Question

I am using Qt and trying to achieve a single instance application by applying this solution in Linux(ubuntu). The problem is that if the application unexpectedly finishes(seg. fault or user kills it) the shared memory remains attached and no other process can create it again. Recall from QSharedMemory doc:

Unix: QSharedMemory "owns" the shared memory segment. When the last
thread or process that has an instance of QSharedMemory attached to a
particular shared memory segment detaches from the segment by
destroying its instance of QSharedMemory, the Unix kernel release the
shared memory segment. But if that last thread or process crashes
without running the QSharedMemory destructor, the shared memory
segment survives the crash.

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

    // Ensure single instanse of Cevirgec application
    QSharedMemory shared(ApplicationConstants::

    if( !shared.create( 512, QSharedMemory::ReadWrite) )
    {
      // QMessageBox msgBox;
      QMessageBox::critical(0, QObject::tr("application is already running!"), QObject::tr("application is already running!"), QMessageBox::Ok, QMessageBox::Ok);
      qCritical() << "application is already running!";

      exit(0);
    }
    else {
        qDebug() << "application staring...";
    }
    return a.exec(); 
}

What solutions can you suggest here? How can I assure that the shared memory is cleared(or whatever verb used generally) after the process eventually finishes. I need something like finally in java all around the main function :/

EDIT: (Solution)

I have achieved the desired behavior by using QSharedMemory and catching SIGSEGV signal then call sharedMemory.detach() in the signal handler.

Answer 1

You can catch the signals that crash your program and use a handler that calls the QSharedMemory destructor.