Special Character Kill

I'm studying a few chapters from The Linux Programming Interface book, and I'm learning about how the terminal driver can interpret special characters.

The KILL character (typically sent by Control-U) should make the terminal driver to delete the current line on the terminal.

We can use stty -a to see what KILL is bound to:

speed 9600 baud; 102 rows; 271 columns;
lflags: icanon isig iexten echo echoe -echok echoke -echonl echoctl
	-echoprt -altwerase -noflsh -tostop -flusho pendin -nokerninfo
	-extproc
iflags: -istrip icrnl -inlcr -igncr ixon -ixoff ixany imaxbel iutf8
	-ignbrk brkint -inpck -ignpar -parmrk
oflags: opost onlcr -oxtabs -onocr -onlret
cflags: cread cs8 -parenb -parodd hupcl -clocal -cstopb -crtscts -dsrflow
	-dtrflow -mdmbuf
cchars: discard = ^O; dsusp = ^Y; eof = ^D; eol = <undef>;
	eol2 = <undef>; erase = ^?; intr = ^C; kill = ^U; lnext = ^V;
	min = 1; quit = ^\; reprint = ^R; start = ^Q; status = ^T;
	stop = ^S; susp = ^Z; time = 0; werase = ^W;

Normally, the shell will also implement a kill-line behaviour on Control-U though, so if the terminal driver doesn't clear it, at least the shell will:

  bindkey | grep '\^U' # zsh
  # "^U" kill-whole-line

If I unbind the key sequence ^U in the shell, I should still expect the line to be cleared when I send ^U because the terminal driver should've handled the special character before it reaches the shell.

Hint: The key sequence can be unbound on zsh with bindkey -r '^U'.

Earlier I mentioned how the terminal driver should handle ^U, but this is only when the terminal is set in canonical mode.

In the stty -a dump, we could see that the terminal is in canonical mode because there's no - in front of icanon.

But this is a lie! The zsh shell is enabling canonical mode when we enter a command, and turns it off during line editing!

We can find the shell's pseudoterminal slave file descriptor, and inspect it in using another shell to see its terminal's attribute in a normal line editing context:

speed 9600 baud; 100 rows; 271 columns;
lflags: -icanon isig iexten -echo echoe -echok echoke -echonl echoctl
	-echoprt -altwerase -noflsh -tostop -flusho -pendin -nokerninfo
	-extproc
iflags: -istrip icrnl inlcr -igncr -ixon -ixoff ixany imaxbel iutf8
	-ignbrk brkint -inpck -ignpar -parmrk
oflags: opost onlcr -oxtabs -onocr -onlret
cflags: cread cs8 -parenb -parodd hupcl -clocal -cstopb -crtscts -dsrflow
	-dtrflow -mdmbuf
cchars: discard = <undef>; dsusp = <undef>; eof = ^D; eol = <undef>;
	eol2 = <undef>; erase = ^?; intr = ^C; kill = ^U;
	lnext = <undef>; min = 1; quit = <undef>; reprint = ^R;
	start = <undef>; status = ^T; stop = <undef>; susp = <undef>;
	time = 0; werase = ^W;

Let's see a diff!

2,3c2,3
< lflags: icanon isig iexten echo echoe -echok echoke -echonl echoctl
< 	-echoprt -altwerase -noflsh -tostop -flusho pendin -nokerninfo
---
> lflags: -icanon isig iexten -echo echoe -echok echoke -echonl echoctl
> 	-echoprt -altwerase -noflsh -tostop -flusho -pendin -nokerninfo
5c5
< iflags: -istrip icrnl -inlcr -igncr ixon -ixoff ixany imaxbel iutf8
---
> iflags: -istrip icrnl inlcr -igncr -ixon -ixoff ixany imaxbel iutf8
10,13c10,14
< cchars: discard = ^O; dsusp = ^Y; eof = ^D; eol = <undef>;
< 	eol2 = <undef>; erase = ^?; intr = ^C; kill = ^U; lnext = ^V;
< 	min = 1; quit = ^\; reprint = ^R; start = ^Q; status = ^T;
< 	stop = ^S; susp = ^Z; time = 0; werase = ^W;
---
> cchars: discard = <undef>; dsusp = <undef>; eof = ^D; eol = <undef>;
> 	eol2 = <undef>; erase = ^?; intr = ^C; kill = ^U;
> 	lnext = <undef>; min = 1; quit = <undef>; reprint = ^R;
> 	start = <undef>; status = ^T; stop = <undef>; susp = <undef>;
> 	time = 0; werase = ^W;

Wow, some considerable differences.

In order to confirm our findings, we can re-enable some of the required terminal attributes and see that ^U will cause a line deletion operation from the terminal driver.

In another shell, we run stty -f /dev/ttys001 icanon echo inlcr.

In our original shell, we can now use ^U and see the terminal driver delete the line.

Hint: If you press Enter, then zsh will have an opportunity to reset the terminal attributes to its likings.