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.