listener是libevent封装的一个方便生成监听者的一组结构和函数，其中包括：
1 /*
2 * Copyright (c) 2000-2007 Niels Provos <[email protected]>
3 * Copyright (c) 2007-2012 Niels Provos and Nick Mathewson
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions
7 * are met:
8 * 1. Redistributions of source code must retain the above copyright
9 * notice, this list of conditions and the following disclaimer.
10 * 2. Redistributions in binary form must reproduce the above copyright
11 * notice, this list of conditions and the following disclaimer in the
12 * documentation and/or other materials provided with the distribution.
13 * 3. The name of the author may not be used to endorse or promote products
14 * derived from this software without specific prior written permission.
15 *
16 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS‘‘ AND ANY EXPRESS OR
17 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
18 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
19 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
20 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
21 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
22 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
23 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
24 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
25 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
26 */
27 #ifndef EVENT2_LISTENER_H_INCLUDED_
28 #define EVENT2_LISTENER_H_INCLUDED_
29
30 #include <event2/visibility.h>
31
32 #ifdef __cplusplus
33 extern "C" {
34 #endif
35
36 #include <event2/event.h>
37
38 struct sockaddr;
39 struct evconnlistener;
40
41 /**
42 A callback that we invoke when a listener has a new connection.
43
44 @param listener The evconnlistener
45 @param fd The new file descriptor
46 @param addr The source address of the connection
47 @param socklen The length of addr
48 @param user_arg the pointer passed to evconnlistener_new()
49 */
50 typedef void (*evconnlistener_cb)(struct evconnlistener *, evutil_socket_t, struct sockaddr *, int socklen, void *);
51
52 /**
53 A callback that we invoke when a listener encounters a non-retriable error.
54
55 @param listener The evconnlistener
56 @param user_arg the pointer passed to evconnlistener_new()
57 */
58 typedef void (*evconnlistener_errorcb)(struct evconnlistener *, void *);
59
60 /** Flag: Indicates that we should not make incoming sockets nonblocking
61 * before passing them to the callback. */
62 #define LEV_OPT_LEAVE_SOCKETS_BLOCKING (1u<<0)
63 /** Flag: Indicates that freeing the listener should close the underlying
64 * socket. */
65 #define LEV_OPT_CLOSE_ON_FREE (1u<<1)
66 /** Flag: Indicates that we should set the close-on-exec flag, if possible */
67 #define LEV_OPT_CLOSE_ON_EXEC (1u<<2)
68 /** Flag: Indicates that we should disable the timeout (if any) between when
69 * this socket is closed and when we can listen again on the same port. */
70 #define LEV_OPT_REUSEABLE (1u<<3)
71 /** Flag: Indicates that the listener should be locked so it‘s safe to use
72 * from multiple threadcs at once. */
73 #define LEV_OPT_THREADSAFE (1u<<4)
74 /** Flag: Indicates that the listener should be created in disabled
75 * state. Use evconnlistener_enable() to enable it later. */
76 #define LEV_OPT_DISABLED (1u<<5)
77 /** Flag: Indicates that the listener should defer accept() until data is
78 * available, if possible. Ignored on platforms that do not support this.
79 *
80 * This option can help performance for protocols where the client transmits
81 * immediately after connecting. Do not use this option if your protocol
82 * _doesn‘t_ start out with the client transmitting data, since in that case
83 * this option will sometimes cause the kernel to never tell you about the
84 * connection.
85 *
86 * This option is only supported by evconnlistener_new_bind(): it can‘t
87 * work with evconnlistener_new_fd(), since the listener needs to be told
88 * to use the option before it is actually bound.
89 */
90 #define LEV_OPT_DEFERRED_ACCEPT (1u<<6)
91 /** Flag: Indicates that we ask to allow multiple servers (processes or
92 * threads) to bind to the same port if they each set the option.
93 *
94 * SO_REUSEPORT is what most people would expect SO_REUSEADDR to be, however
95 * SO_REUSEPORT does not imply SO_REUSEADDR.
96 *
97 * This is only available on Linux and kernel 3.9+
98 */
99 #define LEV_OPT_REUSEABLE_PORT (1u<<7)
100
101 /**
102 Allocate a new evconnlistener object to listen for incoming TCP connections
103 on a given file descriptor.
104
105 @param base The event base to associate the listener with.
106 @param cb A callback to be invoked when a new connection arrives. If the
107 callback is NULL, the listener will be treated as disabled until the
108 callback is set.
109 @param ptr A user-supplied pointer to give to the callback.
110 @param flags Any number of LEV_OPT_* flags
111 @param backlog Passed to the listen() call to determine the length of the
112 acceptable connection backlog. Set to -1 for a reasonable default.
