Which file descriptor should be used in ioctl to know terminal screen size? - rust

Question

I want to reliably know terminal size even when the application is running in a subshell.
It seems like the only working way is to use stdin fd, but I would like to know why have links to documentation that proves it.
There's a lot of questions like why would one even use stdout (or stdin for that matter) fd to know terminal size?
fn main() {
let mut size = winsize {
ws_row: 0,
ws_col: 0,
ws_xpixel: 0,
ws_ypixel: 0,
};
let fd = std::fs::OpenOptions::new()
.read(true)
.write(true)
.open("/dev/tty")
.map(|file| file.as_raw_fd())
.unwrap()
;
unsafe { ioctl(fd, TIOCGWINSZ.into(), &mut size) };
println!("/dev/tty cols: {}, lines: {}", size.ws_row, size.ws_col);
unsafe { ioctl(STDOUT_FILENO, TIOCGWINSZ.into(), &mut size) };
println!("stdout cols: {}, lines: {}", size.ws_row, size.ws_col);
unsafe { ioctl(STDERR_FILENO, TIOCGWINSZ.into(), &mut size) };
println!("stderr cols: {}, lines: {}", size.ws_row, size.ws_col);
unsafe { ioctl(STDIN_FILENO, TIOCGWINSZ.into(), &mut size) };
println!("stdin cols: {}, lines: {}", size.ws_row, size.ws_col);
}
I ran echo (cargo run 2>&1) in the fish shell and got this:
Compiling test_term_size v0.1.0 (/home/m/code/test_term_size) warning: unused import: `File` --> src/main.rs:2:15 | 2 | use std::fs::{File}; | ^^^^ | = note: `#[warn(unused_imports)]` on by default warning: unused imports: `O_RDWR`, `open` --> src/main.rs:4:5 | 4 | O_RDWR, | ^^^^^^ 5 | open, ioctl, winsize, STDIN_FILENO, | ^^^^ Finished dev [unoptimized + debuginfo] target(s) in 0.21s Running `target/debug/test_term_size` /dev/tty cols: 0, lines: 0 stdout cols: 0, lines: 0 stderr cols: 0, lines: 0 stdin cols: 52, lines: 106
strace -e trace=ioctl,open target/debug/test_term_size
ioctl(3, TIOCGWINSZ, 0x7ffdb9805578) = -1 EBADF (Bad file descriptor)
/dev/tty cols: 0, lines: 0
ioctl(1, TIOCGWINSZ, {ws_row=52, ws_col=213, ws_xpixel=3834, ws_ypixel=2028}) = 0
stdout cols: 52, lines: 213
ioctl(2, TIOCGWINSZ, {ws_row=52, ws_col=213, ws_xpixel=3834, ws_ypixel=2028}) = 0
stderr cols: 52, lines: 213
ioctl(0, TIOCGWINSZ, {ws_row=52, ws_col=213, ws_xpixel=3834, ws_ypixel=2028}) = 0
stdin cols: 52, lines: 213
+++ exited with 0 +++
strace -e trace=ioctl,open cargo run
ioctl(2, TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(2, TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(2, TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(2, TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(2, TIOCGWINSZ, {ws_row=52, ws_col=213, ws_xpixel=3834, ws_ypixel=2028}) = 0
ioctl(2, TIOCGWINSZ, {ws_row=52, ws_col=213, ws_xpixel=3834, ws_ypixel=2028}) = 0
ioctl(2, TIOCGWINSZ, {ws_row=52, ws_col=213, ws_xpixel=3834, ws_ypixel=2028}) = 0
Finished dev [unoptimized + debuginfo] target(s) in 0.01s
Running `target/debug/test_term_size`
ioctl(3, TIOCGWINSZ, 0x7ffea819b008) = -1 EBADF (Bad file descriptor)
/dev/tty cols: 0, lines: 0
ioctl(1, TIOCGWINSZ, {ws_row=52, ws_col=213, ws_xpixel=3834, ws_ypixel=2028}) = 0
stdout cols: 52, lines: 213
