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

View Code
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 : 1;
};

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 > 0) {
if (listen(fd, backlog) < 0)
return NULL;
} else if (backlog < 0) {
if (listen(fd, 128) < 0)
return NULL;
}

lev = mm_calloc(1, 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 = 1;

lev->base.accept4_flags = 0;
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 (1) {
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 < 0)
break;
if (socklen == 0) {
/* 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 == 1) {
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。