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Thread: Conversion of QString to char array yields garbage when in debug mode

  1. 1st April 2016, 02:00 #1
    antimatter
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    Question Conversion of QString to char array yields garbage when in debug mode

    Hi,

    during debug mode (F5 with default keys) the conversion from QString to char* fails and produces garbage data. That way, I cannot debug my program properly. When running my program normally (Ctrl+R) it's working just fine.

    Below you will find a small example, that determines the type of drives in Windows. I tested with Qt 5.5.1 and Qt 5.4.2. Qt Creator is at version 3.6.1.

    Qt Code: 
    1. #include <iostream>
    2. #include <windows.h>
    3. #include <winbase.h>
    4. #include <QtCore>
    5. using namespace std;
    6.  
    7. int main(void)
    8. {
    9.     QFileInfoList localDrives = QDir::drives();
    10.     for (QFileInfo info : localDrives)
    11.     {
    12.         const char *drive = QDir::toNativeSeparators(info.path()).toStdString().c_str();
    13.         UINT drivetype = GetDriveTypeA(drive);
    14.         qDebug() << info.path().left(1) << QString::fromLocal8Bit(drive) << "is" << drivetype;
    15.     }
    16.     return 0;
    17. }
    To copy to clipboard, switch view to plain text mode 

    Here is the output during debug mode. Of course, GetDriveTypeA(...) returns always 1 here. Sh*t in, sh*t out.
    Qt Code: 
    1. Debugging starts
    2. "C" "««««««««îþîþ\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000ma?\u0003OÇ\u0000\u0018\u0001ð\u00ADº\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000««««««««\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000la?\u0002OÇ\u0000\u001CÈ\u00ADÆk««««««««îþîþ\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000la?\u0002NÇ\u0000\u0018ȬÆk\u0001\u0000\u0000\u0000««««««««\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000ka<\u0006NÇ\u0000\u0000\u0010G€\u0000P\u0019€\u0000îþîþîþîþla?\u0002IÇ\u0000\u0018ȬÆk\u0000\u0000\u0000\u0000««««««««\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000ma?\u0003NÇ\u0000\u001C(¼Æk\u0002\u0000\u0000\u0000\u0000\u0000\u0000\u0000««««««««îþîþ\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000na?\u0000OÇ\u0000$(¼Æk\u0003\u0000\u0000\u0000\u0000\u0000\u0000\u0000««««««««îþîþîþîþîþîþ\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000la?\u0002LÇ\u0000\u0018ȬÆk\u0002\u0000\u0000\u0000««««««««\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000la?\u0002NÇ\u0000\u0018ȬÆk\u0003\u0000\u0000\u00ò¥êBio
    3. "D" "îþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþoa?\u0001VÇ" is 1
    4. "E" "îþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþoa?\u0001VÇ" is 1
    5. "F" "îþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþoa?\u0001VÇ" is 1
    6. "G" "îþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþoa?\u0001VÇ" is 1
    7. "I" "îþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþoa?\u0001VÇ" is 1
    8. "P" "îþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþoa?\u0001VÇ" is 1
    9. "Q" "îþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþoa?\u0001VÇ" is 1
    10. "R" "îþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþoa?\u0001VÇ" is 1
    11. "W" "îþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþîþoa?\u0001VÇ" is 1
    12. Debugging has finished
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    And here's the output from normal run:
    Qt Code: 
    1. Starting D:\Andreas\Qt\build-StringTest-Desktop_Qt_5_5_1_MinGW_32bit2-Debug\debug\StringTest.exe...
    2. "C" "C:\\" is 3
    3. "D" "D:\\" is 3
    4. "E" "E:\\" is 5
    5. "F" "F:\\" is 2
    6. "G" "G:\\" is 2
    7. "I" "I:\\" is 3
    8. "P" "P:\\" is 5
    9. "Q" "Q:\\" is 5
    10. "R" "R:\\" is 5
    11. "W" "W:\\" is 3
    12. D:\Andreas\Qt\build-StringTest-Desktop_Qt_5_5_1_MinGW_32bit2-Debug\debug\StringTest.exe exited with code 0
    To copy to clipboard, switch view to plain text mode 

    What could be the problem and can I do something to solve it?

    Merci.
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  2. 1st April 2016, 08:56 #2
    anda_skoa
    anda_skoa is offline Administrator
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    Default Re: Conversion of QString to char array yields garbage when in debug mode

    That this works in normal run mode is just conincidence, it is always wrong.

    "drive" becomes an invalid pointer after that line of code has executed, i.e. the std::string returned by toStdString() is a temporary object and goes out of scope right after you have accessed c_str().
    Since c_str() returns a pointer to the std::string's internal data, which is deleted when the std::string is destroyed, the pointer you have in "drive" is now invalid.

    You could just keep the std::string and call c_str() when you need it to pass to GetDriveTypeA().

    Better yet would be to use QFile::decodeName() instead of toStString() to get the local representation.

    Cheers,
    _
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  3. The following user says thank you to anda_skoa for this useful post:

    antimatter (5th April 2016)
  4. 5th April 2016, 11:57 #3
    antimatter
    antimatter is offline Novice
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    Default [SOLVED] Re: Conversion of QString to char array yields garbage when in debug mode

    Ah, that's why.
    Thanks for that explanation. Now I see it, too.
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