Question

I know this has been asked before but none of the cases I’ve seen here are like this one.
I am importing some API functions at runtime, the general declaration on those functions would be like:

// Masks for UnmapViewOfFile and MapViewOfFile
typedef BOOL (WINAPI *MyUnmapViewOfFile)(LPCVOID);
typedef LPVOID (WINAPI *MyMapViewOfFile)(HANDLE, DWORD, DWORD, DWORD, SIZE_T);

// Declarations
MyUnmapViewOfFile LoadedUnmapViewOfFile;
MyMapViewOfFile LoadedMapViewOfFile;

I then call a generic “load” function where it calles GetProcAddress to get the address of the exported function from the proper DLL. That address is returned on a void**. This void** is one of the parameters in the generic load, something like:

int GenericLoad(char* lib, void** Address, char* TheFunctionToLoad)

and I would call this function:

void *Address;
GenericLoad("kernel32.dll", &Address, "UnmapViewOfFile");
LoadedUnmapViewOfFile = (MyUnmapViewOfFile) Address;

Or something similar to this.
Now, of course the compiler complains about trying to cast a data void* to a function pointer. How do I do this then?

I’ve read countless sites and all kinds of nasty casts, so I’d appreciate it if you add code to the explanation.

Thanks
Jess

Answer 1

The correct code would be this one line:

GenericLoad("kernel32.dll", (void**)&LoadedUnmapViewOfFile, "UnmapViewOfFile");

What is done here basically is this: the address of the pointer variable (the one in which you want the address of the function to be put) is passed to GenericLoad – which is basically what it expects. void** was to denote “give me the address of your pointer”. All the type casting is a magic around it. C would not allow specifying “a pointer to any function pointer”, so the API author preferred void**.