Problem with xTaskCreate in C++...

[stefanoxjx]

Hi, I'm trying to write a class, but I've a compilation error.
Anyone can tell me why?
This is the interested code...

led.h

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#ifndef LED_H
#define LED_H

#include <stdint.h>
#include "freertos/FreeRTOS.h"
#include "freertos/task.h"
#include "driver/gpio.h"

class LED
{
   public:
   LED();

   private:
   void blinkLedTask(void *pvParameter);

   ...
};

#endif //LED_H

led.cpp

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#include "led.h"

LED::LED()
{
    ...

    xTaskCreate(&LED::blinkLedTask, "blinkLedTask", 2048, NULL, 5, NULL);
}

void LED::blinkLedTask(void *pvParameter)
{
     ...
}

During compilation, I've this error:

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 error: invalid use of non-static member function xTaskCreate(blinkLedTask, "blinkLedTask", 2048, NULL, 5, NULL);

If I try to declare blinkLedTask to static, I've this error:

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error: cannot declare member function 'static void LED::blinkLedTask(void*)' to have static linkage [-fpermissive]
 static void LED::blinkLedTask(void *pvParameter)

I'm not very good C++ developer, I'm trying to learn it.

Thanks.
Regards.

Stefano

[kolban]

In your code, the function blinkLedTask is not a static function. Putting this another way, the function can't be called directly outside of the context in which the "class" in which it exists is passed. Think of a C++ class as being a container that hides iplementation details. An instance of the class holds state and provides methods you can invoke to set/get the state and perform actions. For example, if I had a C++ class that models "Salary" then I might have a method called void increase(double percent). I can use it like:

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Salary mySalary(1000);
mySalary.increase(10.0);

to give myself a 10% increase.

Now imagine a function (X) which takes a function pointer (Y) as a parameter. When we call X, I can then invoke Y. For example:

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void X((void *Y)()) {
   Y();
}

but I can't call:

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X(Salary::increase)

Why not? ... because a C++ method outside of the class context in which it exists doesn't mean anything.

[WiFive]

You should put the static keyword in led.h not led.cpp

[jeanleflambeur]

Do this:

led.h

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#ifndef LED_H
#define LED_H

#include <stdint.h>
#include "freertos/FreeRTOS.h"
#include "freertos/task.h"
#include "driver/gpio.h"

class LED
{
   public:
   LED();

   private:
   static void staticBlinkLedTask(void *pvParameter);
   void blinkLedTask();

   ...
};

#endif //LED_H

led.cpp

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#include "led.h"

LED::LED()
{
    ...

    xTaskCreate(&LED::blinkLedTask, "blinkLedTask", 2048, this, 5, NULL); //note that I'm passing the instance pointer ('this') to the function
}

void LED::staticBlinkLedTask(void *pvParameter)
{
	LED* led = reintrepret_cast<LED*>(pvParameter); //obtain the instance pointer
	led->blinkLedTask(); //dispatch to the member function, now that we have an instance pointer
}

void LED::blinkLedTask() //this method has a hidden argument - the 'this' pointer
{
     ...
}

The idea is that in order to call a class member function you need an instance pointer.
xTaskCreate gives you the ability to do this using the pvParameter. So we're passing the 'this' ptr to it and we're binding it to a static function.
In the static function we interpret the pvParameter to the instance ptr (a LED*) and call the corret member function.

This indirection is needed when you want to pass a member function as a callback to a C system.

[stefanoxjx]

Hi, thanks at all for answers.
I'm was wrong because I was putting "static" in header and cpp files.
With "static" only to header file I can compile, but I've a "panic" messages.

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Guru Meditation Error: Core  0 panic'ed (IllegalInstruction)
. Exception was unhandled.
Core 0 register dump:
PC      : 0x400d234c  PS      : 0x00060530  A0      : 0x00000000  A1      : 0x3ffb6900  
0x400d234c: LED::blinkLedTask(void*) at /home/stefano/ESP32/ld/main/./led.cpp:88

A2      : 0x00000000  A3      : 0x00000000  A4      : 0x00000000  A5      : 0x00000000  
A6      : 0x00000000  A7      : 0x00000000  A8      : 0x800d234c  A9      : 0x3ffb68e0  
A10     : 0x000000c8  A11     : 0x00000001  A12     : 0x3ffb106c  A13     : 0x3ffb33c4  
A14     : 0x00000000  A15     : 0x00000000  SAR     : 0x00000000  EXCCAUSE: 0x00000000  
EXCVADDR: 0x00000000  LBEG    : 0x00000000  LEND    : 0x00000000  LCOUNT  : 0x00000000  

Backtrace: 0x400d234c:0x3ffb6900 0x7ffffffd:0x3ffb6920
0x400d234c: LED::blinkLedTask(void*) at /home/stefano/ESP32/ld/main/./led.cpp:88


Rebooting...
ets Jun  8 2016 00:22:57

If I try to use jeanleflambeur's solutions, I've compilation errors
I don't know reintrepret_cast.

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/home/stefano:~/ESP32/ld$ make
CXX build/main/led.o
/home/stefano/ESP32/ld/main/./led.cpp: In static member function 'static void LED::staticBlinkLedTask(void*)':
/home/stefano/ESP32/ld/main/./led.cpp:93:15: error: 'reintrepret_cast' was not declared in this scope
    LED* led = reintrepret_cast<LED*>(pvParameter); //obtain the instance pointer
               ^
/home/stefano/ESP32/ld/main/./led.cpp:93:35: error: expected primary-expression before '*' token
    LED* led = reintrepret_cast<LED*>(pvParameter); //obtain the instance pointer
                                   ^
/home/stefano/ESP32/ld/main/./led.cpp:93:36: error: expected primary-expression before '>' token
    LED* led = reintrepret_cast<LED*>(pvParameter); //obtain the instance pointer
                                  ^
[code]

Stefano

[WiFive]

Probably you didn't call vtaskdelete at the end of your task

[stefanoxjx]

No, I don't call vTaskDelete, because I don't need to delete this task.

Stefano

[kolban]

In your post, we see that the exception was caught at led.cpp line 88. However, I couldn't tell what source statement line 88 was at. It might be beneficial to post a copy of the source to pastebin to allow us to see the offending statement.

[cpersoz]

@stefanoxjx, did you end up with a solution to have a running wrapper for xTaskCreate inside a C++ class?
I'm getting the same error as you and I didn't find a way to use xTaskCreate inside a C++ class yet.

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libs/Channel/src/Channel.cpp:279:25: error: 'reintrepret_cast' was not declared in this scope
libs/Channel/src/Channel.cpp:279:49: error: expected primary-expression before '*' token
libs/Channel/src/Channel.cpp:279:50: error: expected primary-expression before '>' token

Thanks

[markkuk]

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libs/Channel/src/Channel.cpp:279:25: error: 'reintrepret_cast' was not declared in this scope

That's simply a typing error, it's reinterpret_cast, not reintrepret_cast