c-ares何时在单一通道中使用多个并行套接字？

c-ares 在默认情况下使用单个套接字来处理所有的DNS查询请求。但是，c-ares可以配置为使用多个并行套接字来提高并发性能。具体来说，当使用轮询模式（AresOptions.ARES_OPT_SOCK_STATE_CB）时，c-ares将尝试根据需要创建并行套接字，以便同时发送多个查询请求和接收应答。如果启用了并行线程模式（AresOptions.ARES_OPT_NPARALLEL），c-ares将使用该模式来并行查询多个DNS服务器。示例代码如下：

#include <stdio.h>
#include <ares.h>

void callback(void *arg, int status, int timeouts, struct hostent *host)
{
    // do something when callback is triggered
}

int main()
{
    ares_channel channel;
    ares_options options;
    struct ares_addr_node servers[2];

    // initialize options and servers
    ares_init_options(&options);
    options.flags |= ARES_FLAG_NOCHECKRESP;
    options.sock_state_cb = NULL;
    options.ndots = 1;
    options.timeout = 5000;
    options.tries = 2;
    servers[0].family = AF_INET;
    inet_pton(AF_INET, "8.8.8.8", &(servers[0].addr.addr4));
    servers[1].family = AF_INET;
    inet_pton(AF_INET, "8.8.4.4", &(servers[1].addr.addr4));
    ares_set_servers(channel, servers);

    // enable parallel mode
    options.flags |= ARES_FLAG_NPAREQ;
    options.nparallel = 2;

    int status = ares_init_options(&channel, &options, ARES_OPT_FLAGS | ARES_OPT_SERVERS | ARES_OPT_NFLAGS);

    ares_query(channel, "example.com", ns_c_in, ns_t_a, callback, NULL);
    ares_query(channel, "example.net", ns_c_in, ns_t_a, callback, NULL);    

    ares_destroy(channel);
    ares_library_cleanup();
    return 0;
}