I'm not even sure what the advantage might be to using the same filename for input and output.
I certainly don't suggest (either here or in the book) that you should
use the same file for input and output. I merely observe that, if you allow people to independently specify input and output filenames on the command-line, some of them inevitably will
use the same name for both files (either intentionally or accidentally). So you have to be aware of the possibility and cope with it somehow.
In the book, I suggest two solutions, one of which is more efficient but apparently fails on certain obscure operating systems (and, yes, I will definitely update the book to reflect that limitation, as soon as I can).