How do I print the reading serial data sending from Arduino
You have no warranty that Qt will receive the whole message in a single
chunk. It may receive "no se" as the first chunk, and then "q pasa" as
the second chunk. Since dataBuffer is dynamically allocated, it
initially contains garbage, and you are printing the part of this
garbage that you did not overwrite.

An easy fix would be to simply NULL-terminate the buffer:

dataBuffer = new char[size + 1];            // + 1 byte for '\0'
c = m_serialPort-&gt;read(dataBuffer, size);
dataBuffer[c] = '\0';                       // terminate the string