Thread: prevent compiler warning: 'X' may be used uninitialized in this function

Hello,

Thanking the people who answer in advance. =)
I fixed my problem by the line from char * mac_str = NULL; to char mac_str[1];

My questions:
1) With * mac_str = NULL I get an segmentation error. Why is that? Why can't the variable be overwritten by the C function ether_ntoa_r ??

2) my fix seems odd for some way: char mac_str[1] can only hold one character.
why can it store my whole MAC address? I am puzzled.

My program which these questions are related to. It's to retrieve the IP and MAC addres from my networkcard. I have to type sudo ./ipaddr eth0 to get it to work

Code:

// Headers needed
#include <net/if.h>
#include<netinet/if_ether.h>
#include <net/if_arp.h>
#include <netpacket/packet.h>
#include <net/ethernet.h>     /* the L2 protocols */
#include <asm/types.h>
#include <linux/sockios.h>
#include<stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <arpa/inet.h> //htons
#include <sys/ioctl.h>
#include <netinet/ether.h> //ether_ntoa_r

 
int main(int argc, char *argv[])
{
      struct ifreq ifr;
      int fd;
      memset(&ifr, 0, sizeof(ifr));
 
      
      if(argc == 1)
      {
          printf("U moet een netwerk interface naam als urgument meegeven. Bijv. eth0\n");
          exit(1);
      }
      
      // setup ifr for ioctl 
      strncpy (ifr.ifr_name, argv[1], sizeof(ifr.ifr_name) - 1);
      ifr.ifr_name[sizeof(ifr.ifr_name)-1] = '\0';
 
      // create socket
      fd = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
      if ( fd == -1) {
             fprintf(stderr," Kan socket niet aanmaken. errno code:  %s \n", strerror(errno));
             return -1;
      }
 
      // IP adres ophalen
      if (ioctl(fd, SIOCGIFADDR, &ifr) == -1)
      {
          perror("Fout ophalen IP adres");     
          close(fd);
          return (-1);
      }
      
      // MAC adres ophalen
      if (ioctl(fd, SIOCGIFADDR, &ifr) == -1)
      {
          perror("Fout ophalen MAC adres");    
          close(fd);
          return (-1);
      }
      
      // close socket if created locally
      close(fd);
 
      printf("IP adres van %s is %s\n",argv[1],inet_ntoa(((struct sockaddr_in    
      *)&ifr.ifr_addr)->sin_addr));
      

     char mac_str[1];    
     
     // mac adres omzetten naar colon format
     ether_ntoa_r((const struct ether_addr *)&(ifr.ifr_hwaddr.sa_data),mac_str);
     printf("En het Mac adres is %s\n",mac_str);
      exit(0);
}

Great! Thanks again Hauzer and Salem.