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[SOLVED] Calling QString::toWCharArray causes segmentation fault

  • Hi,
    I've strange problem in my app: I'm getting SIGSEGV inside the instruction calling QString::toWCharArray.

    This is the used code:

    Help::Help(QWidget *parent) :
        ui(new Ui::Help)
        wchar_t __lbl[33];
        QString lbl = ui->label->text();
        lbl.toWCharArray(__lbl); //here the SIGSEGV bug occurs

    This is the output of the GNU debugger:

    Program received signal SIGSEGV, Segmentation fault.
    0x0000000000405b30 in load<int> (_q_value=@0x3c0000003e: <error reading variable>)
    at /opt/Qt5.4.2/5.4/gcc_64/include/QtCore/qgenericatomic.h:90
    90 return _q_value;

    I'm currently unable to fix this bug.
    Thanks for your help.

  • Hi and welcome to devnet,

    are you sure lbl length is less or equal to 33??

    from Qt docs

    array has to be allocated by the caller and contain enough space to hold the complete string (allocating the array with the same length as the string is always sufficient).

  • @mcosta I tried with wchar_t __lbl[255], but the problem is the same.

  • Whats the length of lbl???

    I tried this program

    #include <QCoreApplication>
    #include <QtDebug>
    int main(int argc, char *argv[])
        QCoreApplication a(argc, argv);
        QString s = "Super Huge Long long long log string";
        const int LABEL_SIZE = 100;
        wchar_t myLabel [LABEL_SIZE];
        qDebug() << s.toWCharArray(myLabel);

    it works, if I change LABEL_SIZE to 20 it crashes.

    Which platform?? I'm on OS X

  • @mcosta Ah, I was misunderstanding .. I was unaware of the length of lbl which was 365: So the cause of the segmentation fault is clear here. It is not an OS problem (I'm on GNU/Linux). Changing the length of __lbl to 399 solved the problem.

  • Moderators

    Why are you hardcoding the length anyway? What if you decide to use 400 characters one day? You're gonna revisit that code? Just check the length of the string before conversion and allocate enough space for it.

  • @Chris-Kawa Yes, that's true .. using that code instead __lbl = (wchar_t *)calloc(this->ui->label->text().count(), sizeof(wchar_t)); for allocation is the best way. Thank you for this notice.

  • Moderators

    Since you're using c++ I would consider using new/delete instead of calloc/free, or, better yet, a unique_ptr to stay safe of exception problems. Also, according to the standard, names starting with double underscores (and single underscore in global namespace) are reserved for internal compiler/stl implementation. You're stepping on enemy territory using these.

  • @Chris-Kawa I'm mainly a C programmer, I don't like using C++ if not for widget programming .. anyway, what does "reserved for internal compiler/stl implementation" mean?

  • Moderators

    anyway, what does "reserved for internal compiler/stl implementation" mean?

    Means that such names are not to be used by users (us). They are reserved for compilers and STL.
    For example the standard __LINE__, __FILE__ and __func__ macros, or the vendor specific keywords like MSVC's __super.

    This restriction is not enforced by any compiler I know, but technically you shouldn't name your stuff this way and implementation is free to do with that code whatever it wants (undefined behavior in user space).

  • @Chris-Kawa Yes, that's true .. but I'm quite careful about that, and I have enough experience in C Programming with GNU development tools such as GCC.

  • Moderators

    Yeah, and I'm not saying that it will ever break. I can't imagine any compiler doing anything malicious to such code. I'm just saying it's standard non-conforming code.

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