How to Convert Address to Function Pointer to Call Method

Question

Hi,

I wanted to call Test1() Method Within WaitAndCallFunc() Function.

Code:

typedef void (*func)();

void StartTimer(void* pFuncAddr);
void WaitAndCallFunc(void* pPtr);

void WaitAndCallFunc(void* pPtr)
{
    int i = 0;
    int nWaitTime = 3;

    while(1)
    {
     Sleep(1000);
    // I want pPtr to call Test1 Function;
     if(i == nWaitTime)
      break;
    }

    _endthread();
}
void StartTimer(void* pFuncAddr)
{
    _beginthread(WaitAndCallFunc, 0, pFuncAddr);
}
void Test1();
int main(int argc, char* argv[])
{

    StartTimer(Test1);

    Sleep(5000);

    return 0;
}

void Test1()
{
    cout << "Testing Thread\n";
}

Answer 1

Cast and call:

typedef void (*func)();

void WaitAndCallFunc(void* pPtr)
{
    int i = 0;
    int nWaitTime = 3;

    while(1)
    {
        Sleep(1000);

        func f=(func)pPtr;   // cast to correct pointer to function type
        f();                 // and call!

        if(i == nWaitTime)
                break;
    }

    _endthread();
}

Answer 2

I'm not sure I understand what your question is exactly, but try this:

((func)pPtr)();

Answer 3

Strictly in C you're not supposed to convert between function pointers and other types of pointers. It's not guaranteed to work how you expect.

So a more pedantically correct version would look something like:

struct hook {
    void (*func)();
};

void StartTimer(void* pFuncAddr);
void WaitAndCallFunc(void* pPtr);

void WaitAndCallFunc(void* pPtr)
{
    struct hook *hook_ptr = pPtr;

    hook_ptr->func();

    _endthread();
}

void StartTimer(void* pFuncAddr)
{
    _beginthread(WaitAndCallFunc, 0, pFuncAddr);
}

void Test1();

int main(int argc, char* argv[])
{
    struct hook hook_test1 = { &Test1 };

    StartTimer(&hook_test1);

    Sleep(5000);

    return 0;
}

Note that in this it's the struct pointer that's cast to and from void *, not the function pointer itself. This also has the advantage that you can stuff some more values into the struct, if you need them to be passed down to Test1().

Answer 4

Actually, converting a function pointer to a void* or a void* to a function pointer is not directly allowed in current C or C++ - even though most compilers compile it.

There are two ways to convert back and forth (using C syntax) on compilers that don't compile the direct cast:

Method 1 (convert first to an integral intermediary)

((func) (intptr_t) pPtr)();  // call the void*

StartTimer( (void*) (intptr_t) &Test1); // pass function pointer to void*

Method 2 (use void**)

func f = 0;
*((void**)&f) = pPtr;
f();  

StartTimer( *((void**) &Test1)); // pass function pointer to void*

You can refer to the following thread for more of an explanation: http://stackoverflow.com/questions/1096341/function-pointers-casting-in-c/1096349#1096349