How to avoid copying during PInvoke marshalling when struct contains array of integers?

Question

I have a "C" struct which is defined as:

typedef unsigned char tUI8;

typedef struct
{
    tUI8 Mode;
    tUI8 Data[16];
} TestStruct;

And a function which takes a pointer to this structure and fill the data:

void FillTest(tUI8 Mode, TestStruct *s);

To PInvoke to this function, I wrote the C# code as:

[StructLayout(LayoutKind.Sequential)]
struct TestStruct
{
    public byte Mode;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 16)]
    public byte[] Data;
}

static class NativeTest
{
    [DllImport("Native.dll")]
    public static extern void FillTest(byte mode, ref TestStruct s);
}

This works but I suspect that during the PInvoke marshaller is copying the struct during call and return instead of pinning it. I can say that because even if I don't initialize the struct, it works fine.

TestStruct s;
//here s.Data == null!
NativeTest.FillTest(10, ref s); //<<< I expected an error here
//here s.Data points to a valid byte[] of length 16
Console.WriteLine(BitConverter.ToString(s.Data));

My question is how can I define a PInvoke signature, using either struct or class, which avoid copying the data during marshalling?

Jon Skeet · Accepted Answer · 2012-02-24 07:35:45Z

2

I suspect you want a fixed size buffer, which will inline the data in your struct:

[StructLayout(LayoutKind.Sequential)]
unsafe struct TestStruct
{
    public byte Mode;
    public fixed byte Data[16];
}

You should now be able to pass that by reference directly to your unmanaged code. (You'll also need to explicitly allow unsafe code.)

I don't know what attributes you'll then need to use for marshalling, if any, but it's worth a try...

edited Feb 24, 2012 at 7:35

answered Feb 24, 2012 at 7:04

Jon Skeet

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8 Comments

linquize Over a year ago

Note that it is unsafe, mark the struct unsafe, and in project properties, check allow unsafe code.

Hemant Over a year ago

Yeah. I tried it and it works but I had to declare multiple times that I am a bad person (by marking unsafe). I am also not sure whether marshaller will automatically pin the struct during the call or I have to do it explicitly.

David Heffernan Over a year ago

@Hemant if you want to avoid copying, then you will have to use unsafe. If you want to avoid unsafe then you have to let the marshaler copy. What's so bad about copying data?

Hans Passant Over a year ago

No pinning is required, the struct lives on the stack where it is unmovable. Getting the C code wrong will blow the stack. This is the infamous buffer overflow attack vector. Thus the unsafe keyword.

David Heffernan Over a year ago

Hundreds of packets each second. That doesn't sound very demanding. You might be well advised to profile before you decide where your bottlenecks are.

|

Mathew Sachin · Accepted Answer · 2016-02-22 11:30:39Z

1

This can be done without using unsafe code.

By marshalling as ByValArray

[StructLayout(LayoutKind.Sequential)]
struct TestStruct
{
    public byte Mode;

    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 16)]
    public byte[] Data;
}

answered Feb 22, 2016 at 11:30

Mathew Sachin

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