Passing C++ class member function as a paremeter to C function

Question

I am using a c dll in a c++ class. One of the member functions should call the following function that generates an interrupt and calls a service routine:

signed short __stdcall SetIrqConditions
(
    signed short DevNum,
    unsigned short bEnable,
    unsigned int dwIrqMask,
    void (__stdcall * funcExternalIsr)
    (
        signed short DevNum,
        unsigned int dwIrqStatus
    )
);

I am trying to call another member function of the same class as the last parameter of this function.(funcExternalIsr) When I tried to do this, the compiler complained that the function is not static. So I defined the callee function as a static function, but when I do that I cannot access other members of the class.

class myClass
{
public:
   int counter;
   void func1();
   static void __stdcall func2(signed short DevNum, unsigned int Status);
};

void myClass::func1()
{
    ...
    Result = SetIrqConditions(DevNum, TRUE, Mask, func2); --> no error here once func2 is static
}

void myClass::func2(signed short DevNum, unsigned int Status)
{
    counter++; --> invalid use of member 'counter' in static member function
}

I tried many different ways and did some research but I can't seem to get this working, any pointers in the right direction would be appreciated.

Unfortunately, because of the way this API is designed, this is not possible. It is not compatible with C++. C++ simply does not work this way. The only possible workaround is to use a static thread local variable to store the object pointer, and use a trampoline wrapper to fetch it out and invoke the class method. — Sam Varshavchik, Aug 21 '20 at 11:56
your error from above has nothing to do with C DLL. you are incrementing non-static counter within static func2 — StPiere, Aug 21 '20 at 11:56

score 0 · Answer 1 · answered Aug 21 '20 at 12:03

Passing C++ class member function as a paremeter to C function

There is no way to do this with a non-static function, because C doesn't have such thing as a pointer to member function. C has only pointers to functions, and those cannot point to non-static member functions.

What you can do instead is to write a separate free wrapper function (or a static member function) where you call that non-static member function. Example:

void wrapper_function(demo args)
{
    instance.member_function(args);
}

The remaining question is where to get the class instance to call within the wrapper. The options are in general:

Pass it as an argument to the wrapper. This is quite often an option with well designed C style callbacks that have a void* argument. That appears to not be the case here.
Use a variable with static storage. This is not ideal because global state is evil.
Create a local automatic variable within the wrapper. This is often not useful when the state of the instance needs to be accessed elsewhere.

What did you expect from an interrupt request service routine? This must be part of an OS API for interrupt handling. And it is a very common approach to only provide 1 or 2 integer parameters for the service routine - provided by underlying HAL. Calling it 'bad design' needs really deep insight. Other than that, this would make a good answer. — Red.Wave, Aug 21 '20 at 14:37

Passing C++ class member function as a paremeter to C function

1 Answers1