SchOS/docs/architecture/terminals.md

<!-- SPDX-License-Identifier: Apache-2.0 -->
# Virtual terminals

Six of them, `/dev/tty1` through `/dev/tty6`.

| | |
|---|---|
| **AltGr+F1 .. F6** | go to that terminal |
| **AltGr+-** / **AltGr+=** | previous / next terminal *in use* |
| `chvt N` | the same, from a script or a serial console |

The chord is settable — `chvt -c` — for the reasons below. Each one is an independent terminal: its own screen, its own
cursor, its own line discipline, its own foreground process group and its own
session. Two shells on two terminals are two unrelated processes that happen to
share a keyboard and a screen.

A seventh is deliberately not bound. That is where a display server goes,
and it will claim switching with `VT_SETMODE` rather than a chord.

---

## What makes switching possible

A backing store, and nothing else.

Before this, the console drew straight into the framebuffer and scrolled by
moving pixels. That works exactly as long as nothing else ever wants the
screen: there was nowhere to put terminal 2 while terminal 1 was visible, and
nothing to repaint terminal 1 from when it came back.

So each terminal owns a grid of cells and the framebuffer shows whichever grid
is active. `vt_switch()` is three lines — set the active index, repaint from
the grid, return — because nothing has to be *saved* on the way out. The
outgoing terminal's grid has been current all along.

The rule the whole layer is built on:

> **A terminal does not know whether it is visible.**

Writes land in the grid, the cursor moves, the grid scrolls, all of it
unconditionally. Drawing is a separate question, asked once, at the bottom of
`kernel/dev/vt.c`, answered by comparing against `vt_active`. Every alternative
spreads "am I on screen" through the escape parser, and then a terminal
switched away from mid-sequence comes back wrong.

## A cell

Four bytes:

| | |
|---|---|
| `ch` | the glyph the font draws — **not** a codepoint |
| `fg`, `bg` | palette indices, 0-15 ANSI plus two theme defaults |
| `attr` | reverse video; room for underline and the rest |

`ch` stores the glyph rather than the codepoint because the console maps UTF-8
to CP437 before it gets here, and that is what `include/font.h` contains.
Storing the codepoint would mean re-running the mapping on every repaint and
getting a different answer for anything the font cannot draw.

The colours are indices rather than RGB because a cell holding two 32-bit
colours would be twelve bytes instead of four, and the grids are static: six
terminals at 240x75 is 432 KB of BSS, paid once at link time rather than failed
to pay at run time. There is no contiguous multi-page allocator to ask.

## What is per-terminal

Everything that used to be a single global:

| Was | Now |
|---|---|
| `int tty_fg_pgrp = 1;` | `v->fg_pgrp`, one per terminal |
| one `struct termios tio` | `v->tio` |
| one 256-byte input ring | `v->in` |
| one escape-parser state | `v->esc_*` |
| one UTF-8 decoder | `v->u8_*` |
| one cursor, in pixels, in `gfx.c` | `v->cx`, `v->cy`, in cells |

`TIOCSCTTY` used to be `return 0` — it claimed to make a terminal the caller's
controlling terminal and did nothing, because there was one terminal and
nothing to choose between. It records `proc->ctty` now, which is what
`/dev/tty` resolves through.

## The devices

| Path | Minor | What |
|---|---|---|
| `/dev/console` | 1 | the **active** terminal, so kernel messages follow a switch |
| `/dev/tty` | 10 | the caller's **controlling** terminal, via `proc->ctty` |
| `/dev/tty1`..`tty6` | 11-16 | that terminal, whether or not it is on screen |

`/dev/tty` was an alias for `/dev/console` with the same major and minor. They
are different questions and now have different answers.

## The ioctls

Linux's numbers, because they do Linux's job.

| | |
|---|---|
| `VT_ACTIVATE` | switch. Argument is a 1-based number **by value**, not a pointer |
| `VT_WAITACTIVE` | returns immediately: `vt_switch` repaints synchronously, so there is nothing left to wait for |
| `VT_GETSTATE` | active terminal and a bitmask of those in use |
| `VT_OPENQRY` | the first terminal nobody has open |
| `VT_GETMODE`/`VT_SETMODE` | accepted and ignored — the kernel drives switching and there is no process mode to set |

## The keyboard

`kbd.c` recognises Ctrl+Alt+Shift+F1..F6 and returns `KEY_VT_SWITCH + n`, which is above
0xFF on purpose: every value that fits in a byte is already either a character
or a UTF-8 continuation byte, and a chord that collided with one would be typed
into the terminal instead of switching away from it.

What the chord *means* is decided in `console.c`, where the terminals are. The
keyboard driver names it and stops.

