Below is a demo program that has been posted a few times previously.

/* display info about network interfaces */

#define _BSD_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <net/if.h>
#include <net/if_arp.h>
#include <arpa/inet.h>

#define inaddrr(x) (*(struct in_addr *) &ifr->x[sizeof sa.sin_port])
#define IFRSIZE   ((int)(size * sizeof (struct ifreq)))

static int
get_addr(int sock, char * ifname, struct sockaddr * ifaddr) {

  struct ifreq *ifr;
  struct ifreq ifrr;
  struct sockaddr_in sa;

  ifr = &ifrr;
  ifrr.ifr_addr.sa_family = AF_INET;
  strncpy(ifrr.ifr_name, ifname, sizeof(ifrr.ifr_name));

  if (ioctl(sock, SIOCGIFADDR, ifr) < 0) {
    printf("No %s interface.\n", ifname);
    return -1;
  }

  *ifaddr = ifrr.ifr_addr;
  printf("Address for %s: %s\n", ifname, inet_ntoa(inaddrr(ifr_addr.sa_data)));
  return 0;

int
main(void)
{
  unsigned char      *u;
  int                sockfd, size  = 1;
  struct ifreq       *ifr;
  struct ifconf      ifc;
  struct sockaddr_in sa;

  if (0 > (sockfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_IP))) {
          fprintf(stderr, "Cannot open socket.\n");
    exit(EXIT_FAILURE);
  }

  ifc.ifc_len = IFRSIZE;
  ifc.ifc_req = NULL;

  do {
    ++size;
    /* realloc buffer size until no overflow occurs  */
    if (NULL == (ifc.ifc_req = realloc(ifc.ifc_req, IFRSIZE))) {
      fprintf(stderr, "Out of memory.\n");
      exit(EXIT_FAILURE);
    }
    ifc.ifc_len = IFRSIZE;
    if (ioctl(sockfd, SIOCGIFCONF, &ifc)) {
      perror("ioctl SIOCFIFCONF");
      exit(EXIT_FAILURE);
    }
  } while  (IFRSIZE <= ifc.ifc_len);

  /* this is an alternate way to get info... */
  {
    struct sockaddr ifa;
    get_addr(sockfd, "ppp0", &ifa);
  }

  ifr = ifc.ifc_req;
  for (;(char *) ifr < (char *) ifc.ifc_req + ifc.ifc_len; ++ifr) {

    if (ifr->ifr_addr.sa_data == (ifr+1)->ifr_addr.sa_data) {
      continue;  /* duplicate, skip it */
    }

    if (ioctl(sockfd, SIOCGIFFLAGS, ifr)) {
      continue;  /* failed to get flags, skip it */
    }

    printf("Interface:  %s\n", ifr->ifr_name);
    printf("IP Address: %s\n", inet_ntoa(inaddrr(ifr_addr.sa_data)));

    /*
      This won't work on HP-UX 10.20 as there's no SIOCGIFHWADDR ioctl. You'll
      need to use DLPI or the NETSTAT ioctl on /dev/lan0, etc (and you'll need
      to be root to use the NETSTAT ioctl. Also this is deprecated and doesn't
      work on 11.00).

      On Digital Unix you can use the SIOCRPHYSADDR ioctl according to an old
      utility I have. Also on SGI I think you need to use a raw socket, e.g. s
      = socket(PF_RAW, SOCK_RAW, RAWPROTO_SNOOP)

      Dave

      From: David Peter <dave.pe...@eu.citrix.com>
     */

    if (0 == ioctl(sockfd, SIOCGIFHWADDR, ifr)) {

      /* Select which  hardware types to process.
       *
       *    See list in system include file included from
       *    /usr/include/net/if_arp.h  (For example, on
       *    Linux see file /usr/include/linux/if_arp.h to
       *    get the list.)
       */
      switch (ifr->ifr_hwaddr.sa_family) {
      default:
        printf("\n");
        continue;
      case  ARPHRD_NETROM:  case  ARPHRD_ETHER:  case  ARPHRD_PPP:
      case  ARPHRD_EETHER:  case  ARPHRD_IEEE802: break;
      }

      u = (unsigned char *) &ifr->ifr_addr.sa_data;

      if (u[0] + u[1] + u[2] + u[3] + u[4] + u[5]) {
        printf("HW Address: %2.2x.%2.2x.%2.2x.%2.2x.%2.2x.%2.2x\n",
             u[0], u[1], u[2], u[3], u[4], u[5]);
      }
    }

    if (0 == ioctl(sockfd, SIOCGIFNETMASK, ifr) &&
        strcmp("255.255.255.255", inet_ntoa(inaddrr(ifr_addr.sa_data)))) {
      printf("Netmask:    %s\n", inet_ntoa(inaddrr(ifr_addr.sa_data)));
    }

    if (ifr->ifr_flags & IFF_BROADCAST) {
      if (0 == ioctl(sockfd, SIOCGIFBRDADDR, ifr) &&
          strcmp("0.0.0.0", inet_ntoa(inaddrr(ifr_addr.sa_data)))) {
        printf("Broadcast:  %s\n", inet_ntoa(inaddrr(ifr_addr.sa_data)));
      }
    }

    if (0 == ioctl(sockfd, SIOCGIFMTU, ifr)) {
      printf("MTU:        %u\n",  ifr->ifr_mtu);
    }

    if (0 == ioctl(sockfd, SIOCGIFMETRIC, ifr)) {
      printf("Metric:     %u\n",  ifr->ifr_metric);
    }
    printf("\n");
  }

  close(sockfd);
  return EXIT_SUCCESS;

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