getaddr.c

#define _GNU_SOURCE
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int
main(int argc, char *argv[])
{
    int i, ret;
    struct gaicb *reqs[argc - 1];
    char host[NI_MAXHOST];
    struct addrinfo *res;

    reqs[0] = malloc(sizeof(*reqs[0]));
    memset(reqs[0], 0, sizeof(*reqs[0]));

    reqs[0]->ar_name = NULL; // localhost
    reqs[0]->ar_service = "http";

   ret = getaddrinfo_a(GAI_WAIT, reqs, 1, NULL);
    if (ret != 0) {
        fprintf(stderr, "getaddrinfo_a() failed: %s\n",
                gai_strerror(ret));
        exit(EXIT_FAILURE);
    }

    struct gaicb const *wait_reqs[1];
    wait_reqs[0] = reqs[0];
    ret = gai_suspend(wait_reqs, 1, NULL);
    // unless you replace this with if (ret && ret != EAI_ALLDONE),
    // it will always fail with EAI_ALLDONE because resolving localhost is fast
    // by the time gai_suspend is called, the address is already resolved.
    if (ret) {
        printf("gai_suspend(): %s\n", gai_strerror(ret));
        return 1;
    }

        printf("%s: ", reqs[0]->ar_name);
        ret = gai_error(reqs[0]);
        if (ret == 0) {
            res = reqs[0]->ar_result;

           ret = getnameinfo(res->ai_addr, res->ai_addrlen,
                    host, sizeof(host),
                    NULL, 0, NI_NUMERICHOST);
            if (ret != 0) {
                fprintf(stderr, "getnameinfo() failed: %s\n",
                        gai_strerror(ret));
                exit(EXIT_FAILURE);
            }
            puts(host);

       } else {
            puts(gai_strerror(ret));
        }
    }
    exit(EXIT_SUCCESS);
}