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#include <stdio.h>
#include <string.h>
#include <net/if.h>
#include <sys/ioctl.h>

#define PUB_TOPIC   "adbiot/changestatus/deviceupdate/matteo/notebook/12345678901234567890123456789012"

char *deviceupdate = PUB_TOPIC;

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

    int s = socket(PF_INET, SOCK_DGRAM, 0);
    struct ifreq ifr = { .ifr_name = "eth0" };
    ioctl(s, SIOCGIFHWADDR, &ifr);

    sprintf(strrchr(deviceupdate, '/') + 1, "%02x%02x%02x%02x%02x%02x",
        (unsigned char)ifr.ifr_hwaddr.sa_data[0] & 0xff,
        (unsigned char)ifr.ifr_hwaddr.sa_data[1] & 0xff,
        (unsigned char)ifr.ifr_hwaddr.sa_data[2] & 0xff,
        (unsigned char)ifr.ifr_hwaddr.sa_data[3] & 0xff,
        (unsigned char)ifr.ifr_hwaddr.sa_data[4] & 0xff,
        (unsigned char)ifr.ifr_hwaddr.sa_data[5] & 0xff
    );

    printf("topic: %s\n", deviceupdate);
}