A little Googling turns up that the author of the Unix library (Xavier, IIRC) provided support for termios(3). So you can already do what you want in Ocaml with no extra C ugly bits.

Here's a little ocaml program to demonstrate, and after it, some strace output showing the way it calls ioctl(2) to manipulate the line discipline (relevant bits bolded in HTML format mail).

let main () =

let open Unix in

let tios = tcgetattr stdin in

Printf.printf "c_echo: %b\n" tios.c_echo ;

Printf.printf "c_echoe: %b\n" tios.c_echoe ;

Printf.printf "c_echok: %b\n" tios.c_echok ;

Printf.printf "c_echonl: %b\n" tios.c_echonl ;

flush Pervasives.stdout ;

tios.c_echo <- false ;

tcsetattr stdin TCSANOW tios ;

let tios = tcgetattr stdin in

Printf.printf "AFTER c_echo: %b\nSleeping 10 sec ....\n" tios.c_echo ;

flush Pervasives.stdout ;

Unix.sleep 10;

tios.c_echo <- true ;

tcsetattr stdin TCSANOW tios ;

()

;;

main() ;;

=====================

$ strace -eioctl ./noecho

ioctl(0, TCGETS, {B38400 opost isig icanon echo ...}) = 0

c_echo: true

c_echoe: true

c_echok: true

c_echonl: false

ioctl(0, TCGETS, {B38400 opost isig icanon echo ...}) = 0

ioctl(0, SNDCTL_TMR_START or TCSETS, {B38400 opost isig icanon -echo ...}) = 0

ioctl(0, TCGETS, {B38400 opost isig icanon -echo ...}) = 0

AFTER c_echo: false

Sleeping 10 sec ....

ioctl(0, TCGETS, {B38400 opost isig icanon -echo ...}) = 0

ioctl(0, SNDCTL_TMR_START or TCSETS, {B38400 opost isig icanon echo ...}) = 0

ioctl(0, TCGETS, {B38400 opost isig icanon echo ...}) = 0

+++ exited with 0 +++