[c++] Calling functions in a DLL from C++

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?

The following are the 5 steps required:

1. declare the function pointer
2. Load the library
3. Get the procedure address
4. assign it to function pointer
5. call the function using function pointer

You can find the step by step VC++ IDE screen shot at http://www.softwareandfinance.com/Visual_CPP/DLLDynamicBinding.html

Here is the code snippet:

int main()
{
/***
__declspec(dllimport) bool GetWelcomeMessage(char *buf, int len); // used for static binding
 ***/
    typedef bool (*GW)(char *buf, int len);

    HMODULE hModule = LoadLibrary(TEXT("TestServer.DLL"));
    GW GetWelcomeMessage = (GW) GetProcAddress(hModule, "GetWelcomeMessage");

    char buf[128];
    if(GetWelcomeMessage(buf, 128) == true)
        std::cout << buf;
        return 0;
}

Presuming you're talking about dynamic runtime loading of DLLs, you're looking for LoadLibrary and GetProAddress. There's an example on MSDN.

Might be useful: https://www.codeproject.com/Articles/6299/Step-by-Step-Calling-C-DLLs-from-VC-and-VB-Part-4

For the example above with "GetWelcomeMessage" you might need to specify "__stdcall" in the typedef field before the function name if getting error after calling imported function.

You can either go the LoadLibrary/GetProcAddress route (as Harper mentioned in his answer, here's link to the run-time dynamic linking MSDN sample again) or you can link your console application to the .lib produced from the DLL project and include the hea.h file with the declaration of your function (as described in the load-time dynamic linking MSDN sample)

In both cases, you need to make sure your DLL exports the function you want to call properly. The easiest way to do it is by using __declspec(dllexport) on the function declaration (as shown in the creating a simple dynamic-link library MSDN sample), though you can do it also through the corresponding .def file in your DLL project.

For more information on the topic of DLLs, you should browse through the MSDN About Dynamic-Link Libraries topic.

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.

When the DLL was created an import lib is usually automatically created and you should use that linked in to your program along with header files to call it but if not then you can manually call windows functions like LoadLibrary and GetProcAddress to get it working.