### The chord is a setting, and that is the point

**There is no combination that is free for everyone on every host**, and a
guest cannot find out what its host takes: a key the host swallowed is
indistinguishable from a key nobody pressed. Three parties have a claim on
every candidate.

| | |
|---|---|
| **The host takes it, and the guest never sees it** | |
| `Alt+F4`, `Alt+F2` | window manager |
| `Ctrl+Alt+F1..F6` | display manager — and it matches **loosely**, so adding Shift does not get past it |
| `Ctrl+Alt+`digit | QEMU's own console tabs |
| **Applications want it, and lose it permanently if the kernel eats it** | |
| `Alt+`anything | Meta. zsh binds Meta-digit as `digit-argument`; so does every readline editor |
| `Ctrl+`letter | control characters |
| `Ctrl+Shift+`digit | types a character — `Ctrl+Shift+2` is `^@` |
| `F1..F6` alone | help, and whatever a full-screen program wants |
| **The keyboard layout wants it** | |
| `AltGr+`letter | Polish diacritics — ą ć ę ł ń ó ś ź ż — and on some setups AltGr arrives as Ctrl+Alt |

So the kernel ships a default and lets it be moved:

```sh
chvt                        # report the active terminal and the current chord
chvt -c altgr+F             # AltGr+F1..F6        (the default)
chvt -c ctrl+shift+F        # Ctrl+Shift+F1..F6
chvt -c alt+1               # Alt+1..6
chvt -c off                 # no chord; chvt N is then the only way
```

`off` is a legitimate setting, not a failure mode. On a machine whose host
grabs everything worth binding, `chvt N` from a shell is the honest answer.

The default is **AltGr+F1..F6**, and the `AltGr+`letter row above needs reading
carefully: what a Polish layout wants is AltGr with a **letter**. AltGr with a
**function key** produces nothing on any layout, no window manager binds it,
and no application asks for it. It is the one combination all three parties
leave alone.

That distinction is only expressible because Right Alt is tracked as its own
modifier bit. It used to share `ALT`, which is harmless for typing — neither is
used to look up a character — and made this impossible to say.

Verified rather than argued: with the default set, `AltGr+F2` switches,
`AltGr+F1` returns to a pixel-identical tty1, and `AltGr+a` still reaches the
shell, so ą is untouched.

Whatever the modifiers are, they also bind **minus** and **equals** to the
previous and next terminal. Those step only through terminals that are *in
use* — the same `opened` count `VT_GETSTATE` reports — because six terminals
with two shells on them means four blank screens in between, and cycling past
them is worse than not having the key.

Which chords reach a guest at all was **measured, not assumed**: a temporary
probe in `kbdgetc` logged every modified keypress, and QEMU's `sendkey` fired
each candidate at it. Everything arrived, `Ctrl+Alt+F2` included — so that
conflict is the host's display manager, not QEMU, and no amount of extra
modifiers gets around a grab that matches loosely.

The modifier test is an **equality**, not a mask:

```c
if (vt_chord_mods != 0 &&
    (shift & (CTL | SHIFT | ALT | RALT)) == vt_chord_mods && ...)
```

A mask fires on any superset, so a chord aimed at the host would switch
terminals behind your back. Right Alt is tracked as its own bit (`RALT`) for
the same reason; it used to share `ALT`, which is harmless for typing — neither
is used to look up a character — and wrong for chords.

`include/posix.h`'s `VTC_*` bits mirror `include/kbd.h`'s modifier bits, and a
`_Static_assert` in `kbd.c` holds them together. A mismatch would be silent:
a chord set from userspace would simply never match, and the only symptom
would be a key that does nothing.

### Taking the chord back

Three layers, in increasing order of how much a program is claiming:

| | |
|---|---|
| **`VT_SETMODE(VT_PROCESS)`** | the terminal's owner handles switching itself. The kernel stops acting on the chord entirely. This is what a display server uses. |
| **`EVIOCGRAB` on `/dev/input/event0`** | an exclusive keyboard grab. Checked in `kbdgetc` *before* the chord is even recognised, so a grabbing program receives every scancode including this one. |
| nothing | the kernel switches, which is the default and right for a plain terminal |

`VT_SETMODE` used to be accepted and ignored — a program could ask for the
chord, be told yes, and still not get it. It works now.

What `VT_PROCESS` does **not** do yet is the release handshake: Linux signals
the owning process on every switch request and blocks until it answers
`VT_RELDISP`, which is how a display server releases the GPU before the screen
changes hands. Nothing here has anything to release, so the kernel simply
stops switching rather than negotiating. `struct vt_mode` already carries
`relsig`/`acqsig` for when Schcom needs it.

Input goes to whichever terminal is on screen. That is what makes a terminal a
terminal: the keyboard follows the eyes.

