Calling unmanaged function which takes a pointer to pointer parameter

Question

I am trying to call a function in C from my .Net Core application. To dive right in, the C function comes from libmpv render.h and the head of the functions looks like this:

int mpv_render_context_create(mpv_render_context **res, mpv_handle *mpv, mpv_render_param *params);

The problem is I have no clue how to call the function from C#. So I've tried the following:

[DllImport("mpv-1.dll", CallingConvention = CallingConvention.Cdecl)]
private static extern int mpv_render_context_create([Out] out IntPtr renderContext, IntPtr mpvHandle,
        MpvRenderParam[] parameters);

Since the **res parameter should be updated by the function I thought this would make sense. This I tried to call with the following:

var ptr = IntPtr.Zero;
var apiTypePtr = Marshal.StringToHGlobalAuto("opengl");
var i = mpv_render_context_create(out ptr, _mpvHandle, new []
{
    new MpvRenderParam(MpvRenderParamType.ApiType, apiTypePtr),
    new MpvRenderParam(MpvRenderParamType.Invalid, IntPtr.Zero
});

This threw me an AccessViolationException, In fact every time I called the mpv_render_context_create Method I got the same Exception, so when I say "It didn't work" I mean the call threw that exception.

So I then thought maybe I would have to make it a ref parameter:

[DllImport("mpv-1.dll", CallingConvention = CallingConvention.Cdecl)]
private static extern int mpv_render_context_create(ref IntPtr renderContext, IntPtr mpvHandle,
        MpvRenderParam[] parameters);

Which caused the same error when calling it.

I then read in another stackoverflow question, that I should pass the bare IntPtr like the following:

[DllImport("mpv-1.dll", CallingConvention = CallingConvention.Cdecl)]
private static extern int mpv_render_context_create(IntPtr renderContext, IntPtr mpvHandle,
        MpvRenderParam[] parameters);

But first, it didn't work again, and second how would I even get the result from this?

So how should I create and call the function so I can create the render context?

The MpvRenderParam is declared as followed:

[StructLayout(LayoutKind.Sequential)]
public struct MpvRenderParam
{
    public MpvRenderParamType Type { get; }
    public IntPtr Data { get; }

    public MpvRenderParam(MpvRenderParamType type, IntPtr data)
    {
        Type = type;
        Data = data;
    }
}

And The MpvRenderParamType:

public enum MpvRenderParamType
{
    Invalid = 0,
    ApiType = 1,
    InitParams = 2,
    Fbo = 3,
    FlipY = 4,
    Depth = 5,
    IccProfile = 6,
    AmbientLight = 7,
    X11Display = 8,
    WlDisplay = 9,
    AdvancedControl = 10,
    NextFrameInfo = 11,
    BlockForTargetTime = 12,
    SkipRendering = 13,
    DrmDisplay = 14,
    DrmDrawSurfaceSize = 15,
    DrmDisplayV2 = 15
}

Update

I have taken into account all of the resources provided by Mattias Santoro I am just not entirely sure how to translate them into C# as it is a managed language. My Method for calling the mpv function looks like this now:

OpenGlControl.OpenGL.MakeCurrent();
IntPtr ptr;
var apiTypePtr = Marshal.StringToHGlobalAuto("opengl");
var opengl = new MpvOpenGlInitParams {get_proc_address = getProcAdress};
var size = Marshal.SizeOf(opengl);
var arr = new byte[size];

fixed (byte* arrPtr = arr)
{
    Marshal.StructureToPtr(opengl, (IntPtr)arrPtr, true);
    var parameters = new MpvRenderParam[]
    {
        new MpvRenderParam {Type = MpvRenderParamType.ApiType, Data = &apiTypePtr},
        new MpvRenderParam {Type = MpvRenderParamType.OpenGlInitParams, Data = &arrPtr},
        new MpvRenderParam {Type = MpvRenderParamType.Invalid, Data = null}
    };
    var i = mpv_render_context_create(&ptr, _mpvHandle, parameters);
}

The MpvOpenGlInitParams:

[StructLayout(LayoutKind.Sequential)]
public struct MpvOpenGlInitParams
{
    public delegate IntPtr GetProcAdressDelegate(IntPtr ctx, string name);

    public GetProcAdressDelegate get_proc_address;
    public IntPtr get_proc_address_ctx;
    public string extra_exts;
}

Sadly it still throws the same error, I am not sure what I am doing wrong and it's really frustrating.

