Hi <@UJRUJP0F9>, I want C++ code invoke a callback...

# multiplatform

[Piasy]

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.

[Artyom Degtyarev [JB]]

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.

[Piasy]

Thanks for your quick reply! But if I don't use memScoped , how could I convert String to CPointer<ByteVar> ?

[Artyom Degtyarev [JB]]

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.

[Piasy]

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 .

[msink]

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

[Piasy]

@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*.

[msink]

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

[Piasy]

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

[msink]

.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.

[Piasy]

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

[msink]

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.

[Piasy]

Oh yes, that's a great idea!