## Two rules that are easy to get wrong

**The grid is the only source of truth for the size.** `TIOCGWINSZ` reports
`v->cols`/`v->rows`, not a fresh `gfx_cols()`/`gfx_rows()`. When it computed
its own answer there were two, and they disagreed: the grids are sized at
`vt_init()`, which runs before the display driver is found on the PCI bus, so
they were 80x24 while `TIOCGWINSZ` said 160x50. Text stopped two thirds of the
way down the screen, `ls` formatted for a width the grid did not have and split
words across the wrap, and zsh's cursor tracking was wrong on every line --
three symptoms, one disagreement. `vt_resize()` now corrects the grids when a
scanout arrives, and the ioctl reads them.

**A glyph in the last column does not wrap.** It sets `wrap_pending`, and the
wrap happens when the *next* glyph arrives. Every real terminal behaves this
way and none of them document it clearly. Wrapping immediately makes a line
exactly as wide as the screen take two rows instead of one, and makes it
impossible to write the bottom-right cell without scrolling -- and zsh's
partial-line marker depends on the rule directly: it writes exactly
`COLUMNS - 1` spaces and expects the cursor to still be on that line. With
immediate wrap, a reverse-video `#` appeared above every prompt.

Anything that moves the cursor deliberately -- CR, LF, backspace, an escape
sequence -- clears the debt.

## Ctrl-C used to kill the shell

Worth its own note, because the cause was one character and the effect was a
machine you could not use.

When Ctrl-C arrives, `consoleread()` discards the partial line and breaks off
the read in progress. It returned **`-1`** to do so — and `-1` is `-EPERM`. So a
program that handles `SIGINT` by carrying on saw a permanent error instead of
an interrupted call, and exited:

```
zsh: error on TTY read: operation not permitted
schinitd: restarting os.schos.console-shell
```

and then the next Ctrl-C did the same thing to the replacement. A shell that
cannot survive Ctrl-C is a shell nobody can use, and the supervisor faithfully
restarting it made it a loop.

It returns **`EINTR`** now, which is what a signal-interrupted read is required
to give: the whole convention — retry, or return to the caller — is built on
receiving that and not something else.

## Editing a line you have already typed

Two bugs, both in the same half-second of using a shell, and both worth
recording because the symptom pointed away from the cause.

**Arrow keys corrupted the next two keystrokes.** They arrive from the keyboard
as single keycodes -- `KEY_UP` is `0xE2` -- and were pushed into the input
buffer exactly like that. No terminal has ever worked that way, and `0xE2` is
the worst byte to have picked: it is a UTF-8 lead byte for a three-byte
character, so a reader decoding UTF-8 swallowed the *next two keys* waiting for
continuations that never came. `consoleintr()` translates them to the DEC/xterm
sequences now -- `ESC[A`, `ESC[D`, `ESC[5~` and the rest -- which is what every
terminfo entry and every line editor already expects.

**Then the end of the line vanished as you typed into it.** With the arrows
working, editing mid-line left the buffer perfectly correct and the screen
wrong, which is a maddening pair. The cause is one line in `vt_putc()`:
backspace *erased the cell it moved onto*.

That is what a line-discipline erase does. It is not what the BS control
character does — a program that wants to rub a character out sends `\b \b`,
backspace, space, backspace, and the separation matters because programs also
use plain `\b` simply to go back. zsh does, on every mid-line edit:

```
echo alpha  \b\b  zha  \b\b
```

Move back two, rewrite the tail, move back two again to leave the cursor where
the user expects it. Against a destructive backspace that last pair deleted the
`ha` it had just drawn.

`\b` moves and nothing else now; `0x7f` is ignored, as it is on a real
terminal; and `consputc()` spells its own erase out as `\b \b`, exactly as it
already did for the serial line. `ESC[@`, `ESC[P` and `ESC[X` — insert, delete
and erase characters — were added at the same time, because `TERM=linux`
declares them and a terminal that drops them silently is the same class of bug
waiting for a different editor.

## What it does not do

- **No scrollback.** The grid is exactly the visible screen. Shift+PageUp does
  nothing because there is nothing above the top row to show.
- **No reflow on resize.** The geometry is read once at `vt_init()`. Nothing
  resizes a framebuffer console today; a compositor will, and that is when this
  has to grow.
- **No `VT_RELDISP` handshake.** `VT_PROCESS` stops the kernel switching, but
  the kernel does not signal the owning process and wait for an answer. A
  display server that needs to release the GPU *before* the screen changes
  hands will need that, and `struct vt_mode` already has the fields.
- **No terminal past 6.** Linux allows 63. Six is what the chords reach and
  what the grids are sized for.
- **Only 240x75 of any screen.** Larger framebuffers work; the grid is clamped
  and the remainder stays background. Raising `VT_MAXCOLS`/`VT_MAXROWS` is the
  fix, and costs BSS.
- **No UTF-8 the font lacks.** CP437 has ~256 glyphs. Anything else draws as
  `?`, which is deliberate: drawing a *different* character would be worse.