getifaddrs, freeifaddrs - get interface addresses - http://www.kernel.org/doc/man-pages/online/pages/man3/getifaddrs.3.html

getifaddrs.c

#include <arpa/inet.h>
#include <sys/socket.h>
#include <netdb.h>
#include <ifaddrs.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int
main(int argc, char *argv[])
{
  struct ifaddrs *ifaddr, *ifa;
  int family, s;
  char host[NI_MAXHOST];

  if (getifaddrs(&ifaddr) == -1) {
    perror("getifaddrs");
    exit(EXIT_FAILURE);
  }

    /* Walk through linked list, maintaining head pointer so we can free list later */

  for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) {
    if (ifa->ifa_addr == NULL)
      continue;

    family = ifa->ifa_addr->sa_family;

    /* Display interface name and family (including symbolic form of the latter for the common families) */

    printf("%s  address family: %d%s\n",
      ifa->ifa_name, family,
      (family == AF_PACKET) ? " (AF_PACKET)" :
      (family == AF_INET) ?   " (AF_INET)" :
      (family == AF_INET6) ?  " (AF_INET6)" : "");

    /* For an AF_INET* interface address, display the address */

    if (family == AF_INET || family == AF_INET6) {
      s = getnameinfo(ifa->ifa_addr,
        (family == AF_INET) ? sizeof(struct sockaddr_in) : sizeof(struct sockaddr_in6),
        host, NI_MAXHOST, NULL, 0, NI_NUMERICHOST);
      if (s != 0) {
        printf("getnameinfo() failed: %s\n", gai_strerror(s));
        exit(EXIT_FAILURE);
      }
      printf("\taddress: <%s>\n", host);
    }
  }

  freeifaddrs(ifaddr);
  exit(EXIT_SUCCESS);
}