Question

I have code that looks like this:

extern "C" __declspec(dllexport)  myInterface(int id, void** pFunction)
{
   ...
}

I need to make the void** pFunction argument point to a function so that the caller can use this function via the pFunction pointer. This function gets called through a DLL, I don’t want to do it this way but for a lot of reasons I have no choice. I know that COM is made for this but I can not use it, the reasons come down to management.

At this point I have no idea how to do this, everything I have tried to do gives me cast problems. Do anyone have any idea how I can do this? I can post more if this is unclear.

Thanks.

Answer 1

As Jonathan Leffler and David Thornley mentioned, you aren’t guaranteed that a function pointer can be converted to void* and back. A portable workaround would be to package the function pointer into a struct and to pass a pointer to that.

(Be aware that void** itself might have its own issues. You can avoid this too.)

For example:

typedef int (*SomeFuncType)(int);

struct FuncWrapper
{
    SomeFuncType func;
    void* output;
};

...
FuncWrapper funcWrapper;
funcWrapper.func = ...;

myInterface(id, &funcWrapper);

and then myInterface could be implemented as:

void myInterface(int id, FuncWrapper* funcWrapper)
{
    funcWrapper->func(...);
    funcWrapper->output = ...;
}