example to list network interfaces on linux/bsd/osx/etc

showif.c

/* showif.c : PUBLIC DOMAIN - Jon Mayo - August 22, 2006
 * - You may remove any comments you wish, modify this code any way you wish,
 *   and distribute any way you wish.*/
/* finds all network interfaces and shows a little information about them.
 * some operating systems list interfaces multiple times because of different
 * flags, modes, etc. if you want to use this code you should be aware that
 * duplicate interfaces is a possibility */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <net/if.h>
#include <netdb.h>

static void show_interface(int fd, const char *name) {
	int family;
	struct ifreq ifreq;
	char host[128];
	memset(&ifreq, 0, sizeof ifreq);
	strncpy(ifreq.ifr_name, name, IFNAMSIZ);
	if(ioctl(fd, SIOCGIFADDR, &ifreq)!=0) {
		/* perror(name); */
		return; /* ignore */
	}
	switch(family=ifreq.ifr_addr.sa_family) {
		case AF_UNSPEC:
			return; /* ignore */
		case AF_INET:
		case AF_INET6:
			getnameinfo(&ifreq.ifr_addr, sizeof ifreq.ifr_addr, host, sizeof host, 0, 0, NI_NUMERICHOST);
			break;
		default:
			sprintf(host, "unknown (family: %d)", family);
	}
	printf("%-24s%s\n", name, host);
}

static void list_interfaces(int fd, void (*show)(int fd, const char *name)) {
	struct ifreq *ifreq;
	struct ifconf ifconf;
	char buf[16384];
	unsigned i;
	size_t len;

	ifconf.ifc_len=sizeof buf;
	ifconf.ifc_buf=buf;
	if(ioctl(fd, SIOCGIFCONF, &ifconf)!=0) {
		perror("ioctl(SIOCGIFCONF)");
		exit(EXIT_FAILURE);
	}

	ifreq=ifconf.ifc_req;
	for(i=0;i<ifconf.ifc_len;) {
		/* some systems have ifr_addr.sa_len and adjust the length that
		 * way, but not mine. weird */
#ifndef linux
		len=IFNAMSIZ + ifreq->ifr_addr.sa_len;
#else
		len=sizeof *ifreq;
#endif
		if(show) {
			show(fd, ifreq->ifr_name);		
		} else {
			printf("%s\n", ifreq->ifr_name);
		}
		ifreq=(struct ifreq*)((char*)ifreq+len);
		i+=len;
	}
}

void show_all_interfaces(int family) {
	int fd;

	fd=socket(family, SOCK_DGRAM, 0);
	if(fd<0) {
		perror("socket()");
		exit(EXIT_FAILURE);
	}
	list_interfaces(fd, show_interface);
	close(fd);
}

int main() {
	show_all_interfaces(PF_INET); /* IPv4 */
	show_all_interfaces(PF_INET6); /* IPv6 */
	return EXIT_SUCCESS;
}