Is it possible to call Kotlin code from a callback...

# kotlin-native

[napperley]

Is it possible to call Kotlin code from a callback on the C side (especially in a concurrent environment), and if so what are the key things to watch out for? The cinterop tool has a severe bug ( https://youtrack.jetbrains.com/issue/KT-55030 ) where it doesn't support C structs/unions that contain bit fields. I am trying to work around the issue which is proving to be very difficult to deal with.

[Ignat Beresnev]

Is it possible to call Kotlin code from a callback on the C side

Haven't tried it, but I remember reading about it in the docs.

Looks like you can convert a Kotlin function to a C pointer and use it as a callback

https://kotlinlang.org/docs/native-c-interop.html#callbacks

[napperley]

That won't do much good since it is done on the Kotlin side. I am trying to do the inverse if that makes sense.

[Jeff Lockhart]

You can use staticCFunction. The lambda can't capture any values though.

[Jeff Lockhart]

Ah, that's what Ignat mentioned. I guess I don't understand your question.

[napperley]

With the C side I am talking about C code inside a def file.

[Jeff Lockhart]

So you're defining a static function inside the .def file and want to be able to call Kotlin code from there? You might try declaring the Kotlin function's C function signature with extern and calling that.

[vbsteven]

You want to have a kotlin function that is passed as a function pointer to a C library so the C library can call it?

Then you likely want staticCFunction as mentioned by others. Use staticCFunction on the kotlin side and put it into a val, and then pass yourVal.CPointer() as the "callback" function pointer.

The lambda cannot capture values, but typically a C function for configuring a callback will take a void * data pointer which will be passed as an argument to the callback. You can use a cinterop StableRef to create a pointer to your Kotlin data, and in the staticCFunction lambda, get your kotlin data back.

[vbsteven]

Hmm, I might have also understood your question wrong. I think what you are looking for is more something like this https://kotlinlang.org/docs/native-dynamic-libraries.html#use-generated-headers-from-c