轉載: http://www.verydemo.com/demo_c167_i125773.html
sendto-->sys_sendto(socket.c)-->sock_sendmsg(socket.c)-->__sock_sendmsg(socket.c)-->sendmsg(socket.c)這個調用是通過sock->ops->sendmsg實現的，sock是socket結構類型，
socket->ops(inet_dgram_ops)->sendmsg(Af_inet.c上定義) UDP報文是sendmsg = inet_sendmsg
調用inet_sendmsg--> (調用sk->sk_prot->sendmsg(udp.c)) sk->sk_prot->sendmsg在udp.c上定義，此處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，路由頂層？？ (route.c 定義了output = ip_output input = ip_forward) 此處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中實現)-->(neigh->ops->queue_xmit)實際queue_xmit = dev_queue_xmit(arp.c)
neigh_hh_output-->hh->hh_output = dev_queue_xmit(arp.c，dev.c實現)
dev_queue_xmit(dev.c)-->dev_hard_start_xmit-->ops->ndo_start_xmit(ops->ndo_start_xmitstruct net_device_ops *ops = dev->netdev_ops)
到達網卡將數據包發送出去