gethostbyname vs. getaddrinfo

After started coding in C again I've had a look at some basic network stuff like: socket(), connect() etc. In order to resolve domain names, one was used to use gethostbyname(). I've used this function in all my previous C projects but this one seems to be out-dated as the main page states:

The gethostbyname*(), gethostbyaddr*(), herror(), and hstrerror() functions are obsolete.  
Applications should use getaddrinfo(3), getnameinfo(3), and gai_strerror(3) instead.

The really bad thing about gethostbyname is the fact that it doesn't support IPv6. Besides that you would then load the results into a struct sockaddr_in and use it for your futher calls. A better option is getaddrinfo() which supports IPv4 and IPv6 as well, does the DNS lookup and fills your structures. Perfect!

So what do you need? First you'll need some variables:

int status;
char ip_addr[INET6_ADDRSTRLEN];
struct sockaddr_in *remote;
struct addrinfo hints, *res, *p;

And then the call:

if ((status=getaddrinfo(server_hostname, port, &hints, &res)) != 0) {
    log_err("getaddrinfo failed: %s", gai_strerror(status));
    return -1;

res is a linked list which has to be looped to fetch the results:

while (p = res; p != NULL; p = p->ai_next) {
    void *addr;
    remote = (struct sockaddr_in *)p->ai_addr;
    addr = &(remote->sin_addr);

    // Convert IP to string
    inet_ntop(p->ai_family, addr, ip_addr, sizeof(ip_addr));

    printf("%s\n", ip_addr);

Now you could create your socket, connect to the host, read data and so on:

if ((socketfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol)) < 0) {
    log_err("Failed creating socket!");
    return -1;

Prev: Coming back to C/C++ after 2 years
Next: Adding concurrency to smalisca

comments powered by Disqus
2015-04-30 00:00