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QString optimized out, causes segmentation fault

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  • M mchinand

    You've created a vector with no elements, so it's going to crash when you try to write to a non-existent 4th element.

    P Offline
    P Offline
    Publicnamer
    wrote on last edited by
    #3

    @mchinand Another variant of this code that also crashes is:

    mystruct *v = new mystruct[100];
    v[3].s = "abc";
    v[3].i = 123;
    
    M 1 Reply Last reply
    0
    • P Publicnamer

      @mchinand Another variant of this code that also crashes is:

      mystruct *v = new mystruct[100];
      v[3].s = "abc";
      v[3].i = 123;
      
      M Offline
      M Offline
      mpergand
      wrote on last edited by
      #4

      @Publicnamer said in QString optimized out, causes segmentation fault:

      @mchinand Another variant of this code that also crashes is:

      mystruct *v = new mystruct[100];
      v[3].s = "abc";
      v[3].i = 123;
      

      No, it doesn't.

      With QVector you can do:

      QVector<mystruct> v(10,{"aa",100});
      qDebug()<<v[5].s<<v[3].s;
      
      P 1 Reply Last reply
      0
      • M mpergand

        @Publicnamer said in QString optimized out, causes segmentation fault:

        @mchinand Another variant of this code that also crashes is:

        mystruct *v = new mystruct[100];
        v[3].s = "abc";
        v[3].i = 123;
        

        No, it doesn't.

        With QVector you can do:

        QVector<mystruct> v(10,{"aa",100});
        qDebug()<<v[5].s<<v[3].s;
        
        P Offline
        P Offline
        Publicnamer
        wrote on last edited by Publicnamer
        #5

        @mpergand
        It really does crash for me compiling with G++ and with clang.

        0x0000007ff6fd8fc0 in QString::operator=(QString const&) () from /usr/lib/aarch64-linux-gnu/libQt5Core.so.5
        

        It also crashes if I assign the QString like so:

        QString otherString = "xyz";
        std::string str = otherString.toString();
        const char *cstr = str.c_str();
        v[3].s = QString(cstr);
        

        As well as:

        v[3].s = QString(strdup(cstr)); // double free error
        

        My struct has many elements. It makes no sense to init it using a single-line expresson.

        JonBJ 2 Replies Last reply
        0
        • P Publicnamer

          @mpergand
          It really does crash for me compiling with G++ and with clang.

          0x0000007ff6fd8fc0 in QString::operator=(QString const&) () from /usr/lib/aarch64-linux-gnu/libQt5Core.so.5
          

          It also crashes if I assign the QString like so:

          QString otherString = "xyz";
          std::string str = otherString.toString();
          const char *cstr = str.c_str();
          v[3].s = QString(cstr);
          

          As well as:

          v[3].s = QString(strdup(cstr)); // double free error
          

          My struct has many elements. It makes no sense to init it using a single-line expresson.

          JonBJ Offline
          JonBJ Offline
          JonB
          wrote on last edited by
          #6

          @Publicnamer
          Instead of stating that your code is correct and the C++ optimizer is wrong, show your code for others to be the judge.

          There are a lot of programs out there, Qt or otherwise, using compiler optimization flags. If that regularly caused crashes there would be an uproar. Do you know better than the people who wrote the compilers?

          P 1 Reply Last reply
          1
          • JonBJ JonB

            @Publicnamer
            Instead of stating that your code is correct and the C++ optimizer is wrong, show your code for others to be the judge.

            There are a lot of programs out there, Qt or otherwise, using compiler optimization flags. If that regularly caused crashes there would be an uproar. Do you know better than the people who wrote the compilers?

            P Offline
            P Offline
            Publicnamer
            wrote on last edited by
            #7

            @JonB
            The code I'm showing is virtually identical to the real code. It really is crashing with these simple operations.

            JonBJ 1 Reply Last reply
            0
            • M Offline
              M Offline
              mchinand
              wrote on last edited by
              #8

              @Publicnamer said in QString optimized out, causes segmentation fault:

              struct mystruct {
              QString s;
              int i;
              }

              Is your real code missing a semicolon at the end of this? I wouldn't think it would compile without it.

              struct mystruct {
              QString s;
              int i;
              };
              
              1 Reply Last reply
              0
              • P Publicnamer

                @JonB
                The code I'm showing is virtually identical to the real code. It really is crashing with these simple operations.

