The "Downloads" link there returns a 404

Do you know what's with the ␣ (open box, U+2423) character in his first example?

  printf("element␣%zu␣is␣%g,␣\tits␣square␣is␣%g\n"

I have a feeling the PDF file is messed up...Or is he showing you a literal space?

This representation for the space U+0020 was very common historically, because a space doesn't look like anything at all, especially if sometimes it's important whether there is no space, one space or two spaces, explicitly showing each space can help.

It seems very rare these days.

He's using that to represent a space character. I guess it forces you to pay more attention to the examples rather than copy-pasting.

Thanks!