Hello! Is there any way to make `staticCFunction` ...

# kotlin-native

[Zhang Zihan]

Hello! Is there any way to make staticCFunction access non-global variables/functions?
For example, in this function, I had to use Mutex, which is ugly and poorly readable 😢

private val tempList = mutableListOf<String>()
private val mutex = Mutex()
suspend fun getWindowTexts(): List<String> {
    return mutex.withLock {
        EnumWindows(
            lpEnumFunc = staticCFunction { hWnd, lParam ->
                val text = memScoped {
                    val textVar = allocArray<UShortVar>(256)
                    GetWindowText!!(hWnd, textVar, 256)
                    textVar.toKString()
                }
                tempList.add(text)
                1
            }, 0L
        )
        val list = tempList.toList()
        tempList.clear()
        list
    }
}

[ephemient]

no, because plain C function pointers are not closures

[Thomas]

You can use StableRef to pass a Kotlin object (like this). Pass it to lParam parameter and you can access it from the callback. Make sure to dispose the StableRef when no longer needed, but not any earlier.

[Thomas]

I think this will work (untested)

val list = mutableListOf<String>()
val stableRef = StableRef.create(list)
EnumWindows(
    lpEnumFunc = staticCFunction { hWnd, pParam ->
        val stableRef2 =   pParam.toCPointer<CPointed>()!!.asStableRef<MutableList<String>>()
        val list2 = stableRef2.get()
        // TODO use list2
        1
    },
    lParam = stableRef.asCPointer().toLong()
)
stableRef.dispose()
// TODO use list

[ephemient]

right, the callback's lParam can be anything you want, and a StableRef is a great way to get pointers to Kotlin objects across the C interface

[ephemient]

it could even be a Kotlin lambda,

val cb: (String) -> Boolean = { list.add(it) }
val stableRef = StableRef.create(cb)
EnumWindows(
    lpEnumFunc = staticCFunction { hWnd, param ->
        val cb: (String) -> Unit = pParam.toCPointer<CPointed>()!!.asStableRef()
        val text = memScoped { ... }
        if (cb(text)) 1 else 0.also { SetLastError(ERROR_something) }
    },
    lParam = stableRef.asCPointer().toLong(),
)