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a9ff1166b5
The second of the two things the stack owed. recvfrom accepted the address arguments and ignored them, so every caller was told nothing about where a datagram came from -- which for UDP is most of the point: a server that cannot see the sender cannot reply to it. Fixing it properly meant fixing something worse that was sitting underneath. UDP payloads went into the same byte ring TCP uses, with no record of where one datagram ended and the next began, so two datagrams that arrived together came back from one read as one. That is a bug whose appearance depends on timing and whose blame lands on the sender. Datagrams now carry a header alongside the bytes: source address, source port, length. Sixteen of them per socket, in a ring beside the existing one. Whole buffers would have been 16 sockets x 16 datagrams x an MTU of static memory for something almost always empty; only the boundaries need keeping, and those are eight bytes each. A datagram too big for the buffer it is read into is truncated and the remainder discarded, rather than left in the ring to be delivered as the head of the next one. One that arrives with the queue full is dropped whole -- that is UDP's own contract, and half a datagram in the ring would corrupt every one after. An empty queue and a zero-length datagram are kept apart, because a sender really can send zero bytes and the right answer to that is 0 rather than "nothing has arrived yet". sendto grew the mirror image: a destination address on a datagram socket, which is what a server replying to whoever just wrote to it needs. On TCP a destination is ignored rather than refused -- the connection decides where the bytes go, and programs pass one out of habit. For a stream, recvfrom's source is read from the pcb: it is the peer, and it is the same answer for every byte. Verified: bootcheck -Net now covers UDP too, 45 [ok] and no failures. The address check is real rather than cosmetic -- nettest deliberately leaves its sockaddr uninitialised, so a kernel that filled in nothing would print stack rubbish instead of 10.0.2.2 and the port the responder is actually on. The boundaries are checked by byte count: the responder sends 20 bytes and then 16, and the guest reports reading 20 and then 16. Treated as a stream the first read would have taken all 36. Left for later, and deliberately: accept() still does not report the peer's address, and getsockname/getpeername do not exist. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> |
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| run.ps1 | ||
A 64-bit operating system, written from the metal up.
SMP · copy-on-write filesystem · PCI, USB and virtio drivers · TrueType text · Linux syscall ABI
SchOS began as xv6 and is now mostly something else. The scheduler, the memory manager and the trap path have been rewritten for x86-64 and for more than one processor; the filesystem has been replaced outright; the syscall table speaks Linux numbering so that real software — musl, toybox, mksh — runs unmodified.
It is a hobby system built by one person, and it is honest about that: what works below has been run, not inferred, and the gaps are listed as plainly as the features.
The pieces
| Layer | Name | What it is |
|---|---|---|
| System | SchOS | The distribution — kernel, userland, tooling |
| Kernel | SchIFT | Multi-core x86-64 kernel with a modular VFS |
| Filesystem | SchFS | Transactional, copy-on-write, the root filesystem |
| API | SchAPI | System programming interface — the stable surface |
| Compositor | SchCom | Display compositor (planned) |
| Window manager | SchWM | (planned) |
| Desktop | SchDE | (planned) |
uname -s reports SchIFT, the way every Unix reports its kernel rather than
its distribution.
What works
Kernel — Long mode, 4-level paging, preemptive multitasking. SMP: one, two and four processors all boot, with CPU enumeration from the ACPI MADT rather than the 1994 MP table. Memory sized from the E820 map, so it runs in tens of megabytes rather than demanding hundreds.
Filesystem — SchFS is the root: a copy-on-write B-tree with a
uberblock ring, so a crash leaves either the old tree or the new one and there
is nothing to repair. Free space is derived from the tree rather than stored
beside it, which removes a whole class of disagreement. /proc/schfs shows the
generation counter advancing as writes commit.
Processes — fork, exec, wait, pipes, signals, sessions and process groups,
mmap (anonymous and file-backed), POSIX shared memory, Unix domain sockets,
poll.
Devices — PCI enumeration with a bus-agnostic device model at
/proc/devices. Drivers for e1000 networking, virtio-gpu and stdvga
displays, xHCI USB with HID, hub and mass-storage class drivers, IDE, PS/2,
serial. /dev/fb0 speaks the Linux fbdev interface.
Userland — musl, toybox and mksh, built from source. ls, cat, ps,
top, wc, cp, mount and the rest behave the way you expect, because they
are the real programs.
Text — a TrueType rasteriser (glyf outlines, composite glyphs, cmap
format 4) replacing the 8×16 cell grid. JetBrains Mono for the terminal, Roboto
for the desktop layer to come.
When it dies — PSOGM, the Purple Screen of Guru Meditation: stop code, fault address, call chain, and a QR code containing a text copy of the whole report.
Quick start
make # kernel + UEFI loader + root filesystem
make qemu # build it and boot it
make iso # a bootable ISO for VMware or real hardware
Builds on Linux, macOS, FreeBSD/OpenBSD/NetBSD and Windows (Git Bash). The toolchain is clang + lld everywhere — it is a cross-compiler by construction, so one LLVM install targets the bare-metal kernel, the 16-bit boot code and musl userland from any host. See docs/building.md for prerequisites per platform, and docs/moving-to-linux.md for moving a checkout between machines.
Testing
Booting to a prompt proves nothing: a filesystem can mount and still fail the first real command, and that has happened here. So the test harness drives the serial console and asserts on what comes back.
tools/windows/bootcheck.ps1 # boot, run real commands, check the answers
tools/windows/bootcheck.ps1 -Tty -Ps # terminal behaviour, process enumeration
make schfs-check # SchFS B-tree, object and image suites
bootcheck is PowerShell because pwsh runs on every supported host.
Layout
kernel/ arch/ core/ dev/ drivers/ fs/ net/ proc/ sys/
fs/schfs/ the filesystem -- same source the host tools compile
include/ kernel and userland headers
boot/ uefi/ (PE loader) common/ (shared loader UI)
lib/ cmd/ first-party userland runtime and programs
ports/ third-party software built from source
tools/ windows/ (.ps1) posix/ (.sh) python/ (.py) and .c/.pl
docs/ architecture, roadmap, licensing, building
One rule worth knowing: the kernel and the host tools compile the same SchFS
source. mkschfs cannot disagree with the kernel about the on-disk format,
because there is only one implementation — found and fixed on the host, where a
test takes a second instead of a boot.
Another: syscalls are declared in exactly one place,
kernel/sys/table.inc. Everything else is generated from it.
Not there yet
- No TCP/IP. The decision is made — port lwIP rather than write one — but it is not done. The e1000 driver works; nothing speaks to it yet.
- No compositor, window manager or desktop. That is the next large piece.
- SchFS has no Linux driver yet; the plan is on-disk format version first, then FUSE, then a kernel module.
- CFF/OpenType outlines and TrueType hinting are deliberately out of scope.
Licensing
Apache-2.0, copyright Nexoniarz. See LICENSE.
The kernel derives from xv6, which is MIT — see LICENSE.MIT. Third-party code redistributed in a built image (musl, toybox, mksh, the fonts) keeps its own licence; NOTICE carries the attributions.
Drivers may be adapted from BSD sources, which are compatible. Linux drivers are not: GPL-2.0-only and Apache-2.0 cannot be combined and redistributed. Reading Linux to understand hardware is fine; translating a driver is not. Best of all is the public specification — xHCI, virtio, AHCI, e1000 — which carries no licence on what you write from it.