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  4. Basic application crashes in QGuiApplication constructor if QCoreApplication::instance is called.

Basic application crashes in QGuiApplication constructor if QCoreApplication::instance is called.

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  • hskoglundH Offline
    hskoglundH Offline
    hskoglund
    wrote last edited by
    #2

    If you omit the "-std=c++23 -" spec when compiling manually, does it still crash?

    1 Reply Last reply
    0
    • J Offline
      J Offline
      Jakob Kenda
      wrote last edited by Jakob Kenda
      #3

      Yes.

      $ make -j2
g++ -I/Qt/6.10.1/gcc_64/include -L/Qt/6.10.1/gcc_64/lib -o crash crash.cpp -lQt6Core -lQt6Gui
g++ -I/Qt/6.10.1/gcc_64/include -L/Qt/6.10.1/gcc_64/lib -o nocrash nocrash.cpp -lQt6Core -lQt6Gui
$ LD_LIBRARY_PATH=~/Qt/6.10.1/gcc_64/lib ./crash
Segmentation fault (core dumped)

      Here's the backtrace from GDB:

      #0  doActivate<false> (sender=0x0, signal_index=9, argv=0x7fffffffd3d0) at /home/qt/work/qt/qtbase/src/corelib/kernel/qobject.cpp:4137
#1  0x00007ffff79e0867 in QMetaObject::activate (sender=<optimized out>, m=m@entry=0x7ffff76bb2a0 <QGuiApplication::staticMetaObject>, local_signal_index=local_signal_index@entry=1,
    argv=argv@entry=0x7fffffffd3d0) at /home/qt/work/qt/qtbase/src/corelib/kernel/qobject.cpp:4317
#2  0x00007ffff6ddef16 in QMetaObject::activate<void, QScreen*> (ret=0x0, local_signal_index=1, mo=0x7ffff76bb2a0 <QGuiApplication::staticMetaObject>, sender=<optimized out>)
    at /home/qt/work/qt/qtbase/src/corelib/kernel/qobjectdefs.h:319
#3  QGuiApplication::screenAdded (this=<optimized out>, _t1=<optimized out>) at /home/qt/work/qt/qtbase_build/src/gui/Gui_autogen/include/moc_qguiapplication.cpp:303
#4  0x00007ffff6e5d05f in QWindowSystemInterface::handleScreenAdded (platformScreen=platformScreen@entry=0x555555595ae0, isPrimary=isPrimary@entry=false)
    at /home/qt/work/qt/qtbase/src/gui/kernel/qwindowsysteminterface.cpp:730
#5  0x00007ffff371674c in QtWaylandClient::QWaylandDisplay::handleScreenInitialized (this=0x555555589ee0, screen=<optimized out>, screen@entry=0x555555595ae0)
    at /home/qt/work/qt/qtbase/src/plugins/platforms/wayland/qwaylanddisplay.cpp:609
#6  0x00007ffff373fae3 in QtWaylandClient::QWaylandScreen::maybeInitialize (this=0x555555595ae0) at /home/qt/work/qt/qtbase/src/plugins/platforms/wayland/qwaylandscreen.cpp:82
#7  QtWaylandClient::QWaylandScreen::maybeInitialize (this=0x555555595ae0) at /home/qt/work/qt/qtbase/src/plugins/platforms/wayland/qwaylandscreen.cpp:73
#8  0x00007ffff3659b16 in ffi_call_unix64 () at ../src/x86/unix64.S:104
#9  0x00007ffff36563ef in ffi_call_int (cif=cif@entry=0x7fffffffd610, fn=<optimized out>, rvalue=<optimized out>, avalue=<optimized out>, closure=closure@entry=0x0) at ../src/x86/ffi64.c:673
#10 0x00007ffff36590be in ffi_call (cif=0x7fffffffd610, fn=<optimized out>, rvalue=<optimized out>, avalue=<optimized out>) at ../src/x86/ffi64.c:710
