Linux Kernel 2.6.9源码分析 – listen
先来看下listen的原型:int listen(int sockfd, int backlog);
参数说明:
sockfd:要监听的socket描述字
backlog:为相应socket可以排队的最大连接个数
返回值:返回错误码
对于有链接的socket对子,系统如何来区分谁是server,谁是client呢 ?只要在创建socket的时候有调用listen接口,此socket就成为了server socket.凡是server socket都不能主动去与别的socket建立连接,而只能被动地通过accept()来接受来自client socket的请求.而client socket刚好相反,不能调用accept()来接受连接请求,而只能主动地通过connect()提出连接请求.
下面来看下系统调用sys_listen如何实现这一功能 ?
asmlinkage long sys_listen(int fd, int backlog)
{
struct socket *sock;
int err;
if ((sock = sockfd_lookup(fd, &err)) != NULL) {
if ((unsigned) backlog > sysctl_somaxconn)
backlog = sysctl_somaxconn;
err = security_socket_listen(sock, backlog);
if (err) {
sockfd_put(sock);
return err;
}
err=sock->ops->listen(sock, backlog);
sockfd_put(sock);
}
return err;
}
其中主要是调用sock->ops->listen,其对应的是inet_listen函数
int inet_listen(struct socket *sock, int backlog)
{
struct sock *sk = sock->sk;
unsigned char old_state;
int err;
lock_sock(sk);
err = -EINVAL;
if (sock->state != SS_UNCONNECTED || sock->type != SOCK_STREAM)
goto out;
old_state = sk->sk_state;
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 = tcp_listen_start(sk);
if (err)
goto out;
}
sk->sk_max_ack_backlog = backlog;
err = 0;
out:
release_sock(sk);
return err;
}
1.将backlog 赋值给sk->sk_max_ack_backlog
2.调用tcp_listen_start函数:
sock->tcp_opt 赋值,并将sock的状态修改:sk->sk_state = TCP_LISTEN;
int tcp_listen_start(struct sock *sk)
{
struct inet_opt *inet = inet_sk(sk);
struct tcp_opt *tp = tcp_sk(sk);
struct tcp_listen_opt *lopt;
sk->sk_max_ack_backlog = 0;
sk->sk_ack_backlog = 0;
tp->accept_queue = tp->accept_queue_tail = NULL;
tp->syn_wait_lock = RW_LOCK_UNLOCKED;
tcp_delack_init(tp);
lopt = kmalloc(sizeof(struct tcp_listen_opt), GFP_KERNEL);
if (!lopt)
return -ENOMEM;
memset(lopt, 0, sizeof(struct tcp_listen_opt));
for (lopt->max_qlen_log = 6; ; lopt->max_qlen_log++)
if ((1 << lopt->max_qlen_log) >= sysctl_max_syn_backlog)
break;
get_random_bytes(&lopt->hash_rnd, 4);
write_lock_bh(&tp->syn_wait_lock);
tp->listen_opt = lopt;
write_unlock_bh(&tp->syn_wait_lock);
/* 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;
if (!sk->sk_prot->get_port(sk, inet->num)) {
inet->sport = htons(inet->num);
sk_dst_reset(sk);
sk->sk_prot->hash(sk);
return 0; //******为何要在这里返回 ?***********
}
sk->sk_state = TCP_CLOSE;
write_lock_bh(&tp->syn_wait_lock);
tp->listen_opt = NULL;
write_unlock_bh(&tp->syn_wait_lock);
kfree(lopt);
return -EADDRINUSE;
}