asmlinkage long sys_connect(int fd, struct sockaddr __user *uservaddr,
int addrlen)
{
struct socket *sock;
char address[MAX_SOCK_ADDR];
int err, fput_needed;
//根據文件描述符獲取套接口指針，并且返回是否需要減少對文件引用計數標志。
sock = sockfd_lookup_light(fd, &err, &fput_needed);
if (!sock)
goto out;
//將用戶空間的uservaddr數據復制到內核空間的address
err = move_addr_to_kernel(uservaddr, addrlen, address);
if (err < 0)
goto out_put;

err =
security_socket_connect(sock, (struct sockaddr *)address, addrlen);
if (err)
goto out_put;

//通過套接口系統調用的跳轉表proto_ops，調用connect操作。TCP 中為 inet_stream_connect(), UDP 為 inet_dgram_connect()
err = sock->ops->connect(sock, (struct sockaddr *)address, addrlen,
sock->file->f_flags);
out_put:
// 根據fput_needed標志，調用fput_light減少對文件引用計數操作
fput_light(sock->file, fput_needed);
out:
return err;
}

int inet_stream_connect(struct socket *sock, struct sockaddr *uaddr,
int addr_len, int flags)
{
struct sock *sk = sock->sk;
int err;
long timeo;

lock_sock(sk);
/* socket的協議族錯誤 */
if (uaddr->sa_family == AF_UNSPEC) {
/* 如果使用的是TCP，則sk_prot為tcp_prot，disconnect為tcp_disconnect() */
err = sk->sk_prot->disconnect(sk, flags);
sock->state = err ? SS_DISCONNECTING : SS_UNCONNECTED;
goto out;
}

switch (sock->state) {
default:
err = -EINVAL;
goto out;
case SS_CONNECTED:
err = -EISCONN;
goto out;
case SS_CONNECTING:
err = -EALREADY;
/* Fall out of switch with err, set for this state */
break;
case SS_UNCONNECTED: /* 此套接口尚未連接對端的套接口，即連接尚未建立 */
err = -EISCONN;
if (sk->sk_state != TCP_CLOSE)
goto out;

/* 如果使用的是TCP，則sk_prot為tcp_prot，connect為tcp_v4_connect() */
err = sk->sk_prot->connect(sk, uaddr, addr_len); /* 發送SYN包 */
if (err < 0)
goto out;

/* 發出SYN包后socket狀態設為正在連接 */
sock->state = SS_CONNECTING;

/* Just entered SS_CONNECTING state; the only
* difference is that return value in non-blocking
* case is EINPROGRESS, rather than EALREADY.
*/
err = -EINPROGRESS;
break;
}

/* sock的發送超時時間，非阻塞則為0 */
timeo = sock_sndtimeo(sk, flags & O_NONBLOCK);

/* 發出SYN包后，等待后續握手的完成 */
if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
/* Error code is set above */
/* 如果是非阻塞的，那么就直接返回錯誤碼-EINPROGRESS。
* socket為阻塞時，使用inet_wait_for_connect()來等待協議棧的處理：
* 1. 使用SO_SNDTIMEO，睡眠時間超過timeo就返回0，之后返回錯誤碼-EINPROGRESS。
* 2. 收到信號，就返回剩余的等待時間。之后會返回錯誤碼-ERESTARTSYS或-EINTR。
* 3. 三次握手成功，被sock I/O事件處理函數喚醒，之后會返回0。
*/
if (!timeo || !inet_wait_for_connect(sk, timeo))
goto out;

err = sock_intr_errno(timeo);
/* 進程收到信號，如果err為-ERESTARTSYS，接下來庫函數會重新調用connect() */
if (signal_pending(current))
goto out;
}

/* Connection was closed by RST, timeout, ICMP error
* or another process disconnected us.
*/
if (sk->sk_state == TCP_CLOSE)
goto sock_error;

/* sk->sk_err may be not zero now, if RECVERR was ordered by user
* and error was received after socket entered established state.
* Hence, it is handled normally after connect() return successfully.
*/
/* 更新socket狀態為連接已建立 */
sock->state = SS_CONNECTED;
/* 清除錯誤碼 */
err = 0;
out:
release_sock(sk);
return err;

sock_error:
err = sock_error(sk) ? : -ECONNABORTED;
sock->state = SS_UNCONNECTED;
/* 如果使用的是TCP，則sk_prot為tcp_prot，disconnect為tcp_disconnect() */
if (sk->sk_prot->disconnect(sk, flags))
sock->state = SS_DISCONNECTING;
goto out;
}

static long inet_wait_for_connect(struct sock *sk, long timeo)
{
/* 初始化等待任務 */
DEFINE_WAIT(wait);
/* 把等待任務加入到socket的等待隊列頭部，把進程的狀態設為TASK_INTERRUPTIBLE */
prepare_to_wait(sk->sk_sleep, &wait, TASK_INTERRUPTIBLE);

/* Basic assumption: if someone sets sk->sk_err, he _must_
* change state of the socket from TCP_SYN_*.
* Connect() does not allow to get error notifications
* without closing the socket.
*/
/* 完成三次握手后，狀態就會變為TCP_ESTABLISHED，從而退出循環 */
while ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
release_sock(sk);
/* 進入睡眠，直到超時或收到信號，或者被I/O事件處理函數喚醒。
* 1. 如果是收到信號退出的，timeo為剩余的jiffies。
* 2. 如果使用了SO_SNDTIMEO選項，超時退出后，timeo為0。
* 3. 如果沒有使用SO_SNDTIMEO選項，timeo為無窮大，即MAX_SCHEDULE_TIMEOUT，
* 那么返回值也是這個，而超時時間不定。為了無限阻塞，需要上面的while循環。
*/
timeo = schedule_timeout(timeo);
/* 被喚醒后重新上鎖 */
lock_sock(sk);
/* 如果進程有待處理的信號，或者睡眠超時了，退出循環，之后會返回錯誤碼 */
if (signal_pending(current) || !timeo)
break;
/* 繼續睡眠 */
prepare_to_wait(sk->sk_sleep, &wait, TASK_INTERRUPTIBLE);
}
/* 等待結束時，把等待進程從等待隊列中刪除，把當前進程的狀態設為TASK_RUNNING */
finish_wait(sk->sk_sleep, &wait);
return timeo;
}

static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
struct tcphdr *th, unsigned len)
{
...

if (!sock_flag(sk, SOCK_DEAD)) {
/* 指向sock_def_wakeup，會喚醒調用connect()的進程，完成連接的建立 */
sk->sk_state_change(sk);
/* 若采用異步通知，則發送SIGIO通知進程可寫 */
sk_wake_async(sk, 0, POLL_OUT);
}

...
}

static void sock_def_wakeup(struct sock *sk)
{
read_lock(&sk->sk_callback_lock);
/* 有進程阻塞在此socket上 */
if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
/* 喚醒此socket上的所有睡眠進程 */
wake_up_interruptible_all(sk->sk_sleep);
read_unlock(&sk->sk_callback_lock);
}