Getting segfault in qt5serialport.dll with PyQt and signals

Question

I'm currently struggling with a weird segfault I'm getting when using QSerialPort and emitting a signal, with Windows XP SP3, Python 3.3.3 and PyQt5 5.1.1.

I really wonder if this happens in pure Qt with C++ as well, but my C++ is horrible and I don't have a running toolchain set up.

Minimal example code (sorry, couldn't manage to simplify more):

#!/usr/bin/python

import sys

from PyQt5.QtWidgets import QWidget, QApplication, QPushButton
from PyQt5.QtCore import QObject, pyqtSlot, QIODevice, pyqtSignal
from PyQt5.QtSerialPort import QSerialPort

PORT = 'COM59'

class DVIFlasher(QObject):
    ser = None
    flash_done = pyqtSignal(int, int)

    def __init__(self):
        super(DVIFlasher, self).__init__()
        self.ser = QSerialPort(PORT)
        self.ser.readyRead.connect(self.read)

    def flash(self):
        print(self.ser.open(QIODevice.ReadWrite))
        self.ser.setBaudRate(115200)

    def read(self):
        print("Got data")
        self.ser.close()
        self.flash_done.emit(0, 0)

class Widget(QWidget):
    def __init__(self):
        super(Widget, self).__init__()
        self.btn = QPushButton("Start", self)
        self.flasher = DVIFlasher()
        self.btn.clicked.connect(self.flasher.flash)
        self.resize(self.btn.sizeHint())

if __name__ == '__main__':
    app = QApplication(sys.argv)
    win = Widget()
    win.show()
    sys.exit(app.exec_())

When I run this and receive a byte via the serial port, I get an "Access violation" (aka segfault) in qt5serialport.dll. Sometimes it happens when clicking "Start" the first time, sometimes after the second time.

Some things I observed so far:

• The signal needs to have a bool/int as first parameter, and some other parameter -- otherwise it doesn't seem to trigger.

• When I emit the signal before the port has been closed (self.ser.close()), it doesn't seem to trigger.

• On Linux (Archlinux, Python 3.3.2, PyQt 5.1.1) I can't reproduce this, but after starting a second time, I always get QSocketNotifier: Invalid socket specified.

What gives? Am I doing something wrong? Or is this a bug in [Py]Qt somewhere?

Answer 1

You are facing this bug. This is a relative old issue and known. If we are lucky, it can be fixed for 5.2.1. Your workaround about not calling ser.close() in the readyRead signal handler looks reasonable.

Hope, this helps.