Found interesting way to crash QML app.

fcarney

I wanted to provide a function to a function as a parameter. I also wanted the default value to be an empty function. This however will crash if I use parameter=function(){} in the function definition:

import QtQuick 2.15
import QtQuick.Window 2.15

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("Crash QML Default Function Parameter")

    // this crashes
    function crashFunction(functionptr=function(){}){ functionptr(); }

    // this does not
    function emptyfunc(){}
    function nocrashFunction(functionptr=emptyfunc){ functionptr(); }

    // this is how I fixed it
    function nocrashFunction(functionptr=null){
        if(functionptr){
            functionptr();
        }
    }
}

I solved this, but I find it interesting that it just kills the application. I would have expected some runtime error of some sort. I suppose this is in a top level function in the app so that exceptions are not caught by the JS engine?

I don't need a fix, I just find this fascinating.

jeremy_k

Indeed, curious. The crash is in an indirect recursive call to QV4::Compiler::Codegen::defineFunction().

Presumably this means that it is dying in the definition of the default value for functionptr. A simplified case triggers the same behavior:

import QtQuick 2.15
Item {
    function crashFunction(functionptr=function(){}){ }
}

Storing the function in a property is another workaround.

property var crashFunction: function(functionptr=function(){}){ }

JKSH

@fcarney said in Found interesting way to crash QML app.:

I would have expected some runtime error of some sort.

Indeed; crashing is never OK.

Would you be willing to submit a bug report to https://bugreports.qt.io/ ?

fcarney

@JKSH said in Found interesting way to crash QML app.:

Would you be willing to submit a bug report to https://bugreports.qt.io/ ?

Yeah, I can do that.

Edit:
https://bugreports.qt.io/browse/QTBUG-98032