ioctl(2, TIOCGWINSZ, {ws_row=52, ws_col=213, ws_xpixel=3834, ws_ypixel=2028}) = 0
stderr cols: 52, lines: 213
ioctl(0, TIOCGWINSZ, {ws_row=52, ws_col=213, ws_xpixel=3834, ws_ypixel=2028}) = 0
stdin cols: 52, lines: 213
+++ exited with 0 +++
echo (strace -e trace=ioctl,open cargo run 2>&1)
ioctl(2, TCGETS, 0x7ffdae149cb0) = -1 ENOTTY (Inappropriate ioctl for device) ioctl(2, TCGETS, 0x7ffdae149a30) = -1 ENOTTY (Inappropriate ioctl for device) ioctl(2, TCGETS, 0x7ffdae149a30) = -1 ENOTTY (Inappropriate ioctl for device) ioctl(2, TCGETS, 0x7ffdae149540) = -1 ENOTTY (Inappropriate ioctl for device) warning: unused import: `File` --> src/main.rs:2:15 | 2 | use std::fs::{File}; | ^^^^ | = note: `#[warn(unused_imports)]` on by default warning: unused imports: `O_RDWR`, `open` --> src/main.rs:4:5 | 4 | O_RDWR, | ^^^^^^ 5 | open, ioctl, winsize, STDIN_FILENO, | ^^^^ Finished dev [unoptimized + debuginfo] target(s) in 0.02s Running `target/debug/test_term_size` ioctl(3, TIOCGWINSZ, 0x7ffd8c3ae728) = -1 EBADF (Bad file descriptor) /dev/tty cols: 0, lines: 0 ioctl(1, TIOCGWINSZ, 0x7ffd8c3ae728) = -1 ENOTTY (Inappropriate ioctl for device) stdout cols: 0, lines: 0 ioctl(2, TIOCGWINSZ, 0x7ffd8c3ae728) = -1 ENOTTY (Inappropriate ioctl for device) stderr cols: 0, lines: 0 ioctl(0, TIOCGWINSZ, {ws_row=52, ws_col=213, ws_xpixel=3834, ws_ypixel=2028}) = 0 stdin cols: 52, lines: 213 +++ exited with 0 +++
echo (strace -e trace=ioctl,open target/debug/test_term_size 2>&1)
ioctl(3, TIOCGWINSZ, 0x7fffdc82e938) = -1 EBADF (Bad file descriptor) /dev/tty cols: 0, lines: 0 ioctl(1, TIOCGWINSZ, 0x7fffdc82e938) = -1 ENOTTY (Inappropriate ioctl for device) stdout cols: 0, lines: 0 ioctl(2, TIOCGWINSZ, 0x7fffdc82e938) = -1 ENOTTY (Inappropriate ioctl for device) stderr cols: 0, lines: 0 ioctl(0, TIOCGWINSZ, {ws_row=52, ws_col=213, ws_xpixel=3834, ws_ypixel=2028}) = 0 stdin cols: 52, lines: 213 +++ exited with 0 +++
echo (strace -e trace=ioctl,open target/debug/test_term_size 2>&1)
ioctl(3, TIOCGWINSZ, 0x7fffdc82e938) = -1 EBADF (Bad file descriptor) /dev/tty cols: 0, lines: 0 ioctl(1, TIOCGWINSZ, 0x7fffdc82e938) = -1 ENOTTY (Inappropriate ioctl for device) stdout cols: 0, lines: 0 ioctl(2, TIOCGWINSZ, 0x7fffdc82e938) = -1 ENOTTY (Inappropriate ioctl for device) stderr cols: 0, lines: 0 ioctl(0, TIOCGWINSZ, {ws_row=52, ws_col=213, ws_xpixel=3834, ws_ypixel=2028}) = 0 stdin cols: 52, lines: 213 +++ exited with 0 +++

Answer 1

The right source of TTY info is always /dev/tty. Don't use stdin or any other weird hacks. Here's why it looks like that isn't working for you:
let fd = std::fs::OpenOptions::new()
.read(true)
.write(true)
.open("/dev/tty")
.map(|file| file.as_raw_fd())
.unwrap()
;
You're opening /dev/tty and getting its FD, but the file is then getting dropped, and hence closed, before you can use it, so the ioctl on it fails with EBADF "Bad file descriptor". To fix the problem, either keep the file alive until after you're done using the raw FD, or use into_raw_fd instead of as_raw_fd and then close the raw FD yourself when you're done with it.