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

                @Publicnamer said in QString optimized out, causes segmentation fault:

                The code I'm showing is virtually identical to the real code.

                If I had a $ for every time somebody said "this code crashes, but it's not quite the same as my code, but it's really the same, honest", I'd be rich. After wasting my time on code which turns out to be different from what the person really has.

                Be reasonable about what you want other people to comment on. That is, if you want to solve it.

                Start by breaking your problem down.

                • Remove the vector. Does, say, qDebug() << QString(cstr); crash? If yes it has nothing to do with the vector.

                • If it is the vector situation only, look at your vector closely instead.

                If you want anything done: Produce a minimal example which others can compile. Don't give us some code and say your actual code is something different.

                Also, are you saying anything about how the code is compiled affecting the outcome? If you can reproduce the problem compiled for debug, let it crash in the debugger and show us the stack trace.

                1 Reply Last reply
                3
                • P Publicnamer

                  @mpergand
                  It really does crash for me compiling with G++ and with clang.

                  0x0000007ff6fd8fc0 in QString::operator=(QString const&) () from /usr/lib/aarch64-linux-gnu/libQt5Core.so.5
                  

                  It also crashes if I assign the QString like so:

                  QString otherString = "xyz";
                  std::string str = otherString.toString();
                  const char *cstr = str.c_str();
                  v[3].s = QString(cstr);
                  

                  As well as:

                  v[3].s = QString(strdup(cstr)); // double free error
                  

                  My struct has many elements. It makes no sense to init it using a single-line expresson.

                  JonBJ Offline
                  JonBJ Offline
                  JonB
                  wrote on last edited by JonB
                  #10

                  @Publicnamer said in QString optimized out, causes segmentation fault:

                  QString otherString = "xyz";
                  std::string str = otherString.toString();

                  This is part of your code which crashes? There is no method QString::toString(), so it would not even compile, so how can it crash? Do you see the point of producing an actual piece of code which exhbits your problem, rather than assuring us it is "virtually identical to the real code"?

                  P 1 Reply Last reply
                  0
                  • JonBJ JonB

                    @Publicnamer said in QString optimized out, causes segmentation fault:

                    QString otherString = "xyz";
                    std::string str = otherString.toString();

                    This is part of your code which crashes? There is no method QString::toString(), so it would not even compile, so how can it crash? Do you see the point of producing an actual piece of code which exhbits your problem, rather than assuring us it is "virtually identical to the real code"?

                    P Offline
                    P Offline
                    Publicnamer
                    wrote on last edited by Publicnamer
                    #11

                    @JonB Obviously I meant toStdString.
                    The problem seems to be that QString cannot be put into a struct for whatever reason. There is some crazy bug in that class.
                    The fact that I get a double free error when I using strdup is really alarming:

                    v[3].s = QString(strdup(cstr)); // double free error here
                    
                    JonBJ JoeCFDJ W 3 Replies Last reply
                    0
                    • P Publicnamer

                      @JonB Obviously I meant toStdString.
                      The problem seems to be that QString cannot be put into a struct for whatever reason. There is some crazy bug in that class.
                      The fact that I get a double free error when I using strdup is really alarming:

                      v[3].s = QString(strdup(cstr)); // double free error here
                      
                      JonBJ Offline
                      JonBJ Offline
                      JonB
                      wrote on last edited by JonB
                      #12

                      @Publicnamer
                      So you would like us to comment on why your code is crashing when we should guess what is "obviously" in your code? This is getting crazy.

                      If you want actual help with your issue, as opposed to telling us that everything is wrong and does not work, produce an actual example. People here will help.

                      The problem seems to be that QString cannot be put into a struct for whatever reason. There is some crazy bug in that class.

                      Simply not so.

                      Meanwhile I have asked you useful questions like: does it happen when compiling for debug? No answer. Does it crash when you do not use a vector? array? No answer.

