array data not marshaled correctly for structures from c# to c within Linux embedded mono program

Question

I've built on the Mono Embed sample to try and invoke a method in a C# assembly that updates a structure. The structure has 1 int array. This is on a Linux system.

Accessing the int array field in c# results in a segmentation fault. Just checking if the field is null is enough to cause the fault.

When I do internal marshaling simulation within C#, converting the struct to bytes and then back to a struct this works fine.

Mono Version: 3.2.3

I have included the c# and c code below and can furnish more information upon request if need be.

Here's the c code...

#include <mono/jit/jit.h>
#include <mono/metadata/object.h>
#include <mono/metadata/environment.h>
#include <mono/metadata/assembly.h>
#include <mono/metadata/debug-helpers.h>
#include <string.h>
#include <stdlib.h>

#ifndef FALSE
#define FALSE 0
#endif

struct STRUCT_Test
{
    int IntValue1[2];
};

int 
main (int argc, char* argv[]) {
    MonoDomain *domain;
    MonoAssembly *assembly; 
    MonoClass *klass;
    MonoObject *obj;
    MonoImage *image;

    const char *file;
    int retval;

    if (argc < 2){
        fprintf (stderr, "Please provide an assembly to load\n");
        return 1;
    }
    file = argv [1];

    domain = mono_jit_init (file);

    assembly = mono_domain_assembly_open(domain, file);
    if (!assembly)
        exit(2);

    image = mono_assembly_get_image(assembly);

    klass = mono_class_from_name(image, "StructTestLib", "StructReader");
    if (!klass) {
        fprintf(stderr, "Can't find StructTestLib in assembly %s\n", mono_image_get_filename(image));
        exit(1);
    }

    obj = mono_object_new(domain, klass);
    mono_runtime_object_init(obj);

    {
        struct STRUCT_Test structRecord; memset(&structRecord, 0, sizeof(struct STRUCT_Test));
        void* args[2];
        int val = 277001;

        MonoMethodDesc* mdesc = mono_method_desc_new(":ReadData", FALSE);
        MonoMethod *method = mono_method_desc_search_in_class(mdesc, klass);

        args[0] = &val;
        args[1] = &structRecord;

        structRecord.IntValue1[0] = 1111;
        structRecord.IntValue1[1] = 2222;

        mono_runtime_invoke(method, obj, args, NULL);

        printf("IntValue1: %d, %d\r\n", structRecord.IntValue1[0], structRecord.IntValue1[1]);
    }


    retval = mono_environment_exitcode_get ();

    mono_jit_cleanup (domain);
    return retval;
}

Here's the c# code...

 using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;

namespace StructTestLib
{
    [StructLayout(LayoutKind.Sequential, Pack = 4, CharSet = CharSet.Ansi)]
    public struct STRUCT_Test
    {
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 2)]
        public Int32[] IntValue1;
    }

    public class StructReader
    {
        public void ReadData(int uniqueId, ref STRUCT_Test tripRecord)
        {
            if (tripRecord.IntValue1 != null)
                Console.WriteLine("IntValue1: " + tripRecord.IntValue1[0] + ", " + tripRecord.IntValue1[1]);
            else
                Console.WriteLine("IntValue1 is NULL");

            tripRecord.IntValue1[0] = 3333;
            tripRecord.IntValue1[1] = 4444;
        }
    }
}

Jeff Marino · Accepted Answer · 2013-10-23 12:45:22Z

Oops! My Ignorance!

Seems that my understanding of the marshaling was incorrect. Raw array based data types (string, long[]) cannot be marshaled directly. The c structure has to have the Monoxxx* type as the member for the runtime to marshal correctly.

Using MonoString* StringValue1 instead of char StringValue1[31] and MonoArray* IntArray instead of int IntArray[2] allows the marshaling to work correctly.

