C# struct to C++ Marshalling problem

Question

Hi All.

I'm invoking a C++ function from within C#.

This is the function header in C++ :

int src_simple (SRC_DATA *data, int converter_type, int channels) ;

And this is the equivilent C# function :

[DllImport("libsamplerate-0.dll")]
    public static extern int src_simple(ref SRC_DATA sd, int converter_type, int channels);

This is the SRC_DATA structure in C++ and then in C# :

 typedef struct
  {   float  *data_in, *data_out ;

      long   input_frames, output_frames ;
      long   input_frames_used, output_frames_gen ;

      int    end_of_input ;

      double src_ratio ;
  } SRC_DATA ;

This is the C# struct I have defined :

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct SRC_DATA
{

    public IntPtr data_in, data_out;

    public long input_frames, output_frames;
    public long input_frames_used, output_frames_gen;

    public int end_of_input;
    public double src_ratio;

}

The big problem is that the last parameter , src_ratio, doesn't get delivered properly to the C++ function, (it sees it as 0 or something invalid).

Are my declarations correct?

Thanks

Answer 1

You force packing in C# but not in C++. What could be happening is that the C++ compiler is padding the 7 in32's with four additional bytes to ensure the double is 8-byte aligned.

Check the #pragma pack compiler directives.

Answer 2

Check what size an int is in your C++ compiler. A C# int is always 32 bits.

Answer 3

Are you sure that the problem is in src_ratio member? long in C# is 64 bit. In C++ on Win32 platform long is 32 bit, I think you need to use int in C# for long C++ structure members. Also, Pack = 1 looks a bit strange, do you use the same structure member alignment in C++?