Simple C++ DLL Loading a Message Box

This tutorial shows you how to create a C++ Win32 DLL (Dynamic Link Library) which you can then use to call functions from that DLL in your main C++ program. I will be calling a MessageBox from the DLL and load it inside my main C++ program using LoadLibrary and GetProcAddress. I will also exchange integers between the DLL and the main program.

YouTube Tutorial

I also have a video tutorial for this on youtube:

DLL Project

I first start by creating our DLL, using CodeBlocks you can create a new project as a Dynamic Link Library (DLL). It may be different in other IDEs or editors. Somewhere you have to specify it's a DLL project.

Create 2 files, one C++ file (.cpp or .cxx) and one header file (.h).

DLL Header File

This will be our DLL header file:

Basically, there is a definition called __MAIN_H__ which will allow our header file to only be called once, because if it is not defined, I define it and include the code, otherwise I don't repeat the same code.

I include windows.h which has our C++ Win32 API functions and then define DLL_EXPORT as __declspec(dllexport) which just means that the function can be exporting via DLL.

Then I check if it's a C++ compile or a C compile, if it is C++ the preprocessor will use extern "C" so that it works in C programs as well.

Then I define an int DLL_EXPORT function called MsgBox with a parameter (int x). Thus, it will return an int and take an int as an argument.

Main DLL Program

This is our main.cpp file. Our header file was called main.h.

I have included the same function in the header file, with integer parameter and return value as a DLL_EXPORT type.

The default value for x is 0.

Then MessageBox which is a Win32 API function will display a message box with no owner (the first parameter), the message is then written, then the title of the message box, and finally MB_OK which means the messagebox will display an OK button, and MB_ICONINFORMATION which means messagebox will have an ICONINFORMATION icon.

Then I return the integer.

DllMain function is the standard main function for all DLLs. I wasn't doing anything special so I didn't put any real code here.

Now compile this dll as "InfernoDevelopment.dll" (or name your project as InfernoDevelopment) because that is the DLL we call in the code below.

Main C++ Program

This will be our main program that will call our DLL function and load our DLL as well.

I declare my includes, iostream for cout and cin. Windows.h for LoadLibrary, GetProcAddress, and FreeLibrary.

I typedef (define a type) an int returning (*MsgFunction) (the name of the type) and an integer argument. This is my definition for the function I am about to call.

Then I declare MsgFunction type called MsgBox, set it equal to zero so that I can check if the function was actually loaded properly.

HINSTANCE hinstDLL is our Win32 instance of that DLL. I set LoadLibrary("InfernoDevelopment.dll") as hinstDLL, calling our DLL file.

Note: DLL should be in the SAME folder as this exe.

I check if hinstDLL is not 0, and is working. Then I cast the GetProcAddress(hinstDLL, "MsgBox"); as a (MsgFunction) type and set that to MsgBox. The parameter in GetProcAddress will take the instance and find the process address where there is a function called "MsgBox" in the DLL.

Finally we call the function and check if we got the exact same integer as a return or a zero. I also checked if MsgBox was 0, and if so, an error will be displayed noting that the MsgBox wasn't properly loaded by GetProcAddress.

Then I FreeLibrary(hinstDLL) to avoid memory leaks and to let the DLL go, because we already called our function.

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