troubleshooting Question

Calling unamanged function which takes a pointer to pointer parameter

Avatar of Michael Paravicini
Michael ParaviciniFlag for Chile asked on
C.NET ProgrammingC#* dll* PInvoke
5 Comments1 Solution207 ViewsLast Modified:
I am trying to call  a function in C from my .Net Core application. The C function comes from libmpv render.h and the head looks like this:
int mpv_render_context_create(mpv_render_context **res, mpv_handle *mpv, mpv_render_param *params);
The problem is, that I have no clue how to call the function from C#.
First of all I have declared the MpvRenderParam struct as follows in C#:
public unsafe struct MpvRenderParam
    public MpvRenderParamType Type;
    public void* Data;

Previously I have defined the Data field to be of type
but in my opinion it makes more sense when it can be an all purpose pointer, so it matches the definition of the C Library.

Following is the MpvRenderParamType enum:

    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

The actual problem

Now I have explained all the data types that were transcribed to C# but the problem still lies in creating the correct definition of the mpv_render_context_create and calling it.
First I tried to define the method like this:
[DllImport("mpv-1.dll", CallingConvention = CallingConvention.Cdecl)]
private static extern int mpv_render_context_create([Out] out IntPtr renderContext, IntPtr mpvHandle,
        MpvRenderParam[] parameters);

And I called it with:
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

I also tried changing the first parameter to be a ref or IntPtr but no matter what combination of parameter type and calling it, I always got the same exception:
Exception thrown at 0x0000000055C73D60 (mpv-1.dll) in Otchi.App.exe: 0xC0000005: Access violation reading location 0x0000000000000010.
My last attempt was going into the unsafe part of C# and declaring the method like this:
[DllImport(LibName, CallingConvention = CallingConvention.Cdecl)]
private static extern unsafe int mpv_render_context_create(IntPtr* renderContext, IntPtr mpvHandle,
    MpvRenderParam[] parameters);

And calling it with the following piece of code:
IntPtr ptr;
var test = GCHandle.Alloc("opengl");
var test2 = GCHandle.ToIntPtr(test);
var apiTypePtr = Marshal.StringToHGlobalAuto("opengl");
var i = mpv_render_context_create(&ptr, _mpvHandle, new []
    new MpvRenderParam{Type = MpvRenderParamType.ApiType, Data = &apiTypePtr},

    new MpvRenderParam{Type = MpvRenderParamType.Invalid, Data = null}

No matter what I tried, I always get the same exception. How can I call the mpv_render_context_create without it throwing an AccessViolationException?
Eduard Ghergu
Architect - Coder - Mentor

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