                      1 Reply Last reply
                      1
                      • P Publicnamer

                        @JonB Obviously I meant toStdString.
                        The problem seems to be that QString cannot be put into a struct for whatever reason. There is some crazy bug in that class.
                        The fact that I get a double free error when I using strdup is really alarming:

                        v[3].s = QString(strdup(cstr)); // double free error here
                        
                        JoeCFDJ Offline
                        JoeCFDJ Offline
                        JoeCFD
                        wrote on last edited by JoeCFD
                        #13

                        @Publicnamer struct is a class as well. How can it be possible that QString can not be put into struct? Your app may have some corrupt memory and the real problem may be somewhere else.
                        On linux, run valgrind to find out memory issue.

                        1 Reply Last reply
                        1
                        • P Publicnamer

                          @JonB Obviously I meant toStdString.
                          The problem seems to be that QString cannot be put into a struct for whatever reason. There is some crazy bug in that class.
                          The fact that I get a double free error when I using strdup is really alarming:

                          v[3].s = QString(strdup(cstr)); // double free error here
                          
                          W Offline
                          W Offline
                          wrosecrans
                          wrote on last edited by
                          #14

                          @Publicnamer said in QString optimized out, causes segmentation fault:

                          @JonB Obviously I meant toStdString.

                          The thing is, nothing is "obvious" when debugging. You didn't mean to have a bug in the first place, so trying to make people guess about the code you are having problems with isn't productive. People can really only help you debug an actual reproducible test case, not just something "obviously" similar to a test case. Seriously, share a full main() with your actual problem, and people will be much more able to see what's happening.

                          1 Reply Last reply
                          1
                          • C Offline
                            C Offline
                            ChrisW67
                            wrote on last edited by ChrisW67
                            #15

                            @Publicnamer said in QString optimized out, causes segmentation fault:

                            Anyone know why this might happen?

                            It really does crash for me compiling with G++ and with clang.

                            Yes, the code presented in the original post will crash for exactly the reason described by @mchinand. The result almost certainly will not change with compiler or optimization level, but might be more informative if compiled for debug.

                            Here is your example, or something "virtually identical" to it:

                            #include <QCoreApplication>
                            #include <QString>
                            #include <QVector>
                            #include <QDebug>
                            
                            struct mystruct {
                              QString s;
                              int i;
                            };
                            
                            int main (int argc, char **argv) {
                                    QCoreApplication app(argc, argv);
                            
                                    qDebug() << "Starting";
                                    QVector<mystruct> v;  // <<<< this vector has no members
                                    v[3].s = "abc";
                                    v[3].i = 123;
                                    qDebug() << v[3].s << v[3].i;
                                            qDebug() << "Ending";
                                            qDebug() << "Ending";
                            
                                    return 0;
                            }
                            

                            Here is what happens when compiled for release (-O2 optimization, gcc version 9.3.0)

                            chrisw@newton:/tmp/crash$ qmake -v
                            QMake version 3.1
                            Using Qt version 5.12.8 in /usr/lib/x86_64-linux-gnu
                            chrisw@newton:/tmp/crash$ qmake CONFIG+=release
                            Info: creating stash file /tmp/crash/.qmake.stash
                            chrisw@newton:/tmp/crash$ make
                            g++ -c -pipe -O2 -Wall -W -D_REENTRANT -fPIC -DQT_DEPRECATED_WARNINGS -DQT_NO_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -I. -I. -isystem /usr/include/x86_64-linux-gnu/qt5 -isystem /usr/include/x86_64-linux-gnu/qt5/QtGui -isystem /usr/include/x86_64-linux-gnu/qt5/QtCore -I. -I/usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++ -o main.o main.cpp
                            g++ -Wl,-O1 -o crash main.o   /usr/lib/x86_64-linux-gnu/libQt5Gui.so /usr/lib/x86_64-linux-gnu/libQt5Core.so /usr/lib/x86_64-linux-gnu/libGL.so -lpthread   
                            chrisw@newton:/tmp/crash$ ./crash 
                            Starting
                            Segmentation fault (core dumped)
                            

                            ... and for debug (no optimization at all, and debug symbols):

