C++ Cross Platform Dynamic Libraries; Linux and Windows

Question

Hey guys I needed some help on writing cross-platform code but not an application but a library.

I am creating a library both static and dynamic with most of the development done in Linux, I have got the static and shared library generated in Linux but now wanted to generate a windows version of a static and dynamic library in the form of .lib and .dll using the same source code.

Is this possible? I'm a bit worried because I noticed generating windows .dll files required using _dllspec or something similiar in your source code.

If not then can anyone advice me the best and quickest solution to getting my code compiled on windows. I don't need to do the compiling under linux I am happy to do it directly under windows. Also I am using 2 external libraries which are boost and xerces xml which I have installed on both my windows and linux system so hopefully they shouldn't be a problem.

What I really want is to have a single source code copy that can be compiled under both linux and windows to generate libraries specific to each platform. I don't really care if I have to edit my code in favour of windows or linux as long as I can have a single source code copy.

Any help on this is greatly appreciated, thank you.

Answer 1

In general, there are two issues you need to be concerned with:

1. The requirement that, on Windows, your DLL explicitly exports symbols that should be visible to the outside world (via __declspec(dllexport), and
2. Being able to maintain the build system (ideally, not having to maintain a separate makefile and Microsoft Visual C++ Project/Solution)

For the first, you will need to learn about __declspec(dllexport). On Windows only projects, typically this is implemented in the way I describe in my answer to this question. You can extend this a step further by making sure that your export symbol (such as MY_PROJECT_API) is defined but expands to nothing when building for Linux. This way, you can add the export symbols to your code as needed for Windows without affecting the linux build.

For the second, you can investigate some kind of cross-platform build system.

If you're comfortable with the GNU toolset, you may want to investigate libtool (perhaps in conjunction with automake and autoconf). The tools are natively supported on Linux and supported on Windows through either Cygwin or MinGW/MSYS. MinGW also gives you the option of cross-compiling, that is, building your native Windows binaries while running Linux. Two resources I've found helpful in navigating the Autotools (including libtool) are the "Autobook" (specifically the section on DLLs and Libtool) and Alexandre Duret-Lutz's PowerPoint slides.

As others have mentioned, CMake is also an option, but I can't speak for it myself.

Answer 2

You can pretty easily do it with #ifdef's. On Windows _WIN32 should be defined by the compiler (even for 64 bit), so code like

#ifdef _WIN32
#  define EXPORTIT __declspec( dllexport )
#else
#  define EXPORTIT
#endif

EXPORTIT int somefunction();

should work OK for you.

Answer 3

Maybe it's better if you Add extern "C" !!!,

/* file CMakeLists.txt */

 SET (LIB_TYPE SHARED)

 ADD_LIBRARY(MyLibrary ${LIB_TYPE} MyLibrary.h)

/* file MyLibrary.h */

#if defined(_WIN32) || defined(__WIN32__)

  #if defined(MyLibrary_EXPORTS) // add by CMake 

    #define  MYLIB_EXPORT extern "C" __declspec(dllexport)
  #else

    #define  MYLIB_EXPORT extern "C" __declspec(dllimport)

  #endif /* MyLibrary_EXPORTS */

#elif defined(linux) || defined(__linux)

 #define MYLIB_EXPORT

#endif


 MYLIB_EXPORT inline int Function(int a)

 {
    return a  ;
 }