dedicate617 / qsingleinstance

Qt C++ library to check if another instance of an application is running. Disallow multiple application instances.

Usage

Add to project

Add qsingleinstance.pri to your application's project file (Qt's pro file), e.g.

# other stuff

include(./QSingleInstance.git/src/qsingleinstance.pri)

# other stuff

Add to source

An instance of the QSingleInstance class must be added to the main.cpp file of your application. The instance must be added right after the QApplication instantiation, but before the instantiation of any other cusotm widget. E.g.

#include <QApplication>
#include <QMessageBox>
#include "dialog.h"

// Add single instance header
#include <QSingleInstance>

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

    // Apply guard after QApplication instantiation
    // but before instantiating any custom widgets
    QSingleInstance guard("MyQDialog");
    if (!guard.tryToRun())
    {
        QMessageBox::critical(nullptr,
            QObject::trUtf8("Single Instance Guard"),
            QObject::trUtf8("An instance of this application is already running.\nClose existng instance before running a new one."),
            QMessageBox::Ok);
        return -1;
    }

    // Custom application dialog
    Dialog w;
    w.show();
    
    return a.exec();
}

Specifics

The QSingleInstance class creates some temporary files to obtain the single instance locking mechanism. The directory and name of these files are platform dependent.

The directory in Windows is in the system's temporary directory, this is usually the path in the TEMP or TMP environment variable.

In Linux, this is the path in the TMPDIR environment variable or /tmp if TMPDIR is not defined.

An example of the names of thse files are:

qipc_systemsem_beeccbcdeeaefcde296e990973cc975cf92bdb10bdb3bfd583bb2d5d
qipc_systemsem_aefbdbfedfcada7964cb9b08e40a7170e807eac76844af69280cd5
qipc_sharedmemory_aefbdbfedfcada7964cb9b08e40a7170e807eac76844af69280cd5

These files are created when the first instance is created. Then remain through the life span of the application. They must be automatically deleted when the single instance is closed. Any bugs on the single instance behaviour are notmally related to these files.

References

• http://doc.qt.io/qt-5/qdir.html#tempPath

• http://habrahabr.ru/post/173281/