                            chrisw@newton:/tmp/crash$ make distclean
                            rm -f moc_predefs.h
                            rm -f main.o
                            rm -f *~ core *.core
                            rm -f crash 
                            rm -f .qmake.stash
                            rm -f Makefile
                            chrisw@newton:/tmp/crash$ qmake CONFIG+=debug
                            Info: creating stash file /tmp/crash/.qmake.stash
                            chrisw@newton:/tmp/crash$ make
                            g++ -c -pipe -g -Wall -W -D_REENTRANT -fPIC -DQT_DEPRECATED_WARNINGS -DQT_GUI_LIB -DQT_CORE_LIB -I. -I. -isystem /usr/include/x86_64-linux-gnu/qt5 -isystem /usr/include/x86_64-linux-gnu/qt5/QtGui -isystem /usr/include/x86_64-linux-gnu/qt5/QtCore -I. -I/usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++ -o main.o main.cpp
                            g++  -o crash main.o   /usr/lib/x86_64-linux-gnu/libQt5Gui.so /usr/lib/x86_64-linux-gnu/libQt5Core.so /usr/lib/x86_64-linux-gnu/libGL.so -lpthread   
                            chrisw@newton:/tmp/crash$ ./crash 
                            Starting
                            ASSERT failure in QVector<T>::operator[]: "index out of range", file /usr/include/x86_64-linux-gnu/qt5/QtCore/qvector.h, line 437
                            Aborted (core dumped)
                            

                            With/without optimization, same result, ergo not optimization induced.
                            Debug version tells you exactly why it crashed, if only you looked.

                            gdb, on a debug version, will tell you which line of your code triggered it:

                            chrisw@newton:/tmp/crash$ gdb ./crash
                            GNU gdb (Ubuntu 9.2-0ubuntu1~20.04) 9.2
                            ...
                            Reading symbols from ./crash...
                            (gdb) run
                            Starting program: /tmp/crash/crash 
                            [Thread debugging using libthread_db enabled]
                            Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
                            Starting
                            ASSERT failure in QVector<T>::operator[]: "index out of range", file /usr/include/x86_64-linux-gnu/qt5/QtCore/qvector.h, line 437
                            
                            Program received signal SIGABRT, Aborted.
                            __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
                            50      ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
                            (gdb) bt
                            #0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
                            #1  0x00007ffff7694859 in __GI_abort () at abort.c:79
                            #2  0x00007ffff7ae3aad in QMessageLogger::fatal(char const*, ...) const () from /lib/x86_64-linux-gnu/libQt5Core.so.5
                            #3  0x00007ffff7ae2f46 in qt_assert_x(char const*, char const*, char const*, int) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
                            #4  0x0000555555555d85 in QVector<mystruct>::operator[] (this=0x7fffffffdf60, i=3) at /usr/include/x86_64-linux-gnu/qt5/QtCore/qvector.h:437
                            #5  0x0000555555555453 in main (argc=1, argv=0x7fffffffe0b8) at main.cpp:16
                            (gdb) 
                            

                            If you correct your erroneous code to, for example:

                            #include <QCoreApplication>
                            #include <QString>
                            #include <QVector>
                            #include <QDebug>
                            
                            struct mystruct {
                              mystruct(): s(), i(0) { }  // <<<< a  constructor so the int is never undefined
                            
                              QString s;
                              int i;
                            };
                            
                            int main (int argc, char **argv) {
                                    QCoreApplication app(argc, argv);
                            
                                    qDebug() << "Starting";
                                    QVector<mystruct> v(4);  // <<<< this vector actually has 4 default constructed members
                                    v[3].s = "abc";
                                    v[3].i = 123;
                                    qDebug() << v[3].s << v[3].i;
                                    qDebug() << "Ending";
                                    
                                    return 0;
                            }
                            

                            Then, oddly, it does not crash:

                            chrisw@newton:/tmp/crash$ make
                            g++ -c -pipe -O2 -Wall -W -D_REENTRANT -fPIC -DQT_DEPRECATED_WARNINGS -DQT_NO_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -I. -I. -isystem /usr/include/x86_64-linux-gnu/qt5 -isystem /usr/include/x86_64-linux-gnu/qt5/QtGui -isystem /usr/include/x86_64-linux-gnu/qt5/QtCore -I. -I/usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++ -o main.o main.cpp
                            g++ -Wl,-O1 -o crash main.o   /usr/lib/x86_64-linux-gnu/libQt5Gui.so /usr/lib/x86_64-linux-gnu/libQt5Core.so /usr/lib/x86_64-linux-gnu/libGL.so -lpthread   
                            chrisw@newton:/tmp/crash$ ./crash 
                            Starting
                            "abc" 123
                            Ending
                            
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