open an existing file with content as below:

1.  QFile aFile(myFilePath);   
2.  if(!aFile.open(QIODevice::WriteOnly | QIODevice::Truncate)) 
3.   {        qDebug() << aFile.errorString();    }   
4. qDebug() << "Pos " << aFile.pos();
5. qDebug() << "File Size write " << aFile.size();
6. aFile.resize(0);   // bug, the size remain the old size.
7. qDebug() << "Pos " << aFile.pos();
8. qDebug() << "File Size write " << aFile.size();