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SchOS/projects/schcom/schcom.c
Nexoniarz fe6462088a Schcom v0: a compositor that actually composites 
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>
2026-08-09 23:12:14 +02:00

605 lines
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/* SPDX-License-Identifier: Apache-2.0 */
/*
* Schcom -- the SchOS compositor.
*
* Owns the display and the input devices. Clients draw into shared buffers and
* hand them over; Schcom decides what is on top of what, composites, and
* presents. It does not decide where windows go: that is SchWM's job, and the
* moment this file starts making placement decisions the split has failed.
*
* v0, deliberately small but not a toy:
*
* - real framebuffer output through /dev/fb0 and the fbdev ioctls
* - real clients over a Unix socket, with a versioned handshake
* - real shared-memory buffers, one per surface
* - a back buffer, so a half-composited frame never reaches the screen
* - input read from /dev/input/eventN and routed to the top surface
*
* What it does not do yet: damage tracking beyond "repaint everything",
* multiple outputs, subsurfaces, or any notion of focus beyond "topmost".
* Those are additions, not rewrites -- which is the point of committing to the
* surface/commit model now rather than growing into it.
*/
#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <poll.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/shm.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/un.h>
#include <linux/fb.h>
#include <linux/input.h>
#include "proto.h"
#define MAX_CLIENTS 16
#define MAX_SURFACES 32
struct surface {
int used;
uint32_t id;
int client; /* index into clients[] */
int shmid;
void *pixels;
uint32_t w, h, stride, size;
int32_t x, y;
};
struct client {
int fd; /* -1 when the slot is free */
int hello_done;
};
static struct client clients[MAX_CLIENTS];
static struct surface surfaces[MAX_SURFACES];
static uint32_t next_surface_id = 1;
/* The screen. */
static uint8_t *fb;
static uint32_t fb_w, fb_h, fb_stride, fb_bpp;
static size_t fb_size;
static uint8_t *back; /* composited here, then copied out in one go */
/* Pointer position, moved by EV_REL from the mouse. */
static int32_t ptr_x, ptr_y;
static void
die(const char *what)
{
fprintf(stderr, "schcom: %s: %s\n", what, strerror(errno));
exit(1);
}
/* ---------------------------------------------------------------- output */
static int
open_framebuffer(void)
{
int fd = open("/dev/fb0", O_RDWR);
if (fd < 0)
die("open /dev/fb0");
struct fb_var_screeninfo var;
struct fb_fix_screeninfo fix;
if (ioctl(fd, FBIOGET_VSCREENINFO, &var) < 0)
die("FBIOGET_VSCREENINFO");
if (ioctl(fd, FBIOGET_FSCREENINFO, &fix) < 0)
die("FBIOGET_FSCREENINFO");
fb_w = var.xres;
fb_h = var.yres;
fb_bpp = var.bits_per_pixel;
/* The stride comes from the driver, never from xres*bpp: the two differ on
* real hardware and assuming otherwise shears the image. */
fb_stride = fix.line_length;
fb_size = (size_t)fb_stride * fb_h;
if (fb_bpp != 32) {
fprintf(stderr, "schcom: need 32bpp, got %u\n", fb_bpp);
exit(1);
}
fb = mmap(NULL, fb_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (fb == MAP_FAILED)
die("mmap /dev/fb0");
back = malloc(fb_size);
if (!back)
die("malloc back buffer");
printf("schcom: %ux%u, %u bpp, stride %u\n", fb_w, fb_h, fb_bpp, fb_stride);
fflush(stdout);
return fd;
}
static void
composite(void)
{
/* Desktop. A flat colour rather than black, so that "nothing was drawn"
* and "the compositor is not running" cannot look the same. */
for (uint32_t y = 0; y < fb_h; y++) {
