a5b0d5a0ef
Three symptoms, two causes, both in the terminal layer.
**The grid and TIOCGWINSZ disagreed about the size.** vt_init() sizes every
grid from gfx_cols()/gfx_rows(), and it runs at main.c:75 -- while the display
driver attaches its scanout from the PCI bus walk at main.c:99. So the grids
were the 80x24 fallback, and TIOCGWINSZ, which computed its own answer live,
said 160x50.
That one disagreement produced all three things visible in the report: text
stopped two thirds of the way down the screen, `ls` laid out 160 columns into
an 80-column grid so words split across the wrap ("vers" / "ion"), and zsh's
cursor tracking was wrong on every line.
Fixed in two places, because either alone would leave the trap set:
gfx_attach_scanout() now calls vt_resize(), so the grids learn the real screen
the moment there is one; and TIOCGWINSZ reports v->cols/v->rows rather than
measuring again. One number, from the terminal being asked.
**Wrapping was eager.** A glyph printed in the last column moved the cursor to
the next line immediately. Real terminals set a pending-wrap flag and wrap when
the *next* glyph arrives -- which is why a line exactly as wide as the screen
takes one row and not two, and why a program can write the bottom-right cell
without scrolling.
zsh's partial-line marker depends on that rule directly: it writes exactly
COLUMNS-1 spaces and expects to still be on the line. With eager wrap it left a
reverse-video '#' above every prompt, which is the hashtag in the report. Both
are gone.
Verified by screendump: `ls /proc` now fits one line with "version" intact, no
markers, and output fills the screen to the last row. schfs-check passes.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2
152 lines
6.8 KiB
Markdown
152 lines
6.8 KiB
Markdown
<!-- SPDX-License-Identifier: Apache-2.0 -->
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# Virtual terminals
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Six of them, `/dev/tty1` through `/dev/tty6`, reached with **Alt+F1..Alt+F6**
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or with `chvt N`. Each one is an independent terminal: its own screen, its own
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cursor, its own line discipline, its own foreground process group and its own
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session. Two shells on two terminals are two unrelated processes that happen to
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share a keyboard and a screen.
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Alt+F7 is deliberately unused. That is where a display server goes.
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---
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## What makes switching possible
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A backing store, and nothing else.
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Before this, the console drew straight into the framebuffer and scrolled by
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moving pixels. That works exactly as long as nothing else ever wants the
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screen: there was nowhere to put terminal 2 while terminal 1 was visible, and
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nothing to repaint terminal 1 from when it came back.
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So each terminal owns a grid of cells and the framebuffer shows whichever grid
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is active. `vt_switch()` is three lines — set the active index, repaint from
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the grid, return — because nothing has to be *saved* on the way out. The
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outgoing terminal's grid has been current all along.
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The rule the whole layer is built on:
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> **A terminal does not know whether it is visible.**
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Writes land in the grid, the cursor moves, the grid scrolls, all of it
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unconditionally. Drawing is a separate question, asked once, at the bottom of
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`kernel/dev/vt.c`, answered by comparing against `vt_active`. Every alternative
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spreads "am I on screen" through the escape parser, and then a terminal
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switched away from mid-sequence comes back wrong.
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## A cell
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Four bytes:
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| `ch` | the glyph the font draws — **not** a codepoint |
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| `fg`, `bg` | palette indices, 0-15 ANSI plus two theme defaults |
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| `attr` | reverse video; room for underline and the rest |
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`ch` stores the glyph rather than the codepoint because the console maps UTF-8
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to CP437 before it gets here, and that is what `include/font.h` contains.
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Storing the codepoint would mean re-running the mapping on every repaint and
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getting a different answer for anything the font cannot draw.
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The colours are indices rather than RGB because a cell holding two 32-bit
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colours would be twelve bytes instead of four, and the grids are static: six
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terminals at 240x75 is 432 KB of BSS, paid once at link time rather than failed
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to pay at run time. There is no contiguous multi-page allocator to ask.
