libevent 在 Android 上的一个改进

想在 Android 上使用 libevent 做一个下载程序，牵涉到域名解析的问题，发现 libevent 无法获取到 dns 服务器。研究了一下源码，找到了问题所在。

使用 libevent 的异步 dns 解析的一般过程如下：

调用evdns_base_new(struct event_base *event_base, int initialize_nameservers) 生成一个 evdns_base 对象
调用 evdns_getaddrinfo 并提供一个回调进行解析
在回调中处理结果

一般我们给 evdns_base_new 函数的第二个参数传递 1 以便 libevent 从系统配置中初始化 nameservers 。在 windows 上读取注册表，在 Linux 上读取 /etc/resolv.conf 。问题就出在这里，Android 上没有 resolv.conf 文件而 libevent 未做处理。查看 evdns.c 文件中的 evdns_base_new 函数实现可知：

struct evdns_base *evdns_base_new(struct event_base *event_base, int initialize_nameservers){struct evdns_base *base;if (evutil_secure_rng_init() < 0) {log(EVDNS_LOG_WARN, "Unable to seed random number generator; "    "DNS can't run.");return NULL;}/* Give the evutil library a hook into its evdns-enabled * functionality.  We can't just call evdns_getaddrinfo directly or * else libevent-core will depend on libevent-extras. */evutil_set_evdns_getaddrinfo_fn(evdns_getaddrinfo);base = mm_malloc(sizeof(struct evdns_base));if (base == NULL)return (NULL);memset(base, 0, sizeof(struct evdns_base));base->req_waiting_head = NULL;EVTHREAD_ALLOC_LOCK(base->lock, EVTHREAD_LOCKTYPE_RECURSIVE);EVDNS_LOCK(base);/* Set max requests inflight and allocate req_heads. */base->req_heads = NULL;evdns_base_set_max_requests_inflight(base, 64);base->server_head = NULL;base->event_base = event_base;base->global_good_nameservers = base->global_requests_inflight =base->global_requests_waiting = 0;base->global_timeout.tv_sec = 5;base->global_timeout.tv_usec = 0;base->global_max_reissues = 1;base->global_max_retransmits = 3;base->global_max_nameserver_timeout = 3;base->global_search_state = NULL;base->global_randomize_case = 1;base->global_getaddrinfo_allow_skew.tv_sec = 3;base->global_getaddrinfo_allow_skew.tv_usec = 0;base->global_nameserver_probe_initial_timeout.tv_sec = 10;base->global_nameserver_probe_initial_timeout.tv_usec = 0;TAILQ_INIT(&base->hostsdb);if (initialize_nameservers) {int r;#ifdef WIN32r = evdns_base_config_windows_nameservers(base);#elser = evdns_base_resolv_conf_parse(base, DNS_OPTIONS_ALL, "/etc/resolv.conf");#endifif (r == -1) {evdns_base_free_and_unlock(base, 0);return NULL;}}EVDNS_UNLOCK(base);return base;}

就是 #ifdef WIN32 那几行代码。

解决方案是编译时增加一个 ANDROID 宏，针对 Android 平台实现读取 dns 配置的代码，这在我的文章《Android C 语言读取系统属性》中有相关解说。下面是具体的代码：

    if (initialize_nameservers) {int r;#ifdef WIN32r = evdns_base_config_windows_nameservers(base);#elif defined(ANDROID)    {        int add_servers = 0;        char buf[PROP_VALUE_MAX];        r = __system_property_get("net.dns1", buf);        if(r >= 7)        {            add_servers++;            evdns_base_nameserver_ip_add(base, buf);        }        r = __system_property_get("net.dns2", buf);        if(r >= 7)        {            add_servers++;            evdns_base_nameserver_ip_add(base, buf);        }        if(add_servers == 0)        {            evdns_base_nameserver_ip_add(base,"8.8.8.8");        }    }#elser = evdns_base_resolv_conf_parse(base, DNS_OPTIONS_ALL, "/etc/resolv.conf");#endifif (r == -1) {evdns_base_free_and_unlock(base, 0);return NULL;}}

OK ，现在可以正常工作了。

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