#11 0x00007ffff3664bfe in ?? () from /lib/x86_64-linux-gnu/libwayland-client.so.0
#12 0x00007ffff3665473 in ?? () from /lib/x86_64-linux-gnu/libwayland-client.so.0
#13 0x00007ffff366571c in wl_display_dispatch_queue_pending () from /lib/x86_64-linux-gnu/libwayland-client.so.0
#14 0x00007ffff366850f in wl_display_roundtrip_queue () from /lib/x86_64-linux-gnu/libwayland-client.so.0
#15 0x00007ffff37156d8 in QtWaylandClient::QWaylandDisplay::initialize (this=0x555555589ee0) at /home/qt/work/qt/qtbase/src/plugins/platforms/wayland/qwaylanddisplay.cpp:399
#16 QtWaylandClient::QWaylandDisplay::initialize (this=0x555555589ee0) at /home/qt/work/qt/qtbase/src/plugins/platforms/wayland/qwaylanddisplay.cpp:388
#17 0x00007ffff373845d in QtWaylandClient::QWaylandIntegration::init (this=this@entry=0x555555593370) at /home/qt/work/qt/qtbase/src/plugins/platforms/wayland/qwaylandintegration.cpp:108
#18 0x00007ffff7fa7389 in QtWaylandClient::QWaylandIntegrationPlugin::create (this=<optimized out>, system=..., paramList=...) at /home/qt/work/qt/qtbase/src/plugins/platforms/wayland/main.cpp:33
#19 0x00007ffff6de4b1f in init_platform (pluginNamesWithArguments=..., platformPluginPath=..., platformThemeName=..., argc=@0x7fffffffdffc: 1, argv=argv@entry=0x7fffffffe1a8)
    at /home/qt/work/qt/qtbase/src/gui/kernel/qguiapplication.cpp:1308
#20 0x00007ffff6de82a7 in QGuiApplicationPrivate::createPlatformIntegration (this=0x555555581140) at /home/qt/work/qt/qtbase/src/gui/kernel/qguiapplication.cpp:1590
#21 0x00007ffff6de8d18 in QGuiApplicationPrivate::createEventDispatcher (this=<optimized out>) at /home/qt/work/qt/qtbase/src/gui/kernel/qguiapplication.cpp:1608
#22 0x00007ffff798b071 in QCoreApplicationPrivate::init (this=this@entry=0x555555581140) at /home/qt/work/qt/qtbase/src/corelib/kernel/qcoreapplication.cpp:865
#23 0x00007ffff6debd15 in QGuiApplicationPrivate::init (this=0x555555581140) at /home/qt/work/qt/qtbase/src/gui/kernel/qguiapplication.cpp:1636
#24 0x00007ffff6ded524 in QGuiApplication::QGuiApplication (this=0x7fffffffe010, argc=@0x7fffffffdffc: 1, argv=0x7fffffffe1a8) at /home/qt/work/qt/qtbase/src/gui/kernel/qguiapplication.h:175
#25 0x00005555555552d4 in main ()

      Looks like a null pointer dereference to me (sender in doActivate is 0).

      1 Reply Last reply
      0
      • J Jakob Kenda

        This is a minimal reproducible example.

        It's a really simple Qt application that constructs a QGuiApplication and then tries to output QGuiApplication::applicationDisplayName.
        There are two versions - one that crashes and one that doesn't:

        $ cat crash.cpp
#include "QtGui/qguiapplication.h"
#include "QtCore/qdebug.h"

int main(int argc, char** argv)
{
        QGuiApplication app(argc, argv);
        qDebug() << qApp->applicationDisplayName();
}
$ cat nocrash.cpp
#include "QtGui/qguiapplication.h"
#include "QtCore/qdebug.h"

int main(int argc, char** argv)
{
        QGuiApplication app(argc, argv);
        qDebug() << app.applicationDisplayName();
}

        They are compiled using a simple Makefile:

