semaphore reading qstring example error

[SherifOmran]

Hello guys,
i am trying to use the semaphores shared memory to read a QString value. I am attaching my code. I can block the memory and detect it being blocked but can not read the value from the shared memory.

thanks in advance for assistance

#include "mainwindow.h"
#include <QApplication>
#include <QSystemSemaphore>
#include <QSharedMemory>
#include <QMessageBox>
#include <QString>

#include <QBuffer>
#include <QtGlobal>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QSystemSemaphore semaphore("<uniq id>", 1);  // create semaphore
    semaphore.acquire(); // Raise the semaphore, barring other instances to work with shared memory

#ifndef Q_OS_WIN32
    // in linux / unix shared memory is not freed when the application terminates abnormally,
    // so you need to get rid of the garbage
    QSharedMemory nix_fix_shared_memory("<uniq id 2>");
    if(nix_fix_shared_memory.attach()){
        nix_fix_shared_memory.detach();
    }
#endif

    QSharedMemory sharedMemory("<uniq id 2>");  // Create a copy of the shared memory
    bool is_running;            // variable to test the already running application

    QBuffer buffer;
    int x=0; QString ux;
    QDataStream in(&buffer);

    if (sharedMemory.attach()){ // We are trying to attach a copy of the shared memory
                                // To an existing segment
        is_running = true;      // If successful, it determines that there is already a running instance
        sharedMemory.lock();
        buffer.setData((char*)sharedMemory.constData(), sharedMemory.size());
        buffer.open(QBuffer::ReadOnly);
        char *from = (char*)sharedMemory.data();
        memcpy (from, &ux, sharedMemory.size());
        //in >> ux;
        sharedMemory.unlock();

    }else{
        buffer.open(QBuffer::ReadWrite);
        QDataStream out(&buffer);
        sharedMemory.create(2); // Otherwise allocate 1 byte of memory
        out << QString("Y");
        sharedMemory.lock();
        char *to = (char*)sharedMemory.data();
        const char *from = buffer.data().data();
        int size=buffer.size();
        memcpy(to, from, qMin(sharedMemory.size(), size));
        sharedMemory.unlock();

        //out << (int)1;
        is_running = false;     // And determines that another instance is not running
    }
    semaphore.release();

    // If you already run one instance of the application, then we inform the user about it
    // and complete the current instance of the application
    if(is_running){
        QMessageBox msgBox;
        QString u = QString::number(x);
        msgBox.setIcon(QMessageBox::Warning);
        msgBox.setText("The application is already running.\n"
                       "value " + ux);
        msgBox.exec();
        return 1;
    }

    MainWindow w;
    w.show();

    return a.exec();
}

[sierdzio]

You are aware of QtSingleApplication, right? https://github.com/qtproject/qt-solutions/tree/master/qtsingleapplication

And a string can be (simpler) saved to and read from a file (for example using QSettings).

Which is another way of saying "I've never used QSystemSemaphore or QSharedMemory and don't know how to help you" ;-)

[SherifOmran]

@sierdzio I need to exchange data between different applications

[SGaist]

Hi,

What exactly are you trying to exchange between the two applications ?

[SherifOmran]

i am trying to exchange a string. Actually i wanted to create a slot in app B that responds to a signal from App A and tried to do it with semaphore but i think this is not the correct approach. I think doing it with dBus may be better or using local network is also another approach.

[sierdzio]

@SherifOmran said in semaphore reading qstring example error:

I think doing it with dBus may be better or using local network is also another approach.

There is also the (quite new) QtRemoteObjects module.

[SherifOmran]

@sierdzio : do you know if there is an example to do it?
thank you

[sierdzio]

have not used it myself yet, so all I can do is point to the docs. It was developed by a company doing industrial projects and contributed to Qt, I expect it to work quite well for sending data between apps.

[mrjj]

Hi
Just as a note this sample does work
https://doc.qt.io/qt-5/qtcore-ipc-sharedmemory-example.html

[JonB]

@SherifOmran

I can block the memory and detect it being blocked but can not read the value from the shared memory.

If you want to do it your way, and if I read your code right:

QString ux;
        char *from = (char*)sharedMemory.data();
        memcpy (from, &ux, sharedMemory.size());

This writes to the shared memory area, named from, from the QString, which is empty.

If I didn't know better, I'd wonder whether you simply have your arguments to memcpy() the wrong way round; don't you intend:

        memcpy (&ux, from, sharedMemory.size());

This is in the code which attaches to a segment already created by your other, creation route, so I presume it's supposed to be reading into the QString what the other route left there. [Meanwhile I imagine you're not supposed to write directly into a QString like this, so it may well go wrong in that sense, but that's a different matter...]