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Non-Reentrant Class Use In Multithreaded Program

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  • C Crag_Hack

    Thanks JKSH

    Yes, this does happen.

    Such classes must be used from the GUI thread only (e.g. QWidget, QPixmap must only be used in the same thread that created a QApplication)

    I forgot to clarify in the previous post not just two objects isolated to their own threads but two objects of two different classes... But you knew that right? Anything else to know? Are these classes the exception, perhaps only graphical classes?

    1. the fileListCompare doesn't use global data (i.e. static/global variables) - it's reentrant - or if it does, then all accesses to the globals is serialized between different threads (with a mutex) - it's thread-safe.

    It only uses the referance parameters passed to the function.

    And of course this sounds good as well right...

    JKSHJ Offline
    JKSHJ Offline
    JKSH
    Moderators
    wrote on last edited by
    #30

    @Crag_Hack said in Non-Reentrant Class Use In Multithreaded Program:

    I forgot to clarify in the previous post not just two objects isolated to their own threads but two objects of two different classes... But you knew that right?

    Yes, I knew that :)

    The widget example still stands: Suppose you create a very simple program that consists of only 1 QApplication object and 1 QWidget object. These two objects are not allowed to be in different threads.

    Anything else to know? Are these classes the exception, perhaps only graphical classes?

    None that I can think of. I'm quite sure it's only the graphical classes with QApplication/QGuiApplication.

    1. the fileListCompare doesn't use global data (i.e. static/global variables) - it's reentrant - or if it does, then all accesses to the globals is serialized between different threads (with a mutex) - it's thread-safe.

    It only uses the referance parameters passed to the function.

    And of course this sounds good as well right...

    You said: fileListCompare() only uses references parameters. I presume you were confirming that fileListCompare() doesn't use global/static variables.

    As @kshegunov said: If fileListCompare() doesn't use global/static variables, then fileListCompare() is a reentrant function.

    Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

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    • C Offline
      C Offline
      Crag_Hack
      wrote on last edited by
      #31

      A couple more questions creeped into my domain... thankfully they're quick :) is qDebug() reentrant or should it not be used from multiple threads? All STL classes are reentrant right except for <atomic> and std::atomic guys right?

      JKSHJ 1 Reply Last reply
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      • C Crag_Hack

        A couple more questions creeped into my domain... thankfully they're quick :) is qDebug() reentrant or should it not be used from multiple threads? All STL classes are reentrant right except for <atomic> and std::atomic guys right?

        JKSHJ Offline
        JKSHJ Offline
        JKSH
        Moderators
        wrote on last edited by JKSH
        #32

        @Crag_Hack said in Non-Reentrant Class Use In Multithreaded Program:

        A couple more questions creeped into my domain... thankfully they're quick :) is qDebug() reentrant or should it not be used from multiple threads?

        qDebug() itself is thread-safe. It can be called simultaneously from multiple threads, without endangering your application.

        However, qDebug() sends data to stderr by default, which is unbuffered. That means: If you send a long stream of "A"s from one thread and a long stream of "B"s from another thread, you might see interleaved text when viewing stderr:

        "AAAAAAAAAABBBBBBBBBBAAAAAAAAAABBBBBBBBBB"

        To avoid this, call qInstallMessageHandler() to send qDebug() output to a buffered stream (like stdout or a file).

        All STL classes are reentrant right except for <atomic> and std::atomic guys right?

        I'd say so

        Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

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        • C Offline
          C Offline
          Crag_Hack
          wrote on last edited by Crag_Hack
          #33

          Another related question I conjured up - how come some Qt classes aren't reeentrant that would be quite more useful if they were? I.E. QPair, QStorageInfo. My program makes use of drive storage info in both the main GUI thread and the worker thread (I was forced to use the win32 api in the worker thread since QStorageInfo wasn't reentrant) and I could easily foresee QPair being used in such a way as well.
          Thanks ! That might finalize this thread permanently :)

          kshegunovK 1 Reply Last reply
          0
          • C Crag_Hack

            Another related question I conjured up - how come some Qt classes aren't reeentrant that would be quite more useful if they were? I.E. QPair, QStorageInfo. My program makes use of drive storage info in both the main GUI thread and the worker thread (I was forced to use the win32 api in the worker thread since QStorageInfo wasn't reentrant) and I could easily foresee QPair being used in such a way as well.
            Thanks ! That might finalize this thread permanently :)

            kshegunovK Offline
            kshegunovK Offline
            kshegunov
            Moderators
            wrote on last edited by kshegunov
            #34

            QPair is reentrant, if the docs don't state it, then it's a "bug" in the documentation, please file it as such.

