C++ DLL won't Link?

Question

Alright, here's the issue. In an attempt to advance my knowledge of C++ APIs, I attempted to learn more about library file types. I have static libraries figured out quite well. You simply link the library into the project, and its contents are placed inside the binaries when it's compiled and linked. But, I hit an issue when I started trying to use dynamic libraries. I was able to successfully compile a DLL file using the __declspec function, and creating extern blocks(so it can successfully be exported to C++). But, the issue arises when I try to link the file. Based on multiple tutorials I have seen across the web, I create a header with forward definitions, and include it into the executable project. Then, I make sure to add the search directory to the project build settings, along with the path to the library I am importing. Then, when I go to build the project, it produces a linker error(I can tell because the object file is compiled) "cannot find -l[path to file]". Are there more configurations to be made? I assume it has something to do with my compiler(MinGW), because Code::Blocks(I'm currently using the Eclipse CDT plugin) produced a similar link error.

If you need to see the code, I will provide it below.

// EXE: main.cpp
#include <iostream>
#include "DLLTutorial.h"

int main()
{
    Function();
    std::cout << "1 + 3:\t" << Add(1, 3);
}


// DLL: DLLTutorial.cpp
#define DLL_EXPORT
#include <iostream>
#include "DLLTutorial.h"

extern "C"
{
    DLLCOMP int Add(int a, int b)
    {
        return a + b;
    };

    DLLCOMP void Function(void)
    {
        std::cout << "DLL Called!\n";
    };
};


// DLL: DLLTutorial.h
#ifndef DLLTUTORIAL_H_
#define DLLTUTORIAL_H_

#include <iostream>

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

extern "C"
{
    DLLCOMP int Add(int a, int b);
    DLLCOMP void Function(void);
};


#endif /* DLLTUTORIAL_H_ */

It's just very simple example code I used from a website. I cannot get the executable file to link properly, so I would appreciate any advice that could be given to me.

Also, I have a related question. Reading a few of the tutorials online, some made mention about the __declspec function being Windows/Microsoft specific. Is this true? In which case, what would be a nice equivalent to use on other operating systems?

Answer 1

When you build a dll you also create a companying .lib file, the 'implib'. You must statically link this file in your exe.

EDIT: __cdeclspec(...) is indeed Microsoft. Dont't know about other platforms.

Answer 2

You need to tell the compiler where to search for your libraries and includes. In both GCC and MinGW this is achieved by using -I for includes, and -L for libraries. I don't think its necessary for you to generate the .lib (or .a as it usually is in *nix and GCC), GCC should be able to find your function in the dll.

For example, build and link the shared lib:

g++ -shared -olibmy_dll.dll my_dll.cpp

Then, assuming all the files are in the same dir, build and link the exe and tell GCC to use your lib:

g++ -oapp.exe app.cpp -lmy_dll -L.

Note the difference between -l and -L , one tells GCC what libs to use, while the other tells where to look for them. In Eclipse CDT you find these settings in: Project properties -> C/C++ Build -> Settings -> MinGW C++ Linker -> Libraries.

Also, __declspec is recognized by MinGW, but I think its Microsoft specific anyway.