Problem with C style typed pointers

[DarioDaF]

I might be missing some parts, but I have in a header this definition:

typedef struct _MYSTRUCT *MYSTRUCT ;

with no real reference, just a pointer to an empty struct that is never to be "managed" from inside the code and only passed around, sort of like a void*.

This gets mapped to a MYSTRUCT type as it's correct in C#, but I don't have a way to create an instance of this pointing to null (to then pass to the constructor function etc, typed bool CreateMyStruct(MYSTRUCT s);, I tried MYSTRUCT .__CreateInstance(IntPtr.Zero); but this is short circuited and not updated when passed to the create function, is there an intended way to handle this or a workaround to consume this type of create/destroy C style?

[tritao]

From what I understand you have the following code, which has a typedef to an incomplete decl:

struct _MYSTRUCT;
typedef struct _MYSTRUCT *MYSTRUCT ;

And you are not being able to instantiate such a type in C# throuh the generated bindings?

I think you can try to annotate your native function out pointer parameter with CS_OUT: https://github.com/mono/CppSharp/blob/main/docs/UsersManual.md#helper-defines

[DarioDaF]

I would rather not change the source file if possible since it's downloaded from the origin (so can change), I looked at how it's implemented in the pass and I wrote this:

class MyCustomOverridePass: TranslationUnitPass
{
    public override bool VisitFunctionDecl(Function function)
    {
        if (!base.VisitFunctionDecl(function))
        {
            return false;
        }

        if (function.Name == "CreateMyStruct")
        {
            function.Parameters[0].Usage = ParameterUsage.Out;
        }
        return true;
    }
}

This should be similar to what the macro does, unfortunately this way the parameter is marked out correctly, but the code inside the wrapped is not correct anymore:

// Auto generated
var ____arg0 = s is null ? __IntPtr.Zero : s.__Instance; // ERROR: Access to "out" parameter before assignment
var __arg0 = new __IntPtr(&____arg0);
var ___ret = __Internal.CreateMyStruct(__arg0);
// ERORR: No "out" parameter assignement done
return ___ret;

My fixed manually code (that works fine at runtime):

var ____arg0 = __IntPtr.Zero;
var __arg0 = new __IntPtr(&____arg0);
var ___ret = __Internal.CreateMyStruct(__arg0);
s = MYSTRUCT.__CreateInstance(____arg0);
return ___ret;

The direction is good, I'll have to play around and find out how to bypass the normal wrapper generation, or find if there is something already present for that.

[tritao]

Seems like there is a bug, unfortunately this C-style binding approach is not as well tested in CppSharp.

There is another related property, Class.IsOpaque that might be interesting to look at: https://github.com/mono/CppSharp/blob/main/src/AST/Class.cs#L108

If you can come up with a bug fix in the generator then I'd be very welcome to taking a look and merging it.

[DarioDaF]

It's the first time I use this library so currently I have worked around it by redefining the function manually using _Internal parts, if I manage to have more time I'll look into the csharp generator because it seems a bug in the generator step rather than in the transpiration unit processing.

I'll keep this thread updated in case I start working on some changes for a potential PR