CODEWITHSUNDEEP
carrayschar
Tiktac
Tiktac

Reputation: 1058

Pass char pointer to function in C

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?

Upvotes: 1

Views: 241

Answers (3)

Will
Will

Reputation: 842

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.

Upvotes: 1

jaybers
jaybers

Reputation: 2211

If you want to fix it...

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

Upvotes: 2

M.M
M.M

Reputation: 141574

Problem is here:

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

buffer is a local variable to your function. So the pointer you are storing in address points to a local variable which is destroyed when the function returns, leaving a dangling pointer.

Trying to read from this pointer causes undefined behaviour. Your output can be explained depending on whether the memory was re-used for something else already by the time you did the print or not.

Upvotes: 3

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