Why is it necessary to run an event loop within each thread to which you want to connect signals? The reason has to do with the inter-thread communication mechanism used by Qt when connecting signals from one thread to the slot of another thread. When such a connection is made, it is referred to as a queued connection. When signals are emitted through a queued connection, the slot is invoked the next time the destination object's event loop is executed. If the slot had instead been invoked directly by a signal from another thread, that slot would execute in the same context as the calling thread.
I think my question above is solved with that sentence.

But is my data, which will be sent via the "emit sendData(data)", secure?
Will it be stored somewhere temporary , so the producer cannot change it while the signal is still in the queue.