QT Windows GUI - How Open a instance only once per time without qtsingleapplication

Question

I am developing an application in QT for Windows and I have the following problem: 1. Some users are opening multiple instances of the same application, which is not allowed. How can I control that each user only opens the application once, without using qtsingleapplication?

Answer 1

A quick and a dirty way to do it would be to create a lock file. If the lock file exists and the process is active, the new instance will quit. A crude and unchecked example is given below.

bool isProcessRunning( DWORD processID, std::string processName) {

    TCHAR szProcessName[MAX_PATH] = TEXT("<unknown>");

    // Get a handle to the process.

    HANDLE hProcess = OpenProcess( PROCESS_QUERY_INFORMATION | PROCESS_VM_READ, FALSE, processID );

    // Get the process name.

    if ( NULL != hProcess ) {
        HMODULE hMod;
        DWORD cbNeeded;

        if ( EnumProcessModules( hProcess, &hMod, sizeof(hMod), &cbNeeded) )
            GetModuleBaseName( hProcess, hMod, szProcessName, sizeof(szProcessName)/sizeof(TCHAR) );
    }

    // Compare process name with your string
    bool matchFound = !_tcscmp(szProcessName, processName.c_str() );

    // Release the handle to the process.
    CloseHandle( hProcess );

    return matchFound;
};

void lockApp() {
    /*
        *
        * lockApp() -> None
        *
        * Lock app to one instance
        *
        * @return None
        *
    */

    /* Flags */
    int pid = -1;
    bool procRunning = true;

    /* App Lock file: Name and Location */
    QString lockFile = QDir::temp().filePath( "MyAppName.lock" );

    // check for the lock file and see if its stale.
    if ( QFileInfo( lockFile ).exists() ) {

        // OK, lock exists. Now, check if it is stale!
        QFile pidFile( lockFile );
        pidFile.open( QFile::ReadOnly );
        pid = pidFile.readAll().toInt();
        pidFile.close();

        // if /proc/pid doesnot exist, we have a stale lock
        if( isProcessRunning( pid, "MyAppName" ) ) {
            procRunning = false;
            QFile::remove( lockFile );
        }
    }

    else {

        procRunning = false;
    }

    if ( procRunning ) {
        qApp->quit();
        exit( 0 );
    }

    QFile pidFile( lockFile );
    pidFile.open( QFile::WriteOnly );
    pidFile.write( qPrintable( QString::number( qApp->applicationPid() ) ) );
    pidFile.close();
};

The first function was taken from here. https://stackoverflow.com/a/13635127/1233513