Here's what I ultimately ended up with I really needed to pass in the raw structure from c without all the "mono" baggage within the structure, I'm trying to use existing c structures without changing them. I was able to do this by using "unsafe" c# code and allowing the address of the structure itself to be passed into the c# method. This allows the raw memory to be manipulated within c# and gives full freedom for the c# marshaler to convert bytes to struct and vice versa.

c# code

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;
using EmpireCLS.Comm;

namespace StructTestLib
{
    [StructLayout(LayoutKind.Sequential, Pack = 4, CharSet = CharSet.Ansi)]
    public struct STRUCT_Test
    {
        public Int32 IntValue1;
        public Int32 IntValue2;

        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
        public string StringValue1;

    }

    public class StructReader
    {
        unsafe public void ReadDataRaw(int uniqueId, void* tripRecordPtr)
        {
            STRUCT_Test tripRecord = (STRUCT_Test)Marshal.PtrToStructure((IntPtr)tripRecordPtr, typeof(STRUCT_Test));

            tripRecord.IntValue1 = 3333;
            tripRecord.IntValue2 = 4444;

            Console.WriteLine("c# StringValue1: " + tripRecord.StringValue1);
            tripRecord.StringValue1 = "fghij";

            GCHandle pinnedPacket = new GCHandle();
            try
            {
                int structSizeInBytes = Marshal.SizeOf(typeof(STRUCT_Test));

                byte[] bytes = new byte[structSizeInBytes];
                pinnedPacket = GCHandle.Alloc(bytes, GCHandleType.Pinned);

                Marshal.StructureToPtr(tripRecord, pinnedPacket.AddrOfPinnedObject(), true);
                Marshal.Copy(bytes, 0, (IntPtr)tripRecordPtr, bytes.Length);

            }
            finally
            {
                if (pinnedPacket.IsAllocated)
                    pinnedPacket.Free();
            }
        }
    }
}

c code

#include <mono/jit/jit.h>
#include <mono/metadata/object.h>
#include <mono/metadata/environment.h>
#include <mono/metadata/assembly.h>
#include <mono/metadata/debug-helpers.h>
#include <string.h>
#include <stdlib.h>

#ifndef FALSE
#define FALSE 0
#endif

struct STRUCT_Test
{
    int IntValue1;
    int IntValue2;

    char StringValue1[20];
};

int 
main (int argc, char* argv[]) {
    MonoDomain *domain;
    MonoAssembly *assembly; 
    MonoClass *klass;
    MonoObject *obj;
    MonoImage *image;

    const char *file;
    int retval;

    if (argc < 2){
        fprintf (stderr, "Please provide an assembly to load\n");
        return 1;
    }
    file = argv [1];

    domain = mono_jit_init (file);

    assembly = mono_domain_assembly_open(domain, file);
    if (!assembly)
        exit(2);

    image = mono_assembly_get_image(assembly);

    klass = mono_class_from_name(image, "StructTestLib", "StructReader");
    if (!klass) {
        fprintf(stderr, "Can't find StructTestLib in assembly %s\n", mono_image_get_filename(image));
        exit(1);
    }

    obj = mono_object_new(domain, klass);
    mono_runtime_object_init(obj);

    {
        struct STRUCT_Test structRecord; memset(&structRecord, 0, sizeof(struct STRUCT_Test));
        void* args[2];
        int val = 277001;
        char* p = NULL;

        MonoMethodDesc* mdesc = mono_method_desc_new(":ReadDataRaw", FALSE);
        MonoMethod *method = mono_method_desc_search_in_class(mdesc, klass);

        args[0] = &val;
        args[1] = &structRecord;

        structRecord.IntValue1 = 1111;
        structRecord.IntValue2 = 2222;
        strcpy(structRecord.StringValue1, "abcde");

        mono_runtime_invoke(method, obj, args, NULL);

        printf("C IntValue1: %d, %d\r\n", structRecord.IntValue1, structRecord.IntValue2);
        printf("C StringValue: %s\r\n", structRecord.StringValue1);
    }


    retval = mono_environment_exitcode_get ();

    mono_jit_cleanup (domain);
    return retval;
}

MonoString is part of the embedded c API and is used to wrap managed objects within an unmanaged c program.

Robert Harvey · Accepted Answer · 2013-10-21 20:21:20Z

0

Try passing your StringValue1 as an array of characters, since that's what you've actually defined it to be in the C program.

answered Oct 21, 2013 at 20:21

Robert Harvey

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1 Comment

Jeff Marino Over a year ago

thanks for the info but that didn't work. It did prompt me to be able to whittle down my example a little better. Seems that it has to do with arrays in general and has nothing to do with strings.

lupus · Accepted Answer · 2013-10-22 12:46:16Z

0

mono_runtime_invoke() doesn't do any type marshalling (same if you go the other way around and use internal calls).

Only P/Invoke methods perform data marshalling.

answered Oct 22, 2013 at 12:46

lupus

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