gistfile1.m

#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <arpa/inet.h>
#include <net/if.h>

void ip_for_interface(const char *interface_name, char *ip, const int length) {
  *ip = NULL;
	
	int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
	if( sockfd < 0 ) {
		return;
	}
	
	struct ifconf ifc;
	ifc.ifc_len = 1024;
	ifc.ifc_buf = malloc(ifc.ifc_len);
	
	if( ioctl(sockfd, SIOCGIFCONF, &ifc ) < 0 ) {
		free(ifc.ifc_buf);
		return;
	}
	
	for( char *current = ifc.ifc_buf; current < ifc.ifc_buf + ifc.ifc_len; ) {
		
		struct ifreq *request = (struct ifreq *)current;
		current += sizeof(request->ifr_name) +
			(sizeof(struct sockaddr) > request->ifr_addr.sa_len
				? sizeof(struct sockaddr)
				: request->ifr_addr.sa_len);
		
		if( request->ifr_addr.sa_family == AF_INET && strcmp(request->ifr_name, interface_name) == 0 ) {
			struct sockaddr_in *sin = (struct sockaddr_in *)&request->ifr_addr;
			
			strncpy(ip, inet_ntoa(sin->sin_addr), length);
			break;
		}
	}
	
	close(sockfd);
	free(ifc.ifc_buf);
}


-(NSString *)getIPAddress {
	char addr[32];
	ip_for_interface("en0", addr, 32); // public: "pdp_ip0", local: "lo0"
	return addr ? [NSString stringWithUTF8String:addr] : @"error";
}