在使用ares_channel之前,你至少需要初始化它。
if(ares_init(&channel) != ARES_SUCCESS) {
//handle error
}
你还需要一个事件循环来处理ares文件描述符上的事件,并调用
ares_process来处理这些事件(更常见的是在应用程序的事件循环中集成它)。
ares没有使用线程来进行异步处理,因此简单地调用sleep(15);不能让ares在“后台”运行。
如果查找失败,你的回调函数还应该检查
status
变量,否则你无法访问
host->h_name
。
一个完整的示例如下:
#include <time.h>
#include <iostream>
#include <netdb.h>
#include <arpa/inet.h>
#include <ares.h>
void dns_callback (void* arg, int status, int timeouts, struct hostent* host)
{
if(status == ARES_SUCCESS)
std::cout << host->h_name << "\n";
else
std::cout << "lookup failed: " << status << '\n';
}
void main_loop(ares_channel &channel)
{
int nfds, count;
fd_set readers, writers;
timeval tv, *tvp;
while (1) {
FD_ZERO(&readers);
FD_ZERO(&writers);
nfds = ares_fds(channel, &readers, &writers);
if (nfds == 0)
break;
tvp = ares_timeout(channel, NULL, &tv);
count = select(nfds, &readers, &writers, NULL, tvp);
ares_process(channel, &readers, &writers);
}
}
int main(int argc, char **argv)
{
struct in_addr ip;
int res;
if(argc < 2 ) {
std::cout << "usage: " << argv[0] << " ip.address\n";
return 1;
}
inet_aton(argv[1], &ip);
ares_channel channel;
if((res = ares_init(&channel)) != ARES_SUCCESS) {
std::cout << "ares feiled: " << res << '\n';
return 1;
}
ares_gethostbyaddr(channel, &ip, sizeof ip, AF_INET, dns_callback, NULL);
main_loop(channel);
return 0;
}
$ g++ -Wall test_ares.cpp -lcares
$ ./a.out 8.8.8.8
google-public-dns-a.google.com
ares_gethostbyaddr()
中使用4而不是sizeof(in_addr)
作为地址长度? - chrisaycock