Marshaling array of structs from c# to c++ with fixed size buffer

Question

I am using the following code to marshal an array of structs to c++:

[DllImport("IPD.dll", EntryPoint = "process", CallingConvention = CallingConvention.Cdecl)]
public static extern Pixel* process(Pixel* pixels, int numPoints, uint processingFactor);    
[StructLayout(LayoutKind.Sequential)]
public unsafe struct Pixel
{
    public fixed byte x[3];
    public uint numebrOfPixels;
}  
...
Pixel[] pixels = extractPixels(image);
fixed (Pixel* ptr = pixels)
{
            Pixel* result = process(ptr, pixels.Length,processingFactor);
}

In order to populate my struct I am using the following code:

//Looping and populating the pixels    
for(i=0;i<numOfPixels;i++)  
{
   fixed (byte* p = pixels[i].x)
   {
                p[0] = r;
                p[1] = g;
                p[2] = b;
   }
}

The code works just fine with no memory leaks.
How can I be sure the during marshaling the pixels to the native code the CLR doesn't copy the pixels array back and forth?

Cheers,
Doron

Just as a suggestion: if the returned Pixel* are distinct pixels from the one passed as an input parameter, then it is probably better to pass them as an input parameter as a second Pixel* pixelOut, so that C#-side you can have a Pixel[] even for the output pixels. — xanatos
– xanatos, Commented Apr 13, 2015 at 7:59

David Heffernan · Accepted Answer · 2015-04-13 07:34:07Z

2

The way you can be sure that the marshaller doesn't copy members of the array is that the marshaller doesn't know the size of the array. It is simply not capable of marshalling the array contents. You are simply passing the address of a pinned array. No copying of the content of that array is performed.

answered Apr 13, 2015 at 7:34

David Heffernan

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2 Comments

DoronBM Over a year ago

Thanks! As you can see I am use the fixed buffer and therefor when I am populating the struct pixels array I need to use a fixed scope. This has performance penalty. What is the alternative?

David Heffernan Over a year ago

Use a uint in place of that fixed buffer, and form the value with bitwise operations.

mainvoid · Accepted Answer · 2015-04-13 06:42:25Z

0

You can use the In Attribute to specify an argument that needs to be marshaled from the caller to the callee:

[DllImport("IPD.dll", EntryPoint = "process", CallingConvention = CallingConvention.Cdecl)]
public static extern Pixel* process([In] Pixel* pixels, int numPoints, uint processingFactor);

answered Apr 13, 2015 at 6:42

mainvoid

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1 Comment

David Heffernan Over a year ago

[In] is the default for parameters

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