how to create Singleinstance application

[Ratzz]

@Bharth
You can make use of QtSingleApplication https://doc.qt.io/archives/qtextended4.4/qtopiadesktop/qtsingleapplication.html
How to use them here.
Good example by using QLockFile http://blog.aeguana.com/2015/10/15/how-to-run-a-single-app-instance-in-qt/

[J.Hilk]

@Bharth
I do this via a QSharedMemory instance, I'm not sure if thats the best way to do it though.

//insice main.cpp
QSharedMemory _singular("myAppNameInstance");
    if(_singular.attach(QSharedMemory::ReadOnly)){
        //Instance Already running
        _singular.detach();
        //Your MessageBox-code should go here
        return -42;
    }else{
       //Program is not yet running.
        _singular.create(1);
    }

[Bharth]

@Ratzz said in how to create Singleinstance application:

@Bharth
You can make use of QtSingleApplication https://doc.qt.io/archives/qtextended4.4/qtopiadesktop/qtsingleapplication.html
How to use them here.

i checked this one it is showing file not found
#include<QtSingleApplication>

[Ratzz]

@Bharth said in how to create Singleinstance application:

it is showing file not found

Its not part of Qt. You need to download and add them to your project.
You can download from here .

[Bharth]

@J.Hilk said in how to create Singleinstance application:

myAppNameInstance

QMessageBox::information(this,"Application is already running...","appname");

this keyword showing error,,,,invalid use of this

why you are retunring -42 what is the meaning...

thanks for your reply

[J.Hilk]

@Bharth
the main.cpp is not QObject based class, so you can't give QMessageBox this as parent.

why you are retunring -42 what is the meanin

The Answer to the Ultimate Question of Death, The Universe, and Everything ...

The boring answer: an arbitrary return value I choose to identify the exit of the application.

[Bharth]

@Ratzz said in how to create Singleinstance application:

@Bharth said in how to create Singleinstance application:

it is showing file not found

Its not part of Qt. You need to download and add them to your project.
You can download from here .

i downloaded but i dont know which i should add to my project

[Ratzz]

@Bharth said in how to create Singleinstance application:

i dont know which i should add to my project

For me this works

[Bharth]

thanks for your reply ,,,i will try

[Bharth]

its working but not able to show qmessagebox,,,,
again if i click exe file it should open old one now its not opening

[jsulm]

@Bharth said in how to create Singleinstance application:

again if i click exe file it should open old one now its not opening

Do you mean you want to show a message if you try to start a second instance?
Then simply do it inside this block:

if (instance.sendMessage("Wake up!"))
{
    QMessageBox::information(nullptr, "Information", "Already running!", ...);
    return 0;
}

[Bharth]

@jsulm said in how to create Singleinstance application:

QMessageBox::information(nullptr, "Information", "Already running!", ...);

thank you so much its worked....