Hey guys, I'm making Kotlin Multiplatform work on ...

# multiplatform

[Piasy]

Hey guys, I'm making Kotlin Multiplatform work on Windows now, but I have an issue of calling Kotlin back from C++.

Here is my use case: I want use socket.io-client-cpp to connect to a SocketIO server, and get a callback when it connects successfully. But the callback is not invoked on main thread, I do notice the note on the C Interop page, calling kotlin.native.initRuntimeIfNeeded() in my kotlin code before I connect to server, but it still crash my program. If I fire the callback on main thread, e.g. APP C++ code calls kotlinTestFunc , which calls cppSocketIOConnect , and fire the callback directly inside cppSocketIOConnect , it works fine.

Do I miss something?

[Artyom Degtyarev [JB]]

Can you share more details on the error?

[Piasy]

Sorry, visual studio just says Exception thrown:

Exception thrown at 0x004FEFB0 in WindowsExample.exe: 0xC0000005: Access violation executing location 0x004FEFB0.

I write it to crash dump and analyze it with dbghelper, but the stack also doesn't give helpful message, the attachment is the full analyze output.

crash-analyze.txt

[Piasy]

Sorry the previous attachment may contain duplicate content, please check this one, thanks!

crash-analyze-2.txt

[Piasy]

Oh sorry @Artyom Degtyarev [JB], I find the reason! It turns out that I do the wrong capture in C++ lambda, how stupid am I!

Here is the working code snippet:

void SioOn(void* sio, SIOListener2 listener) {
    reinterpret_cast<sio::client*>(sio)->set_socket_open_listener(
        [=](std::string const& nsp) {
            listener();
    });
}

Previously I capture it by reference [&] .

[Piasy]

Oh btw, do you know how to create .pdb when compile kotlin code to .dll on windows? @Artyom Degtyarev [JB]

[Artyom Degtyarev [JB]]

Seems like there is no such option for now. The best you can get here is a dwarf-compatible debug info, see here(https://github.com/JetBrains/kotlin-native/blob/master/DEBUGGING.md).

[Piasy]

Thanks! I'll check it out.

[Piasy]

Hi @Artyom Degtyarev [JB], I have a follow up question in the non-main thread case: I want to pass some data when the callback is fired in C++ code, e.g. a integer, but if I access the Int in kotlin code, my program crashed with error: Exception thrown at 0x6514A0E9 (AvConf.dll) in WindowsExample.exe: 0xC0000005: Access violation reading location 0x00000020. if I don't access the integer, it works fine, if I fire the callback directly in main thread, it also works fine.

[Artyom Degtyarev [JB]]

I’m sorry, but I’m missing something here. Are you sending a Kotlin’s StableRef to the callback or it is a C++ int that the Kotlin function takes as input?

[Piasy]

It is a C++ int that the Kotlin function takes as input.

Here is my C++ code:

typedef void(*SIOListener)(int);
void SioOn(void* sio, const char* event, SIOListener listener) {
    reinterpret_cast<sio::client*>(sio)->set_socket_open_listener(
        [=](std::string const& nsp) {
        listener(0);
    });
}

Here is my kotlin code:

private fun SioListenerFunction2(len: Int) {
  Logging.error("SIO", "SioListenerFunction2")
  Logging.error("SIO", "len $len")
}

NKBundle.SioOn(socket, event, staticCFunction(::SioListenerFunction2))

If I don't log len param, or if I invoke listener directly, it works fine.

[Piasy]

btw, the socket variable is created by a C++ function, so don't worry about it.

[Artyom Degtyarev [JB]]

Try to call initRuntimeIfNeeded() here, before logging.

[Artyom Degtyarev [JB]]

Also, I got some suspicions about passing value like that, maybe one should create a parameter with default value like

[=](std::string const& nsp, int zero = 0) {
        listener(zero);
    }

, but it’s just a guess.

[Piasy]

Hi @Artyom Degtyarev [JB], calling initRuntimeIfNeeded() does the magic! So the note in C Interop page means it should be called inside the callback right?

And regarding your comment on C++ code, actually passing 0 back is only for test purpose, there shall be more data from C++, and the int is the length of an array.

Thank you very much!