送family id 请求消息：

首先向内核发一个family_id的请求消息，根据family name查询family id，在内核注册genl_family demo_family的时候分配一个family id 与family name对应
nlmsg_type = GENL_ID_CTRL
nlmsg_flags = NLM_F_REQUEST

genl_cmd = CTRL_CMD_GETFAMILY

na->nla_type = CTRL_ATTR_FAMILY_NAME

struct nlmsghdr struct genlmsghdr char buf（可以放多个struct nlattr ） 
  1.     struct nlattr *na;
  2.     struct sockaddr_nl nladdr;
  3.     int r, buflen;
  4.     char *buf;
  6.     struct msgtemplate msg;
  8.     /* 填充msg (本函数发送的msg只填充一个attr) */
  9.     msg.n.nlmsg_len = NLMSG_LENGTH(GENL_HDRLEN);
  10.     msg.n.nlmsg_type = GENL_ID_CTRL;             //后面要是在发送其它消息到内核就用到这个消息的响应消息里面的family_id
  11.     msg.n.nlmsg_flags = NLM_F_REQUEST;
  12.     msg.n.nlmsg_seq = 0;
  13.     msg.n.nlmsg_pid = nlmsg_pid;
  14.     msg.g.cmd = CTRL_CMD_GETFAMILY;             //
  15.     msg.g.version = DEMO_GENL_VERSION;
  16.     na = (struct nlattr *) GENLMSG_DATA(&msg);
  17.     na->nla_type = CTRL_ATTR_FAMILY_NAME;         //
  18.     na->nla_len = nla_len + 1 + NLA_HDRLEN;
  19.     memcpy(NLA_DATA(na), nla_data, nla_len);
  20.     msg.n.nlmsg_len += NLMSG_ALIGN(na->nla_len);
  22.     buf = (char *) &msg;
  23.     buflen = msg.n.nlmsg_len;
  24.     memset(&nladdr, 0, sizeof(nladdr));
  25.     nladdr.nl_family = AF_NETLINK;
  27.     /* 循环发送直到发送完成 */
  28.     while ((r = sendto(sd, buf, buflen, 0, (struct sockaddr *) &nladdr,
  29.                sizeof(nladdr))) < buflen) {
  30.         if (r > 0) {
  31.             buf += r;
  32.             buflen -= r;
  33.         } else if (errno != EAGAIN)
  34.             return -1;
  35.     }

发送普通消息：

nlmsg_type = family_id
nlmsg_flags = NLM_F_REQUEST

genl_cmd = DEMO_CMD_ECHO

na->nla_type = DEMO_CMD_ATTR_MESG