libevent源码分析：listener

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：

 /*
  * Copyright (c) 2000-2007 Niels Provos <provos@citi.umich.edu>
  * Copyright (c) 2007-2012 Niels Provos and Nick Mathewson
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  * 3. The name of the author may not be used to endorse or promote products
  *    derived from this software without specific prior written permission.
  *
  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 #ifndef EVENT2_LISTENER_H_INCLUDED_
 #define EVENT2_LISTENER_H_INCLUDED_

 #include <event2/visibility.h>

 #ifdef __cplusplus
 extern "C" {
 #endif

 #include <event2/event.h>

 struct sockaddr;
 struct evconnlistener;

 /**
    A callback that we invoke when a listener has a new connection.

    @param listener The evconnlistener
    @param fd The new file descriptor
    @param addr The source address of the connection
    @param socklen The length of addr
    @param user_arg the pointer passed to evconnlistener_new()
  */
 typedef void (*evconnlistener_cb)(struct evconnlistener *, evutil_socket_t, struct sockaddr *, int socklen, void *);

 /**
    A callback that we invoke when a listener encounters a non-retriable error.

    @param listener The evconnlistener
    @param user_arg the pointer passed to evconnlistener_new()
  */
 typedef void (*evconnlistener_errorcb)(struct evconnlistener *, void *);

 /** Flag: Indicates that we should not make incoming sockets nonblocking
  * before passing them to the callback. */
 #define LEV_OPT_LEAVE_SOCKETS_BLOCKING    (1u<<0)
 /** Flag: Indicates that freeing the listener should close the underlying
  * socket. */
 #define LEV_OPT_CLOSE_ON_FREE        (1u<<1)
 /** Flag: Indicates that we should set the close-on-exec flag, if possible */
 #define LEV_OPT_CLOSE_ON_EXEC        (1u<<2)
 /** Flag: Indicates that we should disable the timeout (if any) between when
  * this socket is closed and when we can listen again on the same port. */
 #define LEV_OPT_REUSEABLE        (1u<<3)
 /** Flag: Indicates that the listener should be locked so it's safe to use
  * from multiple threadcs at once. */
 #define LEV_OPT_THREADSAFE        (1u<<4)
 /** Flag: Indicates that the listener should be created in disabled
  * state. Use evconnlistener_enable() to enable it later. */
 #define LEV_OPT_DISABLED        (1u<<5)
 /** Flag: Indicates that the listener should defer accept() until data is
  * available, if possible.  Ignored on platforms that do not support this.
  *
  * This option can help performance for protocols where the client transmits
  * immediately after connecting.  Do not use this option if your protocol
  * _doesn't_ start out with the client transmitting data, since in that case
  * this option will sometimes cause the kernel to never tell you about the
  * connection.
  *
  * This option is only supported by evconnlistener_new_bind(): it can't
  * work with evconnlistener_new_fd(), since the listener needs to be told
  * to use the option before it is actually bound.
  */
 #define LEV_OPT_DEFERRED_ACCEPT        (1u<<6)
 /** Flag: Indicates that we ask to allow multiple servers (processes or
  * threads) to bind to the same port if they each set the option.
  *
  * SO_REUSEPORT is what most people would expect SO_REUSEADDR to be, however
  * SO_REUSEPORT does not imply SO_REUSEADDR.
  *
  * This is only available on Linux and kernel 3.9+
  */
 #define LEV_OPT_REUSEABLE_PORT        (1u<<7)

 /**
    Allocate a new evconnlistener object to listen for incoming TCP connections
    on a given file descriptor.

    @param base The event base to associate the listener with.
    @param cb A callback to be invoked when a new connection arrives.  If the
       callback is NULL, the listener will be treated as disabled until the
       callback is set.
    @param ptr A user-supplied pointer to give to the callback.
    @param flags Any number of LEV_OPT_* flags
    @param backlog Passed to the listen() call to determine the length of the
       acceptable connection backlog.  Set to -1 for a reasonable default.
       Set to 0 if the socket is already listening.
    @param fd The file descriptor to listen on.  It must be a nonblocking
       file descriptor, and it should already be bound to an appropriate
       port and address.
 */
 EVENT2_EXPORT_SYMBOL
 struct evconnlistener *evconnlistener_new(struct event_base *base,
     evconnlistener_cb cb, void *ptr, unsigned flags, int backlog,
     evutil_socket_t fd);
 /**
    Allocate a new evconnlistener object to listen for incoming TCP connections
    on a given address.

    @param base The event base to associate the listener with.
    @param cb A callback to be invoked when a new connection arrives. If the
       callback is NULL, the listener will be treated as disabled until the
       callback is set.
    @param ptr A user-supplied pointer to give to the callback.
    @param flags Any number of LEV_OPT_* flags
    @param backlog Passed to the listen() call to determine the length of the
       acceptable connection backlog.  Set to -1 for a reasonable default.
    @param addr The address to listen for connections on.
    @param socklen The length of the address.
  */
 EVENT2_EXPORT_SYMBOL
 struct evconnlistener *evconnlistener_new_bind(struct event_base *base,
     evconnlistener_cb cb, void *ptr, unsigned flags, int backlog,
     const struct sockaddr *sa, int socklen);
 /**
    Disable and deallocate an evconnlistener.
  */
 EVENT2_EXPORT_SYMBOL
 void evconnlistener_free(struct evconnlistener *lev);
 /**
    Re-enable an evconnlistener that has been disabled.
  */
 EVENT2_EXPORT_SYMBOL
 int evconnlistener_enable(struct evconnlistener *lev);
 /**
    Stop listening for connections on an evconnlistener.
  */
 EVENT2_EXPORT_SYMBOL
 int evconnlistener_disable(struct evconnlistener *lev);

