libevent2 启用 iocp 的问题

[the_venus]

#include <iostream>
#include <event2/event.h>
#include <event2/thread.h>
#include <csignal>


void log_callback(int security, const char* msg)
{
	std::cout << "log: security=" << security << ", msg=" << msg;
}

void fatal_callback(int err)
{
	std::cout << "fatal: " << err << std::endl;
	abort();
}


void event_callback(evutil_socket_t s, short event, void * arg)
{
	std::cout << "SIGINT triggered, exit loop" << std::endl;
	struct event_base *eb = (struct event_base*)(arg);
	if (eb != NULL)
	{
		event_base_loopexit(eb, NULL);
	}
}

int _tmain(int argc, _TCHAR* argv[])
{
	event_set_log_callback(log_callback);
	event_set_fatal_callback(fatal_callback);
	//event_enable_debug_logging(EVENT_DBG_ALL);
	event_enable_debug_mode();
	evthread_use_windows_threads();

	struct event_config *ec = event_config_new();
	event_config_set_flag(ec, EVENT_BASE_FLAG_STARTUP_IOCP);
	event_config_require_features(ec, EV_FEATURE_ET);

	struct event_base *eb = event_base_new_with_config(ec);
	if (eb != NULL)
	{
		struct event *sigint = evsignal_new(eb, SIGINT, event_callback, eb);

		event_base_dispatch(eb);

		event_base_free(eb);
	}
	event_config_free(ec);

	return 0;

上面这段代码在链接libevent2版本之后，执行event_base_new_with_config(ec)的时候老是返回NULL, log_callback 打印出 no event mechanism available, libevent启用iocp是这么做的呀，请问高手我这段代码哪里出了问题，多谢了