在LINUX下，如果需要獲取本機的IP地址，需要用到以下一個結構體，該結構體在/usr/include/net/if.h檔案中

struct ifreq
  {
# define IFHWADDRLEN    6
# define IFNAMSIZ   IF_NAMESIZE
    union
      {
    char ifrn_name[IFNAMSIZ];   /* Interface 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 int ifru_flags;
    int ifru_ivalue;
    int ifru_mtu;
    struct ifmap ifru_map;
    char ifru_slave[IFNAMSIZ];  /* Just fits the size */
    char ifru_newname[IFNAMSIZ];
    __caddr_t ifru_data;
      } 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 _IOT_ifreq _IOT(_IOTS(char),IFNAMSIZ,_IOTS(char),16,0,0)
# define _IOT_ifreq_short _IOT(_IOTS(char),IFNAMSIZ,_IOTS(short),1,0,0)
# define _IOT_ifreq_int _IOT(_IOTS(char),IFNAMSIZ,_IOTS(int),1,0,0)

從這個結構體，你可以看到ifr_addr就是我們要獲取的IP地址，於是程式設計實現如下

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

#define IF_NAME "eth0"

int get_local_ip(char * ifname, char * ip)
{
    char *temp = NULL;
    int inet_sock;
    struct ifreq ifr;

    inet_sock = socket(AF_INET, SOCK_DGRAM, 0); 

    memset(ifr.ifr_name, 0, sizeof(ifr.ifr_name));
    memcpy(ifr.ifr_name, ifname, strlen(ifname));

    if(0 != ioctl(inet_sock, SIOCGIFADDR, &ifr)) 
    {   
        perror("ioctl error");
        return -1;
    }

    temp = inet_ntoa(((struct sockaddr_in*)&(ifr.ifr_addr))->sin_addr);     
    memcpy(ip, temp, strlen(temp));

    close(inet_sock);

    return 0;
}

int main(void)
{
    char ip[32] = {0};

    get_local_ip(IF_NAME, ip);

    if(0 != strcmp(ip, ""))
        printf("%s ip is %s\n",IF_NAME, ip);

    return 0;
}