Variable length packets?

From: Lucas (a.k.a T-Bird or bsdfan3) (tbird-contact_at_cox.net)
Date: 08/31/04

  • Next message: Subhro: "Re: newest kdelibs fails to build" 
    To: <freebsd-questions@freebsd.org>
Date: Mon, 30 Aug 2004 20:35:07 -0500

    I am trying to implement a custom protocol that sends and receives
    variable-length packets on top of TCP/IPv4. The problem is that the length
    field of the packet is silently being mangled first becoming 0 and then
    getting turned into a very large number (about 2-3 billion). The length
    field is a u_int32_t and I am using the byteorder routines. Source code
    snippets follow:

    --decl of struct packet_t--
    struct packet_t
    {
      u_int16_t num;
      u_int32_t len;
      char data[0];
    };
    --receive code--
          if (read (s, auth_header, sizeof (struct packet_t)))
            {
              perror ("srvrmond read: could not read from socket");
              exit (2);
            }
          auth_header -> num = ntohs (auth_header -> num);

          auth_header -> len = ntohl (auth_header -> len);
    --send code--
      struct packet_t *auth_data2 = malloc (sizeof (struct packet_t) +
    AUTHCOOKIE_S\
    IZE + sizeof (time_t));
      auth_data2 -> num = htons (100);
      auth_data2 -> len = htonl (AUTHCOOKIE_SIZE + sizeof (time_t));
      fprintf (stderr, "Packet payload length: %lu", ntohl (auth_data2 -> len));
      long temp = htonl ((long)time (0) & 0xffffff00);
      memcpy (auth_data2 -> data, cookie, AUTHCOOKIE_SIZE);
      memcpy (auth_data2 -> data + AUTHCOOKIE_SIZE, &temp, sizeof (time_t));
      if (write (s, auth_data2, AUTHCOOKIE_SIZE + sizeof (struct packet_t) +
    sizeof\
    (time_t)) == -1)
        {
          perror ("srvrmon write");
          free (auth_data2);
          exit (1);
        }

    P.S. Please CC me, I am not subscribed to the list

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