113 Set to 0 if the socket is already listening.
114 @param fd The file descriptor to listen on. It must be a nonblocking
115 file descriptor, and it should already be bound to an appropriate
116 port and address.
117 */
118 EVENT2_EXPORT_SYMBOL
119 struct evconnlistener *evconnlistener_new(struct event_base *base,
120 evconnlistener_cb cb, void *ptr, unsigned flags, int backlog,
121 evutil_socket_t fd);
122 /**
123 Allocate a new evconnlistener object to listen for incoming TCP connections
124 on a given address.
125
126 @param base The event base to associate the listener with.
127 @param cb A callback to be invoked when a new connection arrives. If the
128 callback is NULL, the listener will be treated as disabled until the
129 callback is set.
130 @param ptr A user-supplied pointer to give to the callback.
131 @param flags Any number of LEV_OPT_* flags
132 @param backlog Passed to the listen() call to determine the length of the
133 acceptable connection backlog. Set to -1 for a reasonable default.
134 @param addr The address to listen for connections on.
135 @param socklen The length of the address.
136 */
137 EVENT2_EXPORT_SYMBOL
138 struct evconnlistener *evconnlistener_new_bind(struct event_base *base,
139 evconnlistener_cb cb, void *ptr, unsigned flags, int backlog,
140 const struct sockaddr *sa, int socklen);
141 /**
142 Disable and deallocate an evconnlistener.
143 */
144 EVENT2_EXPORT_SYMBOL
145 void evconnlistener_free(struct evconnlistener *lev);
146 /**
147 Re-enable an evconnlistener that has been disabled.
148 */
149 EVENT2_EXPORT_SYMBOL
150 int evconnlistener_enable(struct evconnlistener *lev);
151 /**
152 Stop listening for connections on an evconnlistener.
153 */
154 EVENT2_EXPORT_SYMBOL
155 int evconnlistener_disable(struct evconnlistener *lev);
156
157 /** Return an evconnlistener‘s associated event_base. */
158 EVENT2_EXPORT_SYMBOL
159 struct event_base *evconnlistener_get_base(struct evconnlistener *lev);
160
161 /** Return the socket that an evconnlistner is listening on. */
162 EVENT2_EXPORT_SYMBOL
163 evutil_socket_t evconnlistener_get_fd(struct evconnlistener *lev);
164
165 /** Change the callback on the listener to cb and its user_data to arg.
166 */
167 EVENT2_EXPORT_SYMBOL
168 void evconnlistener_set_cb(struct evconnlistener *lev,
169 evconnlistener_cb cb, void *arg);
170
171 /** Set an evconnlistener‘s error callback. */
172 EVENT2_EXPORT_SYMBOL
173 void evconnlistener_set_error_cb(struct evconnlistener *lev,
174 evconnlistener_errorcb errorcb);
175
176 #ifdef __cplusplus
177 }
178 #endif
179
180 #endif
定义的函数有以下几个：
evconnlistener_cb：函数指针类型，当有一个新连接到来时被回调。
evconnlistener_errorcb：函数指针类型，当有一个错误发生时被回调。
evconnlistener_new：基于一个给定的socket套接字描述符，返回一个struct evconnlistener对象，（要求描述符已经调用bind）
evconnlistener_new_bind：基于一个给定的sockaddr地址，返回一个struct evconnlistener对象；该函数会调用上面的函数。
evconnlistener_free：释放一个struct evconnlistener对象。
evconnlistener_enable：激活一个struct evconnlistener对象。
evconnlistener_disable：关闭一个struct evconnlistener对象。
evconnlistener_get_base：返回与该listener关联的event_base对象。
evconnlistener_get_fd：返回与该listener关联的socket描述符。
evconnlistener_set_cb：设置新连接到来时的回调函数。
evconnlistener_set_error_cb：设置发生错误时的回调函数。
数据结构：
1 struct evconnlistener {
2 const struct evconnlistener_ops *ops;
3 void *lock;
4 evconnlistener_cb cb;
5 evconnlistener_errorcb errorcb;
6 void *user_data;
7 unsigned flags;
8 short refcnt;
9 int accept4_flags;
10 unsigned enabled : 1;
11 };
12
13 struct evconnlistener_event {
14 struct evconnlistener base;
