Error LNK2028 when calling exported C function from C++ wrapper

Question

I have C project from which I export function f() and call it from other C++ project and it works fine. However, when I call some other function g() inside f I get LNK2028 error.

The minimal example of Cproject looks like:

Test.h

#ifndef TEST_H
#define TEST_H

#include "myfunc.h"
#define EXTERN_DLL_EXPORT extern "C" __declspec(dllexport)

EXTERN_DLL_EXPORT void f()
{
    g();    // this will provide LNK2028 if f() is called from other project
}

#endif

myfunc.h

void g();

myfunc.c

#include "myfunc.h"
void g(){}

The project itself is being built. However, when I call this function from other C++/CLIproject

#include "Test.h"
public ref class CppWrapper
{
 public:
    CppWrapper(){ f(); }   // call external function        
};

I get error:

error LNK2028: unresolved token (0A00007C) "void __cdecl g(void)" (?g@@$$FYAXXZ) referenced in function "extern "C" void __cdecl f(void)" (?f@@$$J0YAXXZ)   main.obj    CppWrapper
error LNK2019: unresolved external symbol "void __cdecl g(void)" (?g@@$$FYAXXZ) referenced in function "extern "C" void __cdecl f(void)" (?f@@$$J0YAXXZ)    main.obj    CppWrapper

Additional details:

1. I set x64 platform for whole solution
2. In CppWrapper I include .lib file from C project

Answer 1

Test.h

#ifndef TEST_H
#define TEST_H

#ifdef BUILDING_MY_DLL
#define DLL_EXPORT __declspec(dllexport)
#else
#define DLL_EXPORT __declspec(dllimport)
#endif

#ifdef __cplusplus
extern "C" {
#endif

DLL_EXPORT void f();

#ifdef __cplusplus
}
#endif

#endif

Test.c

#include "Test.h"
#include "myfunc.h"

void f()
{
    g();
}

In your C project you have to add BUILDING_MY_DLL to

Configuration Properties > C/C++ > Preprocessor > Preprocessor Definitions

The only real change is that I added the toggle between __declspec(dllexport) and __declspec(dllimport). Changes required:

• Moved f's body to Test.c because functions imported with __declspec(dllimport) cannot have a definition already.

Other changes:

• Do never write extern "C" without an #ifdef __cplusplus guard, or many C compilers will not compile your code.

Answer 2

I just spent 2 days fighting this exact same problem. Thank you for the solution. I'd like to extend it.

In my case, I am calling a c function from an exported c++ dll function and I was getting the same error. I was able to fix it so (using your example)

#ifndef TEST_H
#define TEST_H

#ifdef BUILDING_MY_DLL
#define DLL_EXPORT __declspec(dllexport)
#else
#define DLL_EXPORT __declspec(dllimport)
#endif

#ifdef __cplusplus
extern "C" {
#endif

#include "myfunc.h"

#ifdef __cplusplus
}
#endif

#endif