如何在使用bufferevents的libevent中写入后关闭套接字

下面是libevent book中echo TCP服务器的一个示例。如何修改它以在每次写入后关闭套接字。我知道我可以通过调用bufferevent_free（）来关闭套接字连接，但我不明白每次向套接字发送回显时如何使用它来关闭连接。

#include <event2/listener.h>
#include <event2/bufferevent.h>
#include <event2/buffer.h>
#include <arpa/inet.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
static void
echo_read_cb(struct bufferevent *bev, void *ctx)
{
        /* This callback is invoked when there is data to read on bev. */
        struct evbuffer *input = bufferevent_get_input(bev);
        struct evbuffer *output = bufferevent_get_output(bev);
        /* Copy all the data from the input buffer to the output buffer. */
        evbuffer_add_buffer(output, input);
}
static void
echo_event_cb(struct bufferevent *bev, short events, void *ctx)
{
        if (events & BEV_EVENT_ERROR)
                perror("Error from bufferevent");
        if (events & (BEV_EVENT_EOF | BEV_EVENT_ERROR)) {
                bufferevent_free(bev);
        }
}
static void
accept_conn_cb(struct evconnlistener *listener,
    evutil_socket_t fd, struct sockaddr *address, int socklen,
    void *ctx)
{
        /* We got a new connection! Set up a bufferevent for it. */
        struct event_base *base = evconnlistener_get_base(listener);
        struct bufferevent *bev = bufferevent_socket_new(
                base, fd, BEV_OPT_CLOSE_ON_FREE);
        bufferevent_setcb(bev, echo_read_cb, NULL, echo_event_cb, NULL);
        bufferevent_enable(bev, EV_READ|EV_WRITE);
}
static void
accept_error_cb(struct evconnlistener *listener, void *ctx)
{
        struct event_base *base = evconnlistener_get_base(listener);
        int err = EVUTIL_SOCKET_ERROR();
        fprintf(stderr, "Got an error %d (%s) on the listener. "
                "Shutting down.n", err, evutil_socket_error_to_string(err));
        event_base_loopexit(base, NULL);
}
int
main(int argc, char **argv)
{
        struct event_base *base;
        struct evconnlistener *listener;
        struct sockaddr_in sin;
        int port = 9876;
        if (argc > 1) {
                port = atoi(argv[1]);
        }
        if (port<=0 || port>65535) {
                puts("Invalid port");
                return 1;
        }
        base = event_base_new();
        if (!base) {
                puts("Couldn't open event base");
                return 1;
        }
        /* Clear the sockaddr before using it, in case there are extra
         * platform-specific fields that can mess us up. */
        memset(&sin, 0, sizeof(sin));
        /* This is an INET address */
        sin.sin_family = AF_INET;
        /* Listen on 0.0.0.0 */
        sin.sin_addr.s_addr = htonl(0);
        /* Listen on the given port. */
        sin.sin_port = htons(port);
        listener = evconnlistener_new_bind(base, accept_conn_cb, NULL,
            LEV_OPT_CLOSE_ON_FREE|LEV_OPT_REUSEABLE, -1,
            (struct sockaddr*)&sin, sizeof(sin));
        if (!listener) {
                perror("Couldn't create listener");
                return 1;
        }
        evconnlistener_set_error_cb(listener, accept_error_cb);
        event_base_dispatch(base);
        return 0;
}