These are blocking. If there is nothing, they wait.

Interestingly, run them on the toplevel followed by CTRL-D

A control signals an EOF condition in standard mode, and we see -1:

?- peek_code(X).
^DX = -1.

But in raw mode:

?- with_tty_raw(peek_code(X)).
X = 4.

ASCII 4 is "end of transmission". The EOT character is received instead of the EOF condition being signaled.

See https://en.wikipedia.org/wiki/End-of-Transmission_character