Answer 1

I've put my code in this repo: (for .net core 2.1 and sdl2 and mpv dll for X64) https://github.com/shodo/MPVCore.git

Update2: DOOOONEEEE! First, this is my (really really ugly) code. I don't know if every thing I've done is needed:

Function defined as:

 [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
 private delegate int MpvRenderContextCreate(ref IntPtr context, IntPtr mpvHandler, IntPtr parameters);
 private MpvRenderContextCreate _mpvRenderContextCreate;

And code (it's part of an hardcoded frankestein of the c++ sdl sample plus winform and other stuff)

if (_mpvHandle != IntPtr.Zero)
            _mpvTerminateDestroy(_mpvHandle);

        LoadMpvDynamic();
        if (_libMpvDll == IntPtr.Zero)
            return;

        _mpvHandle = _mpvCreate.Invoke();
        if (_mpvHandle == IntPtr.Zero)
            return;

        _mpvInitialize.Invoke(_mpvHandle);



        MpvOpenGlInitParams oglInitParams = new MpvOpenGlInitParams();
        oglInitParams.get_proc_address = (ctx, name) => SDL.SDL_GL_GetProcAddress(name);
        oglInitParams.get_proc_address_ctx = IntPtr.Zero;
        oglInitParams.extra_exts = IntPtr.Zero;

        var size = Marshal.SizeOf<MpvOpenGlInitParams>();
        var oglInitParamsBuf = new byte[size];

        fixed (byte* arrPtr = oglInitParamsBuf)
        {
            IntPtr oglInitParamsPtr = new IntPtr(arrPtr);
            Marshal.StructureToPtr(oglInitParams, oglInitParamsPtr, true);

            MpvRenderParam* parameters = stackalloc MpvRenderParam[3];

            parameters[0].type = MpvRenderParamType.ApiType;
            parameters[0].data = Marshal.StringToHGlobalAnsi("opengl");

            parameters[1].type = MpvRenderParamType.InitParams;
            parameters[1].data = oglInitParamsPtr;

            parameters[2].type = MpvRenderParamType.Invalid;
            parameters[2].data = IntPtr.Zero;

            var renderParamSize = Marshal.SizeOf<MpvRenderParam>();

            var paramBuf = new byte[renderParamSize * 3];
            fixed (byte* paramBufPtr = paramBuf)
            {
                IntPtr param1Ptr = new IntPtr(paramBufPtr);
                Marshal.StructureToPtr(parameters[0], param1Ptr, true);

                IntPtr param2Ptr = new IntPtr(paramBufPtr + renderParamSize);
                Marshal.StructureToPtr(parameters[1], param2Ptr, true);

                IntPtr param3Ptr = new IntPtr(paramBufPtr + renderParamSize + renderParamSize);
                Marshal.StructureToPtr(parameters[2], param3Ptr, true);


                IntPtr context = new IntPtr(0);
                _mpvRenderContextCreate(ref context, _mpvHandle, param1Ptr);
            }
        }

And what made the function works is:

parameters[0].data = Marshal.StringToHGlobalAnsi("opengl");

Instead of using

parameters[0].data = Marshal.StringToHGlobalAuto("opengl");

I think it wasn't just recognizing the "opengl" parameter needed to init the mpv player with the opengl render context

Update: in order to call that function you need an initialized opengl context, or, you should pass a parameter of type MPV_RENDER_PARAM_OPENGL_INIT_PARAMS with the callback needed to access open gl functions. Check this example in C++ that uses SDL in order to initialize a window and get an opengl context.

https://github.com/mpv-player/mpv-examples/blob/master/libmpv/sdl/main.c

Without a proper initalized OpenGl context you always receive an ViolationException error regarding of the proper c# Marshaling: github.com/mpv-player/mpv/issues/6249
https://github.com/mpv-player/mpv/issues/6507

-- Old: See this one: https://stackoverflow.com/questions/20419415/c-sharp-call-c-dll-passing-pointer-to-pointer-argument