C#•2y ago

Interoperability issues (PInvoke)

Hi! I'm calling a native CPP library and retrieving a struct by reference which returns a struct containing some pointers.

69 Replies

rick_o_maxOP•2y ago

I first call a CPP method to fill the structure, this is what CPP returns:

rick_o_maxOP•2y ago

rick_o_maxOP•2y ago

This is the struct in C# (returned value)

rick_o_maxOP•2y ago

rick_o_maxOP•2y ago

When using the same struct as a reference to call another CPP method, the values are wrong in the CPP side:

rick_o_maxOP•2y ago

rick_o_maxOP•2y ago

This is my CPP struct:

typedef struct TFBXContext {
    int code;
    FbxManager* manager;
    FbxIOSettings* ios;
    FbxScene* scene;
    FbxImporter* importer;
    MemoryStream* stream;
    FbxEmbeddedFileCallback* fileCallback;
    FbxSimpleMap<FbxString, FbxPair<char*, size_t>, FbxStringCompare>* embeddedResources;
    rapidjson::Document* assetLoaderOptions;
    //const char* status;
} FBXContext;

typedef struct TFBXContext {
    int code;
    FbxManager* manager;
    FbxIOSettings* ios;
    FbxScene* scene;
    FbxImporter* importer;
    MemoryStream* stream;
    FbxEmbeddedFileCallback* fileCallback;
    FbxSimpleMap<FbxString, FbxPair<char*, size_t>, FbxStringCompare>* embeddedResources;
    rapidjson::Document* assetLoaderOptions;
    //const char* status;
} FBXContext;

And this is my C# code with the method called in the second CPP screenshot and the same interface in C#:

        [DllImport(DllPath, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.StdCall)]
        private static extern IntPtr ConvertScene(
            FBXContext context
        );

        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
        private struct FBXContext
        {
           public int code; 
           public IntPtr manager;
           public IntPtr ios;
           public IntPtr scene;
           public IntPtr importer;
           public IntPtr stream;
           public IntPtr fileCallback;
           public IntPtr embeddedResources;
           public IntPtr assetLoaderOptions;
           // public string status;
        }

        [DllImport(DllPath, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.StdCall)]
        private static extern IntPtr ConvertScene(
            FBXContext context
        );

        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
        private struct FBXContext
        {
           public int code; 
           public IntPtr manager;
           public IntPtr ios;
           public IntPtr scene;
           public IntPtr importer;
           public IntPtr stream;
           public IntPtr fileCallback;
           public IntPtr embeddedResources;
           public IntPtr assetLoaderOptions;
           // public string status;
        }

So, both CPP screenshots are meant to have the same FBXContext data, but somehow, it is broken when using the context as a parameter No reason to get 90 in code where the original value was 1 Only if C# is not keeping the original layout order sizeof(void*) and IntPtr.Size are both the same: 8 sizeof(FBXContext) and Marshal.SizeOf<FBXContext>() are both 72

sibber•2y ago

how are you passing the struct to the cpp method and how are you getting it show the signatures of the methods

rick_o_maxOP•2y ago

extern "C" {
    FBX_API FBXContext CALL_CONV ReadStream(void* data, long dataLength, const char* assetLoaderOptions, const char* filename, bool async, FbxProgressCallback onProgress = 0);
    FBX_API RootModel CALL_CONV ConvertScene(FBXContext context);
}

extern "C" {
    FBX_API FBXContext CALL_CONV ReadStream(void* data, long dataLength, const char* assetLoaderOptions, const char* filename, bool async, FbxProgressCallback onProgress = 0);
    FBX_API RootModel CALL_CONV ConvertScene(FBXContext context);
}

First method generates the struct, second one receives the struct c#:

        [DllImport(DllPath, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.StdCall)]
        private static extern FBXContext ReadStream(
            IntPtr data,
            long dataLength,
            [MarshalAs(UnmanagedType.LPStr)]
            string assetLoaderOptions,
            [MarshalAs(UnmanagedType.LPStr)]
            string filename,
            bool async,
            [MarshalAs(UnmanagedType.FunctionPtr)] 
            ProgressCallback onProgress
            );

        [DllImport(DllPath, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.StdCall)]
        private static extern IntPtr ConvertScene(
            FBXContext context
        );

