Interface request structure used for socket ioctl's

/*
 * Interface request structure used for socket
 * ioctl‘s.  All interface ioctl‘s must have parameter
 * definitions which begin with ifr_name.  The
 * remainder may be interface specific.
 */
struct ifreq {
#define IFHWADDRLEN    6
    union
    {
        char    ifrn_name[IFNAMSIZ];        /* if name, e.g. "en0" */
    } ifr_ifrn;
    
    union {
        struct    sockaddr ifru_addr;
        struct    sockaddr ifru_dstaddr;
        struct    sockaddr ifru_broadaddr;
        struct    sockaddr ifru_netmask;
        struct  sockaddr ifru_hwaddr;
        short    ifru_flags;
        int    ifru_ivalue;
        int    ifru_mtu;
        struct  ifmap ifru_map;
        char    ifru_slave[IFNAMSIZ];    /* Just fits the size */
        char    ifru_newname[IFNAMSIZ];
        void *    ifru_data;
        struct    if_settings ifru_settings;
    } ifr_ifru;
};

#define ifr_name    ifr_ifrn.ifrn_name    /* interface name     */
#define ifr_hwaddr    ifr_ifru.ifru_hwaddr    /* MAC address         */
#define    ifr_addr    ifr_ifru.ifru_addr    /* address        */
#define    ifr_dstaddr    ifr_ifru.ifru_dstaddr    /* other end of p-p lnk    */
#define    ifr_broadaddr    ifr_ifru.ifru_broadaddr    /* broadcast address    */
#define    ifr_netmask    ifr_ifru.ifru_netmask    /* interface net mask    */
#define    ifr_flags    ifr_ifru.ifru_flags    /* flags        */
#define    ifr_metric    ifr_ifru.ifru_ivalue    /* metric        */
#define    ifr_mtu        ifr_ifru.ifru_mtu    /* mtu            */
#define ifr_map        ifr_ifru.ifru_map    /* device map        */
#define ifr_slave    ifr_ifru.ifru_slave    /* slave device        */
#define    ifr_data    ifr_ifru.ifru_data    /* for use by interface    */
#define ifr_ifindex    ifr_ifru.ifru_ivalue    /* interface index    */
#define ifr_bandwidth    ifr_ifru.ifru_ivalue    /* link bandwidth    */
#define ifr_qlen    ifr_ifru.ifru_ivalue    /* Queue length     */
#define ifr_newname    ifr_ifru.ifru_newname    /* New name        */
#define ifr_settings    ifr_ifru.ifru_settings    /* Device/proto settings*/

/*
 * Structure used in SIOCGIFCONF request.
 * Used to retrieve interface configuration
 * for machine (useful for programs which
 * must know all networks accessible).
 */

struct ifconf  {
    int    ifc_len;            /* size of buffer    */
    union {
        char *ifcu_buf;
        struct ifreq *ifcu_req;
    } ifc_ifcu;
};
#define    ifc_buf    ifc_ifcu.ifcu_buf        /* buffer address    */
#define    ifc_req    ifc_ifcu.ifcu_req        /* array of structures    */

#include <string.h>
#include <stdio.h>
#include <sys/ioctl.h>
#include <net/if.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>


void get_local_ip(char *g_localIP )
{
    int sock_get_ip;
    struct sockaddr_in *sin;
    struct ifreq ifr_ip;

    if ((sock_get_ip = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
        //todo:
    }
    
    memset(&ifr_ip, 0, sizeof(ifr_ip));
    strncpy(ifr_ip.ifr_name, "eth0", sizeof(ifr_ip.ifr_name)-1);
    if (ioctl(sock_get_ip, SIOCGIFADDR, &ifr_ip) < 0) {
        //todo:
    }
    sin = (struct sockaddr_in*)&ifr_ip.ifr_addr;
    printf("host: %s\r\n", inet_ntoa(sin->sin_addr));
    strcpy(g_localIP, inet_ntoa(sin->sin_addr));
    close(sock_get_ip);
}

int main()
{
    char g_localIP[20]="";
    get_local_ip(g_localIP);
    printf("%s\r\n", g_localIP);
    return 0;
}