Showing warning from a thread

KodzghlyCZ

I am currently stuck at trying to figure out how to display any kind of warning message while in a thread. The problem is that everything I tried leads the app to crash displaying exit code -1073741819 (0xC0000005). This gets even more mysterious when the app sometimes crashes and sometimes it runs just fine. My code is listed down below. I am using a standard implementatioon of the QMainWindow, the class I have problems with is a separate class running an instance of a PHP server, but that's not important. The part where it starts to crash is when I try to verify if there is some other application listening on the port and if so, it shows a warning message. But like I said, the problem is that sometime when I call that warning window and then click the OK button to close it, the whole app crashes or freezes.

I have tried to use QMessageBox and even tkinter windows, but the result is the same.

Is there any way to inform the user of some event while running code in a thread or is it just not possible? Thanks for your replies.

The file where the issue happens

import subprocess
import sys
import threading

from netwTools import netstat

phpProcess = None
phpProcessThread = None


def startPhpServerThread(parent):
    global phpProcess
    global phpProcessThread

    if netstat.checkPortTaken(8001):
        parent.showWarning()

    else:
        cmd = ["C:/Users/kodzghly/PycharmProjects/UURSEM_PY/php/php.exe", "-S", "127.0.0.1:8001"]
        phpProcess = subprocess.Popen(cmd, shell=False, stderr=subprocess.PIPE, universal_newlines=True)
        outBuff = ''
        while True:
            out = phpProcess.stderr.read(1)
            if out == '' and phpProcess.poll() is not None:
                break
            if out == '\n':
                parent.ui.phpConsole.addItem(outBuff)
                outBuff = ''
                sys.stdout.flush()
            if out != '' and out != '\n':
                outBuff += out
                parent.ui.phpConsole.scrollToBottom()


def startPhpServer(parent):
    global phpProcessThread
    phpProcessThread = threading.Thread(target=startPhpServerThread, args=(parent,))
    phpProcessThread.start()


def stopPhpServer(parent):
    parent.ui.phpServerStatusWidget.changeState(2)
    global phpProcess
    if phpProcess is not None:
        phpProcess.terminate()

And the file that runs the whole program and has actually the method to display a warning

import sys

from PySide6 import QtWidgets
from PySide6.QtWidgets import QApplication, QMainWindow, QMessageBox
from guiTools import darkTheme
from phpServer import phpServer
from ui_mainwindow import Ui_MainWindow


class MainWindow(QMainWindow):

    def __init__(self):
        super(MainWindow, self).__init__()
        Ui_MainWindow.__init__(self)
        self.ui = Ui_MainWindow()
        self.ui.setupUi(self)
        self.ui.actionRun.triggered.connect(self.startPhpServer)
        self.ui.actionStop.triggered.connect(self.stopPhpServer)
        self.ui.phpServerStatusWidget.setBetterShape(1)

    def startPhpServer(self):
        phpServer.startPhpServer(self)

    def stopPhpServer(self):
        phpServer.stopPhpServer(self)

    def setPhpServerStatusEnabled(self):
        self.ui.phpServerStatusWidget.changeState(0)

    def setPhpServerStatusDisabled(self):
        self.ui.phpServerStatusWidget.changeState(2)

    def showWarning(self):
        msgBox = QtWidgets.QMessageBox(self)
        msgBox.setWindowTitle("Warning");
        msgBox.setText("Warning test");
        msgBox.exec();
        msgBox.close();


if __name__ == '__main__':
    app = QApplication(sys.argv)
    darkTheme.setDarkTheme(app)
    window = MainWindow()
    try:
        window.show()
        sys.exit(app.exec_())
    finally:
        window.stopPhpServer()

Christian Ehrlicher

@KodzghlyCZ said in Showing warning from a thread:

it shows a warning message

You must not access the ui from outside the main threads. Use signals/slots instead.

KodzghlyCZ

Thanks for your reply, I was not sure if using signals and slots is the right thing to use in this application, but your reply is as clear as it could. I will investigate more in how to use this feature and hopefully I will be able to solve my problem soon.

JKSH

@KodzghlyCZ said in Showing warning from a thread:

Thanks for your reply, I was not sure if using signals and slots is the right thing to use in this application, but your reply is as clear as it could. I will investigate more in how to use this feature and hopefully I will be able to solve my problem soon.

From your thread, emit a signal when you want to raise a warning.

In you GUI thread, implement a slot that displays a warning.

Connect that signal to that slot.

SimonSchroeder

First of all, what the others already suggested is correct and works fine. However, I myself find it to cumbersome to introduce a slot for this. There is a way to execute some function in the context of the event loop of a different thread (ui code has to be executed inside the main thread):

QMetaObject::invokeMethod(qApp, [](){ QMessageBox::warning(...); });

Just make sure to not capture anything by reference in the lambda to avoid lifetime issues.