Get Address of a Network Interface in C

So, here I am searching the internet over and over again to find out how to easily and quickly get the ip address of a local interface, and there isn't an easy way to do it without running `ifconfig | grep | cut...` in some crazy fashion.

So, here I've compiled a simple C script that will iterate through your interfaces and get the IP address of a local network interface if you supply it on the command line.

Feel free to redistribute under GNU.

https://github.com/markizano/scripts/blob/master/getifaddr/getifaddr.c

/*
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.


    I am not the original author of this script. This was inspired by
    the man page at getifaddrs(3).
*/

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

int Usage () {
 printf("Usage: getifaddr [ifaddr] [inet|inet6]\nDefaults to eth0/inet.\n");
 return 8;
}

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

 iface = NULL; inet = AF_INET;
 if ( argc >= 2 ) {
  if ( !strcmp(argv[1], "-h") || !strcmp(argv[1], "--help") ) {
   return Usage();
  }
 } else if ( argc <= 2 ) {
  return Usage();
 }

 if ( argc >= 2 ) {
  iface = strndup(argv[1], strlen(argv[1]));
 }

 if ( argc >= 3 ) {
  if ( !strcmp(argv[2], "inet6") ) {
   inet = AF_INET6;
  }
 }

 if ( iface == NULL ) {
  iface = malloc(4);
  if ( iface == NULL ) return 4;
  strncpy(iface, "eth0", 4);
 }

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

 memset(host, 0, sizeof host);

    /* 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;

  if ( family == AF_PACKET ) continue;

  /* Display interface name and family (including symbolic
   form of the latter for the common families) */
  if ( !strcmp(ifa->ifa_name, iface) ) {
   if ( family == inet ) {
    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) {
     fprintf(stderr, "getnameinfo() failed: %s\n", gai_strerror(s));
     exit(EXIT_FAILURE);
    }
   }

   printf( "%s\n", host );
   freeifaddrs(ifaddr);
   free(iface);
   return 0;
  }
 }

 fprintf(stderr, "Could not find devivce.\n");
 freeifaddrs(ifaddr);
 free(iface);
 exit(EXIT_SUCCESS);
}