- 用户态对accept的标准用法：
- if ((client_fd = accept(sockfd, ( struct sockaddr *)&remote_addr, &sin_size)) == -1)
- {
- //accept()函数让服务器接收客户的连接请求
- perror("accept Errorn");
- continue;
- }
- sockfd是通过socket系统调用，并且经过listen过的套接字：
- sockfd = socket(AF_INET, SOCK_STREAM, 0)
- listen(sockfd, 128)
- remote_addr将会存储远端设备的地址信息。
-
SYSCALL_DEFINE3
(
accept
,
int
,
fd
,
struct
sockaddr __user
*,
upeer_sockaddr
,
-
int
__user
*,
upeer_addrlen
)
-
{
-
return
sys_accept4
(
fd
,
upeer_sockaddr
,
upeer_addrlen
,
0
);
-
}
-
SYSCALL_DEFINE4
(
accept4
,
int
,
fd
,
struct
sockaddr __user
*,
upeer_sockaddr
,
-
int
__user
*,
upeer_addrlen
,
int
,
flags
)
-
{
-
struct
socket
*
sock
,
*
newsock
;
-
struct
file
*
newfile
;
-
int
err
,
len
,
newfd
,
fput_needed
;
-
struct
sockaddr_storage address
;
-
-
if
(
flags
&
~(
SOCK_CLOEXEC
|
SOCK_NONBLOCK
))
-
{
-
return
-
EINVAL
;
-
}
-
-
if
(
SOCK_NONBLOCK
!=
O_NONBLOCK
&&
(
flags
&
SOCK_NONBLOCK
))
-
{
-
flags
=
(
flags
&
~
SOCK_NONBLOCK
)
|
O_NONBLOCK
;
-
}
-
-
sock
=
sockfd_lookup_light
(
fd
,
&
err
,
&
fput_needed
);
-
if
(!
sock
)
-
{
-
goto
out
;
-
}
-
-
err
=
-
ENFILE
;
-
newsock
=
sock_alloc
();
/*! 1.创建新的sock给新的连接 */
-
if
(!
newsock
)
-
{
-
goto
out_put
;
-
}
-
-
newsock
->
type
=
sock
->
type
;
-
newsock
->
ops
=
sock
->
ops
;
-
-
/*
-
* We don‘t need try_module_get here, as the listening socket (sock)
-
* has the protocol module (sock->ops->owner) held.
-
*/
-
__module_get
(
newsock
->
ops
->
owner
);
-
-
newfd
=
get_unused_fd_flags
(
flags
);
/*! 2.分配一个fd给新的连接 */
-
if
(
unlikely
(
newfd
<
0
))
-
{
-
err
=
newfd
;
-
sock_release
(
newsock
);
-
goto
out_put
;
-
}
-
newfile
=
sock_alloc_file
(
newsock
,
flags
,
sock
->
sk
->
sk_prot_creator
->
name
);
/*! 3.为newsock创建一个对应的file结构 */
-
if
(
unlikely
(
IS_ERR
(
newfile
)))
-
{
-
err
=
PTR_ERR
(
newfile
);
-
put_unused_fd
(
newfd
);
-
sock_release
(
newsock
);
-
goto
out_put
;
-
}
-
-
err
=
security_socket_accept
(
sock
,
newsock
);
-
if
(
err
)
-
{
-
goto
out_fd
;
-
}
-
-
err
=
sock
->
ops
->
accept
(
sock
,
newsock
,
sock
->
file
->
f_flags
);
/*! 4.调用Socket层操作函数inet_accept() */
-
if
(
err
<
0
)
-
{
-
goto
out_fd
;
-
}
-
-
if
(
upeer_sockaddr
)
-
{
-
if
(
newsock
->
ops
->
getname
(
newsock
,
(
struct
sockaddr
*)&
address
,
-
&
len
,
2
)
<
0
)
-
{
-
err
=
-
ECONNABORTED
;
-
goto
out_fd
;
-
}
-
err
=
move_addr_to_user
(&
address
,
-
len
,
upeer_sockaddr
,
upeer_addrlen
);
-
if
(
err
<
0
)
-
{
-
goto
out_fd
;
-
}
-
}
-
-
/* File flags are not inherited via accept() unlike another OSes. */
-
-
fd_install
(
newfd
,
newfile
);
-
err
=
newfd
;
-
-
out_put
:
-
fput_light
(
sock
->
file
,
fput_needed
);
-
out
:
-
return
err
;
-
out_fd
:
-
fput
(
newfile
);
-
put_unused_fd
(
newfd
);
-
goto
out_put
;
-
}
3、sock_alloc_file()
-
struct
file
*
sock_alloc_file
(
struct
socket
*
sock
,
int
flags
,
const
char
*
dname
)
-
{
-
struct
qstr name
=
{
.
name
=
""
};
-
struct
path path
;
-
struct
file
*
file
;
-
-
if
(
dname
)
-
{
-
name
.
name
=
dname
;
-
name
.
len
=
strlen
(
name
.
