Calling functions in a DLL from C++

Question

I have a solution in VS 2008 with 2 projects in it. One is a DLL written in C++ and the other is a simple C++ console application created from a blank project. I would like know how to call the functions in the DLL from the application.

Assume I am starting with a blank C++ project and that I want to call a function called int IsolatedFunction(int someParam)

How do I call it?

Answer 1

There are many ways to do this but I think one of the easiest options is to link the application to the DLL at link time and then use a definition file to define the symbols to be exported from the DLL.

CAVEAT: The definition file approach works bests for undecorated symbol names. If you want to export decorated symbols then it is probably better to NOT USE the definition file approach.

Here is an simple example on how this is done.

Step 1: Define the function in the export.h file.

int WINAPI IsolatedFunction(const char *title, const char *test); 

Step 2: Define the function in the export.cpp file.

#include <windows.h>  int WINAPI IsolatedFunction(const char *title, const char *test) {     MessageBox(0, title, test, MB_OK);     return 1; } 

Step 3: Define the function as an export in the export.def defintion file.

EXPORTS    IsolatedFunction          @1 

Step 4: Create a DLL project and add the export.cpp and export.def files to this project. Building this project will create an export.dll and an export.lib file.

The following two steps link to the DLL at link time. If you don't want to define the entry points at link time, ignore the next two steps and use the LoadLibrary and GetProcAddress to load the function entry point at runtime.

Step 5: Create a Test application project to use the dll by adding the export.lib file to the project. Copy the export.dll file to ths same location as the Test console executable.

Step 6: Call the IsolatedFunction function from within the Test application as shown below.

#include "stdafx.h"  // get the function prototype of the imported function #include "../export/export.h"  int APIENTRY WinMain(HINSTANCE hInstance,                      HINSTANCE hPrevInstance,                      LPSTR     lpCmdLine,                      int       nCmdShow) {     // call the imported function found in the dll     int result = IsolatedFunction("hello", "world");      return 0; } 

Answer 2

Can also export functions from dll and import from the exe, it is more tricky at first but in the end is much easier than calling LoadLibrary/GetProcAddress. See MSDN.

When creating the project with the VS wizard there's a check box in the dll that let you export functions.

Then, in the exe application you only have to #include a header from the dll with the proper definitions, and add the dll project as a dependency to the exe application.

Check this other question if you want to investigate this point further Exporting functions from a DLL with dllexport.