What’s the difference between the ^S and ^Z control characters inside a terminal

A tty is a native terminal device, the backend is either hardware or kernel emulated.

A pty (pseudo terminal device) is a terminal device which is emulated by an other program (example: xterm, screen, or ssh are such programs). A pts is the slave part of a pty.

(More info can be found in man pty.)

Short summary:

A pty is created by a process through posix_openpt() (which usually opens the special device /dev/ptmx), and is constituted by a pair of bidirectional character devices:

1. The master part, which is the file descriptor obtained by this process through this call, is used to emulate a terminal. After some initialization, the second part can be unlocked with unlockpt(), and the master is used to receive or send characters to this second part (slave).

2. The slave part, which is anchored in the filesystem as /dev/pts/x (the real name can be obtained by the master through ptsname() ) behaves like a native terminal device (/dev/ttyx). In most cases, a shell is started that uses it as a controlling terminal.

Originally, "tty" had two definitions: the hardware (now the emulator) and the driver (interfaced through /dev/pty* or /dev/tty*).

The hardware/emulator was/is responsible for:

• Taking a stream of data and presenting it; this included interpreting control sequences like "move cursor left", "blinking cursor", "clear-screen" although these control sequences were often different among manufacturers.
• Sending keycodes for keys typed by the user; most of these were standard ASCII characters, but some terminals sent propriety keycodes for even standard keys.

The 'tty' driver was responsible for:

• Managing the buffering, in raw or canonical mode; for example, buffering a line of characters until Enter is pressed.
• Managing the control flow; e.g. being able to stop/continue with Cntl-s/Cntl-q.
• Translating propriety keycodes to standard ASCII, where applicable.
• Intercepting certain control characters (like Cntl-c and Backspace) and processing them appropriately (send SIGINT on a Cntl-c or signal an EOF on Cntl-d.
• Canonical display of characters, for example, if echo is turned off, then do not send feedback (character typed) back to the terminal.

The terminfo and termcap databases managed what terminal control characters should be sent for an operation (like 'clear-screen'). These control sequences were/are not interpreted by the driver, but by the hardware/emulator.