I'd like to pass an objective-c object as a void* ...

# kotlin-native

[Soren Roth]

I'd like to pass an objective-c object as a void* that gets used internally in the c api. My interop is generated to expect a CValuesRef<*> for the object. I've seen the guidance to use StableRef to swim "kotlin to kotlin through c", but I am not invoking a callback, the c api is using the objective-c object itself.

cinterop binding: some_c_fun(objcObj: CValuesRef<*>?)
actual c function: some_c_func(void* objcObj)
what I am trying in kotlin: some_c_func(StableRef.create(myObjcObj).asCPointer())

passing in the CValuesRef cpointer is crashing in the c api which I think I understand, the c api would need to call or unwrap to the obj c object before using it. Is there any advice on how I can either construct the appropriate CValueRef or create some new c api that can bridge the divide?

[jw]

What type is myObjcObj today?

[jw]

As an aside: `StableRef`s must be `dispose()`d so you wouldn't want to create orphaned ones like that.

[Soren Roth]

MTLDeviceProtocol
thanks on the dispose tip, not quite there yet.

[jw]

Is that a struct? So like a type that implements CStructVar?

[Soren Roth]

also MTKView and MTLCommandQueueProtocol. all 3 are passed in. Speaking only of MTLDeviceProtocol first...

[jw]

There's a .ptr extension function on CPointed which CStructVar extends from.

[Soren Roth]

not a struct, it is an instance of the interface MTLDeviceProtocol

@kotlinx.cinterop.ExternalObjCClass public interface MTLDeviceProtocol : platform.darwin.NSObjectProtocol

[jw]

Hmm I don't have a project with a darwin source set open right now. Perhaps all you need to do is then pin it? Like .usePinned { }, and then either .objcPtr or addressOf(0). I've done a bit of objective-c interop, but I usually have to feel my way around as compared to raw C interop.

[Soren Roth]

I still need to pass in CValuesRef<*> to the c api where it expects void* parameter. I am guessing I can add some c api in the def file to expect a native pointer, then call my underlying function.

[jw]

addressOf returns a CPointer which is a CValuesRef

[jw]

i don't know what objcPtr returns off the top of my head

[jw]

I would have to go spelunking in a few projects which have Darwin sources to see if I've ever done this with an NSObject subtype that I referenced from Kotlin

[Soren Roth]

this seems to work, at least doesn't crash immediately

device.objcPtr().usePinned { d ->
        commandQueue.objcPtr().usePinned { q ->
            metalView.objcPtr().usePinned { view ->
                my_c_fun(
                    interpretCPointer<CPointed>(d.get()),
                    interpretCPointer<CPointed>(q.get()),
                    interpretCPointer<CPointed>(view.get())
                )
            }
        }
    }

not sure what will happen when the underlying things are referenced

[jw]

Hmm I would think you would call objcPtr on the Pinned, but maybe pinning an objcPtr and calling get is functionally equivalent

[Soren Roth]

that works too. you could write a book about what I dont know on this topic. thanks!

calling objcPtr on the Pinned<MTLDeviceProtocol> was causing some unknown exception in calling a method on it on objc land. Reverting it back to calling objcPtr() on the actual underlying NSObjects makes it work, which makes sense to me.