        [DllImport(DllPath, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.StdCall)]
        private static extern FBXContext ReadStream(
            IntPtr data,
            long dataLength,
            [MarshalAs(UnmanagedType.LPStr)]
            string assetLoaderOptions,
            [MarshalAs(UnmanagedType.LPStr)]
            string filename,
            bool async,
            [MarshalAs(UnmanagedType.FunctionPtr)] 
            ProgressCallback onProgress
            );

        [DllImport(DllPath, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.StdCall)]
        private static extern IntPtr ConvertScene(
            FBXContext context
        );

c# call:

            var data = stream.ReadBytes();
            var gcHandle = LockGc(data);
            var context = ReadStream(gcHandle.AddrOfPinnedObject(), data.LongLength, assetLoaderContext.Options.Serialize(), filename, assetLoaderContext.Async, null);// todo
            gcHandle.Free();
            if (context.code != 1)
            {
                throw new Exception();
                //throw new Exception(context.status);
            }
            var scene = ConvertScene(context);

            var data = stream.ReadBytes();
            var gcHandle = LockGc(data);
            var context = ReadStream(gcHandle.AddrOfPinnedObject(), data.LongLength, assetLoaderContext.Options.Serialize(), filename, assetLoaderContext.Async, null);// todo
            gcHandle.Free();
            if (context.code != 1)
            {
                throw new Exception();
                //throw new Exception(context.status);
            }
            var scene = ConvertScene(context);

ConvertScene seems to be messing up the context contents when sending it back to cpp

sibber•2y ago

i tested interoping with passing a simple struct with a pointer and an int back and forth, and everything worked fine no idea why this isnt working for you its probably a good idea to write a wrapper in cpp to make interoping with c# easier

rick_o_maxOP•2y ago

Tks. Trying to find the issue source The proble is in marshalling struct conversion, when passing the struct as a pointer, it works as expected But I cannot rely on that bc I need to readback some info from the structs (In C#)

sibber•2y ago

if youre on .net 7 you can use LibraryImport and see what marshaling its doing

rick_o_maxOP•2y ago

I'm on Unity :c (Mono)

sibber•2y ago

oh oof

rick_o_maxOP•2y ago

These issues makes 0 sense

sibber•2y ago

but your struct doesnt require any marshalling its just pointers and an int

rick_o_maxOP•2y ago

Indeed hmm This struct is allocated in the stack when it comes from CPP Maybe that is the issue? Afaik, C# should just copy the struct contents to managed memory, so it doesn't make much sense either I mean, it is not a blittable type bc of the pointers, I guess

sibber•2y ago

no it doesnt copy anything to managed mem

rick_o_maxOP•2y ago

hmm!

sibber•2y ago

its still on the stack in c#

rick_o_maxOP•2y ago

Managed or unmanaged stack?

sibber•2y ago

theres no managed stack

rick_o_maxOP•2y ago

Got it, it shares the same stack as C then

sibber•2y ago

yeah and with stdcall cpp cleans the stack

rick_o_maxOP•2y ago

So that might be the issue? Lemme test it with __cdecl

sibber•2y ago

probably not but what do you have to lose

rick_o_maxOP•2y ago

Indeed Maybe mono works differently YEEEESSS The problem is the calling convention cdecl works as expected Tks for the tip @Cyberrex

sibber•2y ago

huh thats weird np i tested with both cdecl and stdcall and both worked for me as expected

rick_o_maxOP•2y ago

But you're not on Mono, right?

sibber•2y ago

no still

rick_o_maxOP•2y ago

It might be a Mono implementation bug Unity uses an outdated Mono implementation afaik

sibber•2y ago

yeah because the new versions dont support something that they want or whatever

rick_o_maxOP•2y ago

Now I just have to retrieve a list of nested structs which will give me way more headache...lol

sibber•2y ago

marshalling arrays shouldnt be a problem in theory lol consider writing a wrapper in cpp

rick_o_maxOP•2y ago

Yep, I'm planning to use the "count, array" format Bc they are varying in size A wrapper is a good idea, but I'm already late with this project

sibber•2y ago

rick_o_maxOP•2y ago

And I plan to do only a few calls to CPP and return the structs I need

sibber•2y ago

yeah dynamically allocate the array and return a pointer you can use pointers in c# btw

