How do I control which symbols a Windows DLL imports from the application?

Question

I'm trying to build a shared library (DLL) on Windows, using MSVC 6 (retro!) and I have a peculiar link issue I need to resolve. My shared library must access some global state, controlled by the loading application.

Broadly, what I have is this:

application.c:

static int g_private_value;

int use_private_value() {
    /* do something with g_private_value */
}

int main (...) {
    return shared_library_method ();
}

shared_library.c:

__declspec(dllexport) int __stdcall shared_library_method() {
    use_private_value();
}

(Updated - I forgot the __declspec(dllexport) int __stdcall portion, but it's there in the real code)

How do I set up shared_library.dll so that it exports shared_library_method and imports use_private_value?

Please remember that A) I'm a unix programmer, generally, and B) that I'm doing this without Visual Studio; our automated build infrastructure drives MSVC with makefiles. If I'm omitting something that will make it easier to answer the question, please comment and I'll update it ASAP.

Answer 1

I'll start with half of the answer.

In shared_library.c:

__declspec(dllexport) int __stdcall shared_library_method(void)
{


}

The MSDN article about exporting function from DLL:s.

Answer 2

This is actually going to be pretty difficult to get working. On Unix/Linux you can have shared objects and applications import symbols from each other, but on Windows you can't have a DLL import symbols from the application that loads it: the Windows PE executable format just doesn't support that idiom.

I know that the Cygwin project have some sort of work-around to address this problem, but I don't believe that it's trivial. Unless you want to do lots of PE-related hacking you probably don't want to go there.

An easier solution might be to just have some sort of initializer method exported from the DLL:

typedef int (*func_ptr)();
void init_library(func_ptr func);

The application must call this at start-up, passing in the address of the function you want to share. Not exactly elegant, but it should work okay.

Answer 3

For the second half you need to export the functions from your application.c. You can do this in the linker with:

/export:use_private_value@0

This should get you a lib-file that you build with your DLL.

The option to link the lib-file is to use GetProcAddress().

As DavidK noted if you only have a few functions it is probably easier to pass the function pointers in an init function. It is however possible to do what you are asking for.