listen系统调用

对于面向连接的socket，需要会用监听连接的到来，并且使用backlog参数来限制连接数量；具体backlog限制的内容，请参考本博另外一篇文章，本文结尾提供链接地址；

 /*
  *    Perform a listen. Basically, we allow the protocol to do anything
  *    necessary for a listen, and if that works, we mark the socket as
  *    ready for listening.
  */

 SYSCALL_DEFINE2(listen, int, fd, int, backlog)
 {
     struct socket *sock;
     int err, fput_needed;
     int somaxconn;

     /* 查找socket结构 */
     sock = sockfd_lookup_light(fd, &err, &fput_needed);
     if (sock) {
         somaxconn = sock_net(sock->sk)->core.sysctl_somaxconn;

         /* backlog不能超过设置值 */
         if ((unsigned int)backlog > somaxconn)
             backlog = somaxconn;

         /* 安全模块检查 */
         err = security_socket_listen(sock, backlog);
         if (!err)
             /* 调用对应类型的listen函数，只有SOCK_STREAM支持listen */
             err = sock->ops->listen(sock, backlog);

         fput_light(sock->file, fput_needed);
     }
     return err;
 }

inet_listen、数对socket类型，连接状态等进行基本的校验，如果已经处于listen状态，则只会调整backlog的值，如果尚未处于listen状态，则调用传输层的listen实现函数进行针对的listen操作；

 /*
  *    Move a socket into listening state.
  */
 int inet_listen(struct socket *sock, int backlog)
 {
     struct sock *sk = sock->sk;
     unsigned char old_state;
     int err;

     lock_sock(sk);

     err = -EINVAL;

     /* 检查socket状态和类型，仅支持SOCK_STREAM */
     if (sock->state != SS_UNCONNECTED || sock->type != SOCK_STREAM)
         goto out;

     /* 记录连接当前状态 */
     old_state = sk->sk_state;

     /* 检查连接状态，需为close或者listen */
     if (!(( << old_state) & (TCPF_CLOSE | TCPF_LISTEN)))
         goto out;

     /* Really, if the socket is already in listen state
      * we can only allow the backlog to be adjusted.
      */
     /* 如果尚未listen过 */
     if (old_state != TCP_LISTEN) {
         /* Enable TFO w/o requiring TCP_FASTOPEN socket option.
          * Note that only TCP sockets (SOCK_STREAM) will reach here.
          * Also fastopen backlog may already been set via the option
          * because the socket was in TCP_LISTEN state previously but
          * was shutdown() rather than close().
          */
         /* TFO相关*/
         if ((sysctl_tcp_fastopen & TFO_SERVER_WO_SOCKOPT1) &&
             (sysctl_tcp_fastopen & TFO_SERVER_ENABLE) &&
             !inet_csk(sk)->icsk_accept_queue.fastopenq.max_qlen) {
             fastopen_queue_tune(sk, backlog);
             tcp_fastopen_init_key_once(true);
         }

         /* 开始监听 */
         err = inet_csk_listen_start(sk, backlog);
         if (err)
             goto out;
     }

     /* 设置backlog，包括已经listen过，调整backlog */
     sk->sk_max_ack_backlog = backlog;
     err = ;

 out:
     release_sock(sk);
     return err;
 }
 EXPORT_SYMBOL(inet_listen);

其中tcp层对于listen的实现，实际上也是调用了inet_csk_get_port函数，与bind()系统调用在tcp层的调用函数一致，listen的时候，如果发现尚未绑定端口，则自动选择端口绑定，已经绑定端口，则进行相关检查；具体请参考bind()系统调用的tcp层实现；请阅读<TCP层bind系统调用的实现分析>；

关于TCP的listen函数和backlog参数说明，请移步以下文章<TCP之listen&backlog>；