Question

Im trying to add some bindings to ARP table in Linux, in C. Im opening a file with “a” (append, right?) and then trying to write some addresses, but I always get segmentation fault. I believe there is something wrong with ARP_table[i].IPaddr, ARP_table[i].MACaddr, ARP_table[i].ARPstatus

struct ARP_entry 
{
  char IPaddr[16];
  char MACaddr[18];
  char ARPstatus;
  int timec;
};

static struct ARP_entry ARP_table[ARP_table_vel];

void copyZIS()
{
  int i=0;
  const char filename[] = "/proc/net/arp";
  FILE *file = fopen(filename, "a");


  for (i; i< i+j; i++)
  {
     fprintf(file, "%c %c %c", ARP_table[i].IPaddr, ARP_table[i].MACaddr, ARP_table[i].ARPstatus);
  }

}

Answer 1

You are not checking the return value of fopen. I am pretty sure you are not allowed to write to that file.

FILE *file = fopen(filename, "a");
if (NULL == file) {
    perror("fopen");
    /* return / exit */
}

Here is /proc/net/arp on my system

[cnicutar@aiur ~]$ ls -l /proc/net/arp
-r--r--r-- 1 root root 0 Jun  8 16:29 /proc/net/arp