uint32_t *row = (uint32_t *)(back + (size_t)y * fb_stride);
for (uint32_t x = 0; x < fb_w; x++)
row[x] = 0x00203040;
}
/* Surfaces, bottom to top. Creation order is stacking order in v0; SchWM
* will reorder later through a protocol request rather than by reaching in
* here. */
for (int i = 0; i < MAX_SURFACES; i++) {
struct surface *s = &surfaces[i];
if (!s->used || !s->pixels)
continue;
for (uint32_t sy = 0; sy < s->h; sy++) {
int32_t dy = s->y + (int32_t)sy;
if (dy < 0 || dy >= (int32_t)fb_h)
continue;
uint32_t *dst = (uint32_t *)(back + (size_t)dy * fb_stride);
uint32_t *src = (uint32_t *)((uint8_t *)s->pixels + (size_t)sy * s->stride);
for (uint32_t sx = 0; sx < s->w; sx++) {
int32_t dx = s->x + (int32_t)sx;
if (dx < 0 || dx >= (int32_t)fb_w)
continue;
dst[dx] = src[sx];
}
}
}
/* A crude pointer, drawn last so it is never under a window. */
for (int32_t py = 0; py < 10; py++) {
int32_t dy = ptr_y + py;
if (dy < 0 || dy >= (int32_t)fb_h)
continue;
uint32_t *dst = (uint32_t *)(back + (size_t)dy * fb_stride);
for (int32_t px = 0; px < 10 - py; px++) {
int32_t dx = ptr_x + px;
if (dx >= 0 && dx < (int32_t)fb_w)
dst[dx] = 0x00FFFFFF;
}
}
/* One copy to the screen. Compositing straight into the framebuffer would
* show every intermediate state -- the flicker would be the whole desktop
* being repainted under the user's eyes. */
memcpy(fb, back, fb_size);
}
/* ---------------------------------------------------------------- clients */
static int
send_all(int fd, const void *buf, size_t n)
{
const uint8_t *p = buf;
while (n) {
ssize_t w = write(fd, p, n);
if (w < 0) {
if (errno == EINTR)
continue;
return -1;
}
p += w;
n -= (size_t)w;
}
return 0;
}
static int
read_all(int fd, void *buf, size_t n)
{
uint8_t *p = buf;
while (n) {
ssize_t r = read(fd, p, n);
if (r == 0)
return -1; /* client went away */
if (r < 0) {
if (errno == EINTR)
continue;
return -1;
}
p += r;
n -= (size_t)r;
}
return 0;
}
static void
send_error(int fd, uint32_t serial, uint32_t code)
{
struct schcom_error e;
memset(&e, 0, sizeof e);
e.hdr.op = SCHCOM_OP_ERROR;
e.hdr.length = sizeof e;
e.hdr.serial = serial;
e.code = code;
send_all(fd, &e, sizeof e);
}
static void
drop_client(int ci)
{
if (clients[ci].fd < 0)
return;
for (int i = 0; i < MAX_SURFACES; i++) {
struct surface *s = &surfaces[i];
if (s->used && s->client == ci) {
if (s->pixels)
shmdt(s->pixels);
/* IPC_RMID once, here: the segment goes when the last attach does,
* so a client that died mid-frame does not leak it. */
shmctl(s->shmid, IPC_RMID, NULL);
memset(s, 0, sizeof *s);
}
}
close(clients[ci].fd);
clients[ci].fd = -1;
clients[ci].hello_done = 0;
printf("schcom: client %d gone\n", ci);
fflush(stdout);
}
static void
handle_create_surface(int ci, const struct schcom_create_surface *req)
{
int fd = clients[ci].fd;
int slot = -1;
for (int i = 0; i < MAX_SURFACES; i++)
if (!surfaces[i].used) { slot = i; break; }
if (slot < 0) {
send_error(fd, req->hdr.serial, ENOSPC);
return;
}
uint32_t w = req->width, h = req->height;
if (w == 0 || h == 0 || w > fb_w || h > fb_h) {
send_error(fd, req->hdr.serial, EINVAL);
return;
}
uint32_t stride = w * 4;
uint32_t size = stride * h;
/* The compositor owns the segment, not the client: it has to survive the
* client crashing mid-frame, and it must not be something a client can
* quietly resize underneath us. */
int shmid = shmget(IPC_PRIVATE, size, IPC_CREAT | 0600);
if (shmid < 0) {
send_error(fd, req->hdr.serial, errno);
return;
}
void *pix = shmat(shmid, NULL, 0);
if (pix == (void *)-1) {
shmctl(shmid, IPC_RMID, NULL);
send_error(fd, req->hdr.serial, errno);
