C++ DLL import class unresolved external symbol

Question

Hi I'm sorry I've seen that there is a lot of "unresolved external symbol error" questions, and I've seen them but none of the answers that I found fixed my error.

I've tested 2 ways to compile the DLL and use the HelloWorld method from SerialPort class. btw I'm using VS2019 community edition

Both ways are throwing the same error :

Error   LNK2019 unresolved external symbol "public: void __thiscall SerialPort::HelloWorld(void)" (?HelloWorld@SerialPort@@QAEXXZ) referenced in function _main Test DriverCore C:\Users\$USER\source\repos\Test DriverCore\Test DriverCore\Main.obj    1   

To what I've understood it's a linker error and the name of the method that I'm using is unresolved (not found) but I have no idea how to fix that (I thought that extern "C" prevented this to happen)

I've also tried to add #pragma comment(lib, "DriverCore.lib")(with DriverCore.lib in the same Dir as DriverCore.h) but still nothing :/

Way 1 using a function to return a pointer to the class

DriverCore.h

#pragma once

#ifdef DRIVERCORE_EXPORTS
#define DLLCALL __declspec(dllexport)

#else 
#define DLLCALL __declspec(dllimport)

#endif

class SerialPort
{
private:
    bool connected = 0;
public:
    SerialPort() {};
    void HelloWorld();
    bool isConnected() { return 0; };
    int readSerialPort(char* buffer, unsigned int buf_size) { return 0; };
    bool writeSerialPort(char* buffer, unsigned int buf_size) { return 0; };
};

extern "C" { 
    DLLCALL SerialPort* __stdcall CreateSerialPort(); 
}; 

DriverCore.cpp

#include "pch.h"
#include "DriverCore.h"
#include <iostream>
#define DRIVERCORE_EXPORTS


BOOL APIENTRY DllMain( HMODULE hModule,DWORD ul_reason_for_call,LPVOID lpReserved)
{
    return TRUE;
}

SerialPort* __stdcall CreateSerialPort()
{
    return new SerialPort();
}

void SerialPort::HelloWorld()
{
    std::cout << "Hello World !";
}

Main.cpp

#include "pch.h"
#include <Windows.h>
#include <iostream>
#include "DriverCore.h"

typedef SerialPort* (__stdcall *SerialPortImported) ();

int main()
{
    // instantiate the dll location
    HINSTANCE hDLL = LoadLibraryW(L"DriverCore.dll"); 
    if (!hDLL) {
        std::cout << "could not load the dynamic library" << std::endl;
        return EXIT_FAILURE;
    }
    
    //Resolve Objects Addr
    SerialPortImported pCSerialPort = (SerialPortImported) GetProcAddress(hDLL, "CreateSerialPort") ;
    SerialPort* CSerialPort = pCSerialPort();
    CSerialPort->HelloWorld();

    return 0;
}

Way 2 without using extern "c" {...} but using __declspec directly onto the class declaration

DriverCore.h

#pragma once

#ifdef DRIVERCORE_EXPORTS
#define DLLCALL __declspec(dllexport)

#else 
#define DLLCALL __declspec(dllimport)

#endif

class DLLCALL SerialPort
{
private:
    bool connected = 0;
public:
    SerialPort() {};
    void HelloWorld();
    bool isConnected() { return 0; };
    int readSerialPort(char* buffer, unsigned int buf_size) { return 0; };
    bool writeSerialPort(char* buffer, unsigned int buf_size) { return 0; };
};

DriverCore.cpp

#include "pch.h"
#include "DriverCore.h"
#include <iostream>
#define DRIVERCORE_EXPORTS


BOOL APIENTRY DllMain( HMODULE hModule,DWORD ul_reason_for_call,LPVOID lpReserved)
{
    return TRUE;
}

void SerialPort::HelloWorld()
{
    std::cout << "Hello World !";
}

Main.cpp

#include "pch.h"
#include <Windows.h>
#include <iostream>
#include "DriverCore.h"

int main()
{
    // instantiate the dll location
    HINSTANCE hDLL = LoadLibraryW(L"DriverCore.dll");
    if (!hDLL) {
        std::cout << "could not load the dynamic library" << std::endl;
        return EXIT_FAILURE;
    }

    //Resolve Objects Addr
    SerialPort* pSerialPort = (SerialPort*) GetProcAddress(hDLL, "SerialPort") ;
    pSerialPort->HelloWorld();

    return 0;
}

Thanks a lot in advance for your help !

Answer 1

You are calling HelloWorld which is missing its implementation in your application.

There is some fundamental misunderstanding about how C++ executables are compiled and linked against DLLs.

No libraries:

• All symbols that the Application needs must be defined in the Application.
• All needed symbol definitions must be available to the linker.

Static libraries:

• All symbols that the Application needs must be defined in the Application or a static library.
• All needed symbol definitions must be available to the linker.
• The symbols are added to the generated Application's executable.

Dynamic libraries:

• All symbols that the Application needs must be defined in the Application or a dynamiclibrary.
• All needed symbol definitions must be available to the linker.
• The symbols remain at their original places and they are loaded only at load time. This allows swap the dynamic libraries with any other ABI-compatible one at load time.

Since you are not linking with the dll and only load it at runtime, the linker correctly complains about the missing HelloWorld method.

Extern "C" is irrelevant here.