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## What is per-terminal
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Everything that used to be a single global:
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| Was | Now |
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| `int tty_fg_pgrp = 1;` | `v->fg_pgrp`, one per terminal |
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| one `struct termios tio` | `v->tio` |
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| one 256-byte input ring | `v->in` |
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| one escape-parser state | `v->esc_*` |
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| one UTF-8 decoder | `v->u8_*` |
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| one cursor, in pixels, in `gfx.c` | `v->cx`, `v->cy`, in cells |
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`TIOCSCTTY` used to be `return 0` — it claimed to make a terminal the caller's
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controlling terminal and did nothing, because there was one terminal and
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nothing to choose between. It records `proc->ctty` now, which is what
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`/dev/tty` resolves through.
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## The devices
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| Path | Minor | What |
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| `/dev/console` | 1 | the **active** terminal, so kernel messages follow a switch |
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| `/dev/tty` | 10 | the caller's **controlling** terminal, via `proc->ctty` |
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| `/dev/tty1`..`tty6` | 11-16 | that terminal, whether or not it is on screen |
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`/dev/tty` was an alias for `/dev/console` with the same major and minor. They
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are different questions and now have different answers.
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## The ioctls
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Linux's numbers, because they do Linux's job.
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| `VT_ACTIVATE` | switch. Argument is a 1-based number **by value**, not a pointer |
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| `VT_WAITACTIVE` | returns immediately: `vt_switch` repaints synchronously, so there is nothing left to wait for |
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| `VT_GETSTATE` | active terminal and a bitmask of those in use |
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| `VT_OPENQRY` | the first terminal nobody has open |
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| `VT_GETMODE`/`VT_SETMODE` | accepted and ignored — the kernel drives switching and there is no process mode to set |
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## The keyboard
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`kbd.c` recognises Alt+F1..F6 and returns `KEY_ALT_F1 + n`, which is above
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0xFF on purpose: every value that fits in a byte is already either a character
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or a UTF-8 continuation byte, and a chord that collided with one would be typed
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into the terminal instead of switching away from it.
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What the chord *means* is decided in `console.c`, where the terminals are. The
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keyboard driver names it and stops.
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Input goes to whichever terminal is on screen. That is what makes a terminal a
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terminal: the keyboard follows the eyes.
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## Two rules that are easy to get wrong
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**The grid is the only source of truth for the size.** `TIOCGWINSZ` reports
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`v->cols`/`v->rows`, not a fresh `gfx_cols()`/`gfx_rows()`. When it computed
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its own answer there were two, and they disagreed: the grids are sized at
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`vt_init()`, which runs before the display driver is found on the PCI bus, so
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they were 80x24 while `TIOCGWINSZ` said 160x50. Text stopped two thirds of the
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way down the screen, `ls` formatted for a width the grid did not have and split
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words across the wrap, and zsh's cursor tracking was wrong on every line --
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three symptoms, one disagreement. `vt_resize()` now corrects the grids when a
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scanout arrives, and the ioctl reads them.
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**A glyph in the last column does not wrap.** It sets `wrap_pending`, and the
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wrap happens when the *next* glyph arrives. Every real terminal behaves this
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way and none of them document it clearly. Wrapping immediately makes a line
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exactly as wide as the screen take two rows instead of one, and makes it
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impossible to write the bottom-right cell without scrolling -- and zsh's
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partial-line marker depends on the rule directly: it writes exactly
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`COLUMNS - 1` spaces and expects the cursor to still be on that line. With
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immediate wrap, a reverse-video `#` appeared above every prompt.
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Anything that moves the cursor deliberately -- CR, LF, backspace, an escape
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sequence -- clears the debt.
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## What it does not do
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- **No scrollback.** The grid is exactly the visible screen. Shift+PageUp does
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nothing because there is nothing above the top row to show.
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- **No reflow on resize.** The geometry is read once at `vt_init()`. Nothing
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resizes a framebuffer console today; a compositor will, and that is when this
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has to grow.
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- **No process-mode switching.** `VT_SETMODE` is accepted and ignored. A
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display server that wants to veto a switch — which is what `VT_PROCESS` is
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for — cannot yet, and will need to before Schcom owns Alt+F7.
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- **No terminal past 6.** Linux allows 63. Six is what the chords reach and
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what the grids are sized for.
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- **Only 240x75 of any screen.** Larger framebuffers work; the grid is clamped
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and the remainder stays background. Raising `VT_MAXCOLS`/`VT_MAXROWS` is the
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fix, and costs BSS.
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- **No UTF-8 the font lacks.** CP437 has ~256 glyphs. Anything else draws as
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`?`, which is deliberate: drawing a *different* character would be worse.
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