c-ares example

async_dns.c

#include <ares.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdarg.h>
#include <string.h>
#include <ctype.h>
#include <unistd.h>

static void
state_cb(void *data, int s, int read, int write)
{
    printf("Change state fd %d read:%d write:%d\n", s, read, write);
}


static void
callback(void *arg, int status, int timeouts, struct hostent *host)
{

    if(!host || status != ARES_SUCCESS){
        printf("Failed to lookup %s\n", ares_strerror(status));
        return;
    }

    printf("Found address name %s\n", host->h_name);
    char ip[INET6_ADDRSTRLEN];
    int i = 0;

    for (i = 0; host->h_addr_list[i]; ++i) {
        inet_ntop(host->h_addrtype, host->h_addr_list[i], ip, sizeof(ip));
        printf("%s\n", ip);
    }
}

static void
wait_ares(ares_channel channel)
{
    for(;;){
        struct timeval *tvp, tv;
        fd_set read_fds, write_fds;
        int nfds;

        FD_ZERO(&read_fds);
        FD_ZERO(&write_fds);
        nfds = ares_fds(channel, &read_fds, &write_fds);
        if(nfds == 0){
            break;
        }
        tvp = ares_timeout(channel, NULL, &tv);
        select(nfds, &read_fds, &write_fds, NULL, tvp);
        ares_process(channel, &read_fds, &write_fds);
    }
}

int
main(void)
{
    ares_channel channel;
    int status;
    struct ares_options options;
    int optmask = 0;

    status = ares_library_init(ARES_LIB_INIT_ALL);
    if (status != ARES_SUCCESS){
        printf("ares_library_init: %s\n", ares_strerror(status));
        return 1;
    }
    //options.sock_state_cb_data;
    options.sock_state_cb = state_cb;
    optmask |= ARES_OPT_SOCK_STATE_CB;

    status = ares_init_options(&channel, &options, optmask);
    if(status != ARES_SUCCESS) {
        printf("ares_init_options: %s\n", ares_strerror(status));
        return 1;
    }

    ares_gethostbyname(channel, "google.com", AF_INET, callback, NULL);
    //ares_gethostbyname(channel, "google.com", AF_INET6, callback, NULL);
    wait_ares(channel);
    ares_destroy(channel);
    ares_library_cleanup();
    printf("fin\n");
    return 0;
}

Why wait_ares(channel) is required?

Because c-ares is acync. wait_ares waits until all responses are ready.

http://c-ares.haxx.se/ares_gethostbyname.html
"Completion or failure of the query may happen immediately, or may happen during a later call to ares_process(3), ares_destroy(3) or ares_cancel(3)."

For example, for "localhost" callback is called immediately, but for internet domains responses are returned in wait_ares loop.

How would you change this code to handle multiple queries in asynchronous fashion?

How would you change this code to handle multiple queries in asynchronous fashion?

You can call ares_gethostbyname multiple times. You'd likely want to attach some details in the user arg to determine which response you're dealing with.

Further usage of ares can be found here at libcurl.

The workflow can be generalized as:

• ares_library_init
• ares_init_options
• ares_fds/ares_getsock (depending on which interface you use to wait on sockets (i.e select/poll/epoll/kqueue)).
• /** select/poll/epoll/kqueue */
• ares_process/ares_process_fd
• ares_destroy_options
• ares_library_cleanup

Further usage of ares can be found here at libcurl.

The workflow can be generalized as:

• ares_library_init
• ares_init_options
• ares_fds/ares_getsock (depending on which interface you use to wait on sockets (i.e select/poll/epoll/kqueue)).
• /** select/poll/epoll/kqueue */
• ares_process/ares_process_fd
• ares_destroy_options
• ares_library_cleanup

What if I have multiple threads making multiple requests, is it ok to repeat the process of ares_gethostbyname -> fds, timeout, select & process for each thread? I am using a different channel for each thread, but I am suffering from crashes due to race conditions.

A specific ares_channel_t is not thread-safe. If you want to do resolution from different threads, you can create one per thread.