Linux C: find the default gateway IP address rather than parsing the proc file system

you can use libnl , which is a library for communicating with the kernel via NETLINK sockets.

a simple example for getting the default route would be (please add error checking and required headers)

   void cb_route (struct nl_object *cb, void *arg) {
        struct rtnl_route *route = (struct rtnl_route*) cb;
        /* do what you need to do with rtnl_route object 
         * e.g. copy it to arg, etc.
         */
        struct nl_addr *tmp_addr;
        tmp_addr = rtnl_route_get_gateway(route);
        len = nl_addr_get_len(tmp_addr);
        memcpy(arg, nl_addr_get_binary_addr(tmp_addr), len);
   }

   int get_route () {
        struct nl_handle *nlh;
        struct nl_cache *route_cache;
        struct rtnl_route *filter;
        struct nl_addr *dst;
        char *gw = malloc(16); //enough room for IPv6 if needed
        char n = '0';

        nlh = nl_handle_alloc();
        if (!nlh)
            return -1;

        nl_connect(nlh, NETLINK_ROUTE);
        route_cache = rtnl_route_alloc_cache();

        if (nl_cache_is_empty(route_cache))
             return 0;

        dst = nl_addr_build(AF_INET, &n, 0);
        filter = rtnl_route_alloc();
        rtnl_route_set_scope(filter, RT_SCOPE_UNIVERSE);
        rtnl_route_set_family(filter, AF_INET);
        rtnl_route_set_table(filter, RT_TABLE_MAIN);
        rtnl_route_set_dst(filter, dst);

        nl_cache_foreach_filter(route_cache, (struct nl_object*) filter, cb_filter_route, gw);
        /* gw should now contain the binary representation of the default gw. use inet_ntop to convert it */

        return 0;
  }