How can I monitor the NIC status(up/down) in a C program without polling the kernel?

Yes, open a netlink socket and listen to the RTMGRP_LINK (network interface create/delete/up/down events) multicast groups.

The netlink man page here has a specific example to do this.

After doing a little research/reading on the web, I managed to cook up a working code to monitor NIC status.

#include <asm/types.h>#include <sys/socket.h>#include <unistd.h>#include <errno.h>#include <stdio.h>#include <string.h>#include <net/if.h>#include <netinet/in.h>#include <linux/netlink.h>#include <linux/rtnetlink.h>#include <stdlib.h>#include <sys/time.h>#include <sys/types.h>intread_event (int sockint){  int status;  int ret = 0;  char buf[4096];  struct iovec iov = { buf, sizeof buf };  struct sockaddr_nl snl;  struct msghdr msg = { (void *) &snl, sizeof snl, &iov, 1, NULL, 0, 0 };  struct nlmsghdr *h;  struct ifinfomsg *ifi;  status = recvmsg (sockint, &msg, 0);  if (status < 0)  {      /* Socket non-blocking so bail out once we have read everything */      if (errno == EWOULDBLOCK || errno == EAGAIN)      return ret;      /* Anything else is an error */      printf ("read_netlink: Error recvmsg: %d\n", status);      perror ("read_netlink: Error: ");      return status;  }  if (status == 0)   {      printf ("read_netlink: EOF\n");   }  // We need to handle more than one message per 'recvmsg'  for (h = (struct nlmsghdr *) buf; NLMSG_OK (h, (unsigned int) status);       h = NLMSG_NEXT (h, status))    {      //Finish reading       if (h->nlmsg_type == NLMSG_DONE)        return ret;      // Message is some kind of error       if (h->nlmsg_type == NLMSG_ERROR)    {          printf ("read_netlink: Message is an error - decode TBD\n");          return -1;        // Error        }      if (h->nlmsg_type == RTM_NEWLINK)        {        ifi = NLMSG_DATA (h);            printf ("NETLINK::%s\n", (ifi->ifi_flags & IFF_RUNNING) ? "Up" : "Down");    }    }  return ret;}intmain (int argc, char *argv[]){  fd_set rfds, wfds;  struct timeval tv;  int retval;  struct sockaddr_nl addr;  int nl_socket = socket (AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);  if (nl_socket < 0)    {      printf ("Socket Open Error!");      exit (1);    }  memset ((void *) &addr, 0, sizeof (addr));  addr.nl_family = AF_NETLINK;  addr.nl_pid = getpid ();  addr.nl_groups = RTMGRP_LINK | RTMGRP_IPV4_IFADDR | RTMGRP_IPV6_IFADDR;//  addr.nl_groups = RTMGRP_LINK;  if (bind (nl_socket, (struct sockaddr *) &addr, sizeof (addr)) < 0)    {      printf ("Socket bind failed!");      exit (1);    }  while (1)    {      FD_ZERO (&rfds);      FD_CLR (nl_socket, &rfds);      FD_SET (nl_socket, &rfds);      tv.tv_sec = 10;      tv.tv_usec = 0;      retval = select (FD_SETSIZE, &rfds, NULL, NULL, &tv);      if (retval == -1)        printf ("Error select() \n");      else if (retval)        {          printf ("Event recieved >> ");          read_event (nl_socket);        }      else        printf ("## Select TimedOut ## \n");    }  return 0;}

Have you tried monitoring the /sys/class/net/eth0/operstate file with select or poll function? As far as I can tell sysfs files should behave the same with respect to polling as regular files: whenever a change occurs you should get a notification on the file handle that something has changed and you should be able to respond accordingly.