        $ cat Makefile
CXX=g++ -std=c++23

Qt6_DIR=${HOME}/Qt/6.10.1/gcc_64
Qt6_LIBS=-lQt6Core -lQt6Gui

QT_INCLUDE=-I${Qt6_DIR}/include -L${Qt6_DIR}/lib
QT_LINK=${Qt6_LIBS}

.PHONY: all clean

all: crash nocrash

crash: crash.cpp
        ${CXX} ${QT_INCLUDE} -o crash crash.cpp ${QT_LINK}

nocrash: nocrash.cpp
        ${CXX} ${QT_INCLUDE} -o nocrash nocrash.cpp ${QT_LINK}

clean:
        rm -f crash nocrash

        Compiling and executing the applications:

        $ make -j2
g++ -std=c++23 -I/Qt/6.10.1/gcc_64/include -L/home/jakobk/Qt/6.10.1/gcc_64/lib -o crash crash.cpp -lQt6Core -lQt6Gui
g++ -std=c++23 -I/Qt/6.10.1/gcc_64/include -L/home/jakobk/Qt/6.10.1/gcc_64/lib -o nocrash nocrash.cpp -lQt6Core -lQt6Gui
$ LD_LIBRARY_PATH=/Qt/6.10.1/gcc_64/lib ./crash
Segmentation fault (core dumped)
$ LD_LIBRARY_PATH=~/Qt/6.10.1/gcc_64/lib ./nocrash
"nocrash"

        To me, this looks like a bug in Qt. Am I missing something?
        The reason I'm not sure is that both versions work fine when compiled with CMake using find_package:

        cmake_minimum_required(VERSION 3.16)
project(qt-crash-minimal)

set(Qt6_DIR ~/Qt/6.10.1/gcc_64/lib/cmake/Qt6)
find_package(Qt6 REQUIRED COMPONENTS Core Gui)

add_library(qt INTERFACE)
target_link_libraries(qt INTERFACE Qt::Core Qt::Gui)

add_executable(crash crash.cpp)
target_link_libraries(crash PUBLIC qt)

add_executable(nocrash nocrash.cpp)
target_link_libraries(nocrash PUBLIC qt)

        $ ./crash
"crash"
$ ./nocrash
"nocrash"

        In my opinion there shouldn't be a difference in Qt's behaviour whether it's compiled with CMake or "manually" so this must be a bug in Qt.

        Christian EhrlicherC Offline
        Christian EhrlicherC Offline
        Christian Ehrlicher
        Lifetime Qt Champion
        wrote last edited by
        #4

        @Jakob-Kenda said in Basic application crashes in QGuiApplication constructor if QCoreApplication::instance is called.:

        LD_LIBRARY_PATH=/Qt/6.10.1/gcc_64/lib ./crash

        You are missing a tilde here

        Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
        Visit the Qt Academy at https://academy.qt.io/catalog

        J 1 Reply Last reply
        0
        • Christian EhrlicherC Christian Ehrlicher

          @Jakob-Kenda said in Basic application crashes in QGuiApplication constructor if QCoreApplication::instance is called.:

          LD_LIBRARY_PATH=/Qt/6.10.1/gcc_64/lib ./crash

          You are missing a tilde here

          J Offline
          J Offline
          Jakob Kenda
          wrote last edited by
          #5

          @Christian-Ehrlicher No, that was just me not wanting to put the path to my home directory on here for some reason. That wouldn't result in a crash but in an error while loading shared libraries message.

          jsulmJ 1 Reply Last reply
          0
          • J Jakob Kenda

            @Christian-Ehrlicher No, that was just me not wanting to put the path to my home directory on here for some reason. That wouldn't result in a crash but in an error while loading shared libraries message.

            jsulmJ Offline
            jsulmJ Offline
            jsulm
            Lifetime Qt Champion
            wrote last edited by
            #6

            @Jakob-Kenda Maybe because of https://doc.qt.io/qt-6/qguiapplication.html#qGuiApp vs https://doc.qt.io/qt-6/qapplication.html#qApp ?

            https://forum.qt.io/topic/113070/qt-code-of-conduct

            1 Reply Last reply
            1
            • J Jakob Kenda

              This is a minimal reproducible example.

