shutdown系统调用

shutdown用来关闭一个全双工连接的部分或者全部，比如关闭读，关闭写，或者读写全部关闭；

 /*
  *    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;
 }

inet_shutdown会根据连接的状态进行不同的处理，比如已连接状态需要调用传输层实现的shutdown，而监听或者发送了syn包会根据情况则调用传输层的disconnect等；

 /* 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;
 }

tcp层shutdown实现请移步下面文章<TCP层shutdown系统调用的实现分析>；