slint::invoke_from_event_loop SegFault (C++)

[gammaEmitter]

Hello!

I'm calling a function in a thread that gets the slint::ComponentHandle<MainApp> as an argument. The function itself works as intended. When the function returns, the program terminates with a SegFault (unfortunately the MacOS debugger doesn't show up anymore, I don't have further description and am not trained to use another debugger ^^).

I narrowed it down to the slint::invoke_from_event_loop causing the SegFault. It is no UAF nor any access to StreamInformation Members that is faulty.

Is this because the std::shared_ptr being held by slint::ComponentHandle<MainApp> is freed at the end of the function scope ? If so, how can I implement my function without that happening?

struct PlotMeterArguments
{
    slint::ComponentHandle<HelloWorld> * App;
    InputStreamValues * streaminformation;
};

pthread_create(&Helper::meter_thread, NULL,PlotMeter,(void*)&arguments);

void * PlotMeter (void * args) {
    pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS,NULL);

    PlotMeterArguments * arguments = (PlotMeterArguments *) args;

    slint::ComponentHandle<HelloWorld> * AppHandle = (slint::ComponentHandle<HelloWorld> *) arguments->App;
    InputStreamValues * StreamInformation = (InputStreamValues * ) arguments->streaminformation;

    Pa_Sleep(250); //wait until other thread opened the stream
    while (StreamInformation->err == Pa_IsStreamActive(StreamInformation->stream))
    {
        
        for (int i = 0; i < FRAMES_PER_BUFFER; i++)
        {
           
            slint::invoke_from_event_loop([&](){(*AppHandle)->fn_set_meter_to_value(0.5f);}); 
        }

    }

    return 0;
}

[ogoffart]

Is this because the std::shared_ptr being held by slint::ComponentHandle is freed at the end of the function scope ? If so, how can I implement my function without that happening?

Probably.
Instead of taking a pointer to slint::ComponentHandle<HelloWorld> * , you should just take the ComponentHandle by value (``slint::ComponentHandlewithout the*`) to make sure the handle stays alive.

[gammaEmitter]

Well, passing slint::ComponentHandle<HelloWorld> by value I need to use slint::ComponentHandle<HelloWorld> App = HelloWorld::create(); as the constructor, because there is no default constructor (compared to the pointer version.
This leads me to the same point that the function is working but the App shuts down after the function returns.

A workaround with passing a slint::ComponentWeakHandle<HelloWorld> then calling lock() ends in a type slint::ComponentWeakHandle<HelloWorld> that does not have the member function anymore.

[ogoffart]

Calling lock() should give back a slint::ComponentHandle<HelloWorld>

[gammaEmitter]

• Calling lock() returned a std::optional<slint::ComponentHandle<HelloWorld>> which my function could not been called from (Member does not exist)
• Calling lock().__get() returned the correct slint::ComponentHandle<HelloWorld> which I could call my function from, yet it terminated the app, wanting me to call the function from slint::invoke_from_event_loop (dispite I did call by it)

Wrong Lines:

slint::ComponentHandle<HelloWorld> * AppHandle = (slint::ComponentHandle<HelloWorld> *) arguments->App;
slint::invoke_from_event_loop([&](){(*AppHandle)->fn_set_meter_to_value(0.5f);}); 

Correct Solution: I was not passing AppHandle in the lambdas capture clause

slint::ComponentHandle<HelloWorld> AppHandle = (slint::ComponentHandle<HelloWorld>) arguments->App;
slint::invoke_from_event_loop([AppHandle](){AppHandle->fn_set_meter_to_value(0.5f);}); 

Thanks again for staying with me!