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Clients connect over a Unix socket, are handed a shared buffer, draw into it and commit; Schcom composites and presents. Both of the hard blockers its README listed -- unix domain sockets and raw input events -- have been done for a while, so nothing was in the way. The model is Wayland's, on purpose. A surface is not a window: it is a rectangle of pixels with no policy attached, because placement belongs to SchWM and the moment this program starts deciding where windows go the split has failed. Nothing is shown until COMMIT, so a client halfway through drawing is never presented. The compositor tells clients when to draw rather than letting them render frames nobody sees. Wayland is MIT-licensed and supporting it later is intended, so the divergences are the ones that are cheap to undo: a buffer handle is a System V shm id rather than an fd over SCM_RIGHTS, and the message carries a handle_kind field so switching is a new value rather than a protocol break. Every message carries its own length, so an unknown message is skipped instead of desynchronising the stream -- which is what lets a v0 compositor survive a v1 client. The compositor owns the shm segment, not the client: it has to survive a client crashing mid-frame, and it must not be something a client can resize underneath it. A dropped connection takes its surfaces and their segments with it. Composites into a back buffer and copies once, because compositing straight into the framebuffer shows every intermediate state -- the flicker would be the whole desktop being repainted under the user's eyes. projects/ gets build glue for the first time: `make projects` builds against the sysroot's libc and stages into sysroot/bin. It is not a dependency of anything above it, and `make` never descends there. Verified by `make schcom-test`, which boots SchOS, runs the compositor and a client, screendumps the framebuffer and asserts the pixels on the host. That last part is the point -- the compositor reporting that it composited something proves nothing, and a screenshot nobody inspects proves less. It checks the client's colour mid-surface, the compositor's background, and the white border exactly at the requested origin, so "arrived" and "arrived at the wrong offset" cannot pass for each other. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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Schcom — the SchOS Compositor
Owns the display. Every pixel that reaches the screen goes through it.
Clients do not draw to the framebuffer; they draw into their own buffers and hand them over. Schcom decides what is on top of what, composites the result, and presents it. It also owns input: it reads the raw event devices and sends each event to whichever client should have it.
That is the whole job. Window placement is not part of it — that is SchWM's, and Schcom exposes a protocol for it rather than implementing policy itself.
Status
v0 runs. A client connects, is given a shared buffer, draws into it and
commits; the result is composited and presented. Verified end to end by
make schcom-test, which boots SchOS, runs the compositor and a client, then
screendumps the framebuffer and asserts the pixels from the host — the
compositor's own report that it composited something proves nothing.
What exists:
- framebuffer output through
/dev/fb0and the fbdev ioctls, double-buffered - a versioned protocol over a Unix socket, with a handshake that must come first
- shared-memory buffers, one per surface, owned by the compositor
- surface / attach / commit semantics: nothing is shown mid-draw
- input from
/dev/input/eventN, routed to the topmost surface - clients that die are cleaned up, including their shm segments
What does not exist yet: damage tracking beyond "repaint everything", multiple outputs, subsurfaces, and any focus model beyond "topmost". Those are additions rather than rewrites, which is the reason for committing to the surface/commit model now instead of growing into it later.
Building
Not part of the OS build. Needs make ports first, for the sysroot's libc:
make projects # builds and stages into sysroot/bin
make # rebuilds the image so the binaries are on it
make schcom-test # boots it and checks the pixels
What it needs from the kernel
| Need | State |
|---|---|
Shared memory (shmget/shmat/shmdt) |
done |
mmap of /dev/fb0 |
done |
| Framebuffer geometry via the fbdev ioctls | done |
| Unix domain sockets | done |
| Raw input events — key down/up with modifiers | done |
SCM_RIGHTS |
done, and not yet used — see below |
| Mode setting and double buffering | partial — virtio-gpu binds, no modeset API yet |
Design notes
- Buffers are shared memory, not pipes. A pipe means copying every frame; at 800×600×32 that is 1.9 MB per frame per client.
- The compositor owns the segment, not the client. It has to survive a client crashing mid-frame, and it must not be something a client can resize underneath the compositor.
- One protocol, versioned from the start. SchWM and SchDE are separate programs, so this is a real compatibility surface on day one. Every message carries its own length, so an unknown message is skipped rather than desynchronising the stream — that is what lets a v0 compositor survive a v1 client.
- Policy belongs to SchWM. If Schcom starts deciding where windows go, the
split has already failed.
CREATE_SURFACEcarries x/y today only because there is no SchWM yet to ask.
On Wayland
The protocol is deliberately Wayland-shaped — surfaces rather than windows,
buffers attached and made visible only on commit, and the compositor telling
clients when to draw. Wayland is MIT-licensed, so it can be linked against
without any conflict with Apache-2.0 (see docs/architecture/licensing.md), and
supporting it later is intended.
The one place v0 knowingly diverges: a buffer handle is a System V shared memory
id passed as an integer, where Wayland passes a file descriptor over
SCM_RIGHTS. The kernel supports SCM_RIGHTS already; an integer simply needs
no ancillary-data handling on either side. struct schcom_surface carries a
handle_kind field precisely so that switching is a new value rather than a
protocol break.