Hi everyone, I'm working on a GIR generator to cre...

# kotlin-native

[Roberto Leinardi]

Hi everyone, I'm working on a GIR generator to create kotlin native bindings for GTK and other GObject based libraries. I reached a stage where I can almost compile the bindings for GLib but I'm stuck with the handling of `vararg`s.

For example g_string_append_printf, which is mapped with this:

@kotlinx.cinterop.internal.CCall public external fun g_string_append_printf(string: kotlinx.cinterop.CValuesRef<native.glib.GString /* = native.glib._GString */>?, @kotlinx.cinterop.internal.CCall.CString format: kotlin.String?, vararg variadicArguments: kotlin.Any?): kotlin.Unit { /* compiled code */ }

It seems to expect a vararg variadicArguments: kotlin.Any?, but if I send that to it like this:

public fun appendPrintf(format: String, vararg variadicArguments: Any): Unit {
        g_string_append_printf(cPointer.reinterpret(), format, variadicArguments)
    }

I get this error when I build the module:

type kotlin.Array<out kotlin.Any>  is not supported here: doesn't correspond to any C type

The GIR parameter only gives a name (...) and has a tag `varargs`:

<parameter name="..." transfer-ownership="none">
            <doc xml:space="preserve"
                 filename="glib-2.0.c"
                 line="32361">the parameters to insert into the format string</doc>
            <varargs/>
          </parameter>

But there are no other information about the type.

I am an Android developer and not very familiar with C Interop. Does anyone have a suggestion of what this vararg variadicArguments: kotlin.Any? should be converted to?

[Adam S]

handling interop for varargs is an open issue https://youtrack.jetbrains.com/issue/KT-56164

There might be some sort of workaround by writing a custom C function, but I haven’t tried it

[Roberto Leinardi]

Oh wow, so are you saying that everything that takes a vararg cannot currently be called by Kotlin Native?

[Adam S]

varargs can be called from Kotlin Native, but not vararg Any - but there’s a workaround (maybe - I’m not 100% sure)

[Roberto Leinardi]

ok, I don't know if I'm saying something completely wrong but, could it be a partial workaround to check if the vararg is one of the CValues<ShortVar>, CValues<ByteVar>(), etc type and forward that to the native call and throw an IllegalArgumentException if is not one of the supported type?
I don't know if this make sense, I just saw that the compiler doesn't throws error if I send cValuesOf(1, 2, 3) instead of the vararg

[Adam S]

yeah exactly, if you tell Kotlin what the type is then it should work

https://stackoverflow.com/a/71442518/4161471

[Roberto Leinardi]

OK, the compiler seems to be happy if I send this values: CValues<out CPrimitiveVar>
Could this be a good option?

[Roberto Leinardi]

Since I'm generating the code with KotlinPoet I can add easily as many variable arguments as I want. Isn't pretty but, if it works...

[Roberto Leinardi]

Isn't pretty, but if it works:

public fun appendPrintf(format: String, vararg variadicArguments: CValues<out CPrimitiveVar>): Unit {
        when (variadicArguments.size) {
            0 -> g_string_append_printf(cPointer.reinterpret(), format)
            1 -> g_string_append_printf(cPointer.reinterpret(), format, variadicArguments[0])
            2 -> g_string_append_printf(cPointer.reinterpret(), format, variadicArguments[0], variadicArguments[1])
            3 -> g_string_append_printf(cPointer.reinterpret(), format, variadicArguments[0], variadicArguments[1], variadicArguments[2])
            else -> error("appendPrintf() can only accept up to 3 variable number of arguments (vararg)")
        }
    }

[Big Chungus]

So am I (as a passion project to learn gtk)! Is your project oss for me to have a peek for ideas? What online resources did you use to get started? I found google very disappointing when trying to surface any gir documentation.

[Roberto Leinardi]

Hey, sure, it's completely OSS! You can find it here and it would make me really happy to get some feedback!
The project includes the latest bindings, but they do not build yet.
As for the documentation, I tried to join the effort to improve the https://gitlab.com/gtk-kt/gir-kt, but the main maintainer lost interest.
I then decided to port the gir generator from https://github.com/bailuk/java-gtk: It seems to me more complete than the one from gir-kt.
I just started looking at how the java code is generated and, from there, I'm now writing the KotlinPoet code to generate the Kotlin Native code.

My goal is to make the gir generator generic enough to be able to port any GObject based library. That's why I called the project KtGObject instead of KtGTK or GTK-Kotlin.

If you want to get your hands dirty, please let me know! 🙂

[Clocks]

Varargs are impossible.

They are done in C at compile time, and C does not support vararg manipulation like JVM does.

[Clocks]

That is why there are numerous notes through gtk-kt & disabled warnings in gir-kt about vararg methods

[Roberto Leinardi]

this is how I'm generating the code now for the varargs, but I doubt that CPointer<out CPrimitiveVar> will cover all the cases. I can't use Any, @Clocks do you have any suggestion for a better type?

[Big Chungus]

Doesn't this only ever pass a single arg to g_string_printf regardless of va size? What am I missing here?

[Roberto Leinardi]

oh damn, I need to sleep 😅 Yep, thanks for pointing that out

[Big Chungus]

Phew, thought I was going bonkers myself 😀

[ephemient]

platform-specific ASM could set up the stack for va_list or C varargs, if you really need dynamic varargs. there's no platform-independent way to do it.

[Roberto Leinardi]

@Big Chungus thanks again, it would have took me some time to notice it

[Roberto Leinardi]

(fixing the one with size 0, just notice that there is a va too much in there)

[ephemient]

maybe just a poor example, but… printf accepts non-pointer arguments as well, which aren't necessarily passed the same way pointers are