Invalid use of non-static member function - Class Member Function Calling Another Class Member Function

Question

I keep getting this message when I try to compile:

task.c++:54:102: error: invalid use of non-static member function
      this->createTask(&otherTask, this->otherMain, mainTask.regs.eflags, (uint32_t*)mainTask.regs.cr3);

Here is my task.c++ function:

#include "task.h"



static task_q *runningTask;
static task_q mainTask;
static task_q otherTask;
static PhyiscalMemoryManager *pmm_task;
static  Heap *heap_task;



extern void switch_task_a();

TaskManager::TaskManager(Heap *heap)
{
 heap_task = heap;
}
TaskManager::~TaskManager()
{}

Task::Task()
{}

Task::~Task()
{}
void TaskManager::otherMain()
{
        printf("Hello multitasking world!"); // Not implemented here...
        preempt();
}


void TaskManager::createTask(task_q* task, void(*task_main)(), uint32_t flags, uint32_t* pageDir)
{
        task->regs.ebx = 0;
        task->regs.ecx = 0;
        task->regs.edx = 0;
        task->regs.esi = 0;
        task->regs.edi = 0;
        task->regs.eflags = flags;
        task->regs.eip = (uint32_t) task_main;
        task->regs.cr3 = (uint32_t) pageDir;
        task->regs.esp = (uint32_t) (heap_task->k_malloc(TASK_STACK_SIZE)) + 0x1000; // Not implemented here
        task->next = 0;
}

void TaskManager::init_tasking()
{
     // Get EFLAGS and CR3
        __asm __volatile("movl %%cr3, %%eax; movl %%eax, %0;":"=m"(mainTask.regs.cr3)::"%eax");
        __asm __volatile("pushfl; movl (%%esp), %%eax; movl %%eax, %0; popfl;":"=m"(mainTask.regs.eflags)::"%eax");

        this->createTask(&otherTask, this->otherMain, mainTask.regs.eflags, (uint32_t*)mainTask.regs.cr3);
        mainTask.next = &otherTask;
        otherTask.next = &mainTask;

        runningTask = &mainTask;
}

void TaskManager::switchTask(Registers *old, Registers *new_)
{
    switch_task_a();
}

void TaskManager::preempt()
{
    task_q *last = runningTask;
    runningTask = runningTask->next;
    switchTask(&last->regs, &runningTask->regs);
}

And here is my task.h:

#ifndef _TASK_H_
#define _TASK_H_ 1

#include <stdarg.h>
#include <stdint.h>
#include "gdt.h"
#include "stdio.h"
#include "heap.h"
#include "pmm.h"

#define TASK_STACK_SIZE 0x2000

typedef struct {
        uint32_t eax, ebx, ecx, edx, esi, edi,
                 esp, ebp, eip, eflags, cr3;
} Registers;

typedef struct task_q {
            Registers regs;
            struct task_q *next;
} task_q;

class Task
{
    friend class TaskManager;
public:
    Task();
    ~Task();
private:
};

class TaskManager
{
public:
    TaskManager(Heap *heap);
    ~TaskManager();
    void init_tasking();
    static void createTask(task_q* task, void(*task_main)(), uint32_t flags, uint32_t* pageDir);
    void preempt();
private:
    void switchTask(Registers *old, Registers *new_);
    void otherMain();
};

#endif

Is this something wrong with calling this->OtherMain() inside this->createTask() ?

Answer 1

void(*task_main)() is expecting a pointer to a function. You attempt to feed it void otherMain(); which is a class method and has a hidden this parameter.

This stuff gets a little whacky. Here is a great write-up on some of the badness and how to avoid it.

You're going to have to do a bit of a rethink on how you do this. You can use a static void task_runner(void * userparam) method (no this ) and a user parameter that can be cast by the static method to provide a TaskManager on which you can invoke otherMain . You can turn void(*task_main)() into void(TaskManager::*task_main)() and you'll still have to provide a TaskManager to invoke the method pointer on.

It's a really nasty business, but can I interest you in exploring std::bind instead?

Edit

Task runner is something along the lines of this:

class taskRunner
{
public:
    virtual execute() = 0;
    static void task_runner(void * userparam)
    {
        taskRunner* task = (taskRunner*)userparam;
        task->execute();
    }
};

Downsides are everything you try to run MUST be a class that inherits taskRunner and implements execute and something must keep track of userparam . Non trivial effort is involved, but what the hell. Looks like you're writing yourself an OS. Non trivial effort all-around.

Probably better for you it to push the abstraction up one level and only accept free functions and static methods. Let the function being run sort out whether it's a class or not.

This means that otherMain cannot be a member of TaskManager unless it's static and this will require a re-write of TaskManager to allow cross-cutting behaviour like a task's ability to sleep, yield its timeslice, and other OS goodies to be called without knowing the internals of TaskManager .

otherMain could just be

void otherMain()
{
    printf("Hello multitasking world!"); 
    yield(); // a free function that calls into the system's TaskManager to preempt
}

Answer 2

The second parameter to createTask() is a:

 void(*task_main)()

This is a pointer to a function that returns void .

You are passing a:

 this->otherMain

otherMain is not a function that returns void . It is a class member that returns void . A class member is not exactly the same as a function.

The correct parameter type for createTask , to match a class method that returns void would be:

void  (TaskManager::*task_main)()

and the otherMain class method can be passed as simply:

&TaskManager::otherMain

This would solve the immediate problem of the compilation error for this function call.

However, you will likely now have a new, different problem. Your existing code attempts to stuff the function pointer into something that resembles a CPU register.

Well, you don't have a function pointer any more. You now have a class method pointer. Whether or not the existing code will work, as is, depends on your C++ implementation. Probably not, but that would be a different question.