Call a C library from C++

#c #cpp #library #beginners

Yesterday I needed to write a small C DLL and use it in my C++ sample application. It took some time to figure out how I must do this on my Windows PC with Visual Studio installed.
This is just a little reminder for me, so don't expect too much :-).

C code

The C app contains just a little function:

// ./code/c/lib.c

#include <stdio.h>

__declspec(dllexport) void f()
{
    printf("\n This is a C code\n");
}

The keyword __declspec(dllexport) is only valid within the Microsoft compiler world and exports that function.

Compile C

Open the Visual Studio Developer Command Prompt and navigate to the folder, where the C project is. Then simply type: cl /LD lib.c. This will create two files. A lib.lib and a lib.dll.

C++ Code

Also nothing special. First we need the header file:

// ./code/cpp/Console/Console/Header.h

#pragma once

extern "C" {
  __declspec(dllimport) void f();
}

You find declspec again, but this time it is specified with dllimport which makes perfectly sense as we are importing our function :-)

Our app:

// ./code/cpp/Console/Console/Console.cpp

#include <iostream>
#include "Header.h"
#pragma comment(lib, "../../../c/lib.lib")

int main()
{
  f();
}

Note the line #pragma comment(lib, "../../../c/lib.lib") which specifies the location of the .lib so the linker is able to reference the function f().

Execute

If you now execute Console.exe you will get an error because the dll can not be find. To fix this, just copy the lib.dll to the folder where Console.exe is.

Found a typo?

As I am not a native English speaker, it is very likely that you will find an error. In this case, feel free to create a pull request here: https://github.com/gabbersepp/dev.to-posts . Also please open a PR for all other kind of errors.

Do not worry about merge conflicts. I will resolve them on my own.