The TTY Demystified (2008)

mixmastamyk•about 2 hours ago

This article caught my eye since I've been playing with kmscon. Learned quite a bit, thanks. Though feel I'm still a bit unclear on how systemd init starts the agettys and that area. Also newer distro versions appear to have moved graphics from vt7 to vt1. Not sure of the reasoning why.

What I'd like to do is keep the boot log at tty1 and not wipe it out when finished.

This answer seems like it might help, going to try it. https://askubuntu.com/a/195926/116108

20after4•1 day ago

The low level details of the Linux TTY driver is a surprisingly deep subject. This very old page taught me a lot about it and a recent HN discussion reminded me to dig up the old link. Hopefully others find it as enlightening as I have.

Joker_vD•about 22 hours ago

Frankly, the way the functionality needed to implement LF-to-CRLF conversion on output/CR-to-LF conversion on input/line editing/session and process management is split between the terminal itself, the kernel, and the application code is quite awkward.

And of course, the concerns of the serial line driver are thrown into the mix too, for the added entertainment.

jonnyasmar•about 21 hours ago

The split-of-responsibilities point really shows up when you try to host a TUI inside a PTY you control. Spawning Claude Code, Codex, and other agents into terminal panes on macOS, I hit this chain of small surprises:

- SIGWINCH doesn't always fire on initial spawn — the TUI starts up thinking it has 0 columns and emits garbage until the first real resize. Fix: synthesize an ioctl(TIOCSWINSZ) before the first read, and re-send on focus events. - xterm.js negotiates dimensions with the PTY backend over a non-obvious dance; off-by-one in the cell math wraps long prompts in the wrong place every time. - Tools that detect "am I in a TTY" via isatty() behave differently from tools that stat() stdin; a few agents fall through to non-TUI mode if the PTY's mode bits aren't quite right.

None of that is reflected in the abstract "PTY is a virtual terminal" mental model. The kernel/terminal/application split is a leaky abstraction in practice — you only find out by hosting one inside the other.

Joker_vD•about 21 hours ago

> Tools that detect "am I in a TTY" via isatty() behave differently from tools that stat() stdin

Wait, how do you even use fstat's output to find out if the file is a tty?

Although in my experience the "funniest" part is deciding whether to use isatty() on stdin or on stdout. I mean, there is no much point enabling line editing/tab completion if stdin is a pipe/regular file, right?

jonnyasmar•about 21 hours ago

Fair pushback — I was being sloppy. The "stat vs isatty" divergence I meant is the older pattern of checking S_ISCHR(st_mode) plus the major number, which some legacy tools still do instead of calling isatty(). Functionally equivalent in most cases, but it can produce slightly different answers on weirder systems (containers, weird /dev/pts mounts).

The stdin-vs-stdout split is where I see the most actual "is this a TTY" mistakes though. Tools that emit JSON-on-stdout-when-piped and TUI-when-not work fine until something stuffs them into a PTY with piped stdin — then they're in TUI mode but can't actually read the user input format they expect.

teddyh•about 22 hours ago

The “index.php” in the URL is redundant.

JdeBP•about 17 hours ago

Looking back over the times that the page has been submitted to Hacker News, it's almost an even split between with 'index.php' and without.

It's an interesting question for CIS URLs whether a url-path with an empty final component, that duplicates a url-path with an actual 'index' component (of some kind), is to be taken as the primary or the secondary form. When a site has 'index.php', 'index.html', 'index.gopher', 'index.gemini', and others, the question is whether the container is the same as the index file.

The TTY Demystified (2008)

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