trying to get this netlink version 3 library code to work

stats.c

/* ipchange.c
 *
 *  Netlink based IP monitor on specific interface.
 *  Executes command on new IPv4 address.
 *
 *  Sebastian Kricner
 *  tuxwave.net
 *  June 2014
 *
*/
#include <libnl3/netlink/netlink.h>
#include <libnl3/netlink/socket.h>
#include <libnl3/netlink/msg.h>
#include <libnl3/netlink/errno.h>
#include <libnl3/netlink/route/link.h>
#include <libnl3/netlink/route/addr.h>
#include <libnl3/netlink/route/qdisc.h>
#include <net/if.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>
#include <syslog.h>
#include <sys/types.h>
#include <string.h>

/*
gcc ipchange.c -o ipchange $(pkg-config --cflags --libs libnl-3.0 libnl-route-3.0)
*/

static char *interface;

struct rtnl_link* m_link;


static void parse_notification(struct nl_object *obj, void *arg) {

    struct rtnl_addr *addr;
    char s_addr[128];

    int flags;
    char s_flags[128];

    unsigned int ifindex;
    char s_ifname[16];

    char pchBuf[80];

    addr = (struct rtnl_addr *) obj;
    flags = rtnl_addr_get_flags(addr);
    ifindex = rtnl_addr_get_ifindex(addr);
    if_indextoname(ifindex, s_ifname);

    nl_addr2str(rtnl_addr_get_local(addr), s_addr, sizeof(s_addr));
    rtnl_addr_flags2str(flags, s_flags, sizeof(s_flags));

    setenv("S_INTERFACE", s_ifname, 0);
    setenv("S_ADDRESS", s_addr, 0);
    setenv("S_FLAGS", s_flags, 0);


    if(strcmp(s_ifname, interface) == 0) {
	sprintf(pchBuf,"%s %s","echo",s_ifname);
        system(pchBuf);
        syslog(LOG_NOTICE, "command executed");
    }
    return;
}

static int process_notification(struct nl_msg *msg, void *arg) {

    struct nlmsghdr *msghdr = nlmsg_hdr(msg);

    // These def/enums are defined in linux/rtnetlink.h
    if(msghdr->nlmsg_type == RTM_NEWADDR) {
	// hits when a second address is added to the interface
        nl_msg_parse(msg, &parse_notification, arg);
    } else if(msghdr->nlmsg_type == RTM_DELADDR) {
	// hits when ip addr is removed.
	printf("addr removed\n");
    } else if(msghdr->nlmsg_type == RTM_NEWLINK) {
        // If in the add membership group, I RTNLGRP_LINK,
	// hits when ethernet cable is inserted/removed
	printf("network cable inserted/removed\n");
    } else if(msghdr->nlmsg_type == RTM_DELLINK) {
        // this never hits
	printf("network cable removed\n");
    } else {
        // this never hits
	printf("some other msg\n");
    }
    return NL_OK;
}

static int start_monitor(char *interface) {


    struct nl_sock *sk;
    int err;

 
    // This entire code seems to be based upon the example given 
    // in section 3.3.1 Multicast Example 
    // https://www.infradead.org/~tgr/libnl/doc/core.html#core_sk_multicast

    // https://www.infradead.org/~tgr/libnl/doc/core.html#core_sockets
    // allocate a new socket
    sk = nl_socket_alloc();
    if(!sk) {
        fprintf(stderr, "Could not allocate netlink socket.\n");
        exit(ENOMEM);
    }

    // https://www.infradead.org/~tgr/libnl/doc/core.html#core_sk_seq_num
    // Notifications do not use sequence numbers, disable sequence number checking
    nl_socket_disable_seq_check(sk);

    // Define a callback function, which will be called for each notification received
    nl_socket_modify_cb(sk, NL_CB_VALID, NL_CB_CUSTOM, process_notification, NULL);