            As for QStorageInfo - I don't know if it's reentrant or not, I would have to check the source to find out for sure, but usually the class not being reentrant is due to the underlying API architecture (e.g. the widget classes).

            Read and abide by the Qt Code of Conduct

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            • C Offline
              C Offline
              Crag_Hack
              wrote on last edited by
              #35

              Thanks. Perhaps they are both bugs - I would think a useful class that would have any possibility of being used in multiple threads like QStorageInfo would most definitely be reentrant. Is there another factor at play here besides utility?

              kshegunovK 2 Replies Last reply
              0
              • C Crag_Hack

                Thanks. Perhaps they are both bugs - I would think a useful class that would have any possibility of being used in multiple threads like QStorageInfo would most definitely be reentrant. Is there another factor at play here besides utility?

                kshegunovK Offline
                kshegunovK Offline
                kshegunov
                Moderators
                wrote on last edited by
                #36

                @Crag_Hack said in Non-Reentrant Class Use In Multithreaded Program:

                utility

                What do you mean by "utility"? Whether it's reentrant or thread-safe (or both or neither) just depends on the specific implementation. As Qt's (mostly) a platform agnostic toolkit, implementations for some platforms may be reentrant/thread-safe, while implementations for other platforms may be not, in that case the worst-case is documented - i.e. non reentrant/not thread-safe.

                Read and abide by the Qt Code of Conduct

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                • C Crag_Hack

                  Thanks. Perhaps they are both bugs - I would think a useful class that would have any possibility of being used in multiple threads like QStorageInfo would most definitely be reentrant. Is there another factor at play here besides utility?

                  kshegunovK Offline
                  kshegunovK Offline
                  kshegunov
                  Moderators
                  wrote on last edited by
                  #37

                  After a quick (not exhaustive) look here:
                  https://code.woboq.org/qt5/qtbase/src/corelib/io/qstorageinfo.h.html
                  https://code.woboq.org/qt5/qtbase/src/corelib/io/qstorageinfo.cpp.html
                  https://code.woboq.org/qt5/qtbase/src/corelib/io/qstorageinfo_p.h.html
                  https://code.woboq.org/qt5/qtbase/src/corelib/io/qstorageinfo_unix.cpp.html
                  https://code.woboq.org/qt5/qtbase/src/corelib/io/qstorageinfo_win.cpp.html

                  It seems to me QStorageInfo is reentrant on *nix, but is not on windows (due to ::SetErrorMode). I didn't look at macos' implementation.

                  Read and abide by the Qt Code of Conduct

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                  • C Offline
                    C Offline
                    Crag_Hack
                    wrote on last edited by
                    #38

                    Why doesn't the Qt Company make a class like QStorageInfo reentrant though? It would seem logical to do so since it's an easy situation to find yourself in needing to use the class in multiple threads.

                    JKSHJ 1 Reply Last reply
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                    • C Crag_Hack

                      Why doesn't the Qt Company make a class like QStorageInfo reentrant though? It would seem logical to do so since it's an easy situation to find yourself in needing to use the class in multiple threads.

                      JKSHJ Offline
                      JKSHJ Offline
                      JKSH
                      Moderators
                      wrote on last edited by
                      #39

                      @Crag_Hack said in Non-Reentrant Class Use In Multithreaded Program:

                      Why doesn't the Qt Company make a class like QStorageInfo reentrant though? It would seem logical to do so since it's an easy situation to find yourself in needing to use the class in multiple threads.

                      No strong reason why.

                      You can post this suggestion to https://bugreports.qt.io/

                      Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

                      kshegunovK 1 Reply Last reply
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                      • JKSHJ JKSH

                        @Crag_Hack said in Non-Reentrant Class Use In Multithreaded Program:

                        Why doesn't the Qt Company make a class like QStorageInfo reentrant though? It would seem logical to do so since it's an easy situation to find yourself in needing to use the class in multiple threads.

                        No strong reason why.