return;
}
memset(pix, 0, size);
struct surface *s = &surfaces[slot];
s->used = 1;
s->id = next_surface_id++;
s->client = ci;
s->shmid = shmid;
s->pixels = pix;
s->w = w;
s->h = h;
s->stride = stride;
s->size = size;
s->x = req->x;
s->y = req->y;
struct schcom_surface rep;
memset(&rep, 0, sizeof rep);
rep.hdr.op = SCHCOM_OP_SURFACE;
rep.hdr.length = sizeof rep;
rep.hdr.serial = req->hdr.serial;
rep.hdr.surface = s->id;
rep.handle_kind = SCHCOM_HANDLE_SHMID;
rep.handle = (uint32_t)shmid;
rep.width = w;
rep.height = h;
rep.stride = stride;
rep.size = size;
if (send_all(fd, &rep, sizeof rep) < 0) {
drop_client(ci);
return;
}
printf("schcom: surface %u %ux%u at %d,%d for client %d (shm %d)\n",
s->id, w, h, s->x, s->y, ci, shmid);
fflush(stdout);
}
static struct surface *
find_surface(uint32_t id)
{
for (int i = 0; i < MAX_SURFACES; i++)
if (surfaces[i].used && surfaces[i].id == id)
return &surfaces[i];
return NULL;
}
/* Returns 1 if the screen needs repainting. */
static int
handle_message(int ci)
{
int fd = clients[ci].fd;
struct schcom_header hdr;
if (read_all(fd, &hdr, sizeof hdr) < 0) {
drop_client(ci);
return 0;
}
if (hdr.length < sizeof hdr || hdr.length > 4096) {
send_error(fd, hdr.serial, EPROTO);
drop_client(ci);
return 0;
}
uint8_t body[4096];
memcpy(body, &hdr, sizeof hdr);
size_t rest = hdr.length - sizeof hdr;
if (rest && read_all(fd, body + sizeof hdr, rest) < 0) {
drop_client(ci);
return 0;
}
if (!clients[ci].hello_done && hdr.op != SCHCOM_OP_HELLO) {
/* Nothing before the handshake. A client that gets this wrong is a
* client built against a protocol we do not speak. */
send_error(fd, hdr.serial, EPROTO);
drop_client(ci);
return 0;
}
switch (hdr.op) {
case SCHCOM_OP_HELLO: {
const struct schcom_hello *h = (const void *)body;
if (hdr.length < sizeof *h || h->version != SCHCOM_VERSION) {
send_error(fd, hdr.serial, EPROTONOSUPPORT);
drop_client(ci);
return 0;
}
struct schcom_hello ack;
memset(&ack, 0, sizeof ack);
ack.hdr.op = SCHCOM_OP_HELLO_ACK;
ack.hdr.length = sizeof ack;
ack.hdr.serial = hdr.serial;
ack.version = SCHCOM_VERSION;
if (send_all(fd, &ack, sizeof ack) < 0) {
drop_client(ci);
return 0;
}
clients[ci].hello_done = 1;
printf("schcom: client %d speaks v%u\n", ci, SCHCOM_VERSION);
fflush(stdout);
return 0;
}
case SCHCOM_OP_CREATE_SURFACE:
if (hdr.length < sizeof(struct schcom_create_surface)) {
send_error(fd, hdr.serial, EINVAL);
return 0;
}
handle_create_surface(ci, (const void *)body);
return 1;
case SCHCOM_OP_COMMIT: {
struct surface *s = find_surface(hdr.surface);
if (!s || s->client != ci) {
send_error(fd, hdr.serial, ENOENT);
return 0;
}
/* The commit is the whole point: the pixels were already there, but
* they were not allowed to be shown until now. */
struct schcom_header fr;
memset(&fr, 0, sizeof fr);
fr.op = SCHCOM_OP_FRAME;
fr.length = sizeof fr;
fr.surface = s->id;
if (send_all(fd, &fr, sizeof fr) < 0) {
drop_client(ci);
return 1;
}
return 1;
}
case SCHCOM_OP_DESTROY: {
struct surface *s = find_surface(hdr.surface);
if (s && s->client == ci) {
if (s->pixels)
shmdt(s->pixels);
shmctl(s->shmid, IPC_RMID, NULL);
memset(s, 0, sizeof *s);
return 1;
}
return 0;
}
default:
/* Unknown but well-formed: skipped, not fatal. That is what the length
* field is for, and it is how a v0 compositor survives a v1 client. */
return 0;
}
}
/* ----------------------------------------------------------------- input */
/* Returns 1 if the screen needs repainting, -1 to quit. */
static int
handle_input(int fd)
{
struct input_event ev;
if (read(fd, &ev, sizeof ev) != (ssize_t)sizeof ev)
return 0;
int repaint = 0;
