How to call MessageBox with GetProcAddress function?

Question

I want to call MessageBox() function in such way:

1). load needed library
2). get the function address
3). call it

So, for such aim as I understand, I should define new type with all types of arguments in MessageBox function.

It returnes INT and accepts: HWND, LPCSTR, LPCSTR, UNIT.

So I registred new type:

typedef int(__stdcall *msgbox)(HWND, LPCSTR, LPCSTR, UINT);

I have problems with calling such function. Does such way work for all functions or only for exported?
How can I call MessageBox exactly in such way?

Full code:

#include <iostream>
#include <windows.h>

using namespace std;

typedef int(__stdcall *msgbox)(HWND, LPCSTR, LPCSTR, UINT);

int main(void)
{
    HINSTANCE__ *hModule = LoadLibrary(L"\\Windows\\System32\\User32.dll");
    msgbox *me = 0;

    if(hModule != 0)
    {
        me = (msgbox*)GetProcAddress(hModule, "MessageBox");
    }

    return 0;
}

Answer 1

Why are you declaring everything as a pointer?

LoadLibrary returns an HMODULE, not an HINSTANCE__ * (it will work with the latter but it's always better to adhere to the documentation).

Similarly, msgbox is typedef'd to a function pointer type, so me is a msgbox, not a msgbox *.

The reason why GetProcAddress fails is because user32.dll exports 2 functions, MessageBoxA and MessageBoxW. When you simply call MessageBox in your code, macros defined in Windows.h replace it with one of the 2 actual function names depending on whether you're compiling for UNICODE or not. But when you're trying to directly access the exported function as you are doing, you need to explicitly specify which one you're trying to get a pointer to.

#include <iostream>
#include <windows.h>

typedef int(__stdcall *msgbox)(HWND, LPCSTR, LPCSTR, UINT);

int main(void)
{
  HMODULE hModule = ::LoadLibrary(L"User32.dll");
  msgbox me = NULL;

  if( hModule != NULL ) {
    me = reinterpret_cast<msgbox>( ::GetProcAddress(hModule, "MessageBoxA") );
  }

  if( me != NULL ) {
    (*me)( NULL, "I'm a MessageBox", "Hello", MB_OK );
  }

  if( hModule != NULL ) {
    ::FreeLibrary( hModule );
  }
}