C++ lnk2005 error on two files with extern “C”, why?

Question

I have a CPP with extern "C" functions. If they are all in a single file, everything works great. I want to split up the functions into different files just for organizational purpose.

So lets say I have these two files:

File_One.cpp

#pragma once
#include "stdafx.h"
#include <windows.h>
#include "Functions.h"
#include "Variables.h"
#include <string>
#include "File_Two.cpp"

extern "C"
{
    __declspec(dllexport) void MethodOne()
    { 
        MethodTwo();
    }
}

File_Two.cpp

#pragma once
#include "stdafx.h"
#include <windows.h>
#include "Functions.h"
#include "Variables.h"
#include <string>

extern "C"
{
    __declspec(dllexport) void MethodTwo()
    { 
    }
}

I have tried rearranging my include headers in different order, and even place no include headers in file_one.cpp other than the include for file_two.cpp but I always get the same errors.

1) error LNK1169: one or more multiply defined symbols found

2) error LNK2005: _MethodTwo already defined in File_One.obj

What exactly am I doing wrong? What should I do to fix it?

Thank you!

Answer 1

You're probably running into issues because you're including the File_two.cpp file in your File_one.cpp file. What is happening is that File_two.cpp and File_one.cpp are getting compiled and linked. But because File_two.cpp is included in File_one.cpp, the linker is seeing two copies of MethodTwo, and can't decide which to use.

You should move the declarations to a header:

File_two.h:

extern "C"
{
    __declspec(dllexport) void MethodOne()
}

And include that instead.

File_one.h:

extern "C"
{
    __declspec(dllexport) void MethodOne();
}

Then define the functions with their body in their respective .cpp files. No need for extern "C" in the source files.