---
name: schcom-v0
description: "Schcom v0 runs — surfaces, shm buffers, commit semantics, input routing; Wayland-shaped on purpose, verified by checking pixels from the host"
metadata: 
  node_type: memory
  type: project
  originSessionId: d4821aa9-2e49-40da-91e7-90ef9ed9997e
  modified: 2026-08-09T21:12:35.466Z
---

**Schcom v0 works** (2026-08-09, commit fe64620). A client connects over a Unix
socket, is handed a shared buffer, draws and commits; the compositor composites
and presents. `projects/schcom/`.

Both blockers its README listed had already been done — AF_UNIX sockets and
`/dev/input/eventN` — so nothing was actually in the way when this started.

## How to build and prove it

```sh
make ports        # once: the sysroot libc it links against
make projects     # builds, stages into sysroot/bin
make              # rebuild the image so the binaries are on it
make schcom-test  # boots, runs it, checks the pixels FROM THE HOST
```

`make schcom-test` is the part that matters. It screendumps the framebuffer
through the QEMU monitor and asserts three pixels: the client's colour in the
middle of the surface, the compositor background away from it, and the white
border exactly at the requested origin — so "the buffer arrived" and "the buffer
arrived at the wrong offset" cannot pass for each other. The compositor's own
report that it composited something proves nothing.

**Run it with one display device.** The test passes `-vga std` and *not*
`-device virtio-gpu-pci`: with two, `screendump` captures whichever QEMU thinks
is active, which need not be the one `/dev/fb0` maps.

## The decisions worth not re-litigating

- **Wayland-shaped from the start.** Surfaces, not windows; nothing visible
  until `COMMIT`; the compositor tells clients when to draw. Wayland is
  **MIT-licensed**, so linking it later is fine under Apache-2.0
  ([[schunix-licensing]]), and Nexoniarz intends to add it.
- **The one deliberate divergence:** a buffer handle is a System V shm id, not
  an fd over `SCM_RIGHTS`. `struct schcom_surface` has a `handle_kind` field so
  the switch is a new value, not a protocol break.
- **The compositor owns the shm segment**, never the client — it must survive a
  client dying mid-frame and must not be resizable underneath it.
- **Every message carries its own length**, so an unknown message is skipped
  rather than desynchronising the stream. That is what lets a v0 compositor talk
  to a v1 client.
- **Back buffer, one copy out.** Compositing straight into the framebuffer shows
  every intermediate state.
- **Placement is not Schcom's.** `CREATE_SURFACE` carries x/y only because there
  is no SchWM yet to ask.

## Not done

Damage tracking (it repaints everything), multiple outputs, subsurfaces, any
focus model beyond "topmost", and mode setting. All additions rather than
rewrites — which was the reason for committing to surface/commit now.

**SchAPI / `libschunix` is still not started.** See
[[schunix-schapi-direction]]: the stable ABI is meant to be a library, not the
syscall table, and Schcom currently talks to musl and raw syscalls like anything
else. Nothing about Schcom blocks it.