    // Connect to a routing netlink protocol
    if(err = nl_connect(sk, NETLINK_ROUTE)) {
        fprintf(stderr, "netlink error: %s\n", nl_geterror(err));
        nl_socket_free(sk);
        sk = NULL;
        exit(err);
    }

    // Subscribe to link notifications group.  
    // The example code use RTNLGRP_LINK.
    //nl_socket_add_memberships(sk, RTNLGRP_IPV4_IFADDR|RTNLGRP_LINK, 0);
    // to add multiple groups, seperate them with , instead of |
    // These groups are defined in linux/rtnetlink.h
    //nl_socket_add_memberships(sk, RTNLGRP_IPV4_IFADDR, RTNLGRP_LINK, RTNLGRP_NOTIFY,0);
    nl_socket_add_memberships(sk, RTNLGRP_IPV4_IFADDR, RTNLGRP_LINK, RTNLGRP_NOTIFY,RTNLGRP_IPV4_RULE,0);

    while (1) {
        nl_recvmsgs_default(sk);
	// Everytime I get a message, I also get a callback
	//printf("recv msg\n");


    }

    // Never reached
    return 0;
}


int main(int argc, char **argv, char **envp) {

    int err;
    struct nl_cache *cache;
    struct rtnl_link *link;
    struct nl_addr *addr;
    struct addrinfo *addr_info;
    char *name;
    uint64_t count;
    struct rtnl_qdisc *qdisc;


    struct nl_sock *sk;

    interface = "enp6s0";





    sk = nl_socket_alloc();
    if (!sk) {
        fprintf(stderr, "Could not allocate netlink socket.\n");
        exit(ENOMEM);
    }

    // https://www.infradead.org/~tgr/libnl/doc/core.html#core_sk_seq_num
    // Notifications do not use sequence numbers, disable sequence number checking
    nl_socket_disable_seq_check(sk);


    // Connect to a routing netlink protocol
    //if(err = nl_connect(sk, NETLINK_SOCK_DIAG)) {
    if(err = nl_connect(sk, NETLINK_ROUTE)) {
        fprintf(stderr, "netlink error: %s\n", nl_geterror(err));
        nl_socket_free(sk);
        sk = NULL;
        exit(err);
    }


    err = rtnl_link_alloc_cache(sk, AF_UNSPEC, &cache);
    if (0 != err) {
        /* error */
	printf("rtnl_link_alloc_cache failed with %d: %s\n", err, nl_geterror(err));
	return(EXIT_FAILURE);
    }



    // returns a link object or NULL is no match found
    link = rtnl_link_get_by_name(cache, interface);
    if (NULL == link) {
        /* link does not exist */
	printf("rtnl_link_get_by_name for interface %s does not exist\n",interface);
	return(EXIT_FAILURE);
    }




    /* do something with link */
    addr = rtnl_link_get_addr(link);
    if (NULL == addr) {
        /* failure to get mac address */
	printf("rtnl_link_get_addr failed\n");
	return(EXIT_FAILURE);
    }

    name = rtnl_link_get_name(link);
    printf("name is %s\n",name);

    // allocation of qdisc object
    qdisc = rtnl_qdisc_alloc();

    // Get some counts
    count = rtnl_tc_get_stat(TC_CAST(qdisc), RTNL_TC_PACKETS);
    printf("Total # of packets transmitted is %ld\n",count);  // is zero.
    // how to specify this is the link for my desired network interface?

    err = nl_addr_info(addr, &addr_info);
    if (0 != err) {
        /* failure to get mac address */
	//printf("nl_addr_info failed. %s\n",strerror(errno));
	printf("nl_addr_info failed with %d: %s\n", err, nl_geterror(err));
	return(EXIT_FAILURE);
    }


    // free the qdisc object
    rtnl_qdisc_put(qdisc);

    // put releases the reference to the link struct
    rtnl_link_put(link);
    // i'm guessing this releases the reference to the cache struct
    nl_cache_put(cache);



    //start_monitor(interface);

    return(EXIT_SUCCESS);
}