Pass char pointer to function in C

debugcn Published at Dev

Tiktac

I am trying to get Mac address by this code:

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;
}

I got a correct result as

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

but when I delete printf snippet code in getMacAddress() function it becomes:

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");
}

I got the wrong result

08 00 00 00 00 00

Can you explain to me why that is and how I can solve this problem?

Will

you cannot point to a stack space to return in a function.

Instead, you can malloc a heap space to store the result you wanted:

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");
}

BTW, don't forget to free the heap space in your main function.

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