/*

  *    Shutdown a socket.

  */

 SYSCALL_DEFINE2(shutdown, int, fd, int, how)

 {

     int err, fput_needed;

     struct socket *sock;

     /* 找到socket */

     sock = sockfd_lookup_light(fd, &err, &fput_needed);

     if (sock != NULL) {

         err = security_socket_shutdown(sock, how);

         if (!err)

             /* 调用对应类型的shutdown */

             err = sock->ops->shutdown(sock, how);

         fput_light(sock->file, fput_needed);

     }

     return err;

 }

 /* 关闭操作 */

 int inet_shutdown(struct socket *sock, int how)

 {

     struct sock *sk = sock->sk;

     int err = ;

     /* This should really check to make sure

      * the socket is a TCP socket. (WHY AC...)

      */

     how++; /* maps 0->1 has the advantage of making bit 1 rcvs and

                1->2 bit 2 snds.

                2->3 */

     /* how值错误 */

     if ((how & ~SHUTDOWN_MASK) || !how)    /* MAXINT->0 */

         return -EINVAL;

     lock_sock(sk);

     /* 正在连接 */

     if (sock->state == SS_CONNECTING) {

         /* syn发送，syn接收，关闭状态 */

         if (( << sk->sk_state) &

             (TCPF_SYN_SENT | TCPF_SYN_RECV | TCPF_CLOSE))

             /* 设置正在关闭连接 */

             sock->state = SS_DISCONNECTING;

         /* 否则为已连接状态 */

         else

             sock->state = SS_CONNECTED;

     }

     switch (sk->sk_state) {

     case TCP_CLOSE:

         err = -ENOTCONN;

         /* Hack to wake up other listeners, who can poll for

            POLLHUP, even on eg. unconnected UDP sockets -- RR */

     default:

         /* 设置how值，并且调用传输层的shutdown */

         sk->sk_shutdown |= how;

         if (sk->sk_prot->shutdown)

             sk->sk_prot->shutdown(sk, how);

         break;

     /* Remaining two branches are temporary solution for missing

      * close() in multithreaded environment. It is _not_ a good idea,

      * but we have no choice until close() is repaired at VFS level.

      */

     case TCP_LISTEN:

         /* 监听状态，如果无接收方向的关闭操作，跳出 */

         if (!(how & RCV_SHUTDOWN))

             break;

         /* 有接收方向的关闭，继续 */

         /* Fall through */

     case TCP_SYN_SENT:

         /* 调用传输层的disconnect断开连接 */

         err = sk->sk_prot->disconnect(sk, O_NONBLOCK);

         /* 调增状态 */

         sock->state = err ? SS_DISCONNECTING : SS_UNCONNECTED;

         break;

     }

     /* Wake up anyone sleeping in poll. */

     /* 状态改变，唤醒等待的进程 */

     sk->sk_state_change(sk);

     release_sock(sk);

     return err;

 }