XBee, external libraries and passing structures as arguments

Question

I have very weird problem with a library I am creating. The library will be used to communicate between Arduino modules using XBee Series 1 modules. Library is very simple wrapper library around Arduino XBee library.

I have one function that reads received packet and sends it back. At the moment it is implemented as a simple "echo" service - the function just displays the data received and sends it back to per-defined address.

At the moment I have three versions of this function, out of which one is not working.

1. A function taking no arguments: void processPacket()
2. A function taking structure as a value as an argument: void processPacket(valuesStruct valuesStructData) - THIS VERSION OF THE FUNCTION IS NOT WORKING!
3. A function taking pointer to the structure as an argument: void processPacket(valuesStruct* valuesStructData)

At this moment I noticed strange behavior in the 2nd version of the function. I do nothing with the passed argument - the content of all three functions is the same. In 2nd case the function reads wrong values from received XBee packet. In the 1st and 3rd case the function performs correctly.

Code:

ExampleLib.h

#ifndef ExampleLib_h
#define ExampleLib_h

#include "Arduino.h"
#include <XBee.h>

#define ADDRESS_BROADCAST 0xffff
#define ADDRESS_PC 0x3333

typedef struct
  {
    int valA;
    int valB;
    int valC;
  } valuesStruct;

class ExampleLib
{
public:
    ExampleLib();
    void setSerial(Stream &serial);
    boolean tryReceivePacket();
    void processPacket();
    // THIS FUNCTION IS NOT WORKING!
    void processPacket(valuesStruct valuesStructData);
    void processPacket(valuesStruct* valuesStructData);
private:
    XBee xbee;
    Rx16Response rx16;
};

#endif

ExampleLib.cpp

The value read in line byte* packetData = rx16.getData(); is wrong when we trigger processPacket(valuesStruct valuesStructData) function. In other cases the behavior is correct.

#include "Arduino.h"
#include <XBee.h>
#include "ExampleLib.h"

ExampleLib::ExampleLib()
{
    xbee = XBee();
    rx16 = Rx16Response();
}

void ExampleLib::setSerial(Stream &serial)
{
    xbee.setSerial(serial);
}

boolean ExampleLib::tryReceivePacket()
{
    xbee.readPacket();

  if (xbee.getResponse().isAvailable()) {
    // got something

    if (xbee.getResponse().getApiId() == RX_16_RESPONSE) {
      // got a rx packet

      xbee.getResponse().getRx16Response(rx16);
      return true;
    } 
    else {
      return false;
    }
  } 
  else if (xbee.getResponse().isError()) {
    //nss.print("Error reading packet.  Error code: ");  
    //nss.println(xbee.getResponse().getErrorCode());
    // or flash error led
    return false;
  } 

  return false;
}
void ExampleLib::processPacket()
{
  byte* packetData = rx16.getData();
  byte dataLength = rx16.getDataLength();
  Serial.print("START L:");
  Serial.println(dataLength);
  for (int i = 0; i < dataLength; i++) { 
    Serial.print(packetData[i]);
    Serial.print(" - "); 
  }
  Serial.println("END");

  //16-bit addressing: Enter address of remote XBee, typically the coordinator
  Tx16Request tx = Tx16Request(ADDRESS_PC, packetData, sizeof(packetData));
  xbee.send(tx);
}

void ExampleLib::processPacket(valuesStruct valuesStructData)
{
  processPacket();
}

void ExampleLib::processPacket(valuesStruct* valuesStructData)
{
  processPacket();
}

Arduino sketch

#include <XBee.h>
#include <ExampleLib.h>

ExampleLib exampleLibObj = ExampleLib();

void setup()
{
  Serial.begin(9600);
  exampleLibObj.setSerial(Serial);
}

void loop()
{

  boolean isPacketReceived = exampleLibObj.tryReceivePacket();

  if (isPacketReceived) {
    // leave only one section, the rest should be commented

    //Section 1: working
    exampleLibObj.processPacket();

    //Section 2: not working
//    valuesStruct test;
//    test.valA = 0;
//    test.valB = 0;
//    test.valC = 0;
//    exampleLibObj.processPacket(test);

    //Section 3: working
//  valuesStruct* test;
//  test->valA = 0;
//  test->valB = 0;
//  test->valC = 0;
//  exampleLibObj.processPacket(test);
  }
}

I am really puzzled why in this one case function is performing differently. Looking forward to any suggestions to that issue.

Thanks!

Answer 1

If the second isn't working, it may be best to use the other 2. Especially on the Arduino with limited RAM, you will not have enough if you pass a full struct to a function. Passing a pointer is less memory-intensive, and if it works, use it!