name
);
-
}
-
else
if
(
sock
->
sk
)
-
{
-
name
.
name
=
sock
->
sk
->
sk_prot_creator
->
name
;
-
name
.
len
=
strlen
(
name
.
name
);
-
}
-
path
.
dentry
=
d_alloc_pseudo
(
sock_mnt
->
mnt_sb
,
&
name
);
-
if
(
unlikely
(!
path
.
dentry
))
-
{
-
return
ERR_PTR
(-
ENOMEM
);
-
}
-
path
.
mnt
=
mntget
(
sock_mnt
);
-
-
d_instantiate
(
path
.
dentry
,
SOCK_INODE
(
sock
));
-
-
file
=
alloc_file
(&
path
,
FMODE_READ
|
FMODE_WRITE
,
-
&
socket_file_ops
);
-
if
(
unlikely
(
IS_ERR
(
file
)))
-
{
-
/* drop dentry, keep inode */
-
ihold
(
path
.
dentry
->
d_inode
);
-
path_put
(&
path
);
-
return
file
;
-
}
-
-
/*! 注意这里的属性设置 */
-
sock
->
file
=
file
;
-
file
->
f_flags
=
O_RDWR
|
(
flags
&
O_NONBLOCK
);
-
file
->
private_data
=
sock
;
-
return
file
;
-
}
4、inet_accept()
-
/*
-
* Accept a pending connection. The TCP layer now gives BSD semantics.
-
*/
-
// <net/ipv4/af_inet.c>
-
int
inet_accept
(
struct
socket
*
sock
,
struct
socket
*
newsock
,
int
flags
)
-
{
-
struct
sock
*
sk1
=
sock
->
sk
;
-
int
err
=
-
EINVAL
;
-
-
/**
-
* 如果使用的是TCP，则sk_prot为tcp_prot，accept为inet_csk_accept()
-
* 获取新连接的sock。
-
*/
-
struct
sock
*
sk2
=
sk1
->
sk_prot
->
accept
(
sk1
,
flags
,
&
err
);
/*! 4.1.获取新连接的sock */
-
-
if
(!
sk2
)
-
{
-
goto
do_err
;
-
}
-
-
lock_sock
(
sk2
);
-
-
sock_rps_record_flow
(
sk2
);
-
WARN_ON
(!((
1
<<
sk2
->
sk_state
)
&
-
(
TCPF_ESTABLISHED
|
TCPF_SYN_RECV
|
-
TCPF_CLOSE_WAIT
|
TCPF_CLOSE
)));
-
-
sock_graft
(
sk2
,
newsock
);
/*! 4.2.把sock和socket嫁接起来，让它们能相互索引 */
-
-
newsock
->
state
=
SS_CONNECTED
;
/*! 4.3.把新socket的状态设为已连接 */
-
err
=
0
;
-
release_sock
(
sk2
);
-
do_err
:
-
return
err
;
-
}
4.2、sock_graft()
-
// <net/Sock.h>
-
static
inline
void
sock_graft
(
struct
sock
*
sk
,
struct
socket
*
parent
)
-
{
-
write_lock_bh
(&
sk
->
sk_callback_lock
);
-
sk
->
sk_wq
=
parent
->
wq
;
-
parent
->
sk
=
sk
; /*! INET层的socket使用下层的sock服务 */
-
sk_set_socket
(
sk
,
parent
);
-
security_sock_graft
(
sk
,
parent
);
-
write_unlock_bh
(&
sk
->
sk_callback_lock
);
-
}
4.1、inet_csk_accept()
/**
* inet_csk_accept()用于从backlog队列（全连接队列）中取出一个ESTABLISHED状态的连接请求块，返回它所对应的连接sock。
* 1. 非阻塞的，且当前没有已建立的连接，则直接退出，返回-EAGAIN。
* 2. 阻塞的，且当前没有已建立的连接：
* 2.1 用户没有设置超时时间，则无限期阻塞。
* 2.2 用户设置了超时时间，超时后会退出。
*/
-
// <net/ipv4/Inet_connection_sock.c>
-
/*
-
* This will accept the next outstanding connection.
-
*/
-
struct
sock
*
inet_csk_accept
(
struct
sock
*
sk
,
int
flags
,
int
*
err
)
-
{
-
struct
inet_connection_sock
*
icsk
=
inet_csk
(
sk
);
-
struct
request_sock_queue
*
queue
=
&
icsk
->
icsk_accept_queue
;
-
struct
sock
*
newsk
;
-
struct
request_sock
*
req
;
-
int
error
;
-
-
lock_sock
(
sk
);
-
-
/* We need to make sure that this socket is listening,
-
* and that it has something pending.
