Libevent官方代码样例学习(二)

连接监听器: 接收TCP连接请求

evconnlistener机制用于监听并接受TCP连接请求. 这些方法在event2/listener.h中声明, 在Libevent 2.0.2-alpha之后的版本中可用.

创建和释放evconnlistener

接口

struct evconnlistener *evconnlistener_new(     struct event_base *base, evconnlistener_cb cb, void *ptr, unsigned flags, int backlog, evutil_socket_t fd);
struct evconnlistener *evconnlistener_new_bind(struct event_base *base, evconnlistener_cb cb, void *ptr, unsigned flags, int backlog, const struct sockaddr *sa, int socklen);
void evconnlistener_free(struct evconnlistener *lev);

这两个 evconnlistener_new*() 方法都会allocate并返回一个新的connection listener对象. connection listener使用event_base 来通知来自于制定的socket上的新的TCP连接请求. 并调用预设的回调方法.

这两个方法相同的参数:
base 是listener用来监听的event_base.
cb 是回调方法, 如果cb is NULL, listener会被视为禁用, 直到回调被设置.
ptr 指针会作为参数被传递给回调方法.
flags 参数控制listener的行为.
backlog 参数控制允许的等待的连接数的上限. 如果backlog是负数, Libevent会自动设置一个合适的值, 如果为0, 则Libevent会认为你已经在socket上调用了listen()方法.

这两个方法中不同的参数, 在于如何初始化listener socket. evconnlistener_new() 方法假定你已经在要监听的端口上绑定了socket, 你只需要传递socket的fd就够. 如果你希望Libevent自己allocate并绑定socket, 就调用evconnlistener_new_bind(), 并传入你想监听的的sockaddr和它的length.

Tip	[使用 evconnlistener_new时要确保监听的socket工作于non-blocking 状态, (用evutil_make_socket_nonblocking 或手工指定正确的socket选项). When the listening socket is left in blocking mode, undefined behavior might occur.]

要释放一个 listener, 使用evconnlistener_free().

创建一个signal events

Libevent can also watch for POSIX-style signals. 要创建一个signal的handler, 使用接口:

#define evsignal_new(base, signum, cb, arg)  event_new(base, signum, EV_SIGNAL|EV_PERSIST, cb, arg)

可以看到其实是调用了event_new, 只不过flag是EV_SIGNAL|EV_PERSIST, 并且不是传fd而是传signum

例子

struct event *hup_event;
struct event_base *base = event_base_new();

/* call sighup_function on a HUP signal */
hup_event = evsignal_new(base, SIGHUP, sighup_function, NULL);

注意: signal回调在signal发生后, 在event loop中执行, 所以一些在常规POSIX信号handler中不能调用的方法是可以在这种情况下调用的.

注意	不要在signal event中设置timeout

另外一些相关的宏设定:

#define evsignal_add(ev, tv) \
    event_add((ev),(tv))
#define evsignal_del(ev) \
    event_del(ev)
#define evsignal_pending(ev, what, tv_out) \
    event_pending((ev), (what), (tv_out))

evsignal_* 宏定义也是在Libevent 2.0.1-alpha之后的版本中可用.

例子

/*
  This example program provides a trivial server program that listens for TCP
  connections on port 9995.  When they arrive, it writes a short message to
  each client connection, and closes each connection once it is flushed.
  Where possible, it exits cleanly in response to a SIGINT (ctrl-c).
*/
#include <string.h>
#include <errno.h>
#include <stdio.h>
#include <signal.h>
#include <event2/bufferevent.h>
#include <event2/buffer.h>
#include <event2/listener.h>

static const char MESSAGE[] = "Hello, World!\n";
static const int PORT = 9995;
static void listener_cb(struct evconnlistener *, evutil_socket_t, struct sockaddr *, int socklen, void *);
static void conn_writecb(struct bufferevent *, void *);
static void conn_eventcb(struct bufferevent *, short, void *);
static void signal_cb(evutil_socket_t, short, void *);

int
main(int argc, char **argv) {
    struct event_base *base;
    struct evconnlistener *listener;
    struct event *signal_event;
    struct sockaddr_in sin;

    base = event_base_new();
    if (!base) {
        fprintf(stderr, "Could not initialize libevent!\n");
        return 1;
    }

    memset(&sin, 0, sizeof(sin));
    sin.sin_family = AF_INET;
    sin.sin_port = htons(PORT);
    listener = evconnlistener_new_bind(
            base, listener_cb, (void *) base, LEV_OPT_REUSEABLE | LEV_OPT_CLOSE_ON_FREE, -1,
            (struct sockaddr *) &sin, sizeof(sin));

    if (!listener) {
        fprintf(stderr, "Could not create a listener!\n");
        return 1;
    }

    signal_event = evsignal_new(base, SIGINT, signal_cb, (void *) base);

    if (!signal_event || event_add(signal_event, NULL) < 0) {
        fprintf(stderr, "Could not create/add a signal event!\n");
        return 1;
    }

    event_base_dispatch(base);
    evconnlistener_free(listener);
    event_free(signal_event);
    event_base_free(base);
    printf("done\n");
    return 0;
}

static void
listener_cb(struct evconnlistener *listener, evutil_socket_t fd, struct sockaddr *sa, int socklen, void *user_data) {
    struct event_base *base = user_data;
    struct bufferevent *bev;

    bev = bufferevent_socket_new(base, fd, BEV_OPT_CLOSE_ON_FREE);
    if (!bev) {
        fprintf(stderr, "Error constructing bufferevent!");
        event_base_loopbreak(base);
        return;
    }
    bufferevent_setcb(bev, NULL, conn_writecb, conn_eventcb, NULL);
    bufferevent_enable(bev, EV_WRITE);
    bufferevent_disable(bev, EV_READ);
    bufferevent_write(bev, MESSAGE, strlen(MESSAGE));
}

static void
conn_writecb(struct bufferevent *bev, void *user_data) {
    struct evbuffer *output = bufferevent_get_output(bev);
    if (evbuffer_get_length(output) == 0) {
        printf("flushed answer\n");
        bufferevent_free(bev);
    }
}

static void
conn_eventcb(struct bufferevent *bev, short events, void *user_data) {
    if (events & BEV_EVENT_EOF) {
        printf("Connection closed.\n");
    } else if (events & BEV_EVENT_ERROR) {
        printf("Got an error on the connection: %s\n", strerror(errno));/*XXX win32*/
    }
    /* None of the other events can happen here, since we haven't enabled
     * timeouts */
    bufferevent_free(bev);
}

static void
signal_cb(evutil_socket_t sig, short events, void *user_data) {
    struct event_base *base = user_data;
    struct timeval delay = {2, 0};
    printf("Caught an interrupt signal; exiting cleanly in two seconds.\n");
    event_base_loopexit(base, &delay);
}