How can I prevent SIOCGIFADDR from failing?

Question

I want to get the eth0's IP. Here is what I've written (maybe there is a way around it?):

int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
check(sockfd > 0, "cannot create socket\n");

#define INTERFACE_NAME "eth0"
#define INTERFACE_NAME_LENGTH 4

char *opt = INTERFACE_NAME;
rc = setsockopt(sockfd, SOL_SOCKET, SO_BINDTODEVICE, opt, INTERFACE_NAME_LENGTH);
check(rc == 0, "BINDTODEVICE failed");

struct ifreq req;
strncpy(req.ifr_name, INTERFACE_NAME, IFNAMSIZ);
rc = ioctl(sockfd, SIOCGIFADDR, (unsigned long)&req);
check(rc == 0, "SIOCGIFADDR failed");
server_ip = ((struct sockaddr_in*)&req.ifr_addr)->sin_addr.s_addr;
char str[50];
inet_ntop(AF_INET, &(server_ip), str, INET_ADDRSTRLEN);
debug("serverip: %s", str);

return sockfd;

error:
if (sockfd) close(sockfd);
exit(1); 

I get the following error:

[ERROR] (src/server/server.c:43: errno: Cannot assign requested address) SIOCGIFADDR failed

If I use the same method with wlan0, I get what I'd expected to see.

Here is the netstat output:

netstat -tulpn:

Proto | Local Address   |  PID

udp   | 0.0.0.0:16313   | 4666/dhclient   
udp   | 0.0.0.0:68      | 4687/dhclient   
udp   | 0.0.0.0:68      | 4666/dhclient 

So, I figure that I can't assign the address because of the dhclients? Why there are so many of them? and why there is one on the 16313 port?

UPD:

I added

auto eth0
iface eth0 inet static
        address 192.168.1.1
        netmask 255.255.255.0

to /etc/network/interfaces and restarted the networking and got some progress:

DEBUG src/server/server.c:50: serverip: 192.168.1.1

and then I can successfully bind the socket, but the connection dies for no reason in a couple of seconds.

Answer 1

Since I was looking for the answer to the original question, I'll officially write up the answer that Mark Plotnick gave above as a comment based on the further research by the person posing the question:

SIOCGIFADDR fails with errno set to EADDRNOTAVAIL if there's no address is assigned to the interface. As the original questioner discovered, to stop it from failing, set the interface's IP address.

strerror() translates EADDRNOTAVAIL to "Cannot assign requested address" which is misleading to the point of being wrong in this situation where you're reading the address, not assigning it.