                        You can post this suggestion to https://bugreports.qt.io/

                        kshegunovK Offline
                        kshegunovK Offline
                        kshegunov
                        Moderators
                        wrote on last edited by
                        #40

                        @JKSH said in Non-Reentrant Class Use In Multithreaded Program:

                        No strong reason why.

                        I mentioned already, that ::SetErrorMode breaks reentrancy.

                        Read and abide by the Qt Code of Conduct

                        JKSHJ 1 Reply Last reply
                        2
                        • kshegunovK kshegunov

                          @JKSH said in Non-Reentrant Class Use In Multithreaded Program:

                          No strong reason why.

                          I mentioned already, that ::SetErrorMode breaks reentrancy.

                          JKSHJ Offline
                          JKSHJ Offline
                          JKSH
                          Moderators
                          wrote on last edited by
                          #41

                          @kshegunov said in Non-Reentrant Class Use In Multithreaded Program:

                          I mentioned already, that ::SetErrorMode breaks reentrancy.

                          Yes, you're right.

                          The current implementation is non-reentrant, but I wonder if it's possible to remove this dependency on SetErrorMode(). MSDN recommends SetThreadErrorMode() instead of SetErrorMode() -- I'm not familiar with the Windows API though, so I don't know the implications of this change.

                          Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

                          kshegunovK 1 Reply Last reply
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                          • JKSHJ JKSH

                            @kshegunov said in Non-Reentrant Class Use In Multithreaded Program:

                            I mentioned already, that ::SetErrorMode breaks reentrancy.

                            Yes, you're right.

                            The current implementation is non-reentrant, but I wonder if it's possible to remove this dependency on SetErrorMode(). MSDN recommends SetThreadErrorMode() instead of SetErrorMode() -- I'm not familiar with the Windows API though, so I don't know the implications of this change.

                            kshegunovK Offline
                            kshegunovK Offline
                            kshegunov
                            Moderators
                            wrote on last edited by
                            #42

                            @JKSH said in Non-Reentrant Class Use In Multithreaded Program:

                            possible to remove this dependency

                            Maybe, maybe not. I don't know what was the intent of the developer that implemented it. He/She may've had good reason to use this, or may not. In any case this may solve the reentrancy only partially. I haven't looked at the macOS's implementation at all, which may very well require a global state, and to be frank only glanced through the *nix and windows ones.

                            Perhaps it's better to start with a question to the mailing/development list why it was implemented like this, if someone remembers. But I guess a suggestion wouldn't hurt, so perhaps do them in parallel?

                            Read and abide by the Qt Code of Conduct

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                            • C Offline
                              C Offline
                              Crag_Hack
                              wrote on last edited by Crag_Hack
                              #43

                              About to go live in the next week with one last thing to do... I am going to use the Win API for file copy instead of QFile since I can turn off write buffering. To track progress of the file copy I need to pass a callback to the copy function. See here (the lpProgressRoutine guy). The callback needs to access two worker thread object member functions. My worker thread is simply an object I pass to moveToThread:

                                  workerThread = new QThread(this);
                                  workerThread->start();
                                  worker = new Worker;
                                  worker->moveToThread(workerThread);
                              
                              class Worker : public QObject
                              {
                                  Q_OBJECT
                              ...
                              all worker thread code here
                              

                              What's the best way to do this in a thread-safe manner? Here Kuba simply makes the callback function a static class member fuction.

                              static DWORD CALLBACK copyProgress(...
                              

                              Is that safe for me to do as long as the static callback function only accesses the reentrant worker thread object? The current code follows. I need to access the setProgressMeters and setTimeLabels functions from the callback function - I can make jobSize and startTime member variables of the worker thread object and readSize and fileSize equivalent variables will be passed to the callback function as part of the CopyFileEx setup.

                              void Worker::jobCopyFiles(BackupJob &job, QTime &startTime)
                              {
                              ...
                                  QByteArray buffer;
                                  for (int count = 0; !(buffer = sourceFile.read(1000000)).isEmpty() && cancel == false; count++)
                                  {
                                      int readSize = buffer.size();
                                      targetTempFile.write(buffer);
                                      setProgressMeters(readSize, fileSize, jobSize);
                                      setTimeLabels(startTime.elapsed(),readSize);
                                  }
                              

                              Please let me know if you need more information. Thanks!