              It's a really simple Qt application that constructs a QGuiApplication and then tries to output QGuiApplication::applicationDisplayName.
              There are two versions - one that crashes and one that doesn't:

              $ cat crash.cpp
#include "QtGui/qguiapplication.h"
#include "QtCore/qdebug.h"

int main(int argc, char** argv)
{
        QGuiApplication app(argc, argv);
        qDebug() << qApp->applicationDisplayName();
}
$ cat nocrash.cpp
#include "QtGui/qguiapplication.h"
#include "QtCore/qdebug.h"

int main(int argc, char** argv)
{
        QGuiApplication app(argc, argv);
        qDebug() << app.applicationDisplayName();
}

              They are compiled using a simple Makefile:

              $ cat Makefile
CXX=g++ -std=c++23

Qt6_DIR=${HOME}/Qt/6.10.1/gcc_64
Qt6_LIBS=-lQt6Core -lQt6Gui

QT_INCLUDE=-I${Qt6_DIR}/include -L${Qt6_DIR}/lib
QT_LINK=${Qt6_LIBS}

.PHONY: all clean

all: crash nocrash

crash: crash.cpp
        ${CXX} ${QT_INCLUDE} -o crash crash.cpp ${QT_LINK}

nocrash: nocrash.cpp
        ${CXX} ${QT_INCLUDE} -o nocrash nocrash.cpp ${QT_LINK}

clean:
        rm -f crash nocrash

              Compiling and executing the applications:

              $ make -j2
g++ -std=c++23 -I/Qt/6.10.1/gcc_64/include -L/home/jakobk/Qt/6.10.1/gcc_64/lib -o crash crash.cpp -lQt6Core -lQt6Gui
g++ -std=c++23 -I/Qt/6.10.1/gcc_64/include -L/home/jakobk/Qt/6.10.1/gcc_64/lib -o nocrash nocrash.cpp -lQt6Core -lQt6Gui
$ LD_LIBRARY_PATH=/Qt/6.10.1/gcc_64/lib ./crash
Segmentation fault (core dumped)
$ LD_LIBRARY_PATH=~/Qt/6.10.1/gcc_64/lib ./nocrash
"nocrash"

              To me, this looks like a bug in Qt. Am I missing something?
              The reason I'm not sure is that both versions work fine when compiled with CMake using find_package:

              cmake_minimum_required(VERSION 3.16)
project(qt-crash-minimal)

set(Qt6_DIR ~/Qt/6.10.1/gcc_64/lib/cmake/Qt6)
find_package(Qt6 REQUIRED COMPONENTS Core Gui)

add_library(qt INTERFACE)
target_link_libraries(qt INTERFACE Qt::Core Qt::Gui)

add_executable(crash crash.cpp)
target_link_libraries(crash PUBLIC qt)

add_executable(nocrash nocrash.cpp)
target_link_libraries(nocrash PUBLIC qt)

              $ ./crash
"crash"
$ ./nocrash
"nocrash"

              In my opinion there shouldn't be a difference in Qt's behaviour whether it's compiled with CMake or "manually" so this must be a bug in Qt.

              JonBJ Offline
              JonBJ Offline
              JonB
              wrote last edited by JonB
              #7

              @Jakob-Kenda said in Basic application crashes in QGuiApplication constructor if QCoreApplication::instance is called.:

              To me, this looks like a bug in Qt. Am I missing something?

              Why do people start out with "this looks like a Qt bug"? 99% of the time it's your code. As @jsulm points out the docs state:

              qApp

              A global pointer referring to the unique application object. It is equivalent to QCoreApplication::instance(), but cast as a QApplication pointer, so only valid when the unique application object is a QApplication.

