flameshot/external/singleapplication/examples/sending_arguments/main.cpp

#include "messagereceiver.h"
#include <singleapplication.h>

int main(int argc, char* argv[])
{
    // Allow secondary instances
    SingleApplication app(argc, argv, true);

    MessageReceiver msgReceiver;

    // If this is a secondary instance
    if (app.isSecondary()) {
        app.sendMessage(app.arguments().join(' ').toUtf8());
        qDebug() << "App already running.";
        qDebug() << "Primary instance PID: " << app.primaryPid();
        qDebug() << "Primary instance user: " << app.primaryUser();
        return 0;
    } else {
        QObject::connect(&app,
                         &SingleApplication::receivedMessage,
                         &msgReceiver,
                         &MessageReceiver::receivedMessage);
    }

    return app.exec();
}