Data structure and stack corruption when using the P/Invoke Interop Assistant

Question

I have a structure in a C library:

#pragma pack(push, packing)
#pragma pack(1)

typedef struct
{
    unsigned int  ipAddress;
    unsigned char aMacAddress[6];
    unsigned int  nodeId;
} tStructToMarshall;

__declspec(dllexport) int SetCommunicationParameters(tStructToMarshall parameters);

This code is compiled with cl /LD /Zi Communication.c to produce a DLL and PDB file for debugging.

To use this code from a .Net app, I used the P/Invoke Interop Assistant to generate C# code for a wrapper DLL:

This results in the displayed C# wrapper, which I modified to use the correct DLL instead of "<unkown>". Also, I do actually want an array of bytes for aMacAddress, not a string (though I understand how this would usually be helpful):

[System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential, CharSet = System.Runtime.InteropServices.CharSet.Ansi)]
public struct tStructToMarshall
{
    /// unsigned int
    public uint ipAddress;
    /// unsigned char[6]
    [System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.ByValTStr, SizeConst = 6)]
    public byte[] aMacAddress;
        // ^^^^^^ Was "string"
    /// unsigned int
    public uint nodeId;
}

public partial class NativeMethods
{
    internal const string DllName = "lib/Communication.dll";

    /// Return Type: int
    ///parameters: tStructToMarshall->Anonymous_75c92899_b50d_4bea_a217_a69989a8d651
    [System.Runtime.InteropServices.DllImportAttribute(DllName, EntryPoint = "SetCommunicationParameters")]
                                                    // ^^^^^^^ Was "<unknown>"
    public static extern int SetCommunicationParameters(tStructToMarshall parameters);
}

I have two problems: 1. When I set the values of the structure to something nonzero and look up the node ID, it is mangled or corrupted. The IP address and MAC address are fine, but any structure members (including other data types) after an array are broken, showing very large numbers in the C output even if I specified single-digit values. 2. When I call the method, I get an error that says:

A call to PInvoke function '' has unbalanced the stack. This is likely because the managed PInvoke signature does not match the unmanaged target signature. Check that the calling convention and parameters of the PInvoke signature match the target unmanaged signature.

Attempting to call methods that do not take parameters does not generate this exception. And I'm pretty sure that it matches the target signature, because that's how I generated it!

How can I fix these issues?

Answer 1

1. Structure corruption

This 'corruption' is caused by alignment issues. The interop assistant is ignoring the #pragma pack(1) directive, and using the default, described here.

The fields of a type instance are aligned by using the following rules:

• The alignment of the type is the size of its largest element (1, 2, 4, 8, etc., bytes) or the specified packing size, whichever is smaller.

• Each field must align with fields of its own size (1, 2, 4, 8, etc., bytes) or the alignment of the type, whichever is smaller. Because the default alignment of the type is the size of its largest element, which is greater than or equal to all other field lengths, this usually means that fields are aligned by their size. For example, even if the largest field in a type is a 64-bit (8-byte) integer or the Pack field is set to 8, Byte fields align on 1-byte boundaries, Int16 fields align on 2-byte boundaries, and Int32 fields align on 4-byte boundaries.

• Padding is added between fields to satisfy the alignment requirements.

You've specified in C that the fields should be aligned on 1-byte boundaries. However, your C# code is assuming that there's padding which isn't there, specifically after your 6-byte struct:

Using IP address 0x01ABCDEF, MAC address {0x01, 0x02, 0x03, 0x04, 0x05, 0x06}, and node ID 0x00000001, memory looks like this (ignoring endian-ness issues, which won't matter if you get the alignment right):

Byte   Value   C expects           .NET Expects:
0      0x01    \                   \
1      0xAB     } IP Address        } IP Address
2      0xCD     |                   |
3      0xEF    /                   /
4      0x01    } aMacAddress[0]    } aMacAddress[0]
5      0x02    } aMacAddress[1]    } aMacAddress[1]
6      0x03    } aMacAddress[2]    } aMacAddress[2]
7      0x04    } aMacAddress[3]    } aMacAddress[3]
8      0x05    } aMacAddress[4]    } aMacAddress[4]
9      0x06    } aMacAddress[5]    } aMacAddress[5]
10     0x00    \                   } Padding
11     0x00     } Node ID          } Padding
12     0x00     |                  \
13     0x01    /                    } Node ID
14     0x??    } Unititialized      |
15     0x??    } Unititialized     /

Notice that .NET expects the Node ID, which is a 4-byte value, to begin on address 12, which is divisible by 4. It's actually using uninitialized memory, which causes your incorrect results.

The fix:

Add the named parameter Pack=1 to your call to StructLayoutAttribute:

[System.Runtime.InteropServices.StructLayoutAttribute(
    System.Runtime.InteropServices.LayoutKind.Sequential, Pack=1, CharSet = System.Runtime.InteropServices.CharSet.Ansi)]
                                                      //  ^^^^^^ - Here

2. Stack Unbalanced

This is caused by the different calling conventions. When you call a method with parameters, those parameters go on the stack. Under some calling conventions, the caller cleans up the stack after the method returns. Under others, the called function cleans up before returning.

When you compile the un-annotated function with cl, it uses the cdecl convention, which states:

The caller cleans the stack. This enables calling functions with varargs, which makes it appropriate to use for methods that accept a variable number of parameters, such as printf.

and is therefore a good default for the C compiler. When you import a function into .NET, it uses the stdcall convention, which states:

The callee cleans the stack. This is the default convention for calling unmanaged functions with platform invoke.

This is used in the Windows API (which is probably the library where P/Invoke is most commonly used), and is therefore a good default for P/Invoke, but the two are not compatible.

This is described a little in several other questions (probably because it has a Googleable error message, unlike your struct corruption) and is answered here.

The fix:

Add CallingConvention = CallingConvention.Cdecl to your DllImportAttribute:

[System.Runtime.InteropServices.DllImportAttribute(DllName, EntryPoint = "SetCommunicationParameters", CallingConvention = CallingConvention.Cdecl)]