15 struct event listener;
16 };
函数的实现：
我认为其中最需要花时间理解的函数只有两个：evconnlistener_new、listener_read_cb。
1 struct evconnlistener *
2 evconnlistener_new(struct event_base *base,
3 evconnlistener_cb cb, void *ptr, unsigned flags, int backlog,
4 evutil_socket_t fd)
5 {
6 struct evconnlistener_event *lev;
7
8 #ifdef _WIN32
9 if (base && event_base_get_iocp_(base)) {
10 const struct win32_extension_fns *ext =
11 event_get_win32_extension_fns_();
12 if (ext->AcceptEx && ext->GetAcceptExSockaddrs)
13 return evconnlistener_new_async(base, cb, ptr, flags,
14 backlog, fd);
15 }
16 #endif
17
18 if (backlog > 0) {
19 if (listen(fd, backlog) < 0)
20 return NULL;
21 } else if (backlog < 0) {
22 if (listen(fd, 128) < 0)
23 return NULL;
24 }
25
26 lev = mm_calloc(1, sizeof(struct evconnlistener_event));
27 if (!lev)
28 return NULL;
29
30 lev->base.ops = &evconnlistener_event_ops;
31 lev->base.cb = cb;
32 lev->base.user_data = ptr;
33 lev->base.flags = flags;
34 lev->base.refcnt = 1;
35
36 lev->base.accept4_flags = 0;
37 if (!(flags & LEV_OPT_LEAVE_SOCKETS_BLOCKING))
38 lev->base.accept4_flags |= EVUTIL_SOCK_NONBLOCK;
39 if (flags & LEV_OPT_CLOSE_ON_EXEC)
40 lev->base.accept4_flags |= EVUTIL_SOCK_CLOEXEC;
41
42 if (flags & LEV_OPT_THREADSAFE) {
43 EVTHREAD_ALLOC_LOCK(lev->base.lock, EVTHREAD_LOCKTYPE_RECURSIVE);
44 }
45
46 event_assign(&lev->listener, base, fd, EV_READ|EV_PERSIST,
47 listener_read_cb, lev);
48
49 if (!(flags & LEV_OPT_DISABLED))
50 evconnlistener_enable(&lev->base);
51
52 return &lev->base;
53 }
总结起来这个函数就做个两件事：分配一个evconnlistener_event对象、初始化base成员和listener成员并激活该事件（evconnlistener_enable）。
这里需要注意到，初始化listener时传入的回调函数是listener_read_cb，而这个函数是在listener中定义的，而我们调用evconnlistener_new时传入的回调函数只是被赋值给了base.cb了，那么这个函数是怎么在新连接到来时被调用的呢？秘密就在listener_read_cb函数中，下面来看一下它的实现：
1 static void
2 listener_read_cb(evutil_socket_t fd, short what, void *p)
3 {
4 struct evconnlistener *lev = p;
5 int err;
6 evconnlistener_cb cb;
7 evconnlistener_errorcb errorcb;
8 void *user_data;
9 LOCK(lev);
10 while (1) {
11 struct sockaddr_storage ss;
12 ev_socklen_t socklen = sizeof(ss);
13 evutil_socket_t new_fd = evutil_accept4_(fd, (struct sockaddr*)&ss, &socklen, lev->accept4_flags);
14 if (new_fd < 0)
15 break;
16 if (socklen == 0) {
17 /* This can happen with some older linux kernels in
18 * response to nmap. */
19 evutil_closesocket(new_fd);
20 continue;
21 }
22
23 if (lev->cb == NULL) {
24 evutil_closesocket(new_fd);
25 UNLOCK(lev);
26 return;
27 }
28 ++lev->refcnt;
29 cb = lev->cb;
30 user_data = lev->user_data;
31 UNLOCK(lev);
32 cb(lev, new_fd, (struct sockaddr*)&ss, (int)socklen,
33 user_data);
34 LOCK(lev);
35 if (lev->refcnt == 1) {
36 int freed = listener_decref_and_unlock(lev);
37 EVUTIL_ASSERT(freed);
38
39 evutil_closesocket(new_fd);
40 return;
41 }
42 --lev->refcnt;
43 }
44 err = evutil_socket_geterror(fd);
45 if (EVUTIL_ERR_ACCEPT_RETRIABLE(err)) {
46 UNLOCK(lev);
47 return;
48 }
49 if (lev->errorcb != NULL) {
50 ++lev->refcnt;
51 errorcb = lev->errorcb;
52 user_data = lev->user_data;
53 UNLOCK(lev);
54 errorcb(lev, user_data);
55 LOCK(lev);
56 listener_decref_and_unlock(lev);
57 } else {
58 event_sock_warn(fd, "Error from accept() call");
59 UNLOCK(lev);
60 }
61 }
首先可以从L29-L32看到，我们在调用evconnlistener_new时传入的回调函数被调用了，这里就解开了上面我说的那个秘密，这里其实是libevent对于回调加了一层处理，通过listener_read_cb间接回调我们设置的回调函数，这样可以在listener_read_cb中处理一些异常情况，例如：当异常发生时回调errorcb。
到这里listener就分析完了，上面说需要理解的就这两个函数是因为其他的函数都是功能函数，实现不复杂，而evconnlistener_new_bind函数其实是调用了evconnlistener_new函数的。
时间： 2024-10-10 09:45:51