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How to release the plugin's dll file when application is running

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  • Q Offline
    Q Offline
    qtyangyong
    wrote on last edited by
    #6

    sometimes, It is necessary to modify the DLL when work app is still running.
    the online update function of plug-in architecture

    1 Reply Last reply
    0
    • Q qtyangyong

      i use pluginloader to load and unload dll file. but the dll file can not be deleted even after QPluginLoader::unload returned true.
      what's the matter with below program?

      QPluginLoader loader("ExtraFiltersPlugin.dll");
      plugin = loader.instance();
      if (plugin)
      {
      qDebug() << "plugin";
      FilterInterface *fff = qobject_cast<FilterInterface *>(plugin);
      fff->test();
      }
      int ret = loader.unload();
      qDebug() << ret;
      qDebug() << QFile::remove("ExtraFiltersPlugin.dll"); // delete failed

      thanks.

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

      @qtyangyong
      Why do you ask this question again, with the same code, as I distinctly recall you asking a week or so ago, to which you got answers?

      OK, hold on: I see you have only made 3 posts. Exactly this question with exactly the same code was asked by somebody a week or so ago. Was it not you? Looks like identical code....

      You might search the form for that thread. The gist was that under Windows you are liable not to be able to delete a DLL once loaded. You need to call setLoadHints() with the QLibrary::PreventUnloadHint flag cleared I think. But even then an answer someone gave to the other thread indicated it may not work, and you're not supposed to unload plugins/delete their DLL file till after program exit, I believe.

      UPDATE
      The other thread in question was https://forum.qt.io/topic/136303/how-to-release-the-plugin-s-dll-file-when-application-is-running. Oh, that is indeed you! What a waste of us responders' time :(
      Your code does not even show what you were told to try from there. So you already asked just this question and got your answer, why do you make people have to answer again in a new thread? Do you think that will generate the answer you want it to now?

      1 Reply Last reply
      3
      • Q Offline
        Q Offline
        qtyangyong
        wrote on last edited by
        #8

        i'am very very sorry,I haven't seen that reply before
        by the way, I tried to use ploader - > setloadhints (0); Still can't succeed
        It is said that it can work normally in the historical version of QT
        please read this: https://ask.csdn.net/questions/1059003

        jsulmJ 1 Reply Last reply
        0
        • Q qtyangyong

          i'am very very sorry,I haven't seen that reply before
          by the way, I tried to use ploader - > setloadhints (0); Still can't succeed
          It is said that it can work normally in the historical version of QT
          please read this: https://ask.csdn.net/questions/1059003

          jsulmJ Offline
          jsulmJ Offline
          jsulm
          Lifetime Qt Champion
          wrote on last edited by
          #9

          @qtyangyong I doubt @JonB can read Chinese, I also can't.
          "historical version of QT" - what does this mean? Do you mean old Qt version? What Qt version do you use?

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

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          0
          • Q Offline
            Q Offline
            qtyangyong
            wrote on last edited by
            #10

            https://ask.csdn.net/questions/1059003
            In that article, it said Qt5 4.2 has no such problems.
            I try to use 5.12.10, 5.12.12 and 5.15.2, all have such problems

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            0
            • Q Offline
              Q Offline
              qtyangyong
              wrote on last edited by JKSH
              #11

              Because of repeated questions, this question is merged into https://forum.qt.io/topic/136392/how-to-release-the-plugin-s-dll-file/7 , please don't reply here, thank you.
              by the way, the dll file can not be deleted event if pluginLoader.setLoadHints(0); is called.

              [EDIT: Both topics are now merged. --JKSH]

              1 Reply Last reply
              0
              • Christian EhrlicherC Offline
                Christian EhrlicherC Offline
                Christian Ehrlicher
                Lifetime Qt Champion
                wrote on last edited by
                #12

                Unloading a plugin will not remove it from the internal structures since there might be still references to this plugin somewhere so it might crash otherwise. There is a lengthy discussion about this on the Qt dev mailing list some years ago. Was in conjunction with QStringLiteral iirc.

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

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                • Q Offline
                  Q Offline
                  qtyangyong
                  wrote on last edited by
                  #13

                  With regard to the use of dynamic libraries, the basic principle is to release when the last user exits. If it is not released, the dynamic library cannot be hot updated. In this way, we need to add a dynamic library version control module in the application, use a new name for each update, and delete useless files when the application restarts.

                  by the way, I tried, as long as loader->instance(); is called, the DLL file is locked. It has nothing to do with the call of loader->unload(). Is it possible that the unload method did not close the handle of the DLL file?

                  1 Reply Last reply
                  0
                  • kkoehneK Offline
                    kkoehneK Offline
                    kkoehne
                    Moderators
                    wrote on last edited by
                    #14

                    @qtyangyong said in How to release the plugin's dll file?:

                    With regard to the use of dynamic libraries, the basic principle is to release when the last user exits. If it is not released, the dynamic library cannot be hot updated. In this way, we need to add a dynamic library version control module in the application, use a new name for each update, and delete useless files when the application restarts.

