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Segfault using modal dialogs in QPlugin

  • Hello everyone,

    I create this thread since nobody's answering on :

    Which is not a thread that I own (and is almost 2 years old).

    My problem is that whenever I call a modal dialog on a re-loaded QPlugin using QWidgets (if I remove the usage everythings works), I get a segfault from the modal dialog at :
    qapplication.cpp : l. 4562
    @ // deliver the event
    bool consumed = receiver->event(e);@

    I created a small testing program that uses two plugins. I can send it by email or on an online storage if someone wants to.
    Here are the relevants functions :
    @void MainWindow::pluginInteract(QString pluginPath)
    InterfaceExample *plugin;
    static QPluginLoader *loader;
    static QPluginLoader *oldloader;

    // Allow plugin reloading
    if (loader) {
    if (oldloader) {
    delete oldloader;

    loader = new QPluginLoader(pluginPath);
    if (loader) {
    if (loader->instance()) {
    plugin = qobject_cast<InterfaceExample *>(loader->instance());
    if (plugin) {
    // Where everything happens
    oldloader = loader;

    void MainWindow::on_b_Action_clicked()
    // Always works... on first time
    // Segfault if this one is using a modal dialog

    Plugin1Widget *gui_plugin = new Plugin1Widget();
    connect(this, SIGNAL(send_right()), gui_plugin, SLOT(do_right()));

    void Plugin1::doSomething()

    void Plugin1::doSomethingRight()
    emit send_right();

    Q_EXPORT_PLUGIN2(PluginExample, Plugin1)@

    @void Plugin1Widget::do_right()
    // Nothing !
    QMessageBox::information(this, "Here is first plugin",
    "First plugin does nothing !");

    Same things on the plugin2widget.cpp:
    @void Plugin2Widget::do_wrong()
    // Uncomment to unleash the fury and segfault every time
    // QFileDialog::getOpenFileName(this,
    // "Please choose a file",
    // qApp->applicationDirPath(),
    // "*");

    So at first I thought it was the call to QFileDialog::getOpenFileName that created the segfault, but now I know that it's the same for all modal dialogs so, did I miss something ? Did I do something wrong ? Or did I find a bug ?

    I use Qt4.8.4 on W7x64.

    Thanks in advance !

  • If no one can help with this issue, shall I report a bug on the tracker and then see what happens ?

  • Posted on the Qt bug tracker here :

  • Hello again, I need some help.
    The bug I reported has been accepted but I'm not really optimistic about it being corrected in the few months I have left for my project.

    So I was wondering how to correct this (mis-)behaviour.

    I had the idea to put signals from the QPlugin to inform my application to display the dialog boxes.
    Do any of you have a better idea ?

    EDIT UPDATE : if someone is reading, deplacing the call from the QPlugin to the main app doesn't solve anything...
    Which means that when switching QPlugins, modal dialogs don't work anymore.

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