ICMP checksum being incorrect

Question

I'm on x64 machine. Here is how I'm calculation the checksum for ICMP:

unsigned short in_checksum(unsigned short *ptr, int n_bytes)
{
  register long sum;
  u_short odd_byte;
  register u_short ret_checksum;

  while (n_bytes > 1)
  {
    sum += *ptr++;
    n_bytes -= 2;
  }

  if (n_bytes == 1)
  {
    odd_byte = 0;
    *((u_char *) & odd_byte) = * (u_char *) ptr;
    sum += odd_byte;
  }

  sum = (sum >> 16) + (sum & 0xffff);
  sum += (sum >> 16);
  ret_checksum = ~sum;

  return ret_checksum;
}

When I sniff the sent packets by wireshark, I always says the checksum is incorrect for each icmp packet. What's up with this?

Answer 1

You forgot to initialize

  register long sum;

to 0. Passing option -W to gcc would have told you.

...: In function 'in_checksum':
...: warning: 'sum' may be used uninitialized in this function