                    I learned recently, that, while you cannot delete a .dll or .exe on Windows that is currently used, you can rename it (as long as the library or executable is still in the same folder).

                    So maybe you can just rename the .dll, extract the new .dll, and after a restart delete the old .dll?

                    Director R&D, The Qt Company

                    Q 1 Reply Last reply
                    3
                    • Christian EhrlicherC Offline
                      Christian EhrlicherC Offline
                      Christian Ehrlicher
                      Lifetime Qt Champion
                      wrote on last edited by
                      #15

                      You should take a look at QLibrary::setLoadHints()

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

                      Q 1 Reply Last reply
                      0
                      • hskoglundH Offline
                        hskoglundH Offline
                        hskoglund
                        wrote on last edited by
                        #16

                        Hi, on Windows removing .dll and .exe files that have been recebtly used is not likely to succeed due tio caching etc.

                        Why don”t you instead try renaming your plugin file to something like DeleteMe.dll? That usually works. After that, you can create a new copy with the original filename, and then you”ll have full read/write/delete acccess.

                        JonBJ 1 Reply Last reply
                        2
                        • kkoehneK kkoehne

                          @qtyangyong said in How to release the plugin's dll file?:

                          With regard to the use of dynamic libraries, the basic principle is to release when the last user exits. If it is not released, the dynamic library cannot be hot updated. In this way, we need to add a dynamic library version control module in the application, use a new name for each update, and delete useless files when the application restarts.

                          I learned recently, that, while you cannot delete a .dll or .exe on Windows that is currently used, you can rename it (as long as the library or executable is still in the same folder).

                          So maybe you can just rename the .dll, extract the new .dll, and after a restart delete the old .dll?

                          Q Offline
                          Q Offline
                          qtyangyong
                          wrote on last edited by
                          #17

                          @kkoehne yes, this is a good idea

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                          0
                          • hskoglundH hskoglund

                            Hi, on Windows removing .dll and .exe files that have been recebtly used is not likely to succeed due tio caching etc.

                            Why don”t you instead try renaming your plugin file to something like DeleteMe.dll? That usually works. After that, you can create a new copy with the original filename, and then you”ll have full read/write/delete acccess.

                            JonBJ Offline
                            JonBJ Offline
                            JonB
                            wrote on last edited by
                            #18

                            @hskoglund said in How to release the plugin's dll file when application is running:

                            Hi, on Windows removing .dll and .exe files that have been recebtly used is not likely to succeed due tio caching etc.

                            Exactly!

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                            0
                            • hskoglundH Offline
                              hskoglundH Offline
                              hskoglund
                              wrote on last edited by hskoglund
                              #19

                              Sorry didn't read the above ...please don't reply here.. (use the other forum post), also I see in that post that @kkoehne already suggested the same thing, i.e. renaming the plugin :-)

                              1 Reply Last reply
                              2
                              • Christian EhrlicherC Christian Ehrlicher

                                You should take a look at QLibrary::setLoadHints()

                                Q Offline
                                Q Offline
                                qtyangyong
                                wrote on last edited by
                                #20

                                @Christian-Ehrlicher
                                thans, I tried it but didn't solve the problem. Maybe the usage is incorrect.
                                such as
                                loader.setloadhints (0);
                                loader.unload();

                                Christian EhrlicherC 1 Reply Last reply
                                0
                                • Q qtyangyong

                                  @Christian-Ehrlicher
                                  thans, I tried it but didn't solve the problem. Maybe the usage is incorrect.
                                  such as
                                  loader.setloadhints (0);
                                  loader.unload();

                                  Christian EhrlicherC Offline
                                  Christian EhrlicherC Offline
                                  Christian Ehrlicher
                                  Lifetime Qt Champion
                                  wrote on last edited by
                                  #21

                                  @qtyangyong said in How to release the plugin's dll file when application is running:

                                  Maybe the usage is incorrect.

                                  For sure - they're load hints, not unload hints... set it before you load the library.

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

                                  1 Reply Last reply
                                  2
                                  • Q Offline
                                    Q Offline
                                    qtyangyong
                                    wrote on last edited by
                                    #22

                                    thank you very much, I moved "loader.setLoadHints(0);" before "loader.instance();", the problem was solved. the program such as below:
                                    int main(int argc, char *argv[])
                                    {
                                    ......
                                    QPluginLoader loader("ExtraFiltersPlugin.dll");
                                    loader.setLoadHints(0);
                                    QObject *plugin = loader.instance();
                                    if (plugin)
                                    {
                                    FilterInterface *fff = qobject_cast<FilterInterface *>(plugin);
                                    fff->test();
                                    }
                                    MainWindow w;
                                    w.ploader = &loader;
                                    ......
                                    }

                                    void MainWindow::on_unloadBtn_clicked()
                                    {
                                    ploader->unload();
                                    }

                                    1 Reply Last reply
                                    0

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