Question

I have written a program for the Arnuino that sends a struct witin a union to a program on the PC. The struct has to integers but i dont’t get the right output. The program on the PC makes use of the boost library for the serial conection. And is build and compiled in 64bit (with vs2010).

The code works if i have a single integer variable within a union. But a struct witin a union doesn’t work. Only one integer gets data and that data is wrong.

I it maby a 64 bit(pc) and 32bit(Ardunio) problem? And can anyone help me with this. Thanks in advance.

The PC code snippet (serial settings are omitted):

union packed{
  struct test{
  unsigned int data;
  unsigned int data2;
} struc;

 unsigned char bytes[8];
}SerialPacked;

  SerialPacked.struc.data = 0;
  SerialPacked.struc.data2 = 0;

  cout << "Data before: " << SerialPacked.struc.data << endl;
  cout << "Data2 before: " << SerialPacked.struc.data2 << endl;

  read(port,buffer((unsigned char*)&SerialPacked.bytes[0], 1));
  read(port,buffer((unsigned char*)&SerialPacked.bytes[1], 1));
  read(port,buffer((unsigned char*)&SerialPacked.bytes[2], 1));
  read(port,buffer((unsigned char*)&SerialPacked.bytes[3], 1));
  read(port,buffer((unsigned char*)&SerialPacked.bytes[4], 1));
  read(port,buffer((unsigned char*)&SerialPacked.bytes[5], 1));
  read(port,buffer((unsigned char*)&SerialPacked.bytes[6], 1));
  read(port,buffer((unsigned char*)&SerialPacked.bytes[7], 1));

  cout << "Data after: " << SerialPacked.struc.data << endl;
  cout << "Data2 after: " << SerialPacked.struc.data2 << endl;

The Arduino code:

int ledPin = 13;


union packed{
    struct test{
      unsigned int data;
      unsigned int data2;
}struc;
    unsigned char bytes[8];
} 
SerialPacked;

void setup() {  
    pinMode(ledPin, OUTPUT); 
    Serial.begin(9600); 

   SerialPacked.struc.data = 0;
   SerialPacked.struc.data2 = 0;
};




void loop() {

while(1){
  digitalWrite(ledPin,HIGH);
  SerialPacked.struc.data = SerialPacked.struc.data + 1;
  SerialPacked.struc.data2 = SerialPacked.struc.data2 + 1;;

  for(int i=0;i <8; i++){ 
    Serial.write(SerialPacked.bytes[i]);
  };

    digitalWrite(ledPin,LOW);
    delay(1000);
 };

}

Answer 1

The issue is that int on Arduino is two bytes, but an int on your PC is probably four bytes. Depending on your compiler, there may be a switch that you can use to set the size for int, or you can just use a more explicit type. The idea with int is that it’s supposed to allow easy adaptation of code from one platform to another by adopting whatever the natural size is for the host platform. For that same reason, though, it’s not a good choice for transferring data between platforms.

To confirm that this is the problem, try reading the bytes out of SerialPacked instead of accessing struc. I’m sure you’ll find that all the data is there — it’s just the way you’re trying to read it that’s the problem.