text: #define _GNU_SOURCE
  #include <net/if.h>
  #include <stdlib.h>
  #include <sys/socket.h>
  #include <sys/ioctl.h>
  #include <errno.h>
  #include "syscall.h"
  
  #include <stdio.h>
  
  static void *do_nameindex(int s, size_t n)
  {
  	size_t i, len, k;
  	struct ifconf conf;
  	struct if_nameindex *idx;
  
  	idx = malloc(n * (sizeof(struct if_nameindex)+sizeof(struct ifreq)));
  	if (!idx) return 0;
  
  	conf.ifc_buf = (void *)&idx[n];
  	conf.ifc_len = len = n * sizeof(struct ifreq);
  	if (ioctl(s, SIOCGIFCONF, &conf) < 0) {
  		free(idx);
  		return 0;
  	}
  	if (conf.ifc_len == len) {
  		free(idx);
  		return (void *)-1;
  	}
  
  	n = conf.ifc_len / sizeof(struct ifreq);
  	for (i=k=0; i<n; i++) {
  		if (ioctl(s, SIOCGIFINDEX, &conf.ifc_req[i]) < 0) {
  			k++;
  			continue;
  		}
  		idx[i-k].if_index = conf.ifc_req[i].ifr_ifindex;
  		idx[i-k].if_name = conf.ifc_req[i].ifr_name;
  	}
  	idx[i-k].if_name = 0;
  	idx[i-k].if_index = 0;
  
  	return idx;
  }
  
  struct if_nameindex *if_nameindex()
  {
  	size_t n;
  	void *p = 0;
  	int s = socket(AF_UNIX, SOCK_DGRAM, 0);
  	if (s>=0) {
  		for (n=0; (p=do_nameindex(s, n)) == (void *)-1; n++);
  		__syscall(SYS_close, s);
  	}
  	errno = ENOBUFS;
  	return p;
  }
  