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sendto-->sys_sendto(socket.c)-->sock_sendmsg(socket.c)-->__sock_sendmsg(socket.c)-->sendmsg(socket.c)这个调用是通过sock->ops->sendmsg实现的，sock是socket结构类型，
socket->ops(inet_dgram_ops)->sendmsg(Af_inet.c上定义) UDP报文是sendmsg = inet_sendmsg
调用inet_sendmsg--> (调用sk->sk_prot->sendmsg(udp.c)) sk->sk_prot->sendmsg在udp.c上定义，此处sendmsg=udp_sendmsg-->udp_push_pending_frames-->ip_push_pending_frames-->
ip_local_out-->dst_output-->(skb_dst(skb)->output(skb))skb_dst(skb)得到struct dst_entry，路由顶层？？ (route.c 定义了output = ip_output input = ip_forward) 此处output = ip_output(Ip_output.c)-->ip_finish_output-->ip_finish_output2-->dst->neighbour->output or neigh_hh_output
dst->neighbour->output = neigh_resolve_output(arp.c指向neighbour中实现)-->(neigh->ops->queue_xmit)实际queue_xmit = dev_queue_xmit(arp.c)
neigh_hh_output-->hh->hh_output = dev_queue_xmit(arp.c，dev.c实现)
dev_queue_xmit(dev.c)-->dev_hard_start_xmit-->ops->ndo_start_xmit(ops->ndo_start_xmitstruct net_device_ops *ops = dev->netdev_ops)
到达网卡将数据包发送出去