CODEWITHSUNDEEP
c++linux
Mendes
Mendes

Reputation: 18451

C++ Linux Getting MAC Address returning different values on same computer

I´d wrapped up the following code to get the computer MAC address (Linux Ubuntu) and to print it using a custom hexadecimal string:

#include <iostream>
#include <string>
#include <cstring>
#include <sstream>
#include <iomanip>
#include <stdexcept>

#include <unistd.h>
#include <net/if.h>
#include <sys/socket.h>
#include <sys/ioctl.h>


std::string convertToHex(std::string str)
{
    std::stringstream outText;

    for(unsigned int i = 0; i < str.size(); i++ )
    {
        outText << std::hex << std::setw(2) << std::setfill('0') << (0xFF & static_cast<char>(str[i]));

        if (i != (str.size() - 1))
            outText << ":";
    }

    return outText.str();
}

std::string getMacId()
{
    struct ifreq ifr;
    struct ifreq *IFR;
    struct ifconf ifc;
    char buf[1024];
    int s, i;
    std::string macAddr("");

    s = socket(AF_INET, SOCK_DGRAM, 0);

    if (s==-1)
    {
        return "";
    }

    ifc.ifc_len = sizeof(buf);
    ifc.ifc_buf = buf;
    ioctl(s, SIOCGIFCONF, &ifc);

    IFR = ifc.ifc_req;
    int ok = 0;
    for (i = ifc.ifc_len / sizeof(struct ifreq); --i >= 0; IFR++)
    {

        strcpy(ifr.ifr_name, IFR->ifr_name);
        if (ioctl(s, SIOCGIFFLAGS, &ifr) == 0)
        {
            if (! (ifr.ifr_flags & IFF_LOOPBACK))
            {
                if (ioctl(s, SIOCGIFHWADDR, &ifr) == 0)
                {
                    ok = 1;
                    break;
                }
            }
        }
    }

    close(s);

    std::stringstream data;

    for (auto &c : ifr.ifr_addr.sa_data)
        data << c;

    std::string ret = data.str();

    std::cout << ret << std::endl;

    return ret;
}


void showMacId()
{
    std::string mac = getMacId();

    std::string hexmac = convertToHex(mac);

    std::cout << hexmac << std::endl;
}

int main()
{
    std::cout << "Pass1: " << std::endl;
    showMacId();

    std::cout << "Pass2: " << std::endl;
    showMacId();

    std::cout << "Pass3: " << std::endl;
    showMacId();

    std::cout << "Pass4: " << std::endl;
    showMacId();
}

The problem is that I´m getting different outputs on every read. This is the result running the program:

Pass1: 
.)� W�� 
2e:02:00:00:04:29:80:20:57:82:42:08:80:20
Pass2: 
p�ЕĿ�wq���Ŀ
70:b7:d0:95:c4:bf:aa:77:71:b7:80:95:c4:bf
Pass3: 
p�ЕĿ�wq���Ŀ
70:b7:d0:95:c4:bf:aa:77:71:b7:80:95:c4:bf
Pass4: 
p�ЕĿ�wq���Ŀ
70:b7:d0:95:c4:bf:aa:77:71:b7:80:95:c4:bf

And if I run the program again I get different results:

Pass1: 
.)� W�� 
2e:02:00:00:04:29:80:20:57:82:42:08:80:20
Pass2: 
q��
���wr�P
��
71:b7:a0:0a:93:bf:aa:77:72:b7:50:0a:93:bf
Pass3: 
q��
���wr�P
��
71:b7:a0:0a:93:bf:aa:77:72:b7:50:0a:93:bf
Pass4: 
q��
���wr�P
��
71:b7:a0:0a:93:bf:aa:77:72:b7:50:0a:93:bf

So, what am I doing wrong here?

Upvotes: 0

Views: 714

Answers (2)

Marcus M&#252;ller
Marcus M&#252;ller

Reputation: 36352

sizeof(buf) looks dangerous. Use 1024 instead; I'm not absolutely certain you don't ask for sizeof(char*) this way.

You must check that 

    ioctl(s, SIOCGIFCONF, &ifc);

doesn't return -1!

All in all, this seems a very old-school method of getting the MAC address.

Why don't you just read /sys/class/net/<devicename>/address? I could pretty much rely on sysfs being there on any linux system I've encountered, and it's the failsafe, clear, portable thing to do. To find your devices, just list the /sys/calls/net/ directory.

EDIT I was asked to give an example; I really don't know how to do this, it seems so straightforward:

#include <iostream>
#include <fstream>


int main (int argc, char** argv)
{

    std::ifstream inp ("/sys/class/net/enp0s25/address", std::ifstream::in);
    for(int bytecounter = 0; bytecounter < 6; bytecounter++)
    {   
        unsigned int byte;
        inp >>std::hex >> byte;
        inp.get(); //drop the :
        std::cout <<byte;
        if(bytecounter < 5)
            std::cout << ":";
    }   

    inp.close();

    return 0;
}

Upvotes: 2

m.s.
m.s.

Reputation: 16334

I am not sure why the "MAC" addresses you are printing are that long (usually they are 48 bit long), but I guess your problem is that you do not initialize your structs and arrays with zeros.

This should help (given that you use C++11 anyway):

struct ifreq ifr = {0};
struct ifconf ifc = {0};
char buf[1024] = {0};

Upvotes: 0

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