Asynchronous dns resolving with libev and the gethostbyname_r function.

libev_async_dns.c

#include <ev.h>
#include <stdint.h>
#include <stdbool.h>
#include <stdio.h>
#include <netdb.h>
#include <memory.h>

typedef struct net_client_t {
    ev_signal resolv_watcher;
    char hostname[64];
    struct gaicb gaicb;
    struct addrinfo hints;
} net_client_t;

static void resolv_cb(struct ev_loop *loop, ev_signal *watcher, int revents)
{
    net_client_t *client = (net_client_t*)watcher;

    if (!client->gaicb.ar_result) {
        printf("resolve result is null\n");
        return;
    }

    printf("Resolved %p\n", client->gaicb.ar_result);
    struct addrinfo *rp;
    for (rp = client->gaicb.ar_result; rp != NULL; rp = rp->ai_next) {
            printf("fam=%d type=%d proto=%d addr=%x len=%d\n", rp->ai_family, rp->ai_socktype, rp->ai_protocol, rp->ai_addr, rp->ai_addrlen);
    }

    freeaddrinfo(client->gaicb.ar_result);
    ev_break(EV_DEFAULT, EVBREAK_ALL);
}

bool net_client_init(net_client_t *client, struct ev_loop *loop, const char *hostname)
{
    memset(client, 0, sizeof(*client));

    ev_signal_init(&client->resolv_watcher, resolv_cb, SIGRTMIN);
    ev_signal_start(loop, &client->resolv_watcher);

    snprintf(client->hostname, sizeof(client->hostname), "%s", hostname);

    client->hints.ai_family = AF_UNSPEC;
    client->hints.ai_socktype = SOCK_STREAM;

    client->gaicb.ar_request = &client->hints;
    client->gaicb.ar_name = client->hostname;

    struct gaicb *host = &client->gaicb;

    struct sigevent sig = {
        .sigev_notify = SIGEV_SIGNAL,
        .sigev_signo = SIGRTMIN,
        .sigev_value.sival_ptr = client,
    };

    int ret = getaddrinfo_a(GAI_NOWAIT, &host, 1, &sig);
    if (ret != 0) {
        printf("Error attempting to resolve host '%s', code=%d\n", client->hostname, ret);
        return false;
    }

    return true;
}

int main(int argc, const char **argv)
{
        if (argc != 2) {
                printf("Expected an argument for hostname to resolve\n");
                return 1;
        }

        struct ev_loop *loop = EV_DEFAULT;
        net_client_t client;

        net_client_init(&client, loop, argv[1]);
        ev_run(loop, 0);

        return 0;
}

getaddrinfo_a of eglibc 2.13 seems to simply open a thread and execute the request there synchronously, this could be reimplemented on platforms that don't have getaddrinfo_a.