superlou superlou - 1 day ago 5x
Python Question

Simple PyQt5 QML application causes segmentation fault

In trying to get a very basic PyQt5 QML example to run, I found that I get a segmentation fault. I verified that it only seems deal with displaying QML since an example without a window runs fine. I tried the following minimial test:

import sys
from PyQt5.QtCore import QUrl
from PyQt5.QtWidgets import QGuiApplication
from PyQt5.QtQml import QQmlApplicationEngine

# Main Function
if __name__ == '__main__':
app = QGuiApplication(sys.argv)
engine = QQmlApplicationEngine("simple.qml")


import QtQuick 2.5
import QtQuick.Controls 1.4

ApplicationWindow {
width: 300
height: 200
title: "Simple"
visible: true

When I run this application, a window appears for a split second before closing like in the more detailed example, and I receive
Segmentation fault
in the console (and nothing more).

Running from GDB shows that the
is receiving the SIGSEGV:

(gdb) run
Starting program: /usr/bin/python3
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/".
[New Thread 0x7fffe912b700 (LWP 17200)]
[New Thread 0x7fffe3dbb700 (LWP 17201)]
[New Thread 0x7fffe1442700 (LWP 17202)]
[New Thread 0x7fffdbfff700 (LWP 17203)]

Thread 5 "QSGRenderThread" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fffdbfff700 (LWP 17203)]
__strstr_sse2 (haystack_start=0x0, needle_start=0x7fffe28c9dd0 "nouveau") at ../string/strstr.c:63
63 ../string/strstr.c: No such file or directory.

The backtrace follows:

#0 __strstr_sse2 (haystack_start=0x0, needle_start=0x7fffe28c9dd0 "nouveau") at ../string/strstr.c:63
#1 0x00007fffe27233ea in QSGRenderContext::initialize(QOpenGLContext*) ()
from /usr/local/lib/python3.5/dist-packages/PyQt5/Qt/qml/QtQuick.2/../../lib/
#2 0x00007fffe273e979 in ?? ()
from /usr/local/lib/python3.5/dist-packages/PyQt5/Qt/qml/QtQuick.2/../../lib/
#3 0x00007ffff56835f9 in ?? () from /usr/local/lib/python3.5/dist-packages/PyQt5/Qt/lib/
#4 0x00007ffff7bc16fa in start_thread (arg=0x7fffdbfff700) at pthread_create.c:333
#5 0x00007ffff78f7b5d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109

If I run the QML file from a C++ application, there is no segmentation fault and the application works. Note that I'm using PyQT without PyOtherside, but the symptoms seem similar.

Is there a way to get more information to continue debugging?

I am running Python 3.5.2 on Linux Mint 18. My QT version is 5.7.0, my PyQt version is 5.7, and my SIP version is 4.18.1.


The rabbit hole went deeper than I thought. There were a few problems.

  1. The version of SIP in the Linux Mint 18 (likely Ubuntu 16.04) repository was too old. I updated to 4.18.1 by installing from source.
  2. With an updated SIP version, I was able to compile the latest PyQt5 from source. Before compiling, however, I had to install libqt5opengl5-dev so that it would compile with the QtOpenGL module. After compiling, I installed using checkinstall to make rolling back easier.

With the latest SIP and PyQt5, I no longer received the segmentation fault. However, I had issues with the OpenGL shaders not being created properly and just received a plain white window.

The workaround for this bug is to include the following before loading PyQt in the Python program:

import ctypes
from ctypes import util
ctypes.CDLL(util.find_library('GL'), ctypes.RTLD_GLOBAL)

I believe this is unique to Ubuntu-based OSs with NVIDIA graphics cards and binary drivers.