如何使用 c-ares 将 IP 解析为主机？

Question

这就是我到目前为止所做的。它可以编译，但当我尝试运行它时会出现段错误。

#include <iostream>
#include <netdb.h>
#include <arpa/inet.h>
#include <ares.h>

void dns_callback (void* arg, int status, int timeouts, struct hostent* host)
  {
    std::cout << host->h_name << "\n";
  }

int main(int argc, char **argv)
  {
    struct in_addr ip;
    char *arg;
    inet_aton(argv[1], &ip);
    ares_channel channel;
    ares_gethostbyaddr(channel, &ip, 4, AF_INET, dns_callback, arg);
    sleep(15);
    return 0;
  }

Answer 1

在使用 ares_channel 之前，您至少必须初始化，

 if(ares_init(&channel) != ARES_SUCCESS) {
   //handle error
  }

您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，您d 将其集成到应用程序的事件循环中）
ares 没有什么神奇之处，它不使用线程来进行异步处理，因此只需调用 sleep(15);不允许 ares 在“后台”运行

您的回调还应该检查 status 变量，如果查找失败，您将无法访问 host->h_name。

一个完整的例子变成：

#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
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