3

In C, if I have the following struct Data:

struct __attribute__((__packed__)) Data
{
    double x;
    int y;
};

And then create an array named val of this struct:

Data val[3];

All values stored in val (i.e. val[0].x, val[0].y, val[1].x, val[1].y, val[2].x and val[2].y are contiguous in memory.

Now, when having the following struct in C#:

[StructLayout(LayoutKind.Sequential, Pack=0)]
struct Data
{
    public double x;
    public int y;
}

And then create an array named val of this struct:

Data val[3];

I would expect that all values stored in val are contiguous in memory, which they are not (based on the tests that I have conducted). In details, val[0].x and val[0].y are both contiguous, but they are not contiguous with val[1] (which, in turn, is not contiguous with val[2]).

My question is: in C#, how can I create a variable (that is an array) of a certain struct that all its elements and members are contiguous in memory?

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6
  • Arrays are stored contigous by default. Except when a reference type is used. In that case only references are contigous. Commented Jan 7, 2022 at 17:27
  • 2
    Can you show us those tests? What you say contradicts the normal implementations of arrays (which is having them contiguous for value type-only arrays). Have you tried pinning a fixed pointer to &val[0]? Commented Jan 7, 2022 at 17:30
  • 3
    @user7698505 no, but Pack=0 means "default padding", and what you are looking at is packing at the least possible. That'd be Pack=1 in C# I believe Commented Jan 7, 2022 at 17:48
  • @user7698505: it solved the issue. Thank you very much for all the comments above! Commented Jan 7, 2022 at 17:50
  • 1
    I'm curious what the use case is behind this question. Setting Pack=1 can cause issues (speed and/or stability). Are you doing the for managed/native interop reasons, or something else? Commented Jan 7, 2022 at 19:15

1 Answer 1

Reset to default
3

Just to try it, I made a very simple project: using Pack=1 seems to pack perfectly fine to the smallest byte.

Reproducing code (on a console app):

[StructLayout(LayoutKind.Sequential, Pack=1)]
struct Data
{
    public double x;
    public int y;
}

class Program {
    static void Main()
    {
        Data[] val = new Data[3] {
            new Data { x = 0, y = 0 },
            new Data { x = 1, y = 1 },
            new Data { x = 2, y = 2 },
        };

        for(var i = 0; i < 3; i++)
        {
            Console.WriteLine("data[" + i + "].x = " + BitConverter.ToString(BitConverter.GetBytes(val[i].x)));
            Console.WriteLine("data[" + i + "].y = " + BitConverter.ToString(BitConverter.GetBytes(val[i].y)));
        }       

        unsafe {
            fixed (Data *p = &val[0])
            {
                byte *c = (byte *)p;
                for(var j = 0; j < 3; j++)
                {
                    for(var i = 0; i < Marshal.SizeOf<double>(); i++, c++)
                    {
                        if(i!=0) Console.Write("-");
                        Console.Write("{0:X2}", *c);
                    }
                    Console.WriteLine();
                    for(var i = 0; i < Marshal.SizeOf<int>(); i++, c++)
                    {
                        if(i!=0) Console.Write("-");
                        Console.Write("{0:X2}", *c);
                    }
                    Console.WriteLine();
                }
            }

        }
    }
}

Output:

data[0].x = 00-00-00-00-00-00-00-00
data[0].y = 00-00-00-00
data[1].x = 00-00-00-00-00-00-F0-3F
data[1].y = 01-00-00-00
data[2].x = 00-00-00-00-00-00-00-40
data[2].y = 02-00-00-00
00-00-00-00-00-00-00-00
00-00-00-00
00-00-00-00-00-00-F0-3F
01-00-00-00
00-00-00-00-00-00-00-40
02-00-00-00

Which looks perfectly continuous to me, unless I'm missing something

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