C++ – Dynamically load a function from a DLL

cdllwinapi

I'm having a little look at .dll files, I understand their usage and I'm trying to understand how to use them.

I have created a .dll file that contains a function that returns an integer named funci()

using this code, I (think) I've imported the .dll file into the project(there's no complaints):

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

int main() {
  HINSTANCE hGetProcIDDLL = LoadLibrary("C:\\Documents and Settings\\User\\Desktop  \\fgfdg\\dgdg\\test.dll");

  if (hGetProcIDDLL == NULL) {
    std::cout << "cannot locate the .dll file" << std::endl;
  } else {
    std::cout << "it has been called" << std::endl;
    return -1;
  }

  int a = funci();

  return a;
}

# funci function 

int funci() {
  return 40;
}

However when I try to compile this .cpp file that I think has imported the .dll I have the following error: 

C:\Documents and Settings\User\Desktop\fgfdg\onemore.cpp||In function 'int main()':|
C:\Documents and Settings\User\Desktop\fgfdg\onemore.cpp|16|error: 'funci' was not     declared in this scope|
||=== Build finished: 1 errors, 0 warnings ===|

I know a .dll is different from a header file so I know I can't import a function like this but it's the best I could come up with to show that I've tried.

My question is, how can I use the hGetProcIDDLL pointer to access the function within the .dll.

I hope this question makes sense and I'm not barking up some wrong tree yet again.

Best Answer

LoadLibrary does not do what you think it does. It loads the DLL into the memory of the current process, but it does not magically import functions defined in it! This wouldn't be possible, as function calls are resolved by the linker at compile time while LoadLibrary is called at runtime (remember that C++ is a statically typed language).

You need a separate WinAPI function to get the address of dynamically loaded functions: GetProcAddress.

Example

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

/* Define a function pointer for our imported
 * function.
 * This reads as "introduce the new type f_funci as the type: 
 *                pointer to a function returning an int and 
 *                taking no arguments.
 *
 * Make sure to use matching calling convention (__cdecl, __stdcall, ...)
 * with the exported function. __stdcall is the convention used by the WinAPI
 */
typedef int (__stdcall *f_funci)();

int main()
{
  HINSTANCE hGetProcIDDLL = LoadLibrary("C:\\Documents and Settings\\User\\Desktop\\test.dll");

  if (!hGetProcIDDLL) {
    std::cout << "could not load the dynamic library" << std::endl;
    return EXIT_FAILURE;
  }

  // resolve function address here
  f_funci funci = (f_funci)GetProcAddress(hGetProcIDDLL, "funci");
  if (!funci) {
    std::cout << "could not locate the function" << std::endl;
    return EXIT_FAILURE;
  }

  std::cout << "funci() returned " << funci() << std::endl;

  return EXIT_SUCCESS;
}

Also, you should export your function from the DLL correctly. This can be done like this:

int __declspec(dllexport) __stdcall funci() {
   // ...
}

As Lundin notes, it's good practice to free the handle to the library if you don't need them it longer. This will cause it to get unloaded if no other process still holds a handle to the same DLL.

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