The TTY Demystified (2008)

a day ago

The TTY subsystem is crucial to Linux/UNIX design but often misunderstood, originating from historical teletypes and stock tickers.
TTY devices combine a UART driver, line discipline (like N_TTY for editing), and TTY driver for session management, allowing line editing and job control.
Modern systems use emulated terminals (like xterm) or pseudo terminals (ptys) instead of physical hardware, but legacy TTY functionality persists.
Process states include running, sleeping, stopped, and zombie, with job control managing foreground/background process groups via signals.
Key signals include SIGINT (^C), SIGTSTP (^Z), SIGTTIN/SIGTTOU (background job I/O), SIGWINCH (terminal resize), and SIGHUP (hangup).
TTY configuration can be modified via ioctl or stty, affecting baud rate, special characters (e.g., intr), canonical mode, and flow control (^S/^Q).
Flow control and blocking I/O regulate data flow between processes and terminals, with kernel buffers managing speed mismatches.
Session leaders (shells) coordinate jobs, TTY foreground settings, and handle signals like SIGCHLD for process state changes.
The article demystifies TTY internals, highlighting its role in terminal emulation, line disciplines, and userland tools like screen or ssh.
Feedback and corrections note nuances like raw vs. cooked mode, and discuss challenges in TTY relay over SSH or with complex terminal states.

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