Linux内核协议栈（7）listen函数分析

来源：互联网发布：衣柜合理布局知乎编辑：程序博客网时间：2024/06/15 08:40

监听函数主要干了两件事:

1.建立接受队列，并为接受队列分配空间

2.将sock对象设置为监听状态，并放入监听哈希表

/* *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. *//*** 就是把套接字挂到监听链表下*/SYSCALL_DEFINE2(listen, int, fd, int, backlog){struct socket *sock;int err, fput_needed;int somaxconn;sock = sockfd_lookup_light(fd, &err, &fput_needed);//查找sockif (sock) {somaxconn = sock_net(sock->sk)->core.sysctl_somaxconn;if ((unsigned)backlog > somaxconn)backlog = somaxconn;err = security_socket_listen(sock, backlog);if (!err)err = sock->ops->listen(sock, backlog);/**/inet_listenfput_light(sock->file, fput_needed);}return err;}

调用链为：sys_listen-->inet_listen-->inet_csk_listen_start

int inet_csk_listen_start(struct sock *sk, const int nr_table_entries){struct inet_sock *inet = inet_sk(sk);struct inet_connection_sock *icsk = inet_csk(sk);int rc = reqsk_queue_alloc(&icsk->icsk_accept_queue, nr_table_entries);//创建请求队列及为队列分配空间，listen_sock + nr_table_entries * request_sockif (rc != 0)return rc;sk->sk_max_ack_backlog = 0;sk->sk_ack_backlog = 0;inet_csk_delack_init(sk);//将sock中的icsk_ack置0 ---> Delayed ACK control data/* There is race window here: we announce ourselves listening, * but this transition is still not validated by get_port(). * It is OK, because this socket enters to hash table only * after validation is complete. */  /* *  要么把自己加入到tcp_hashinfo 中的ehash 中，要么加入到listening_hash 中，这要根据     *  sk_state 的值来操作，如果是LISTEN，就加入后者，如果是除LISTEN 之外的值，那么就加入     *  ehash 表，我们会在研究connect 的代码中看到。 */sk->sk_state = TCP_LISTEN;if (!sk->sk_prot->get_port(sk, inet->num)) {              /*获取可用端口*/inetsw_array  --> tcp_prot --> inet_csk_get_portinet->sport = htons(inet->num);sk_dst_reset(sk);sk->sk_prot->hash(sk);/*------------------------>加入监听哈希表*/ inetsw_array --> tcp_prot -->  __inet_hashreturn 0;}sk->sk_state = TCP_CLOSE;__reqsk_queue_destroy(&icsk->icsk_accept_queue);return -EADDRINUSE;}

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