if (ev.type == EV_REL) {
if (ev.code == REL_X) { ptr_x += ev.value; repaint = 1; }
if (ev.code == REL_Y) { ptr_y += ev.value; repaint = 1; }
if (ptr_x < 0) ptr_x = 0;
if (ptr_y < 0) ptr_y = 0;
if (ptr_x >= (int32_t)fb_w) ptr_x = (int32_t)fb_w - 1;
if (ptr_y >= (int32_t)fb_h) ptr_y = (int32_t)fb_h - 1;
}
if (ev.type == EV_KEY && ev.value == 1 && ev.code == KEY_ESC)
return -1;
/* Route to the topmost surface. "Topmost" is the whole focus model in v0;
* anything cleverer is policy and belongs to SchWM. */
struct surface *top = NULL;
for (int i = 0; i < MAX_SURFACES; i++)
if (surfaces[i].used)
top = &surfaces[i];
if (top && (ev.type == EV_KEY || ev.type == EV_REL)) {
struct schcom_input msg;
memset(&msg, 0, sizeof msg);
msg.hdr.op = SCHCOM_OP_INPUT;
msg.hdr.length = sizeof msg;
msg.hdr.surface = top->id;
msg.type = ev.type;
msg.code = ev.code;
msg.value = ev.value;
msg.x = ptr_x;
msg.y = ptr_y;
if (clients[top->client].fd >= 0)
send_all(clients[top->client].fd, &msg, sizeof msg);
}
return repaint;
}
/* ------------------------------------------------------------------ main */
int
main(int argc, char **argv)
{
int frames_then_exit = 0; /* --frames N: for automated testing */
for (int i = 1; i < argc; i++)
if (!strcmp(argv[i], "--frames") && i + 1 < argc)
frames_then_exit = atoi(argv[++i]);
for (int i = 0; i < MAX_CLIENTS; i++)
clients[i].fd = -1;
open_framebuffer();
ptr_x = (int32_t)fb_w / 2;
ptr_y = (int32_t)fb_h / 2;
unlink(SCHCOM_SOCKET_PATH);
int lfd = socket(AF_UNIX, SOCK_STREAM, 0);
if (lfd < 0)
die("socket");
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 (bind(lfd, (struct sockaddr *)&addr, sizeof addr) < 0)
die("bind " SCHCOM_SOCKET_PATH);
if (listen(lfd, 8) < 0)
die("listen");
int kbd = open("/dev/input/event0", O_RDONLY | O_NONBLOCK);
int mouse = open("/dev/input/event1", O_RDONLY | O_NONBLOCK);
printf("schcom: listening on %s\n", SCHCOM_SOCKET_PATH);
fflush(stdout);
composite();
int frames = 1;
for (;;) {
struct pollfd pfd[MAX_CLIENTS + 3];
int n = 0;
pfd[n].fd = lfd; pfd[n].events = POLLIN; n++;
int kbd_slot = -1, mouse_slot = -1;
if (kbd >= 0) { kbd_slot = n; pfd[n].fd = kbd; pfd[n].events = POLLIN; n++; }
if (mouse >= 0) { mouse_slot = n; pfd[n].fd = mouse; pfd[n].events = POLLIN; n++; }
int first_client = n;
for (int i = 0; i < MAX_CLIENTS; i++)
if (clients[i].fd >= 0) {
pfd[n].fd = clients[i].fd;
pfd[n].events = POLLIN;
n++;
}
if (poll(pfd, n, 100) < 0) {
if (errno == EINTR)
continue;
die("poll");
}
int repaint = 0;
if (pfd[0].revents & POLLIN) {
int cfd = accept(lfd, NULL, NULL);
if (cfd >= 0) {
int slot = -1;
for (int i = 0; i < MAX_CLIENTS; i++)
if (clients[i].fd < 0) { slot = i; break; }
if (slot < 0) {
close(cfd);
} else {
clients[slot].fd = cfd;
clients[slot].hello_done = 0;
printf("schcom: client %d connected\n", slot);
fflush(stdout);
}
}
}
if (kbd_slot >= 0 && (pfd[kbd_slot].revents & POLLIN)) {
int r = handle_input(kbd);
if (r < 0) break;
repaint |= r;
}
if (mouse_slot >= 0 && (pfd[mouse_slot].revents & POLLIN)) {
int r = handle_input(mouse);
if (r < 0) break;
repaint |= r;
}
/* Walk the client slots in the same order they were added to pfd. */
int k = first_client;
for (int i = 0; i < MAX_CLIENTS; i++) {
if (clients[i].fd < 0)
continue;
if (pfd[k].revents & (POLLIN | POLLHUP))
repaint |= handle_message(i);
k++;
}
if (repaint) {
composite();
frames++;
if (frames_then_exit && frames >= frames_then_exit) {
printf("schcom: %d frames, exiting as asked\n", frames);
fflush(stdout);
break;
}
}
}
unlink(SCHCOM_SOCKET_PATH);
return 0;
}
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