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        简单：

#include <iostream>
#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <malloc.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/ioctl.h>
#include <sys/types.h>  
#include <netinet/in.h>  
#include <arpa/inet.h>  
#include <linux/if.h>

#define BUF_SIZE 1024
using namespace std;

static int getIP(string net_name, string &strIP)
{
    int sock_fd;
    struct ifconf conf;
    struct ifreq *ifr;
    char buff[BUF_SIZE] = {0};
    int num;
    int i;

    sock_fd = socket(PF_INET, SOCK_DGRAM, 0);
    if ( sock_fd < 0 )     
		return -1;

    conf.ifc_len = BUF_SIZE;
    conf.ifc_buf = buff;

    if ( ioctl(sock_fd, SIOCGIFCONF, &conf) < 0 )
    {
        close(sock_fd);
        return -1;
    }

    num = conf.ifc_len / sizeof(struct ifreq);
    ifr = conf.ifc_req;

    for(i = 0; i < num; i++)
    {
        struct sockaddr_in *sin = (struct sockaddr_in *)(&ifr->ifr_addr);

        if ( ioctl(sock_fd, SIOCGIFFLAGS, ifr) < 0 )
        {
                close(sock_fd);
                return -1;
        }

        if ( (ifr->ifr_flags & IFF_UP) && strcmp(net_name.c_str(),ifr->ifr_name) == 0 )
        {
                strIP = inet_ntoa(sin->sin_addr);
                close(sock_fd);

                return 0;
        }
		
        ifr++;
    }
	
    close(sock_fd);

    return -1;
}


int main()
{
	string strIP;
	getIP("eth1", strIP);
	cout << strIP << endl;
	return 0;
}

       测试OK