-
*/
-
error
=
-
EINVAL
;
-
if
(
sk
->
sk_state
!=
TCP_LISTEN
)
-
{
-
goto
out_err
;
-
}
-
-
/* Find already established connection */
-
if
(
reqsk_queue_empty
(
queue
))
// 没有ESTABLISHED状态的连接请求块
-
{
-
long
timeo
=
sock_rcvtimeo
(
sk
,
flags
&
O_NONBLOCK
);
-
-
/* If this is a non blocking socket don‘t sleep */
-
error
=
-
EAGAIN
;
-
if
(!
timeo
)
-
{
-
goto
out_err
;
-
}
-
-
/*! 4.1.1 阻塞等待，直到有全连接。如果用户设置有等待时间，超时后会退出 */
-
error
=
inet_csk_wait_for_connect
(
sk
,
timeo
);
-
if
(
error
)
-
{
-
goto
out_err
;
-
}
-
}
-
-
/*! 从全连接队列中取出第一个established状态的连接请求块 */
-
req
=
reqsk_queue_remove
(
queue
);
-
newsk
=
req
->
sk
;
-
-
sk_acceptq_removed
(
sk
);
-
if
(
sk
->
sk_protocol
==
IPPROTO_TCP
&&
queue
->
fastopenq
!=
NULL
)
-
{
-
spin_lock_bh
(&
queue
->
fastopenq
->
lock
);
-
if
(
tcp_rsk
(
req
)->
listener
)
-
{
-
/* We are still waiting for the final ACK from 3WHS
-
* so can‘t free req now. Instead, we set req->sk to
-
* NULL to signify that the child socket is taken
-
* so reqsk_fastopen_remove() will free the req
-
* when 3WHS finishes (or is aborted).
-
*/
-
req
->
sk
=
NULL
;
-
req
=
NULL
;
-
}
-
spin_unlock_bh
(&
queue
->
fastopenq
->
lock
);
-
}
-
out
:
-
release_sock
(
sk
);
-
if
(
req
)
-
{
-
__reqsk_free
(
req
);
-
}
-
return
newsk
;
-
out_err
:
-
newsk
=
NULL
;
-
req
=
NULL
;
-
*
err
=
error
;
-
goto
out
;
-
}
4.1.1 inet_csk_wait_for_connect()
-
// <net/ipv4/Inet_connection_sock.c>
-
/*
-
* Wait for an incoming connection, avoid race conditions. This must be called
-
* with the socket locked.
-
*/
-
static
int
inet_csk_wait_for_connect
(
struct
sock
*
sk
,
long
timeo
)
-
{
-
struct
inet_connection_sock
*
icsk
=
inet_csk
(
sk
);
-
DEFINE_WAIT
(
wait
);
-
int
err
;
-
-
/*
-
* True wake-one mechanism for incoming connections: only
-
* one process gets woken up, not the ‘whole herd‘.
-
* Since we do not ‘race & poll‘ for established sockets
-
* anymore, the common case will execute the loop only once.
-
*
-
* Subtle issue: "add_wait_queue_exclusive()" will be added
-
* after any current non-exclusive waiters, and we know that
-
* it will always _stay_ after any new non-exclusive waiters
-
* because all non-exclusive waiters are added at the
-
* beginning of the wait-queue. As such, it‘s ok to "drop"
-
* our exclusiveness temporarily when we get woken up without
-
* having to remove and re-insert us on the wait queue.
-
*/
-
for
(;;)
-
{
-
/*! 把自己加入到等待队列，并且设置自己的状态是可中断的 */
-
prepare_to_wait_exclusive
(
sk_sleep
(
sk
),
&
wait
,
-
TASK_INTERRUPTIBLE
);
-
release_sock
(
sk
);
-
if
(
reqsk_queue_empty
(&
icsk
->
icsk_accept_queue
))
-
{
-
/**
-
* 用户发起的accept操作就停schedule_timeout中
-
* switch (timeout)
-
* {
-
* case MAX_SCHEDULE_TIMEOUT:
-
* schedule();
-
* goto out;
-
* default:
-
* }
-
* 根据其实现代码，由于我们一般没有设置timeout值，所以是MAX_SCHEDULE_TIMEOUT的情况，这表示立即进入重新调度，
-
* 而当前的进程可以处于睡眠，直到被其它事件唤醒。
-
*/
-
timeo
=
schedule_timeout
(
timeo
);
-
}
-
sched_annotate_sleep
();
-
lock_sock
(
sk
);
-
err
=
0
;
-
if
(!
reqsk_queue_empty
(&
icsk
->
icsk_accept_queue
))
-
{
-
break
;
-
}
-
err
=
-
EINVAL
;
-
if
(
sk
->
sk_state
!=
TCP_LISTEN
)
-
{
-
break
;
-
}
-
err
=
sock_intr_errno
(
timeo
);
-
if
(
signal_pending
(
current
))
-
{
-
break
;
-
}
-
err
=
-
EAGAIN
;
-
if
(!
timeo
)
-
{
-
break
;
-
}
-
}
-
/*! 下面把任务设置成TASK_RUNNING状态，然后把当前sock从等待队列中删除 */
-
finish_wait
(
sk_sleep
(
sk
),
&
wait
);
-
return
err
;
-
}
原文：http://www.cnblogs.com/fengkang1008/p/4688633.html
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