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Schcom v0: a compositor that actually composites

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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>
This commit is contained in:
Nexoniarz 2026-08-09 23:12:14 +02:00
commit fe6462088a
9 changed files with 1127 additions and 18 deletions
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149
projects/schcom/schcom-demo.c Normal file
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/* SPDX-License-Identifier: Apache-2.0 */
/*
* A Schcom client, and the test for the compositor.
*
* Draws a flat rectangle of a known colour at a known position and commits it.
* Both of those are chosen so the result can be checked from outside the
* machine: `make schcom-shot` screendumps the framebuffer and a script on the
* host asserts the pixel at a given coordinate. That is the same oracle
* pattern the SchFS round-trip uses -- something with no connection to the code
* under test has to agree that it worked.
*
* schcom-demo [--colour 0xRRGGBB] [--at X Y] [--size W H] [--wait SECONDS]
*/
#define _GNU_SOURCE
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/shm.h>
#include <sys/socket.h>
#include <sys/un.h>
#include "proto.h"
static int
read_all(int fd, void *buf, size_t n)
{
unsigned char *p = buf;
while (n) {
ssize_t r = read(fd, p, n);
if (r == 0) return -1;
if (r < 0) { if (errno == EINTR) continue; return -1; }
p += r; n -= (size_t)r;
}
return 0;
}
int
main(int argc, char **argv)
{
uint32_t colour = 0x00C04020;
int32_t at_x = 100, at_y = 80;
uint32_t w = 240, h = 160;
int wait_s = 3;
for (int i = 1; i < argc; i++) {
if (!strcmp(argv[i], "--colour") && i + 1 < argc)
colour = (uint32_t)strtoul(argv[++i], NULL, 0);
else if (!strcmp(argv[i], "--at") && i + 2 < argc) {
at_x = atoi(argv[++i]); at_y = atoi(argv[++i]);
} else if (!strcmp(argv[i], "--size") && i + 2 < argc) {
w = (uint32_t)atoi(argv[++i]); h = (uint32_t)atoi(argv[++i]);
} else if (!strcmp(argv[i], "--wait") && i + 1 < argc)
wait_s = atoi(argv[++i]);
}
int fd = socket(AF_UNIX, SOCK_STREAM, 0);
if (fd < 0) { perror("socket"); return 1; }
struct sockaddr_un addr;
memset(&addr, 0, sizeof addr);
addr.sun_family = AF_UNIX;
strncpy(addr.sun_path, SCHCOM_SOCKET_PATH, sizeof addr.sun_path - 1);
if (connect(fd, (struct sockaddr *)&addr, sizeof addr) < 0) {
fprintf(stderr, "schcom-demo: connect %s: %s\n",
SCHCOM_SOCKET_PATH, strerror(errno));
return 1;
}
/* Handshake first: the compositor accepts nothing else before it. */
struct schcom_hello hello;
memset(&hello, 0, sizeof hello);
hello.hdr.op = SCHCOM_OP_HELLO;
hello.hdr.length = sizeof hello;
hello.hdr.serial = 1;
hello.version = SCHCOM_VERSION;
if (write(fd, &hello, sizeof hello) != (ssize_t)sizeof hello) {
perror("write hello"); return 1;
}
struct schcom_hello ack;
if (read_all(fd, &ack, sizeof ack) < 0 || ack.hdr.op != SCHCOM_OP_HELLO_ACK) {
fprintf(stderr, "schcom-demo: no hello ack\n");
return 1;
}
printf("schcom-demo: connected, protocol v%u\n", ack.version);
struct schcom_create_surface req;
memset(&req, 0, sizeof req);
req.hdr.op = SCHCOM_OP_CREATE_SURFACE;
req.hdr.length = sizeof req;
req.hdr.serial = 2;
req.width = w; req.height = h;
req.x = at_x; req.y = at_y;
if (write(fd, &req, sizeof req) != (ssize_t)sizeof req) {
perror("write create"); return 1;
}
struct schcom_surface surf;
if (read_all(fd, &surf, sizeof surf) < 0) {
fprintf(stderr, "schcom-demo: no reply to create\n");
return 1;
}
if (surf.hdr.op != SCHCOM_OP_SURFACE) {
fprintf(stderr, "schcom-demo: create refused (op %u)\n", surf.hdr.op);
return 1;
}
if (surf.handle_kind != SCHCOM_HANDLE_SHMID) {
fprintf(stderr, "schcom-demo: unexpected handle kind %u\n", surf.handle_kind);
return 1;
}
void *pix = shmat((int)surf.handle, NULL, 0);
if (pix == (void *)-1) { perror("shmat"); return 1; }
printf("schcom-demo: surface %u, %ux%u stride %u\n",
surf.hdr.surface, surf.width, surf.height, surf.stride);
/* Draw: a solid block with a one-pixel border, so a test can tell "the
* buffer arrived" from "the buffer arrived at the wrong offset". */
for (uint32_t y = 0; y < surf.height; y++) {
uint32_t *row = (uint32_t *)((unsigned char *)pix + (size_t)y * surf.stride);
for (uint32_t x = 0; x < surf.width; x++) {
int edge = (x == 0 || y == 0 || x == surf.width - 1 || y == surf.height - 1);
row[x] = edge ? 0x00FFFFFF : colour;
}
}
struct schcom_commit commit;
memset(&commit, 0, sizeof commit);
commit.hdr.op = SCHCOM_OP_COMMIT;
commit.hdr.length = sizeof commit;
commit.hdr.serial = 3;
commit.hdr.surface = surf.hdr.surface;
commit.width = surf.width; commit.height = surf.height;
if (write(fd, &commit, sizeof commit) != (ssize_t)sizeof commit) {
perror("write commit"); return 1;
}
printf("schcom-demo: committed 0x%06X at %d,%d\n", colour, at_x, at_y);
fflush(stdout);
/* Stay alive: the surface belongs to this connection, and the compositor
* drops it when the client goes. Exiting immediately would erase the very
* thing the screenshot is meant to catch. */
sleep(wait_s);
return 0;
}