Question

我有这个代码。屏幕显示变量值的两倍并且是不同的。为什么？难道我做错了什么？ Linux用gcc编译。我不明白这个错误。该值不应在函数中更改。

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

int main()
{
    int fd;
    struct ifreq ifr;
    char *iface = "eth0";
    unsigned char *mac, *ip, *mask, *broad;

    fd = socket(AF_INET, SOCK_DGRAM, 0);

    ifr.ifr_addr.sa_family = AF_INET;
    strncpy(ifr.ifr_name , iface , IFNAMSIZ-1);

    //get the ip address
    ioctl(fd, SIOCGIFADDR, &ifr);
    ip= (unsigned char *)inet_ntoa(((struct sockaddr_in *)&ifr.ifr_addr )->sin_addr);
    //display ip
    printf("IP address of %s - %s\n" , iface , ip );
    //get the netmask ip
        ioctl(fd, SIOCGIFNETMASK, &ifr);
    mask = (unsigned char *)inet_ntoa(((struct sockaddr_in *)&ifr.ifr_addr )->sin_addr);
    //display netmask
    printf("Netmask of %s - %s\n" , iface , mask);
        //get the MAC address
    ioctl(fd, SIOCGIFHWADDR, &ifr);
    mac = (unsigned char *)ifr.ifr_hwaddr.sa_data;
    //display mac address
    printf("Mac of %s - %.2x:%.2x:%.2x:%.2x:%.2x:%.2x\n" , iface, mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);
    //get the BroadCast ip
        ioctl(fd, SIOCGIFBRDADDR, &ifr);
    broad = (unsigned char *)inet_ntoa(((struct sockaddr_in *)&ifr.ifr_addr )->sin_addr);
    //display BroadCast
        printf("Broadcast of %s - %s\n" , iface , broad);

    close(fd);

    //display ip
    printf("IP address of %s - %s\n" , iface , ip );
    //display netmask
    printf("Netmask of %s - %s\n" , iface , mask);
    //display mac address
    printf("Mac of %s - %.2x:%.2x:%.2x:%.2x:%.2x:%.2x\n" ,iface, mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);
    //display BroadCast
        printf("Broadcast of %s - %s\n" , iface , broad);

    return 0;
}

Answer 1

每次调用ioctl都会覆盖ifr中的内容。

ip只是指向ifr结构的指针。当您打印ip时，会打印从上次调用ioctl获得的地址。

您可以通过在调用中使用不同的struct ifreq或将要保留的数据复制到另一个内存区域（例如使用strdup）来解决问题。

为什么它会改变变量的值？

1 个答案: