Reference a GNU C (POSIX) DLL built in GCC against Cygwin, from C#/NET

Here is what I want: I have a huge legacy C/C++ codebase written for POSIX, including some very POSIX specific stuff like pthreads. This can be compiled on Cygwin/GCC and run as an executable under Windows with the Cygwin DLL.

What I would like to do is build the codebase itself into a Windows DLL that I can then reference from C# and write a wrapper around it to access some parts of it programatically.

I have tried this approach with the very simple "hello world" example at http://www.cygwin.com/cygwin-ug-net/dll.html and it doesn't seem to work.

#include <stdio.h>
extern "C" __declspec(dllexport) int hello();

int hello()
{
  printf ("Hello World!\n");
 return 42;
}

I believe I should be able to reference a DLL built with the above code in C# using something like:

[DllImport("kernel32.dll")]
public static extern IntPtr LoadLibrary(string dllToLoad);

[DllImport("kernel32.dll")]
public static extern IntPtr GetProcAddress(IntPtr hModule, string procedureName);

[DllImport("kernel32.dll")]
public static extern bool FreeLibrary(IntPtr hModule);


[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
private delegate int hello();

static void Main(string[] args)
{
    var path = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "helloworld.dll");
    IntPtr pDll = LoadLibrary(path);
    IntPtr pAddressOfFunctionToCall = GetProcAddress(pDll, "hello");

    hello hello = (hello)Marshal.GetDelegateForFunctionPointer(
                                                                pAddressOfFunctionToCall,
                                                                typeof(hello));

    int theResult = hello();
    Console.WriteLine(theResult.ToString());
    bool result = FreeLibrary(pDll);
    Console.ReadKey();
}

But this approach doesn't seem to work. LoadLibrary returns null. It can find the DLL (helloworld.dll), it is just like it can't load it or find the exported function.

I am sure that if I get this basic case working I can reference the rest of my codebase in this way. Any suggestions or pointers, or does anyone know if what I want is even possible? Thanks.

Edit: Examined my DLL with Dependency Walker (great tool, thanks) and it seems to export the function correctly. Question: should I be referencing it as the function name Dependency Walker seems to find (_Z5hellov)?

Edit2: Just to show you I have tried it, linking directly to the dll at relative or absolute path (i.e. not using LoadLibrary):

    [DllImport(@"C:\.....\helloworld.dll")]
    public static extern int hello();


    static void Main(string[] args)
    {
        int theResult = hello();
        Console.WriteLine(theResult.ToString());
        Console.ReadKey();
    }

This fails with: "Unable to load DLL 'C:.....\helloworld.dll': Invalid access to memory location. (Exception from HRESULT: 0x800703E6)

*****Edit 3: ***** Oleg has suggested running dumpbin.exe on my dll, this is the output:

Dump of file helloworld.dll

File Type: DLL

Section contains the following exports for helloworld.dll

00000000 characteristics
4BD5037F time date stamp Mon Apr 26 15:07:43 2010
    0.00 version
       1 ordinal base
       1 number of functions
       1 number of names

ordinal hint RVA      name

      1    0 000010F0 hello

Summary

    1000 .bss
    1000 .data
    1000 .debug_abbrev
    1000 .debug_info
    1000 .debug_line
    1000 .debug_pubnames
    1000 .edata
    1000 .eh_frame
    1000 .idata
    1000 .reloc
    1000 .text

Edit 4 Thanks everyone for the help, I managed to get it working. Oleg's answer gave me the information I needed to find out what I was doing wrong.

There are 2 ways to do this. One is to build with the gcc -mno-cygwin compiler flag, which builds the dll without the cygwin dll, basically as if you had built it in MingW. Building it this way got my hello world example working! However, MingW doesn't have all the libraries that cygwin has in the installer, so if your POSIX code has dependencies on these libraries (mine had heaps) you can't do this way. And if your POSIX code didn't have those dependencies, why not just build for Win32 from the beginning. So that's not much help unless you want to spend time setting up MingW properly.

The other option is to build with the Cygwin DLL. The Cygwin DLL needs an initialization function init() to be called before it can be used. This is why my code wasn't working before. The code below loads and runs my hello world example.

