Given the WinAPI `CreateThread` function, how do I...

# kotlin-native

[Edoardo Luppi]

Given the WinAPI CreateThread function, how do I correctly pass a this reference (as a pointer) as a parameter to entryPoint?

CreateThread(
  lpThreadAttributes = null,
  dwStackSize = 0u,
  lpStartAddress = staticCFunction(::entryPoint),
  lpParameter = this,
  dwCreationFlags = 0u,
  lpThreadId = null,
)

...

private fun entryPoint(parameter: COpaquePointer?): UInt {
  TODO()
}

[Landry Norris]

You'll need to tell the gc to mark the Kotlin object as stable (can't be moved or collected) for as long as it's needed. You can use StableRef.create or call 'pin' on the object. Make sure to dispose the StableRef or unpin depending on which you use when it's no longer needed.

[Edoardo Luppi]

@Landry Norris so basically something like

StableRef.create(this).asCPointer()

[Landry Norris]

Yes. When you're done, you can convert the CPointer back to StableRef and dispose it. This tells the GC you don't need the variable to stay in the same place anymore.

[Edoardo Luppi]

@Landry Norris this only thing I'm not sure about is I'm doing all this on this, like I'm creating a stable ref to myself basically

[Edoardo Luppi]

Hopefully it doesn't break somewhere else

[Edoardo Luppi]

In regard to staticCFunction, I suppose that's ok as it is now.

[Landry Norris]

I believe it's fine to create a StableRef on 'this', since it's a form of manual memory management. The memory holding 'this' can't be gc'ed while the StableRef is active, and it's fine if the StableRef gets gc'ed. It's just a binding to some runtime functions. Just make sure to dispose the StableRef when it's not needed anymore.

[Edoardo Luppi]

The object I'm writing exposes a dispose function, to be used by clients.
I suppose I'll dispose the StableRef inside dispose

[Edoardo Luppi]

One last question to complete the puzzle.

parameter: COpaquePointer?

This parameter will end up representing the this instance.
How do I dereference the pointer and change its representation to this class?

[Landry Norris]

If I remember correctly, you can call '.asStableRef().get<Foo>()' on the pointer.

[Edoardo Luppi]

Yup, you're right!

parameter?.asStableRef<MyClass>()?.get()

[Dominaezzz]

Considering the use case, you can just dispose the StableRef in your staticCFunction as the very first thing it does.

[Edoardo Luppi]

Thanks @Dominaezzz, it makes sense, as then everything is handled after the entry point.