Question

Every time I run my project in Qt Creator, it spins up another instance of my app. I have to manually quit the app or else my Dock gets full pretty fast. What a pain. Is there a way around this? It would make a lot more sense if when I run the app again, I could just terminate the already running version. Can this be done?

Answer 1

you can use shared memory to solve your problem. I have used this approach to not start another instance of my program while there is already a running instance. Actually I have implemented this to achieve so called single instance application.

However your case is a bit different from mine, you need to somehow send a signal from 2nd application to the first one to make it close. I think you can achieve this behaviour still using QSharedMemory.

What I’ve done to achive single instance app is to create a shared memory with a universally unique id(UUID) as key and every time my program starts put a lock on it, so if it is already locked my program understands there is already a running instance and closes automatically.

You need to improve this implementation to adapt your requirement. In theory what you need to do is to put a function pointer(or a qt signal) to the shared memory and when another instance come up make your (second) instance fire that function which forces to exit the first instance. Unfortunately I don’t know how to implement this but I hope this would give you an opinion…

The flow should be somewhat like follows:

IN MAIN
   check if shared memory in use
      if yes
         fire the exit function via shared memory to close 1st app
      if no
         put the function pointer which will close the app when another instance come up

    do stuff

To give you a slight hint, my code for single instance is like follows

QSharedMemory shared(AppConstants::UUID); //Global variable

int main(){
// Ensure single instanse of App
if( !shared.create( 512, QSharedMemory::ReadWrite) )
{
    // QMessageBox msgBox;
    QMessageBox::critical(0, QObject::tr("App is already running!"), QObject::tr("App is already running!"), QMessageBox::Ok, QMessageBox::Ok);
    qCritical() << "Cevirgec is already running!";

    exit(0);
}
else {
    qDebug() << "App staring...";
}
}

good luck and don’t forget to share your solution here 😉

EDIT:

If putting a function pointer or Qt signal and then fire it is impossible(I hope not) you can put a variable to shared memory to lets say hold the number of running instances and in your app periodically(in a thread) check it and if it is greater than 1 close the application.

Watch out for race conditions here! You can avoid race conditions by putting a pair of a random number generated by each instance and start time. so before closing, your app ensures that it is the elder one. Ex: QPair<int, QDateTime>