Hi @Artyom Degtyarev [JB], I want C++ code invoke a callback that is implemented in Kotlin, and that callback need to return a string, is that possible? Currently I declare the callback in C header like this:

typedef const char* (*PCClientOnPreferCodecs)(void*, const char*, const char*);

And my Kotlin code implementing it is:

private fun PCClientCallbackOnPreferCodecs(opaque: COpaquePointer?, peerUidPtr: CPointer<ByteVar>?, sdpPtr: CPointer<ByteVar>?): CPointer<ByteVar>? {
  initRuntimeIfNeeded()
  if (opaque == null) {
    return null
  }
  memScoped {
    return opaque.asStableRef<PeerConnectionClientCallback>().get().onPreferCodecs(peerUid, sdp).cstr.ptr
  }
}

I pass it into C++ with

staticCFunction(::PCClientCallbackOnPreferCodecs)

, and pass

opaque

with

StableRef.create(callback).asCPointer()

. It could be compiled and linked, but at runtime the string I get at C++ is scrambled code.

Ugh, maybe the problem here is that you use

memScoped

block here. Pointers allocated like that become invalid as soon as you get out of the block.

Thanks for your quick reply! But if I don't use

memScoped

, how could I convert

String

to

CPointer<ByteVar>

?

There is an example using nativeHeap in the documentation (https://github.com/JetBrains/kotlin-native/blob/master/INTEROP.md#working-with-the-strings). I am thinking on how it should be done most correctly, but it should be enough to proof the idea.

I tried this, the code looks like:

private fun PCClientCallbackOnPreferCodecs(opaque: COpaquePointer?, peerUidPtr: CPointer<ByteVar>?, sdpPtr: CPointer<ByteVar>?): CPointer<ByteVar>? {
  initRuntimeIfNeeded()
  if (opaque == null) {
    return null
  }
  return opaque.asStableRef<PeerConnectionClientCallback>().get().onPreferCodecs(peerUid, sdp).cstr.getPointer(nativeHeap)
}

But it couldn't compile, it fails with

Type mismatch: inferred type is nativeHeap but AutofreeScope was expected

.

Strings in Kotlin are in UTF-16, so if passed directly to C++ it become char16_t[]

@msink I don't think so, my other code, e.g. kotlin call C++ and pass kotlin String into C++, works fine with

const char*

.

Then it is converted on the fly utf8<->utf16, allocating temporary arrays in nativeHeap, as said above.

Okay... then how could I achieve my goal? Could you please help me?

.cstr.getPointer(nativeHeap)

is broken, I use

.cstr.place(nativeHeap.allocArray(buflen))

instead. But you have to somehow calculate or guess

buflen

here. And somewhere later call

nativeHeap.free(it)

, or else you will have memory leak.

thanks man! I'll try that. is it mandatory to free it in kotlin? can I free it in C++?

Not sure, likely it's different heap, so should be called from Kotlin. But nothing prevents you from wrapping it to Kotlin function callable from C.

Oh yes, that's a great idea!