 /** Return an evconnlistener's associated event_base. */
 EVENT2_EXPORT_SYMBOL
 struct event_base *evconnlistener_get_base(struct evconnlistener *lev);

 /** Return the socket that an evconnlistner is listening on. */
 EVENT2_EXPORT_SYMBOL
 evutil_socket_t evconnlistener_get_fd(struct evconnlistener *lev);

 /** Change the callback on the listener to cb and its user_data to arg.
  */
 EVENT2_EXPORT_SYMBOL
 void evconnlistener_set_cb(struct evconnlistener *lev,
     evconnlistener_cb cb, void *arg);

 /** Set an evconnlistener's error callback. */
 EVENT2_EXPORT_SYMBOL
 void evconnlistener_set_error_cb(struct evconnlistener *lev,
     evconnlistener_errorcb errorcb);

 #ifdef __cplusplus
 }
 #endif

 #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：

 struct evconnlistener {
     const struct evconnlistener_ops *ops;
     void *lock;
     evconnlistener_cb cb;
     evconnlistener_errorcb errorcb;
     void *user_data;
     unsigned flags;
     short refcnt;
     int accept4_flags;
     unsigned enabled : ;
 };

 struct evconnlistener_event {
     struct evconnlistener base;
     struct event listener;
 };

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。

 struct evconnlistener *
 evconnlistener_new(struct event_base *base,
     evconnlistener_cb cb, void *ptr, unsigned flags, int backlog,
     evutil_socket_t fd)
 {
     struct evconnlistener_event *lev;

 #ifdef _WIN32
     if (base && event_base_get_iocp_(base)) {
         const struct win32_extension_fns *ext =
             event_get_win32_extension_fns_();
         if (ext->AcceptEx && ext->GetAcceptExSockaddrs)
             return evconnlistener_new_async(base, cb, ptr, flags,
                 backlog, fd);
     }
 #endif

     if (backlog > ) {
         if (listen(fd, backlog) < )
             return NULL;
     } else if (backlog < ) {
         if (listen(fd, ) < )
             return NULL;
     }

     lev = mm_calloc(, sizeof(struct evconnlistener_event));
     if (!lev)
         return NULL;

     lev->base.ops = &evconnlistener_event_ops;
     lev->base.cb = cb;
     lev->base.user_data = ptr;
     lev->base.flags = flags;
     lev->base.refcnt = ;

     lev->base.accept4_flags = ;
     if (!(flags & LEV_OPT_LEAVE_SOCKETS_BLOCKING))
         lev->base.accept4_flags |= EVUTIL_SOCK_NONBLOCK;
     if (flags & LEV_OPT_CLOSE_ON_EXEC)
         lev->base.accept4_flags |= EVUTIL_SOCK_CLOEXEC;

     if (flags & LEV_OPT_THREADSAFE) {
         EVTHREAD_ALLOC_LOCK(lev->base.lock, EVTHREAD_LOCKTYPE_RECURSIVE);
     }

     event_assign(&lev->listener, base, fd, EV_READ|EV_PERSIST,
         listener_read_cb, lev);

     if (!(flags & LEV_OPT_DISABLED))
         evconnlistener_enable(&lev->base);

     return &lev->base;
 }

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：

 static void
 listener_read_cb(evutil_socket_t fd, short what, void *p)
 {
     struct evconnlistener *lev = p;
     int err;
     evconnlistener_cb cb;
     evconnlistener_errorcb errorcb;
     void *user_data;
     LOCK(lev);
     while () {
         struct sockaddr_storage ss;
         ev_socklen_t socklen = sizeof(ss);
         evutil_socket_t new_fd = evutil_accept4_(fd, (struct sockaddr*)&ss, &socklen, lev->accept4_flags);
         if (new_fd < )
             break;
         if (socklen == ) {
             /* This can happen with some older linux kernels in
              * response to nmap. */
             evutil_closesocket(new_fd);
             continue;
         }

         if (lev->cb == NULL) {
             evutil_closesocket(new_fd);
             UNLOCK(lev);
             return;
         }
         ++lev->refcnt;
         cb = lev->cb;
         user_data = lev->user_data;
         UNLOCK(lev);
         cb(lev, new_fd, (struct sockaddr*)&ss, (int)socklen,
             user_data);
         LOCK(lev);
         if (lev->refcnt == ) {
             int freed = listener_decref_and_unlock(lev);
             EVUTIL_ASSERT(freed);

             evutil_closesocket(new_fd);
             return;
         }
         --lev->refcnt;
     }
     err = evutil_socket_geterror(fd);
     if (EVUTIL_ERR_ACCEPT_RETRIABLE(err)) {
         UNLOCK(lev);
         return;
     }
     if (lev->errorcb != NULL) {
         ++lev->refcnt;
         errorcb = lev->errorcb;
         user_data = lev->user_data;
         UNLOCK(lev);
         errorcb(lev, user_data);
         LOCK(lev);
         listener_decref_and_unlock(lev);
     } else {
         event_sock_warn(fd, "Error from accept() call");
         UNLOCK(lev);
     }
 }

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。