OK, all code related to creating connection to database is moved to run() method :
Qt Code: 
  1. void KonwertBJM::run(void)
  2. {
  3.  
  4.     QSettings settings( "dworzec.ini", QSettings::IniFormat);
  5.     settings.beginGroup("DbSetup");
  6.     db_connection_name = UniqueDbName();
  7.     db = QSqlDatabase::addDatabase(settings.value("DbType").toString(),db_connection_name);
  8.  
  9.     db.setHostName(settings.value("HostName").toString());
  10.     db.setDatabaseName(settings.value("DatabaseName").toString());
  11.     db.setUserName(settings.value("UserName").toString());
  12.     db.setPassword(settings.value("Password").toString());
  13.     if( !db.open() )
  14.     {
  15.         qDebug() << "KonwerterBJM"
  16.                     << QObject::tr("Can't connect to database")
  17.                     << db.lastError().text();
  18.         return;
  19.     }
  20.     char miesiac[7];
  21.  
  22.     strcpy_s(miesiac,sizeof(miesiac),bazaDBF.mid(1,6 ).toLocal8Bit().data());
  23.     robKonwersje(katDBF.toLocal8Bit().data(),miesiac);
  24.  
  25.     db.close();
  26. }
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No difference.