将 char 指针传递给 C 中的函数

Question

我正在尝试通过此代码获取 Mac 地址：

void getMacAdress(unsigned char **address)
{
    int s;
    struct ifreq buffer;

    s = socket(PF_INET, SOCK_DGRAM, 0);
    memset(&buffer, 0x00, sizeof(buffer));
    strcpy(buffer.ifr_name, "eth0");
    ioctl(s, SIOCGIFHWADDR, &buffer);
    close(s);
    *address = (unsigned char *)buffer.ifr_hwaddr.sa_data;

    for (s = 0; s < 6; s++)
    {
        printf("%.2X ", *(*address + s));
    }

    printf("\n");
}

int main(int argc, char *argv[])
{
    unsigned char *address;

    getMacAdress(&address);
    int i;

    for (i = 0; i < 6; i++)
    {
        printf("%.2X ", *(address + i));
    }

    printf("\n");
    return 0;
}

我得到了正确的结果

08 00 27 0A 4E 98 
08 00 27 0A 4E 98

但是当我删除 getMacAddress() 函数中的 printf 片段代码时，它变成了：

void getMacAdress(unsigned char **address)
{
    int s;
    struct ifreq buffer;

    s = socket(PF_INET, SOCK_DGRAM, 0);
    memset(&buffer, 0x00, sizeof(buffer));
    strcpy(buffer.ifr_name, "eth0");
    ioctl(s, SIOCGIFHWADDR, &buffer);
    close(s);
    *address = (unsigned char *)buffer.ifr_hwaddr.sa_data;
    printf("\n");
}

我得到了错误的结果

08 00 00 00 00 00

你能解释一下这是为什么吗？我该如何解决这个问题？

Answer 1

问题在这里：

*address = (unsigned char *)buffer.ifr_hwaddr.sa_data;

buffer 是函数的局部变量。因此，您存储在 address 中的指针指向一个局部变量，该变量在函数 returns 时被销毁，留下一个悬空指针。

尝试从此指针读取会导致未定义的行为。您的输出可以根据您打印时内存是否已被重新用于其他用途来解释。

Answer 2

如果你想修复它...

//*address = (unsigned char *)buffer.ifr_hwaddr.sa_data;
*address = malloc (strlen (buffer.ifr_hwaddr.sa_data) + 1);
strcpy (*address, buffer.ifr_hwaddr.sa_data);

Answer 3

您不能在函数中指向堆栈 space 到 return。

相反，您可以 malloc 一个堆 space 来存储您想要的结果：

void getMacAdress(unsigned char **address)
{
    int s;
    struct ifreq buffer;

    s = socket(PF_INET, SOCK_DGRAM, 0);
    memset(&buffer, 0x00, sizeof(buffer));
    strcpy(buffer.ifr_name, "eth0");
    ioctl(s, SIOCGIFHWADDR, &buffer);
    close(s);
    *address = (unsigned char*) malloc(sizeof(buffer.ifr_hwaddr.sa_data));
    memcpy(*address, buffer.ifr_hwaddr.sa_data,sizeof(buffer.ifr_hwaddr.sa_data));

    //for (s = 0; s < 6; s++)
    //{
    //    printf("%.2X ", *(*address + s));
    //}

    //printf("\n");
}

顺便说一句，不要忘记在主函数中释放堆 space。

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