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);
}

Well there's always the option of reading the manual.
ether_ntoa_r(3) - Linux man page

The functions ether_ntoa_r() and ether_aton_r() are re-entrant threadsafe versions of ether_ntoa() and ether_aton() respectively, and do not use static buffers.

OK, so now read

ether_aton() converts the 48-bit Ethernet host address asc from the standard hex-digits-and-colons notation into binary data in network byte order and returns a pointer to it in a statically allocated buffer, which subsequent calls will overwrite. ether_aton() returns NULL if the address is invalid.

OK, so the output would appear to be HH:HH:HH:HH:HH:HH, which would appear to be 18 chars long (including the \0).

> char mac_str[1];
So how about making the length 18 instead of 1?

> 1) With * mac_str = NULL I get an segmentation error. Why is that?
Because you lied to the function.
The function expect a pointer to some memory it can write to.
You gave it NULL
Answer - BBBBBBOOOOOOOMMMMMMM!!!!!!!!!!!!

When you're programming in C, you need to think about how things are going to be used.
You can't just go around declaring variables with the right type, so the compiler stops complaining. No - that's not even half the problem.

If you declare something as a pointer it needs to point to valid memory or to NULL, else it's a dangling pointer and dereferencing it, that is getting the value at which it points, is unexpected, often leading to a crash. To "make" valid memory, that is to allocate it, you have to call malloc(), free()ing it after it's use. In this case you need to allocate a string of 18 characters, 17 for a valid MAC address and one more for the leading '\0' which completes a null terminated string.

Code:

#define MAC_STR_LEN 17

...

char *mac_str = malloc(sizeof(char) * (MAC_STR_LEN + 1));

... use mac_str ...

free(mac_str);

...

The compiler gives a sound warning telling you that you haven't assigned aything useful to mac_str. char mac_str[1] is an ugly hack which just might burn your computer up just like a dangling pointer if you store anything more than one character in it. The sole reason why it didn't is because you were lucky; unclaimed memory after it wasn't used by anything and it wasn't write-protected.

Great! Thanks again Hauzer and Salem.