rick_o_maxOP•2y ago

I see, but I can't force the Unity user to allow unsafe code So.. :/ I also hate to work with pointers in C# Prone to a lot of bugs

sibber•2y ago

if your lib uses unsafe it doesnt mean that projects that use must be unsafe what youre doing is already prone to bugs and youre interoping with cpp

rick_o_maxOP•2y ago

If I do what you're saying, I'll have to pre-compile two different C# lib versions Bc for some platforms, Mono won't use the DLL name when calling the DLL

sibber•2y ago

avoiding pointers isnt a good idea

rick_o_maxOP•2y ago

And uses __Internal instead

sibber•2y ago

if you do what

rick_o_maxOP•2y ago

If I use unsafe code On C#

sibber•2y ago

but unsafe is just a compiler flag afaik

rick_o_maxOP•2y ago

Bc I can't force users enabling unsafe code in the Unity project, so I have to move the unsafe part to a managed DLL But depending on the platform, the managed DLL has to find the native library using the library name or __Internal __Internal is used in platforms like WebGL, iOS On Unity

sibber•2y ago

wdym move to a managed dll its already managed its c#

rick_o_maxOP•2y ago

In Unity, you can use unsafe code, but you have to change a compiler setting. And since I'm developing a plugin, I can't force users enabling this option So I can compile a managed DLL and add it to the Unity project

sibber•2y ago

you dont have to expose pointers to the public api of your plugin

rick_o_maxOP•2y ago

But then comes the problem that Unity uses different DllPath values depending on the platform And I can't change these values inside a compiled DLL, got it?

#if ((UNITY_WEBGL || UNITY_IOS) && !UNITY_EDITOR)
        private const string DllPath = "__Internal";
#else
        private const string DllPath = "TriLibCoreFbxNative";
#endif

#if ((UNITY_WEBGL || UNITY_IOS) && !UNITY_EDITOR)
        private const string DllPath = "__Internal";
#else
        private const string DllPath = "TriLibCoreFbxNative";
#endif

I can't do that on a compiled library And using a compiled library, I can enable unsafe code only for the library itself

sibber•2y ago

i dont get why you need 2 dlls but it doesnt matter do what suits you :) idk how unity works

rick_o_maxOP•2y ago

Bc I can't change #define after the library is already compiled But I can include a different library version depending on the platform the user is compiling to It is complicated

sibber•2y ago

yeah i get that but what does using unsafe in c# change why would you suddenly need to reference a different version for different platforms

rick_o_maxOP•2y ago

If the library is compiled by Unity itself, users have to enable unsafe code in their projects. I can't do that If I put a precompiled managed DLL inside Unity, the DLL itself can use unsafe code

sibber•2y ago

but you dont ship the code you ship the dll dont you?

rick_o_maxOP•2y ago

I can do both

sibber•2y ago

so ship the dll so the users dont have to enable unsafe

rick_o_maxOP•2y ago

But this precompiled DLL would be using the managed code from the lib I was having issues with

sibber•2y ago

and shipping the source doesnt?

rick_o_maxOP•2y ago

But depending on the platform, the name of the called DLL changes, because Mono can't use dynamic C libraries in WebGL and iOS, so it links to static The __Internal library name makes Mono search for the function in static libraries when linking But there comes the problem, I can't change the __Internal to LibraryName stuff to work on all platforms inside an already compiled managed DLL

sibber•2y ago

ohhh so you letting the user compile i dont know what took me so long to get that

rick_o_maxOP•2y ago

Yes, that is why I'm putting all the managed source inside my plugin Np But I swear I found something once that could actually change a #define inside a precompiled managed DLL Or maybe I was just on a bad trip

sibber•2y ago

#define is preprocessor how would something change it after compilation

rick_o_maxOP•2y ago

Yes, but I found something once that seem to do that!? Maybe I just made some confusion back in time

sibber•2y ago

yeah that doesnt make sense

rick_o_maxOP•2y ago

When it gets compiled to ILCode, it loses all info that wasn't precompiled, right?

sibber•2y ago

yeah thats the point

rick_o_maxOP•2y ago

I guess I made some confusion then Anyway. I appreciate a lot bro

Gaming

Programming

Interoperability issues (PInvoke)