c++ - gai_cancel() 需要很长时间才能成功

Question

我正在尝试在 C++ 中异步查找域。原因是我希望能够有效地添加一个超时期限，以防系统无法查找域。我遇到了 getaddrinfo_a() 命令，所以我决定试一试。然而，取消任何不会成功的 dns 查找(例如当没有互联网连接时)在我的机器上永远不会少于 20 秒。这是一个简单的例子:

#include <iostream>
#include <netdb.h>
#include <string.h>
#include <unistd.h>

using namespace std;

int main() {


        int ret;
        gaicb* reqs;

        reqs = new gaicb;

        memset(reqs, 0, sizeof (gaicb));
        reqs->ar_name = "google.com";

        ret = getaddrinfo_a(GAI_NOWAIT, &reqs, 1, NULL);
        if (ret != 0) {
                cout << "something went wrong" << endl;
                return false;
        }

        while (1) {
                ret = gai_cancel(reqs);
                if (ret == EAI_CANCELED || ret == EAI_ALLDONE) {
                        break;
                }
                usleep(100 * 1000); //sleep for 100 milliseconds
        }

        cout << "finished cancellation" << endl;


        return 0;
}

像这样编译:

g++ -o main main.cpp -lanl

然后在没有互联网连接的基于 linux 的系统上运行命令，如下所示:

time ./main

您会发现程序总是需要 20 秒左右才能关闭。任何帮助将不胜感激!

Answer 1

好的，如果您需要短于 20 秒的异步 dns 查找，答案是不要使用 getaddrinfo_a(3)。同样根据 Martin Sustrik 对 http://sourceware-org.1504.n7.nabble.com/getaddrinfo-a-memory-leaks-td233794.html 的第二条评论以及我在这里的经验，它看起来像 getaddrinfo_a 是相当实验性的，无论如何都不应该使用。我实际上最终使用了 dns.c ( https://github.com/wahern/dns )。这是供任何人查看的示例:

#include "dns.c"

uint8_t getaddrinfo_k(
uint32_t* ip_addr, const char* dns_address, uint32_t timeout) {
    int32_t                 rc;
    struct addrinfo         hints;
    struct dns_resolv_conf* dns_conf;
    struct dns_hosts*       dns_hosts;
    struct dns_hints*       dns_hints;
    struct dns_resolver*    resolver;
    struct dns_addrinfo*    ai;
    struct addrinfo*        it;

    dns_conf    = dns_resconf_local(&rc);
    assert(dns_conf);
    dns_hosts   = dns_hosts_local(&rc);
    assert(dns_hosts);
    dns_hints   = dns_hints_local(dns_conf, &rc);
    assert(dns_hints);
    resolver    = dns_res_open(
    dns_conf, dns_hosts, dns_hints, NULL, dns_opts(), &rc);
    assert(resolver);
    it          = NULL;

    memset(&hints, 0, sizeof(hints));
    hints.ai_family = PF_INET;

    ai = dns_ai_open(dns_address, "80", DNS_T_A, &hints, resolver, &rc);
    assert(ai);

    while (timeout-- > 0) {
        rc = dns_ai_nextent(&it, ai);
        switch(rc) {
                case 0:
                        *ip_addr = (
                        (struct sockaddr_in *) it->ai_addr)->sin_addr.s_addr;
                        free(it);
                        goto exit_loop;
                case EAGAIN:
                        rc = dns_ai_poll(ai, 1);
                        assert(rc == 0);
                        break;
                default:
                        goto exit_loop;
        }
        usleep(100 * 1000);
    }

    exit_loop:

    dns_ai_close        (ai);
    dns_res_close       (resolver);
    dns_hints_close     (dns_hints);
    dns_hosts_close     (dns_hosts);
    dns_resconf_close   (dns_conf);

    switch(rc) {
        case 0:
            return 1;
        case EAGAIN:
            printf("DNS_WRAPPER: timed out\n");
            break;
        case ENOENT:
            printf("DNS_WRAPPER: file doesn't exist\n");
            break;
        default:
            printf("DNS_WRAPPER: unknown error\n");
            break;
    }

    return 0;
}