网络函数listen源码分析

static int sock_listen(int fd, int backlog)
{
	struct socket *sock;

	if (fd < 0 || fd >= NR_OPEN || current->files->fd[fd] == NULL)
		return(-EBADF);
	if (!(sock = sockfd_lookup(fd, NULL))) 
		return(-ENOTSOCK);

	if (sock->state != SS_UNCONNECTED) 
	{
		return(-EINVAL);
	}

	if (sock->ops && sock->ops->listen)
		sock->ops->listen(sock, backlog);
	// 设置socket的监听属性，accept函数时用到
	sock->flags |= SO_ACCEPTCON;
	return(0);
}
```
```c
static int inet_listen(struct socket *sock, int backlog)
{
	struct sock *sk = (struct sock *) sock->data;
	// 如果没有绑定端口则绑定一个，并把sock加到sock_array中
	if(inet_autobind(sk)!=0)
		return -EAGAIN;

	/* We might as well re use these. */ 
	/*
	 * note that the backlog is "unsigned char", so truncate it
	 * somewhere. We might as well truncate it to what everybody
	 * else does..
	 */
	if ((unsigned) backlog > 128)
		backlog = 128;
	// 设置syn+已连接队列的最大长度，在tcp.c中用到
	sk->max_ack_backlog = backlog;
	// 防止多次调用listen
	if (sk->state != TCP_LISTEN)
	{	
		// syn+已连接队列长度
		sk->ack_backlog = 0;
		sk->state = TCP_LISTEN;
	}
	return(0);
}
```
```c
// 绑定一个随机的端口，更新sk的源端口字段，并把sk挂载到端口对应的队列中
static int inet_autobind(struct sock *sk)
{
	// 已经绑定了端口则直接返回，否则获取一个随机的端口
	if (sk->num == 0) 
	{
		sk->num = get_new_socknum(sk->prot, 0);
		if (sk->num == 0) 
			return(-EAGAIN);
		put_sock(sk->num, sk);
		sk->dummy_th.source = ntohs(sk->num);
	}
	return 0;
}
listen函数主要的工作包括，设置socket和sock结构体的标记和状态，设置syn和已连接队列的上限。