FreeRTOS creating a task within a C++ class

Question

I am writing a C++ code to handle my temperature controller. I have decided to learn more object oriented programming and learn more about classes therefore I have chose C++ this time ( I usually code in C ). I have created a simple Controller class where I have my public and private functions. In my controller.h file:

#include "stdint.h"
#include "stdio.h"
#include "params.h"
#include "freertos/FreeRTOS.h"
#include "freertos/task.h"

#ifdef __cplusplus
extern "C" {
#endif

class Controller
{
    private://only accesible for class
        int test1;
   

    public: 
    //accesible outside class
    
        Controller();                                                   // INIT OBJECT
        void begin();                                                   // DECLARE DEFAULT 
        e_thermostat_state Get_state();    
        void normal_operation_task(void *argument);    // FREERTOS TASK FUNCTION PROTOTYPE     
};


#ifdef __cplusplus
}
#endif

#endif

And in my controller.c:

#include "Controller.h"


Controller controller_obj;
uint8_t app_temp = 10;

Controller::Controller(){
    printf("Controller object created \n");
}

void Controller::begin(){
    printf("Initialise the object with the default values \n");
    xTaskCreate(this->normal_operation_task,"controller normal operation task",10000,NULL,1,NULL);
    
        
}

void Controller::normal_operation_task(void *argument){
    for(;;)
    {   
        printf("normal_operation_task\n");
        vTaskDelay(1000/portTICK_RATE_MS);
    }
}

As you can see when I call begin method, I want to simply start the task. The reason why I want the freertos task to be a part of my controller class is because inside that task I will need access to variables and functions of my controller class.

The error that I am getting:

../components/Controller/Controller.cpp: In member function 'void Controller::begin()':
../components/Controller/Controller.cpp:20:98: error: invalid use of non-static member function 'void Controller::normal_operation_task(void*)'
     xTaskCreate(this->normal_operation_task,"thermostat normal operation task",10000,NULL,1,NULL); // receiving commands from main uart

Could someone give me some advice regarding this? I think I have 2 options:

1. In my class, declare all variables and methods as public so I can access them even if the freertos task is not part of the class.

2. Create task as part of Controller class so It can use private methods and variables.

UPDATE

I have managed to do it when declared the static function normal_operation_task as been suggested for me. in my controller.h I have declared:

static void Thermostat::normal_operation_task2(Thermostat* thermostat_obj){
    for(;;)
    {   
        printf("normal_operation_task\n");
        vTaskDelay(1000/portTICK_RATE_MS);
    }
}

I am now trying to understand the following:

1. Why this function must be declared the static
2. Why do I need to pass the pointer to my controller object? Since it is declared as class method, it automatically has access to class function and variables

ISSUE

What is the issue with the following and why is this wrong?

void Controller::begin(){

    printf("Initialise the object with the default values \n");
    xTaskCreate(this->normal_operation_task3,"thermostat normal operation task",10000,&controller_obj,1,NULL); //receiving commands from main uart
}

 void Controller::normal_operation_task3(void* parameters)
 {
            Controller controller_obj = *((Controller*)parameters);
            for(;;)
            {   
                printf("normal_operation_task\n");
                vTaskDelay(1000/portTICK_RATE_MS);
            }
}

Answer 1

In regard to "Why do I need to pass the pointer to my controller object? Since it is declared as class method, it automatically has access to class function and variables", static methods only have access to static members of a class. In other words, static methods and members belong to the class, not to instances of the class. You need to pass your static method a pointer to your instance so access the instance and its members.

As to why it must be a static method, I wonder that myself.

Answer 2

The FreeRTOS headers have the needed code (the conditional extern “C”) to make them interface with C++ code - but any callback that FreeRTOS calls by name will also need to be declared as extern “C”. So calling xTaskCreate from C++ works, but FreeRTOS cannot call your class method as it is not declared with extern "C". You declared the class as extern "C", but not the methods (which you cannot declare as extern "C"). The compiler "detects" this mismatch via name mangling (google this for further details).

You could make a bridging function outside the class (e.g. as part of a C-struct) which holds the pointer to your controller class and which calls your member-methods with the help of the pointer whenever FreeRTOS calls this bridging function. You could also use the argument passed to xTaskCreate to hold the instance pointer.