They are not (generally) thread safe. Quoting http://doc.qt.io/qt-4.8/threads-reentrancy.html http://doc.qt.io/qt-4.8/threads-reentrancy.html > Many Qt classes

They are not (generally) thread safe. Quoting http://doc.qt.io/qt-4.8/threads-reentrancy.html http://doc.qt.io/qt-4.8/threads-reentrancy.html > Many Qt classes are reentrant, but they are not made thread-safe, because making them thread-safe would incur the extra overhead of repeatedly locking and unlocking a QMutex. ... > Some Qt classes and functions are thread-safe. These are mainly the thread-related classes (e.g. QMutex) and fundamental functions (e.g. QCoreApplication::postEvent()).