call dll in a c++ file - c++

Question

I created a DLL file (helloWorld.dll):
#define WIN32_LEAN_AND_MEAN
#include <windows.h>
#define DLL_FUNC extern "C" __declspec(dllexport)
DLL_FUNC int __stdcall Hello() {
MessageBox(HWND_DESKTOP, "Hello, world", "MEssage", MB_OK);
return 0;
}
After that I created a cpp where I would like to call (useDLL.cpp)
#include <windows.h>
#include <stdio.h>
int main () {
typedef void (*pfunc)();
HINSTANCE hdll = LoadLibrary("HelloWorld.dll");
pfunc Hello;
Hello = (pfunc)GetProcAddress(hdll, "hello");
Hello();
return 0;
}
How can I call the Hello() function?

Answer 1

The code in the question contains a number of errors:
LoadLibrary returns HMODULE and not HINSTANCE
The function pointer has the wrong return value and an incorrect calling convention.
Function names are case sensitive and you must account for name decoration.
You did no error checking at all. Your code probably fails on the call to GetProcAddress, returns NULL and then bombs when you try to call the function at NULL.
So you need something like this:
typedef int (__stdcall *HelloProc)();
....
HMODULE hdll = LoadLibrary("HelloWorld.dll");
if (hdll == NULL)
// handle error
HelloProc Hello = (HelloProc)GetProcAddress(hdll, "_Hello#0");
if (Hello == NULL)
// handle error
int retval = Hello();
The function name is decorated because you used __stdcall. If you had used __cdecl, or a .def file, then there would have been no decoration. I'm assuming MSVC decoration. It seems that decoration differs with your compiler, mingw, and the function is named "Hello#0".
Frankly it's much easier to do it with a .lib file instead of calling LoadLibrary and GetProcAddress. If you can, I'd switch to that way now.

Answer 2

You need to specifically search and find specific functions you are lookins for, check out this link:
Calling functions in a DLL from C++