如何编组包含 function 指针的 ac/cpp 结构？

Question

Good Morning, I am attempting to create a C# dot net core wrapper for the Audio functionality of SDL. In particular, I am attempting to create a wrapper for https://wiki.libsdl.org/SDL_LoadWAV

The existing code is:

To represent the non basic types being marshalled:

    public delegate void SDL_AudioCallback(IntPtr userdata, byte[] stream,out int len);
    [StructLayout(LayoutKind.Sequential)]
    public struct SDL_AudioSpec
    {
        public int freq;
        public ushort format;
        public byte channels;
        public byte silence;
        public ushort samples;
        public UInt32 size;
        public IntPtr callback;
        IntPtr userdata;
    }
    public class SDL_AudioSpecPtr
    {
        public readonly IntPtr NativePointer;

        public SDL_AudioSpecPtr(IntPtr pointer)
        {
            NativePointer = pointer;
        }

        public static implicit operator IntPtr(SDL_AudioSpecPtr Sdl2Window) => Sdl2Window.NativePointer;
        public static implicit operator SDL_AudioSpecPtr(IntPtr pointer) => new SDL_AudioSpecPtr(pointer);
    }

Importing the Method signature:

[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
private delegate SDL_AudioSpec SDL_LoadWAV_t(string file, IntPtr spec, byte[] audio_buf, out UInt32 audio_len);
private static SDL_LoadWAV_t s_sdl_LoadWAV = LoadFunction<SDL_LoadWAV_t>("SDL_LoadWAV");
public static void SDL_LoadWAV(string file, IntPtr spec, byte[] audio_buf, out UInt32 audio_len) => s_sdl_LoadWAV(file,spec,audio_buf,out audio_len);

Attempting to call the function yields null exception:

Sdl2Native.SDL_Init(SDLInitFlags.Audio);
SDL_AudioSpec spec = new SDL_AudioSpec();
byte[] bytes=new byte[20000];
uint len;
SDL_AudioSpec* specptr = &spec;
Sdl2Native.SDL_LoadWAV(@"G:\Assets\sfx_movement_jump2.wav", (IntPtr)specptr, bytes, out len);
            

Answer 1

You should rely more heavily on the CLR's marshalling rather than doing your own, it's a lot easier to make serious errors.

Furthermore:

• Strings need marshalling to ANSI with [MarshalAs(UnmanagedType.LPStr)]

• You shouldn't just convert & pointer to * like that, the struct needs marshalling.

• A callback given to unmanaged code needs to be held onto, so that the GC doesn't sweep it away.

It's unclear why you have a LoadFunction setup rather than just using standard P/Invoke, but I've run with that.

So the delegate should be

private static SDL_AudioSpec SDL_LoadWAV_t(
    [MarshalAs(UnmanagedType.LPStr)] string file,
    in SDL_AudioSpec spec,
    ref byte[] audio_buf,
    out UInt32 audio_len);

and then SDL_AudioSpec struct should declare the callback

public SDL_AudioCallback callback;

The callback is wrong also: len is not a out parameter, it is by-value; and it's unclear if stream should be ref or out or in , I think out .

public delegate void SDL_AudioCallback(IntPtr userdata, out byte[] stream, int len);

Depending on how long the callback is being used for, you need to hold onto it

• If it is just used within a single call, and is not held by unmanaged code, then place GC.KeepAlive(callback); after the call
• If unmanaged code holds it further then you would need to store it in a field somewhere, where you can guarantee it will not be deleted by GC before the last callback

Your calling code is then:

var callbackDlgt = new SDL_AudioCallback(myCallbackFunction);
SDL_AudioSpec spec = new SDL_AudioSpec{
    callback = callbackDlgt,
// set other values here
};
byte[] bytes = new byte[20000];
Sdl2Native.SDL_LoadWAV(@"G:\Assets\sfx_movement_jump2.wav", in specptr, ref bytes, out uint len);
GC.KeepAlive(callbackDlgt);