networking - How to get my non-loopback network ip address in C?

Question

For a communication between two hosts, I need to send the IP address of my host to the other site. The problem is that if I request my IP address, it might be that I get back my local loopback IP addres (127.x.x.x) , not the network (ethernet) IP address.

I use the following code:

char myhostname[32];


gethostname(myhostname, 32);
hp = gethostbyname(myhostname);
unsigned my_ip = *(unsigned*)(hp->h_addr);

if( (my_ip % 256) == 127) {
  /* Wrong IP adress as it's 127.x.x.x */
  printf("Error, local IP address!");
  return;
}

The only way to solve it is to make sure my hostname in /etc/hosts is behind the real network address, not the local loopback (the default for e.g. Ubuntu).

Is there a way to solve this without relying on the content of /etc/hosts?

Edit: I changed the above code so it makes use of getaddrinfo, but I still get back the loopback device's number (127.0,0,1) instead of the real IP address:

struct addrinfo hint = {0};
struct addrinfo *aip = NULL;
unsigned ip = 0;
struct sockaddr_in *sinp = NULL;

hint.ai_family = AF_INET; /* IPv4 */
hint.ai_socktype = SOCK_STREAM;

if(getaddrinfo(hostname, NULL, &hint, &aip) != 0) {
    return 0;
}
sinp = (struct sockaddr_in *) aip->ai_addr;
ip   = *(unsigned *) &sinp->sin_addr;

(I used to get back a list of 3 addrinfo's with the three SOCK_STREAM,SOCK_DGRAM and SOCK_RAW, but the hint prevents that)

So my question still stands...

Answer 1

There is POSIX function getaddrinfo() that returns linked list of addresses for given hostname, so you just need to go through that list and find non-loopback address.

See man getaddrinfo.