Socket listen 简要分析

#include <sys/types.h> /* See NOTES */
#include <sys/socket.h>
int listen(int sockfd, int backlog);

· 参数 int sockfd ：成功创建的 TCP 套接字。
·int backlog ：定义 TCP 未处理连接的队列长度。该队列虽然已经完成了三次握手，但服务器端还没 有执行 accept 的连接。 APUE 中说， backlog 只是一个提示，具体的数值实际上是由系统来决定的。

Now it specifies the queue length for completely established sockets waiting to be accepted,instead of the number of incomplete connection requests. The maximum length of the queue
for incomplete sockets can be set using the tcp_max_syn_backlog sysctl.

全连接队列的最大长度：backlog保存的是完成三次握手、等待accept的全连接，而不是半连接

min(backlog, somaxconn)，net.core.somaxconn默认为128。

这个值最终存储于sk->sk_max_ack_backlog

半连接队列的最大长度：
tcp_max_syn_backlog默认值为256。(For incomplete connections)
当使用SYN Cookie时，这个参数变为无效。
半连接队列的最大长度为backlog、somaxconn、tcp_max_syn_backlog的最小值

/*
 *	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;

	sock = sockfd_lookup_light(fd, &err, &fput_needed);
	if (sock) {
		somaxconn = sock_net(sock->sk)->core.sysctl_somaxconn;
		if ((unsigned int)backlog > somaxconn)
			backlog = somaxconn;

		err = security_socket_listen(sock, backlog);
		if (!err)
			err = sock->ops->listen(sock, backlog);

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

/*
 *	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;
	/* 此时套接口状态需为SS_UNCONNECTED，套接口类型需为SOCK_STREAM */
	if (sock->state != SS_UNCONNECTED || sock->type != SOCK_STREAM)
		goto out;

	old_state = sk->sk_state; /* 当前的连接需为CLOSED或LISTEN状态 */
	if (!((1 << 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.
	 */
	if (old_state != TCP_LISTEN) {
		err = inet_csk_listen_start(sk, backlog);/* 启动监听 */
		if (err)
			goto out;
	}
	sk->sk_max_ack_backlog = backlog;/* 最大全连接队列长度 */
	err = 0;

out:
	release_sock(sk);
	return err;
}

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);
///* 初始化全连接队列，创建半连接队列的实例 */
	int rc = reqsk_queue_alloc(&icsk->icsk_accept_queue, nr_table_entries);

	if (rc != 0)
		return rc;

	sk->sk_max_ack_backlog = 0;/* 最大的backlog，最大全连接队列长度 初始化为0*/
	sk->sk_ack_backlog = 0;/* 当前的backlog，当前全连接队列长度 先设置为0*/
	inet_csk_delack_init(sk);

	/* 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.
	 */
	sk->sk_state = TCP_LISTEN;
	//检查端口是否可用，防止bind()后其它进程修改了端口信息
	if (!sk->sk_prot->get_port(sk, inet->inet_num)) {
		inet->inet_sport = htons(inet->inet_num);

		sk_dst_reset(sk);//clear 路由
		sk->sk_prot->hash(sk);//把sock链接进入监听哈希表listening_hash中。

		return 0;
	}

	sk->sk_state = TCP_CLOSE;
	  /* 如果端口不可用，则释放半连接队列 */
	__reqsk_queue_destroy(&icsk->icsk_accept_queue);
	return -EADDRINUSE;
}

listen_sock结构用于保存SYN_RECV状态的连接请求块，所以也叫半连接队列。