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How should I declare this C struct for interop?

Answer 1

A:

I am not sure this is what you meant but macros can make your life easier

Create a file that contains a description of the struct's members with enough information to rebuild it correctly:

struct.h
------- 
MEMBER(char, xumrversaocomc01)
ARRAY(char, xumrretcomc02, 2)

Then, include the list file with the correct declaration for the required format:

struct_c.h
----------
#define MEMBER(type, name) type name;
#define ARRAY(type, name, size) type name[size];
typedef struct PXUCAMR
{
#include "struct.h"
}
#undef MEMBER
#undef ARRAY

The same goes for C#

eyalm 2009-11-09 21:10:46

Are you sure? I could be wrong, but judging by this link (http://msdn.microsoft.com/en-us/library/ed8yd1ha%28VS.80%29.aspx), I can't use #define as you've shown, and looks like C# lacks the #include directive too.

Ricardo Nolde 2009-11-10 11:29:59

I am not sure, I am not working with c#. I thought this generic code may you help you. I guess I was wrong.

eyalm 2009-11-10 17:54:47

Answer 2

+1 A:

When you tried with "strings to no avail" what happened? It crashed? Didn't map properly? Missed some fields?

I've used the following constructs in the past...

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct PXUCAMR
{
    [MarshalAs(UnmanagedType.I1)]
    public sbyte xumrversaocomc01;

    // null terminated string?
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 5)]
    public string xumrcodfalhac05;

    // or just raw data?
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 5)]
    public sbyte[] xumrcodfalhac05;
}

The CharSet.Ansi is needed for the ByValTStr setting. If xumrcodfalhac05 is really data and not a string, use the sbyte array.

Justin Rudd 2009-11-10 04:19:21

Well, it has been a while since I have tried all the variants, but now there is a need to speed up the application, so I am once again diving into this routine. But I remember having more than one problem: wrong mapping of fields, exceptions from Marshal.AllocHGlobal telling me it could not "measure" the correct size of the struct, the extern C routine returning errors in the format of the fields... I never had the time to document every case properly.

Ricardo Nolde 2009-11-11 11:39:41

Check the edit for some feedback to your answer! ;)

Ricardo Nolde 2009-11-11 12:08:11

The reason you're having problems after the change is that C# `char` doesn't map to C++ `char`, even with `CharSet.Ansi` (the latter only applies to `string`/char array mappings). A single `char` field either has to be mapped to `string` with `SizeConst=1`, or to `sbyte`. Also, don't map pointers as `int` - use `IntPtr`, it's intended specifically for that.

Pavel Minaev 2009-11-12 23:39:07

@Pavel - indeed, C/C++ char does not map to C# char, as the C# one is an unicode char. And the pointer is mapped as an int because I don't really need it, considering the way I use the struct. So, I didn't care about using an IntPtr.

Ricardo Nolde 2009-11-16 13:11:40

Answer 3

A:

I ended up using a byte array as the representation of the struct, and using a builder to set the values. The performance gain is not big, but it is there. And there is memory advantages too, as I can pin it in memory and send it directly to the unmanaged routine. It actually helps me with another problem: this struct has two versions, with diferent xumrdadosc10000 sizes... the original one is 10000 bytes, but the new one is 200000 bytes, but the name is the same for compatibility purposes. So, I can create the right size in the constructor, and off I go.

/// <summary>Builds PXUCAMR/PXUCAMRV3 as a byte array.</summary>
internal class PxucamrBuilder
{
    internal byte[] Pxucamr;

    /// <summary>Get/set field xumrversaocomc01, offset 0, size 1 byte.</summary>
    internal string StructureVersion
    {
        get { return Encoding.ASCII.GetString(this.Pxucamr, 0, 1); }
        set { Encoding.ASCII.GetBytes(value, 0, 1, this.Pxucamr, 0); }
    }

    /// <summary>Get/set field xumrambientec01, offset 12, size 1 byte.</summary>
    internal string Context
    {
        get { return Encoding.ASCII.GetString(this.Pxucamr, 12, 1); }
        set { Encoding.ASCII.GetBytes(value, 0, 1, this.Pxucamr, 12); }
    }

    /// <summary>Get/set field xumrcodfalhac05, offset 5, size 5 byte.</summary>
    internal string ErrorCode
    {
        get { return Encoding.ASCII.GetString(this.Pxucamr, 5, 5); }
        set { Encoding.ASCII.GetBytes(value, 0, 5, this.Pxucamr, 5); }
    }

    /// <summary>Get/set field xumridservidorc30, offset 130, size 30 bytes.</summary>
    internal string ServerId
    {
        get { return Encoding.ASCII.GetString(this.Pxucamr, 130, 30); }
        set { Encoding.ASCII.GetBytes(value, 0, value.Length, this.Pxucamr, 130); }
    }

    /// <summary>Get/set field xumrtamdadosb31, offset 24, size 4 byte.</summary>
    internal int DataSize
    {
        get
        {
            byte[] bytes = new byte[4];
            Array.Copy(this.Pxucamr, 24, bytes, 0, 4);
            return this.ByteArrayToInt32(bytes);
        }
        set
        {
            byte[] bytes = this.Int32ToByteArray(value);
            Array.Copy(bytes, 0, this.Pxucamr, 24, 4);
        }
    }

    /* ... same pattern to remaining fields ... */   
}

The offsets can be easily found with the Marshal.OffsetOf method.

As I am using this one, I will mark this answer as the correct one. But I can change it if a better answer comes up. I am still willing to try new ideas!

Ricardo Nolde 2009-11-16 13:30:45

Consider evaluating the performance tips on the site "Improving .NET Application Performance and Scalability", at http://msdn.microsoft.com/en-us/library/ms978943.aspx

lsalamon 2010-02-25 12:27:18

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How should I declare this C struct for interop?

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