              Since you don't have a QApplication, qApp will be nullptr, which you could easily test before dereferencing and then wondering why there is a crash. Not meaning to be rude, just saying RTFM ;-)

              J 1 Reply Last reply
              1
              • JonBJ JonB

                @Jakob-Kenda said in Basic application crashes in QGuiApplication constructor if QCoreApplication::instance is called.:

                To me, this looks like a bug in Qt. Am I missing something?

                Why do people start out with "this looks like a Qt bug"? 99% of the time it's your code. As @jsulm points out the docs state:

                qApp

                A global pointer referring to the unique application object. It is equivalent to QCoreApplication::instance(), but cast as a QApplication pointer, so only valid when the unique application object is a QApplication.

                Since you don't have a QApplication, qApp will be nullptr, which you could easily test before dereferencing and then wondering why there is a crash. Not meaning to be rude, just saying RTFM ;-)

                J Offline
                J Offline
                Jakob Kenda
                wrote last edited by
                #8

                @JonB @jsulm It's true that I'm creating a QGuiApplication and thus technically using qApp wrong but if that were the main problem then the application built with CMake would be crashing too. At one point I had a QApplication there but I wanted to get rid of as many dependencies as possible (for instance QtWidgets) so I used QGuiApplication instead of QApplication and I guess I overlooked qApp.

                Substatuting either QGuiApplication for QApplication or qApp for qGuiApp yields the same result - a crash in the QGuiApplication constructor.

                JonBJ 1 Reply Last reply
                0
                • J Jakob Kenda

                  @JonB @jsulm It's true that I'm creating a QGuiApplication and thus technically using qApp wrong but if that were the main problem then the application built with CMake would be crashing too. At one point I had a QApplication there but I wanted to get rid of as many dependencies as possible (for instance QtWidgets) so I used QGuiApplication instead of QApplication and I guess I overlooked qApp.

                  Substatuting either QGuiApplication for QApplication or qApp for qGuiApp yields the same result - a crash in the QGuiApplication constructor.

                  JonBJ Offline
                  JonBJ Offline
                  JonB
                  wrote last edited by JonB
                  #9

                  @Jakob-Kenda said in Basic application crashes in QGuiApplication constructor if QCoreApplication::instance is called.:

                  Substatuting either QGuiApplication for QApplication or qApp for qGuiApp yields the same result - a crash in the QGuiApplication constructor.

                  If you really mean this can you provide a crashing example (verbatim repro code + crash stack trace) which does not use QGuiApplication + qApp->, which we know should crash.... You say

                          QGuiApplication app(argc, argv);
        qDebug() << app.applicationDisplayName();

                  works fine, so....?

                  J 1 Reply Last reply
                  0
                  • J Offline
                    J Offline
                    Jakob Kenda
                    wrote last edited by Jakob Kenda
                    #10

                    Turns out I was missing the -fPIC flag when compiling the executable. Now the application runs as expected (like it does on Windows). I guess I now have to use -fPIC everywhere or compile Qt with no-direct-extern-access on.

                    1 Reply Last reply
                    0
                    • JonBJ JonB

                      @Jakob-Kenda said in Basic application crashes in QGuiApplication constructor if QCoreApplication::instance is called.:

                      Substatuting either QGuiApplication for QApplication or qApp for qGuiApp yields the same result - a crash in the QGuiApplication constructor.

                      If you really mean this can you provide a crashing example (verbatim repro code + crash stack trace) which does not use QGuiApplication + qApp->, which we know should crash.... You say

                              QGuiApplication app(argc, argv);
        qDebug() << app.applicationDisplayName();

                      works fine, so....?

                      J Offline
                      J Offline
                      Jakob Kenda
                      wrote last edited by
                      #11

                      @JonB said in Basic application crashes in QGuiApplication constructor if QCoreApplication::instance is called.:

                      which we know should crash

                              QGuiApplication app(argc, argv);
        qDebug() << qGuiApp->applicationDisplayName();

                      shouldn't crash.

                      1 Reply Last reply
                      0
                      • J Jakob Kenda has marked this topic as solved

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