I guess it isn't - prints can get overlayed with printfs from other tasks, but would still like to confirm if I'm right or wrong..
The printf is not muti-task safe. Please see the code of _io_printf, please notice that no any sync method(sem, event, etc) used within it. The call stack was listed to easy the code reading.
_io_printf -> _io_doprintf->_io_serila_poller_write.
Retrieving data ...