golang qml (go-qml) cgo argument has Go pointer to Go pointer

Question

Hi I am using the qml library for go to create UI's. I am trying to learn how to pass information from the UI (qml) to go to then "do something" with. QML is working if it is just a UI. I can run that fine when I do:

func main() {
    if len(os.Args) != 2 {
        fmt.Fprintf(os.Stderr, "usage: %s <qml file>\n", os.Args[0])
        os.Exit(1)
    }
    if err := qml.Run(run); err != nil {
        fmt.Fprintf(os.Stderr, "error: %v\n", err)
        os.Exit(1)
    }
}

func run() error {
    engine := qml.NewEngine()

    engine.On("quit", func() { os.Exit(0) })

    component, err := engine.LoadFile(os.Args[1])
    if err != nil {
        return err
    }
    window := component.CreateWindow(nil)
    window.Show()
    window.Wait()
    return nil
}

However when I add some code, to try and "learn" something from the UI I get the run time error:

panic: runtime error: cgo argument has Go pointer to Go pointer

The code I am adding is:

window.On("visibleChanged", func(visible bool) {
    if (visible) {
            fmt.Println("Width:", window.Int("width"))
    }
})

I am running "go version go1.6 darwin/amd64" on OSX El Capitan

Any ideas why? A google suggests this was an error in Go 1.6 Beta, but I am running the latest stable version (installed a couple of days ago).

If it's not a simple fix, can someone explain "why" this is occuring?

Answer 1

If you're just playing around, I suggest trying with go 1.5.3. Go 1.6 introduced a different set of constraints on pointers to memory when using cgo, a more restrictive set, and it's possible some go packages that were designed for the older version of go are now breaking a go rule or two.

If this is the case, getting the older package to work with go 1.6, where C is allowed to call go closures, could be harder to fix. But I don't have first hand experience with that yet.