Question

I have the following code that gets the MAC address of an interface:

static struct ifreq get_mac( int socket_desc, char *if_name ) {                                                                                                                                                                         
      struct ifreq if_mac;                                                                                                                                                                                                                
      memset( &if_mac, 0, sizeof( struct ifreq ) );                                                                                                                                                                                       
      strncpy( if_mac.ifr_name, if_name, IFNAMSIZ - 1 );                                                                                                                                                                                  
      ...                                                                                                                                                                                                                                  
      return if_mac;                                                                                                                                                                                                                      
}

My C experience is limited to what I did in college. I roughly get pointers but I understand that returning large structs by value is a bad idea because you can run out of stack space (stackoverflow!). How can I change the above code to return a pointer to if_mac instead ? It’s just a bit confusing because there’s a struct and an ‘address of’ operator :S.

Answer 1

Give a pointer to the expected structure in parameter:

static int ifreq get_mac(int socket_desc, char *if_name, struct ifreq **if_mac)
{
      if (NULL == (*ifmac = malloc(sizeof (struct ifreq))) {
          return -1;                                        
      }                                                                                                                                                                                                                                                                                                                            
      memset(*if_mac, 0, sizeof(struct ifreq));                                                                                                                                                                                       
      strncpy((*if_mac)->ifr_name, if_name, IFNAMSIZ - 1);                                                                                                                                                                                  
      ...                                                                                                                                                                                                                                  
      return 0;                                                                                                                                                                                                                      
}

If the caller has already allocated the space for the structure, the function prototype becomes:

static int ifreq get_mac(int socket_desc, char *if_name, struct ifreq *if_mac)