Question

I send a struct with one field (unsigned int) to a network emulator and it outputs the unsigned int but it is incorrect.

If I do NOT re-run the network emulator, if I resend the struct with the same number, it will be the same random integer. If I send that number +1, I get the random integer -1.

If i do re-run the network emulator and resend the struct with the same number, it shows a different random integer.

This is the stuct:

struct pkt_INIT {
  unsigned int router_id;
};

This is the code to send that struct:

  int sock;
  struct sockaddr_in server_addr;
  struct hostent *host;
  char send_data[1024];

  host= (struct hostent *) gethostbyname(emulator_host);

  if ((sock = socket(AF_INET, SOCK_DGRAM, 0)) == -1)
  {
    perror("socket");
    exit(1);
  }

  server_addr.sin_family = AF_INET;
  server_addr.sin_port = htons(emulator_port);
  server_addr.sin_addr = *((struct in_addr *)host->h_addr);
  bzero(&(server_addr.sin_zero),8);

  struct pkt_INIT init;
  init.router_id = router_id;
  strcpy(send_data,(char *)&init);

  sendto(sock, send_data, strlen(send_data), 0,
         (struct sockaddr *)&server_addr, sizeof(struct sockaddr));

I don’t have any of the code for the network emulator.

Answer 1

Don’t use strcpy() to put a binary struct into a binary array. Likewise, do not use strlen() to get the length of a struct. Both functions are designed for strings, and will copy/read bytes until they encounter a null byte. Use memcpy() and sizeof() instead:

memcpy(send_data, &init, sizeof(init)); 
sendto(sock, send_data, sizeof(init), 0, (struct sockaddr *)&server_addr, sizeof(struct sockaddr)); 

In which case, just get rid of the array and pass the struct directly to sendto() instead:

sendto(sock, (char*)&init, sizeof(init), 0, (struct sockaddr *)&server_addr, sizeof(struct sockaddr)); 

Now, with that said, there is an issue with the router_id. Most network protocols require network byte ordering, so you may or may not need to use htonl(), depending on what the emulator is actually expecting:

init.router_id = htonl(router_id);