Exporting functions from a DLL with dllexport

Question

I'd like a simple example of exporting a function from a C++ windows DLL.

I'd like to see the header, the cpp file, and the def file (if absolutely required).

I'd like the exported name to be undecorated. I'd like to use the most standard calling convention (stdcall?). I'd like the use **__declspec(dllexport) and not have to use a DEF file.

For example:

  //header
  extern "C"
  {
   __declspec(dllexport) int __stdcall foo(long bar);
  }

  //cpp
  int __stdcall foo(long bar)
  {
    return 0;
  }

I'm trying to avoid the linker added underscores and/or numbers (byte counts?) to the name.

I'm OK with not supporting dllimport and dllexport using the same header. I don't want any information about exporting C++ class methods, just c-style global functions.

UPDATE

Not including the calling convention (and using extern "C") gives me the export names as I like, but what does that mean? Is whatever default calling convention I'm getting what pinvoke (.NET), declare (vb6), and GetProcAddress would expect? (I guess for GetProcAddress it would depend on the function pointer the caller created).

I want this DLL to be used without a header file, so I don't really need the a lot of the fancy #defines to make the header usable by a caller.

I'm OK with an answer being that I have to use a DEF file.

Thanks for the help!!!

Answer 1

If you want plain C exports, use a C project not C++. C++ DLLs rely on name-mangling for all the C++isms (namespaces etc...). You can compile your code as C by going into your project settings under C/C++->Advanced, there is an option "Compile As" which cooresponds to the compiler switches /TP and /TC.

Exporting/Importing DLL Libs in VC++

What you really want to do is define a conditional macro in a header that will be included in all of the source files in your DLL project:

#ifdef LIBRARY_EXPORTS
#    define LIBRARY_API __declspec(dllexport)
#else
#    define LIBRARY_API __declspec(dllimport)
#endif

Then on a function that you want to be exported you use LIBRARY_API:

LIBRARY_API int GetCoolInteger();

In your library build project create a define LIBRARY_EXPORTS this will cause your functions to be exported for your DLL build.

Since LIBRARY_EXPORTS will not be defined in a project consuming the DLL, when that project includes the header file of your library all of the functions will be imported instead.

If your library is to be cross-platform you can define LIBRARY_API as nothing when not on Windows:

#ifdef _WIN32
#    ifdef LIBRARY_EXPORTS
#        define LIBRARY_API __declspec(dllexport)
#    else
#        define LIBRARY_API __declspec(dllimport)
#    endif
#elseif
#    define LIBRARY_API
#endif

When using dllexport/dllimport you do not need to use DEF files, if you use DEF files you do not need to use dllexport/dllimport. The two methods accomplish the same task different ways, I believe that dllexport/dllimport is the recommended method.

Exporting unmangled functions from a C++ DLL for LoadLibrary/PInvoke

If you need this to use LoadLibrary and GetProcAddress, or maybe doing PInvoke from .NET you can use extern "C" inline with your dllexport. And since we are using GetProcAddress instead of dllimport we don't need to do the ifdef dance from above, just a simple dllexport:

The Code:

#define EXTERN_DLL_EXPORT extern "C" __declspec(dllexport)

EXTERN_DLL_EXPORT int getEngineVersion() {
  return 1;
}

EXTERN_DLL_EXPORT void registerPlugin(Kernel &K) {
  K.getGraphicsServer().addGraphicsDriver(
    auto_ptr<GraphicsServer::GraphicsDriver>(new OpenGLGraphicsDriver())
  );
}

And here's what the exports look like with Dumpbin /exports:

  Dump of file opengl_plugin.dll

  File Type: DLL

  Section contains the following exports for opengl_plugin.dll

    00000000 characteristics
    49866068 time date stamp Sun Feb 01 19:54:32 2009
        0.00 version
           1 ordinal base
           2 number of functions
           2 number of names

    ordinal hint RVA      name

          1    0 0001110E getEngineVersion = @ILT+265(_getEngineVersion)
          2    1 00011028 registerPlugin = @ILT+35(_registerPlugin)

So this code works fine:

m_hDLL = ::LoadLibrary(T"opengl_plugin.dll");

m_pfnGetEngineVersion = reinterpret_cast<fnGetEngineVersion *>(
  ::GetProcAddress(m_hDLL, "getEngineVersion")
);
m_pfnRegisterPlugin = reinterpret_cast<fnRegisterPlugin *>(
  ::GetProcAddress(m_hDLL, "registerPlugin")
);

Answer 2

I think _naked might get what you want, but it also prevents the compiler from generating the stack management code for the function. extern "C" causes C style name decoration. Remove that and that should get rid of your _'s. The linker doesn't add the underscores, the compiler does. stdcall causes the argument stack size to be appended.

For more, see: http://en.wikipedia.org/wiki/X86_calling_conventions http://www.codeproject.com/KB/cpp/calling_conventions_demystified.aspx

The bigger question is why do you want to do that? What's wrong with the mangled names?