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 <provos@citi.umich.edu> 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。