Qt single instance app how to Manage on crash

haris123

Hi,

I want to execute my application as single instance, currently I am using QSharedMemory and working fine. I am using Qt5.2.1 on Ubuntu 12.04. below is my test code,

@ QApplication a(argc, argv);
a.processEvents();

const char* MEM_KEY = "56";
QSharedMemory sharedMem(MEM_KEY);

if( !sharedMem.create( 512, QSharedMemory::ReadWrite) )
{
  QMessageBox msgBox;
  msgBox.setText( QObject::tr("Can't start more than one instance of the application.") );
  msgBox.setIcon( QMessageBox::Critical );
  msgBox.exec&#40;&#41;;
  exit(0);
}


MainWindow w;
w.show();

int p=0;
//p=p/0; // create exception here

return a.exec&#40;&#41;;@

But the if I made the application crash(as shown in above code) and start the application again it showing @Can't start more than one instance of the application@ means the previous instance is still there even if it's crashed. It should not happened in my case, how can I restart my application in such a situation?.

Thanks
Haris

sierdzio

Try using QtSingleApplication: "link":https://qt.gitorious.org/qt-solutions/qt-solutions/source/38e79e3f04d6fd5a3df585a60b2aec95e8e68368:qtsingleapplication

It is not part of Qt, but is created an maintained by Qt Company, so it's very close.

Francknos

Or you can do like me.
Check if .exe is running ... :

@
//in main => MainWidget m; m.show();
MainWidget::MainWidget(int argc, char** argv, QWidget *parent) :
QWidget(parent),
ui(new Ui::MainWidget)
{
initialisations();

#ifdef ONLY_ONE_INSTANCE
checkOnlyOneInstance();
#endif
}
@
checkOnlyOneInstance
@
void MainWidget::checkOnlyOneInstance()
{
QProcess x;
x.start("tasklist");
x.waitForStarted(200); //1000 avant
x.waitForReadyRead(200);
x.waitForFinished(200);
QString result(x.readAllStandardOutput());
if(result.count("MON APPLICATION.exe",Qt::CaseInsensitive) > 1 )
{
popupMessage(tr("L'Application est déjà en cours d'execution!"),2000,this);
QTimer::singleShot(2000, qApp, SLOT(quit()));
}
}
@

It works for me.

SGaist

Hi,

Beware, you are doing your initialization twice. You're application should not start any initialization before checking it's not already running. Also that's a Windows specific implementation and the OP is running Ubuntu. sierdzio's suggestion is good. It works very well.

Francknos

My Initialisation is just to config the GUI ... (stylesheet ...)

sierdzio

[quote author="Francknos" date="1424334416"]My Initialisation is just to config the GUI ... (stylesheet ...)[/quote]

Yes, but it does start the application. So, for a tiny bit of time, you do have 2 instances of your application launched.

Francknos

Yes I have 2 instances of my aplpication during 2000 ms.
But it's not a problem for me.

I can check this in main so ...

Francknos

[quote author="SGaist" date="1424300270"]Hi,

Beware, you are doing your initialization twice. You're application should not start any initialization before checking it's not already running. Also that's a Windows specific implementation and the OP is running Ubuntu. sierdzio's suggestion is good. It works very well.[/quote]

I have tried this but it doesn't work with me.

SGaist

What doesn't work ?

? Offline

You could also use D-Bus in stead of shared memory.

jerome_isAviable

you also can try to create and use a specific singleton class.
look at C++ codes/exemples for learn how to use well singleton class.
i do it for check login and users access/privileges.
after that, you can call your singleton by handle the "get_instance" method (for exemple).

sierdzio

[quote author="jerome_isAviable?" date="1424746717"]you also can try to create and use a specific singleton class.[/quote]

That won't work for this use case. Singleton has a single instance of a class per application - but there can be many applications launched. The OP wants to run only one instance of the application.

jerome_isAviable

exact.