Not able to call any function or return anything from other class into a callback method of PubSubClient.h

[ashrafulkhn]

Platform: Arduino -WiFi-rev2
Framewrok: Arduino
Compiler: AVR-GCC
Build System: PlatformIO

SoW:
Use MQTT protocol. Capture topic and payload to do some operation. The operation which I want to do here is calling a function from other class inside or outside the callback function to do some optiation based on the payload recieved from MQTT broker.

Problem:

• Not able to call any function inside a callback function of PubSubClient library.
• Next method I have tried is to assign data to a variable inside the callback method and return them. But didn't work either, as I am not able to capture the return values outside the fucntion.

Code snippet:

mqtt.cpp: callback method is mentioned as processData

void processData(char* topic, byte* payload, unsigned int length) {
  // This just marks the termination of the char array.
  payload[length] = '\0';
  
  // Dumping the information into the log file
  // *getLogFile() << "Topic received    : " << topic   << std::endl;
  // *getLogFile() << "Message received  : " << payload << std::endl;
  // *getLogFile() << "Length of message : " << length  << std::endl;
  
  Serial.println("Topic received    : " + String(topic));
  Serial.println("Message received  : " + String((char*)(payload))); 
  Serial.println("Length of message : " + String(length));

  // Based on the message received from the broker, we either switch
  // OFF or switch ON the led of the board
  if((strcmp((char *)payload, "START")) == 0) {
    digitalWrite(25, HIGH);
    shields->setMotorMode(START_MOTOR);

        //Debug message
    Serial.println("Comapred Output for START: " + String((strcmp((char *)payload, "START"))));
    Serial.println("Pin Number using getLed Method: ");
   // Serial.println(led->getLedPin());
    
    //Debug message ends

  } else if ((strcmp((char *)payload, "STOP")) == 0) {
    digitalWrite(25, LOW);
    shields->setMotorMode(STOP_MOTOR);
    
    //Debug message
    Serial.println("Comapred Output for STOP: " + String((strcmp((char *)payload, "STOP"))));
    Serial.println("Pin Number using getLed Method: ");
    //Serial.println(led->getLedPin());
    //Debug message ends
  }
  else{
    //digitalWrite(led->getLedPin(), LOW);
    Serial.println("Comapred Output for STOP: " + String((strcmp((char *)payload, "STOP"))));
  }
}

mqtt.cpp: (mqttSetup method)

void Mqtt::setupMqttClient() {
  // *getLogFile() << "Broker address : " << getMqttServerAddress() << std::endl;
  // *getLogFile() << "Port address   : " << getMqttPortAddress()   << std::endl;
  // Connecting the client with the broker 
/*
   *getLogFile() << "Setting up the boards's server configuration for \
connecting with the broker" << std::endl;
*/
  Serial.print("Broker address : ");
  Serial.println(String(getMqttServerAddress()));
  Serial.print("Port address   : ");
  Serial.println(getMqttPortAddress());
  Serial.println("Setting up the boards's server configuration for \
connecting with the broker");

  mqttClient.setServer(getMqttServerAddress(), getMqttPortAddress());

  // Callback for receiving messages from broker
/*
  *getLogFile() << "Setting up the boards's callback function information for \
each message received by the subscriber" << std::endl;
*/
  Serial.println("Setting up the boards's callback function information for \
each message received by the subscriber");

//   mqttClient.setCallback([this] (char* topic, byte* payload,
//                                 unsigned int length) {
//    this->processData (topic, payload, length);
//   });
//
  //void (*ptr)(char*, byte*, unsigned int, Mqtt*) = &processData;
  Serial.println(led->getLedPin());
  void (*ptr)(char*, byte*, unsigned int) = &processData;
  mqttClient.setCallback(ptr);

  // Converting the std::string into char array since setServer
  // accepts only char[]. For some reason, it does not accept
  // char* which we can get using std::string.c_str()
  if (mqttClient.connect(getMqttClientId())) {
    // *getLogFile() << "Connected the subscriber with the broker";
    Serial.println("Connected the subscriber with the broker");

    // Converting the std::string into char array since setServer
    // accepts only char[]. For some reason, it does not accept
    // char* which we can get using std::string.c_str()
    // Subscribe to topic
    mqttClient.subscribe(getMqttTopicName());
    // *getLogFile() << "Subscriber subscribed to the topic " << TOPIC_NAME << std::endl;
    Serial.println("Subscriber subscribed to the topic: " + String(getMqttTopicName()));
  } else {
    Serial.println("Connection failed while connecting the subscriber \
with the broker.");
/*
    *getLogFile() << "Connection failed while connecting the subscriber \
with the broker." << std::endl;
*/
  }
}

The program compiles successfully.

Problem/Error:

shields->setMotorMode(STOP_MOTOR);

This function is from the Motor drive library.

Serial.println(led->getLedPin());

In this method the getLedPin() should return the pinnumber 25 which I have set in the literals. But actually return 49.

So my program is never working as expected.

Help me to find a way to capture the topic and the payoad from the callbackfunction and use it with some other class.