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  1. Qt
  2. QTBUG-127706

Memory leaks even in the simplest Qt programs

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    • Icon: Bug Bug
    • Resolution: Unresolved
    • Icon: P3: Somewhat important P3: Somewhat important
    • None
    • 6.5.3
    • QPA: X11/XCB
    • None

      Consider this very basic program:

      main.cpp:

      #include <QApplication>
      #include <QtWidgets>
      
      int main(int argc, char **argv)
      {
          QApplication app(argc, argv);
      
          QLabel label("Hello World!");
          label.show();
      
          return app.exec();
      }
      

      untitled.pro:

      QT += widgets
      
      CONFIG += c++17
      
      SOURCES += main.cpp
      

      If I build this with debug on Ubuntu and then run valgrind --num-callers=500 --leak-check=full ./untitled, I get a memory leak:

      ==161281== Memcheck, a memory error detector
      ==161281== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
      ==161281== Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
      ==161281== Command: ./untitled
      ==161281==
      qt.qpa.plugin: Could not load the Qt platform plugin "wayland" in "" even though it was found.
      ==161281==
      ==161281== HEAP SUMMARY:
      ==161281==     in use at exit: 96,433 bytes in 961 blocks
      ==161281==   total heap usage: 15,756 allocs, 14,795 frees, 3,535,593 bytes allocated
      ==161281==
      ==161281== 352 bytes in 1 blocks are possibly lost in loss record 114 of 141
      ==161281==    at 0x484DA83: calloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
      ==161281==    by 0x40147D9: calloc (rtld-malloc.h:44)
      ==161281==    by 0x40147D9: allocate_dtv (dl-tls.c:375)
      ==161281==    by 0x40147D9: _dl_allocate_tls (dl-tls.c:634)
      ==161281==    by 0x590F7B4: allocate_stack (allocatestack.c:430)
      ==161281==    by 0x590F7B4: pthread_create@@GLIBC_2.34 (pthread_create.c:647)
      ==161281==    by 0x52671D4: QThread::start(QThread::Priority) (in /Qt/6.5.3/gcc_64/lib/libQt6Core.so.6.5.3)
      ==161281==    by 0x8C195EB: QDBusConnection::sessionBus() (in /Qt/6.5.3/gcc_64/lib/libQt6DBus.so.6.5.3)
      ==161281==    by 0x6152DFC: QGenericUnixServices::QGenericUnixServices() (in /Qt/6.5.3/gcc_64/lib/libQt6Gui.so.6.5.3)
      ==161281==    by 0x97DBB09: QXcbIntegration::QXcbIntegration(QList<QString> const&, int&, char**) (in /Qt/6.5.3/gcc_64/lib/libQt6XcbQpa.so.6.5.3)
      ==161281==    by 0x977C3EE: ??? (in /Qt/6.5.3/gcc_64/plugins/platforms/libqxcb.so)
      ==161281==    by 0x5C5A86E: ??? (in /Qt/6.5.3/gcc_64/lib/libQt6Gui.so.6.5.3)
      ==161281==    by 0x5C5E2CB: QGuiApplicationPrivate::createPlatformIntegration() (in /Qt/6.5.3/gcc_64/lib/libQt6Gui.so.6.5.3)
      ==161281==    by 0x5C5ECEF: QGuiApplicationPrivate::createEventDispatcher() (in /Qt/6.5.3/gcc_64/lib/libQt6Gui.so.6.5.3)
      ==161281==    by 0x50D8645: QCoreApplicationPrivate::init() (in /Qt/6.5.3/gcc_64/lib/libQt6Core.so.6.5.3)
      ==161281==    by 0x5C61FDF: QGuiApplicationPrivate::init() (in /Qt/6.5.3/gcc_64/lib/libQt6Gui.so.6.5.3)
      ==161281==    by 0x49E0D48: QApplicationPrivate::init() (in /Qt/6.5.3/gcc_64/lib/libQt6Widgets.so.6.5.3)
      ==161281==    by 0x109353: main (main.cpp:6)
      ==161281==
      ==161281== 456 (24 direct, 432 indirect) bytes in 1 blocks are definitely lost in loss record 116 of 141
      ==161281==    at 0x4849013: operator new(unsigned long) (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
      ==161281==    by 0x6152E21: QGenericUnixServices::QGenericUnixServices() (in /Qt/6.5.3/gcc_64/lib/libQt6Gui.so.6.5.3)
      ==161281==    by 0x97DBB09: QXcbIntegration::QXcbIntegration(QList<QString> const&, int&, char**) (in /Qt/6.5.3/gcc_64/lib/libQt6XcbQpa.so.6.5.3)
      ==161281==    by 0x977C3EE: ??? (in /Qt/6.5.3/gcc_64/plugins/platforms/libqxcb.so)
      ==161281==    by 0x5C5A86E: ??? (in /Qt/6.5.3/gcc_64/lib/libQt6Gui.so.6.5.3)
      ==161281==    by 0x5C5E2CB: QGuiApplicationPrivate::createPlatformIntegration() (in /Qt/6.5.3/gcc_64/lib/libQt6Gui.so.6.5.3)
      ==161281==    by 0x5C5ECEF: QGuiApplicationPrivate::createEventDispatcher() (in /Qt/6.5.3/gcc_64/lib/libQt6Gui.so.6.5.3)
      ==161281==    by 0x50D8645: QCoreApplicationPrivate::init() (in /Qt/6.5.3/gcc_64/lib/libQt6Core.so.6.5.3)
      ==161281==    by 0x5C61FDF: QGuiApplicationPrivate::init() (in /Qt/6.5.3/gcc_64/lib/libQt6Gui.so.6.5.3)
      ==161281==    by 0x49E0D48: QApplicationPrivate::init() (in /Qt/6.5.3/gcc_64/lib/libQt6Widgets.so.6.5.3)
      ==161281==    by 0x109353: main (main.cpp:6)
      ==161281==
      ==161281== LEAK SUMMARY:
      ==161281==    definitely lost: 24 bytes in 1 blocks
      ==161281==    indirectly lost: 432 bytes in 5 blocks
      ==161281==      possibly lost: 352 bytes in 1 blocks
      ==161281==    still reachable: 95,625 bytes in 954 blocks
      ==161281==         suppressed: 0 bytes in 0 blocks
      ==161281== Reachable blocks (those to which a pointer was found) are not shown.
      ==161281== To see them, rerun with: --leak-check=full --show-leak-kinds=all
      ==161281==
      ==161281== For lists of detected and suppressed errors, rerun with: -s
      ==161281== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 0 from 0)
      

      If I create more complicated programs with Qt, I get even more memory leaks.

      Please test your Qt functions in Valgrind to eliminate these kinds of memory leaks.

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            liaqi Liang Qi
            donaldduck Donald Duck
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              Created:
              Updated:

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