c-ares DNS域名异步解析成IP

//build1: g++ dns_parse_cares.cc /usr/local/lib/libcares.a -lrt  -o test 
//build2: g++ dns_parse_cares.cc -lcares -lrt -o test

#include 
#include 
#include 
#include 
#include 
#include 
#include 

#define IP_LEN 32

typedef struct {
	char host[64];
	char ip[5][IP_LEN]; 
	int count;   
}IpList;

void dns_callback (void* arg, int status, int timeouts, struct hostent* hptr)  //ares  处理完成，返回DNS解析的信息
{
    IpList *ips = (IpList*)arg;
	if( ips == NULL ) return;
    if(status == ARES_SUCCESS){
		strncpy(ips->host, hptr->h_name, sizeof(ips->host));
        char **pptr=hptr->h_addr_list;
		for(int i=0; *pptr!=NULL && i<5; pptr++,++i){
            inet_ntop(hptr->h_addrtype, *pptr, ips->ip[ips->count++], IP_LEN);
        } 
    }else{
        std::cout << "lookup failed: " << status << std::endl;
    }
}

void dns_resolve(const char* host, IpList &ips)
{
    ares_channel channel;    // 创建一个ares_channel
    
    int res;
    if((res = ares_init(&channel)) != ARES_SUCCESS) {     // ares 对channel 进行初始化
        std::cout << "ares feiled: " << res << std::endl;
        return;
    }
	//set dns server
    ares_set_servers_csv(channel, "114.114.114.114");
    //get host by name
    ares_gethostbyname(channel, host, AF_INET, dns_callback, (void*)(&ips));

    int nfds;  
    fd_set readers, writers;  
    timeval tv, *tvp;  
    while (true) 
    {  
        FD_ZERO(&readers);  
        FD_ZERO(&writers);  
        nfds = ares_fds(channel, &readers, &writers);   //获取ares channel使用的FD  
        if (nfds == 0) break;  
        tvp = ares_timeout(channel, NULL, &tv);      
        select(nfds, &readers, &writers, NULL, tvp);   //将ares的SOCKET FD 加入事件循环  
        ares_process(channel, &readers, &writers);  // 有事件发生 交由ares 处理
    }
    ares_destroy(channel);
}

int main(int argc, char **argv)
{
    int res;
    if(argc < 2 ) {
        std::cout << "usage: " << argv[0] << " ip.address"<< std::endl;
        return 1;
    }

	IpList ips;
	do{
		memset(&ips, 0, sizeof(ips));
        dns_resolve(argv[1],ips);
		std::cout <<"hostname:"<< ips.host << std::endl;
	    for(int i=0; i < ips.count; ++i){
			std::cout<< "address ip:" << ips.ip[i] << std::endl;
		}
		std::cout<< std::endl;
	}while(true);
    return 0;
}