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                              • SGaistS Offline
                                SGaistS Offline
                                SGaist
                                Lifetime Qt Champion
                                wrote on last edited by
                                #44

                                What parameters can you pass to that callback ?

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                                Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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                                • C Offline
                                  C Offline
                                  Crag_Hack
                                  wrote on last edited by
                                  #45

                                  You can pass via the lpData parameter to the CopyFileEx function. lpDada is LPVOID which is this according to msdn:

                                  A pointer to any type.
                                  This type is declared in WinDef.h as follows:
                                  typedef void *LPVOID;
                                  
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                                  • SGaistS Offline
                                    SGaistS Offline
                                    SGaist
                                    Lifetime Qt Champion
                                    wrote on last edited by
                                    #46

                                    So you can pass a structure or an object that will be available in your callback after casting the pointer to a suitable class.

                                    You can then use QMetaObject::invokeMethod to avoid trouble with respect to the event loop.

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                                    Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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                                    • C Offline
                                      C Offline
                                      Crag_Hack
                                      wrote on last edited by
                                      #47

                                      Do I make the callback function a static worker object class member? Or do I declare it as a global outside of the class? I can't do a normal worker object class member function and still pass it as a callback to the CopyFileEx function right? What's the best way?

                                      Can you show me some code? I'm not terribly well rehearsed in these realms...
                                      Thanks

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                                      • SGaistS Offline
                                        SGaistS Offline
                                        SGaist
                                        Lifetime Qt Champion
                                        wrote on last edited by
                                        #48

                                        A function outside your class, that callback is an "implementation detail", it should go into your implementation file. Pass your object as parameter to it.
                                        WARNING: Not tested, nor compiled

                                        DWORD CALLBACK CopyProgressRoutine(
                                        	LARGE_INTEGER TotalFileSize,
                                        	LARGE_INTEGER TotalBytesTransferred,
                                        	LARGE_INTEGER StreamSize,
                                        	LARGE_INTEGER StreamBytesTransferred,
                                        	DWORD dwStreamNumber,
                                        	DWORD dwCallbackReason,
                                        	HANDLE hSourceFile,
                                        	HANDLE hDestinationFile,
                                        	LPVOID lpData
                                        	)
                                        {
                                            int percentage = (double(TotalBytesTransferred.QuadPart) / double(TotalFileSize.QuadPart)) * 100;
                                            MyCoolClass *object = qobject_cast<MyCoolClass *>(lpData);
                                            if(object) {
                                                QMetaObject::invokeMethod(object, 
                                                                          "nameOfYourMethod", 
                                                                          Qt::QueuedConnection,
                                                                          Q_ARG(int, percentage));
                                            }
                                            return PROGRESS_CONTINUE;
                                        }
                                        
                                        
                                        void YouCoolClass::copyFile(const QString& source, const QString& destination)
                                        {
                                                QByteArray srcUtf8 = QDir::toNativeSeparator(source).toUtf8();
                                                char *srcFilename = srcUtf8.data();
                                                QByteArray dstUtf8 = QDir::toNativeSeparator(destination).toUtf8();
                                                char *dstFilename = dstUtf8.data();
                                               
                                        	bool returnVal;
                                        
                                        	returnVal = CopyFileEx(
                                        		srcFilename,
                                        		dstFilename,
                                        		(LPPROGRESS_ROUTINE)CopyProgressRoutine,
                                        		this,
                                        		NULL,
                                        		COPY_FILE_NO_BUFFERING);
                                        
                                        	if(returnVal){
                                        		printf("%s copied to current directory.\n", filename);
                                        	} else {
                                        		printf("%s not copied to current directory.\n", filename);
                                        		printf("Error %u.\n", GetLastError());
                                        	}
                                        }
                                        

                                        Interested in AI ? www.idiap.ch
                                        Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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                                        • C Offline
                                          C Offline
                                          Crag_Hack
                                          wrote on last edited by Crag_Hack
                                          #49

                                          Thanks SGaist! The thing other than static/global/worker member I wasn't sure about was how to get that callback function to call those worker thread object functions. So we have to use QMetaObject::invokeMethod? We can't call the functions directly? What's the reason for that? Is CopyFileEx blocking synchronous? If so wouldn't that prevent queued events from being processed?

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