[Mingw-users] Using a mingw DLL from C#

[Jef D. <jef...@ho...>]

I'm building a DLL with mingw32. It works perfect when used in a C/C++ 
application, but something goes wrong when used in a C# project. 
Ordinary functions work, but with callback functions it starts to go wrong.

I'm able to reproduce the problem with the following code sample for the 
DLL:

typedef void (*callback_t) (unsigned int size, void *userdata);

void
myfunction (callback_t callback, void *userdata)
{
     for (unsigned int i = 0; i < 100; ++i) {
        printf ("iteration %u\n", i);

        if (callback) callback (i, userdata);
     }
}

Now I create a simple C# console project (with MSVC), and try to use 
this DLL with the following code:

[DllImport("mydll.dll")]
static extern void myfunction (callback_t callback, IntPtr userdata);

static void test_callback (UInt32 size, IntPtr userdata)
{
     Console.WriteLine (size);
}

static void Main(string[] args)
{
     callback_t mycallback = new callback_t (test_callback);
     myfunction (mycallback, IntPtr.Zero);
}

It runs fine for the first few iterations, but than something goes
wrong. I get this output:

iteration 0
0
iteration 1
1
iteration 2
2
iteration 3
3
iteration 4
0
iteration 1
1697911994

Unhandled Exception: System.AccessViolationException: Attempted to read
or write protected memory. This is often an indication that other memory
is corrupt.
     at test.Class1.myfunction(callback_t callback, IntPtr userdata)
     at test.Class1.Main(String[] args)

Any idea what is going wrong here? If I compile the DLL with MSVC, 
everything is fine.