QSharedMemory: IPC problem between apps which built on Qt 4.6 and Qt 5.6

[VMironiuk]

Hi!
I have small application (was wrote on Qt 5.6.0) which creates shared memory segment and starts external program which I got from examples of Qt 4.6.0 (/mainwindows/application). Here the main class of my app:

#define QT_56

#include <QtWidgets>

class SMTest : public QWidget
{
    Q_OBJECT

public:
    explicit SMTest(QWidget *parent = Q_NULLPTR)
        : QWidget(parent), _sharedMemory("hello_world", this)
    {
        _sharedMemory.create(sizeof(double));

        QPushButton *goButton = new QPushButton("Go!");
        connect(goButton, SIGNAL(clicked()), this, SLOT(go()));

        setLayout(new QHBoxLayout);
        layout()->addWidget(goButton);

        resize(200, 100);
    }

private slots:
    void go()
    {
        QString pathChunk;
#ifdef QT_56
        pathChunk = "/SharedMemoryTesting/application_qt56/application";
#else
        pathChunk = "/SharedMemoryTesting/application_qt46/application";
#endif
        QProcess *process = new QProcess(this);
        process->start(QDir::homePath() + pathChunk);
    }

private:
    QSharedMemory _sharedMemory;
};

In Qt's example I just added a function for attaching to the shared memory segment and writing some info to a log file (this function is called in the constructor of main window). You can see the code below:

void MainWindow::connectToSegment()
{
    QFile file(QDir::homePath() + "/sharedMem_qt46.txt");
    file.open(QIODevice::WriteOnly | QIODevice::Text);
    QTextStream out(&file);

    QSharedMemory *sm = new QSharedMemory("hello_world", this);
    if (!sm->attach()) {
        out << sm->errorString() << "\n";
    } else {
        out << "Everything good!\n";
    }
}

When I run an example from my application it doesn't attach to the shared memory segment and the log file ( "sharedMem_qt46.txt") has the following error: QSharedMemory::attach (shmget): doesn't exist.
But if I take the same example from Qt 5.6.0 and run with my app, it works fine.
Maybe is this Qt's bug?

P.S.
I use Debian 8 and don't know whether this problem will occur on other platforms.

Up.
On Windows 8.1 x64 works fine.

[SGaist]

Hi and welcome to devnet,

Qt 4.6 being pretty old, I'd recommend moving to at least 4.8.7 and check again.

[VMironiuk]

@SGaist
Ok, thank you for response.

[VMironiuk]

Qt 4.8.6 has the same problem.
On Qt 5.4.2 works fine.
I think, the best solution is the moving to 5.6.0 version.