    //[DllImport(@"hello.dll", EntryPoint = "#1",SetLastError = true)]
    //static extern int helloworld(); //don't do this! cygwin needs to be init first

    [DllImport("kernel32", CharSet = CharSet.Ansi, ExactSpelling = true, SetLastError = true)]
    static extern IntPtr GetProcAddress(IntPtr hModule, string procName);

    [DllImport("kernel32", SetLastError = true)]
    static extern IntPtr LoadLibrary(string lpFileName);


    public delegate int MyFunction();

    static void Main(string[] args)
    {
        //load cygwin dll
        IntPtr pcygwin = LoadLibrary("cygwin1.dll");
        IntPtr pcyginit = GetProcAddress(pcygwin, "cygwin_dll_init");
        Action init = (Action)Marshal.GetDelegateForFunctionPointer(pcyginit, typeof(Action));
        init(); 

        IntPtr phello = LoadLibrary("hello.dll");
        IntPtr pfn = GetProcAddress(phello, "helloworld");
        MyFunction helloworld = (MyFunction)Marshal.GetDelegateForFunctionPointer(pfn, typeof(MyFunction));

        Console.WriteLine(helloworld());
        Console.ReadKey();
    }

Thanks to everyone that answered~~

The main problem which you has is following. Before you can use your helloworld.dll a cygwin environment must be initialized (see http://cygwin.com/faq/faq.programming.html#faq.programming.msvs-mingw). So the following code in native C++ will works:

#include <windows.h>

typedef int (*PFN_HELLO)();
typedef void (*PFN_CYGWIN_DLL_INIT)();

int main()
{
    PFN_HELLO fnHello;
    HMODULE hLib, h = LoadLibrary(TEXT("cygwin1.dll")); 
    PFN_CYGWIN_DLL_INIT init = (PFN_CYGWIN_DLL_INIT) GetProcAddress(h,"cygwin_dll_init");
    init(); 

    hLib = LoadLibrary (TEXT("C:\\cygwin\\home\\Oleg\\mydll.dll"));
    fnHello = (PFN_HELLO) GetProcAddress (hLib, "hello");
    return fnHello();
}

Of cause the path to cygwin1.dll must be found. You can set C:\cygwin\bin as a current directory, use SetDllDirectory function or easy include C:\cygwin\bin in the global PATH environment variable (click on right mouse button on Computer, choose Properties then "Advanced System Settings", "Environment variables...", then choose system variable PATH and append it with ";C:\cygwin\bin").

Next if you compile you DLL, you should better to use DEF-file to define BASE address of DLL during compiling and makes all function names, which you exported more clear readable (see http://www.redhat.com/docs/manuals/enterprise/RHEL-4-Manual/gnu-linker/win32.html)

You can verify results with dumpbin.exe mydll.dll /exports, if you have Visual Studio installed. (don't forget start command promt from "Visual Studio Command Prompt (2010)" to have all Visual Studio set).

UPDATED: Because you don't write about the success I think there are exist some problems. In Win32/Win64 world (unmanaged world) it works. The code which I posted I have tested. Loading of CygWin DLLs in .NET can have some problem. In http://cygwin.com/faq/faq.programming.html#faq.programming.msvs-mingw one can read "Make sure you have 4K of scratch space at the bottom of your stack". This requirement can be wrong in .NET. A stack is a part of thread and not a process. So you can try to use CygWin DLLs in the new .NET Thread. Since .NET 2.0 one can define the maximum stack size for the thread. One other way is trying to understand http://cygwin.com/cgi-bin/cvsweb.cgi/~checkout~/src/winsup/cygwin/how-cygtls-works.txt?rev=1.1&content-type=text/plain&cvsroot=src and the code described in http://old.nabble.com/Cygwin-dll-from-C--Application-td18616035.html#a18616996. But the really interesting I find two ways without any tricks:

1. Compiling the DLL with respect of MinGW tools instead of CygWin tools. MinGW produce code which are much more Windows compatible. I am not using CygWin or MinGW myself, so I am not sure, that you will be able to compile all you existing code used POSIX function in MinGW. If it is do possible, this way can have more success. You can look at http://www.adp-gmbh.ch/csharp/call_dll.html for example, to see, that MinGW DLL can be called from C# exactly like a Windows DLL.
2. Usage of CygWin DLL inside of unmanaged process or unmanaged thread. This is a standard way described in CygWin documentation and it works (see example from my first post).

P.S. Please write short in the text of your question if you have success in one of this or in another way which you choose at the end. It's interesting for me independent on reputation and bounty.