Sample C code to obtain the IP address attached to a selected network interface under Linux.

get_ip_from_nic.c

/*
 * Sample C code to obtain the IP address attached to
 * a selected network interface under Linux.
 *
 * Found here: http://www.geekpage.jp/en/programming/linux-network/get-ipaddr.php
 *
 * Missing includes added.
 */

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <netinet/in.h>
#include <net/if.h>
#include <unistd.h>
#include <arpa/inet.h>

int main() {
  int fd;
  struct ifreq ifr;

  fd = socket(AF_INET, SOCK_DGRAM, 0);

  /* I want to get an IPv4 IP address */
  ifr.ifr_addr.sa_family = AF_INET;

  /* I want IP address attached to "eth0" */
  strncpy(ifr.ifr_name, "eth0", IFNAMSIZ - 1);

  ioctl(fd, SIOCGIFADDR, &ifr);

  close(fd);

  /* display result */
  printf("%s\n", inet_ntoa(((struct sockaddr_in *) &ifr.ifr_addr)->sin_addr));

  return 0;
}