Convert to and from QDataStream into struct of arrays results in sigfault?

cdwijs

Hi All,
I would like to let two Qt programs exchange information via the network. Therefore I would like to encode the data on the transmitter side into a QDataStream, and decode it on the receiver side. I am now trying to get the encoding and decoding part correct, the next step is to use the QDataStreams like in the network/fortune example. I've made the following program, but the data is not decoded correctly, and I see a segmentation fault during the last for loop. I suspect the problem is between the chair and the keyboard, but I don't see it yet :-) Cheers,
Cedric

#ifndef OVERIDE_H
#define OVERIDE_H

#include <QObject>
#include <QDataStream>

#define NUMLIGHTS 64
#define STREAMVERSION QDataStream::Qt_4_0

struct msgOveride
{
    qint8 overridden[NUMLIGHTS];
    float lightvalue[NUMLIGHTS];
};

class overide : public QObject
{
    Q_OBJECT
public:
    explicit overide(QObject *parent = 0);

signals:

public slots:
private:
    msgOveride myOverrides;
    msgOveride myRxOverrides;
    qint16 myBlocksizeRx;
};

#endif // OVERIDE_H

#include "overide.h"
#include <QByteArray>

overide::overide(QObject *parent) : QObject(parent)
{
    for (int i = 0; i < NUMLIGHTS; ++i)
    {
       myOverrides.lightvalue[i]=i;
       myOverrides.overridden[i]=false;
    }

    QByteArray block;
    QDataStream out(&block, QIODevice::WriteOnly);
    out.setVersion(STREAMVERSION);
    out << (quint16)0;

    for (int i = 0; i < NUMLIGHTS; ++i)
    {
        out << myOverrides.lightvalue[i];
        out << myOverrides.overridden[i];
    }
    out.device()->seek(0);
    out << (quint16)(block.size() - sizeof(quint16));

    QDataStream in(&block, QIODevice::ReadOnly);
    in.setVersion(STREAMVERSION);
    //emulate transmission via network:
    //in.device()->seek(0);
    //now consume the datastream at the receiving end

    in >> myBlocksizeRx;
    //check rx size here
    for (int i = 0; i < NUMLIGHTS; ++i)
    {
        in >> myRxOverrides.lightvalue[i];  //segfault when i=56, and value[3]and [54] is incorrect
        in >> myRxOverrides.overridden[i];
    }
}

cdwijs

I forgot to give my version numbers:
Windows 7 enterprise service pack 1, 64 bits
Qt Creator 3.6.0
Based on Qt 5.5.1 (MSVC 2013, 32 bit)
Built on Dec 15 2015 01:01:38
From revision b52c2f91f5
C:/Qt/Qt5.5/Tools/mingw492_32/bin/mingw32-make -f Makefile.Debug

Changing this line still generates a segfault:
#define STREAMVERSION QDataStream::Qt_DefaultCompiledVersion

Cheers,
Cedric

cdwijs

Hi All,
I have also tried it in a virtual machine running linux mint. There it works without any problems.
The version of Qt is a bit older, so I had to change this line:
#define STREAMVERSION QDataStream::Qt_4_8
My versions:
$ uname -a
Linux cedric-VirtualBox 3.19.0-32-generic #37~14.04.1-Ubuntu SMP Thu Oct 22 09:41:40 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
Qt Creator 3.0.1
Based on Qt 5.2.1 (GCC 4.8.2, 64 bit)
Built on Apr 9 2014 at 09:12:59
g++ -c -m64 -pipe -g -Wall -W -D_REENTRANT -DQT_CORE_LIB -DQT_SHARED -I/usr/share/qt4/mkspecs/linux-g++-64 -I../overide -I/usr/include/qt4/QtCore -I/usr/include/qt4 -I. -I../overide -I. -o main.o ../overide/main.cpp

I have also tried streamversion 4.8 in windows, but it still segfaults. This tells me there is something wrong with the Qt version (or compiler) I use on windows. I will try to update Qt on both platforms, and see what happens.

Cheers,
Cedric

VRonin

is myBlocksizeRx 320 (it should)?
did you try uncomment //in.device()->seek(0);?

cdwijs

Hi Vronin,
myBlocksizeRx is 576 bytes. As far as I know it should be (1 byte per qint8 + 8 bytes per float)*64 = 576 bytes.

But it's very strange, I have done the following steps, and now the problem does not occur anymore. I don't like this, as I have not adjusted my code, so I cannot be sure the problem does not re-appear when I expand my code.
-> rename c:\Qt to C:\Qt-moved
->install the latest version of Qt (5.7) in c:\Qt
->try to build the application with Qt 5.7. (did not work, as I don't have a debugger out-of-the-box)
->rename c:\Qt into Qt-new (containing Qt5.7)
->rename C:\qt-moved into c:\qt (containing Qt 5.5)
->remove the build folder and the .user file of my program
->build and debug the program.
Now the program works without problems.

For now the problem is gone, but I hope it does not return. Thanks for the help,
Cedric