docs: how to build on each supported host
Prerequisites per platform, why the toolchain is clang rather than a cross-gcc, how to force GNU, and the host differences already handled -- listed because they are the shape of the next such bug. Records the two things a second machine needs to know: ports cannot be built on a Windows host (sysroot/ is used as staged, so build them once on a Unix), and pwsh runs on Linux and macOS so the bootcheck suite is not Windows-only.
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<!-- SPDX-License-Identifier: Apache-2.0 -->
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# Building SchOS
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Supported hosts: **Linux, macOS, FreeBSD/OpenBSD/NetBSD, and Windows** (under
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Git Bash or MSYS2). The build is developed from both Windows and Linux, so a
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change that only works on one of them is a bug.
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```sh
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make # kernel + bootloader + root filesystem -> schos.img
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make qemu # build and boot it
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```
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## Prerequisites
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| Need | Why | Windows | Linux | macOS | BSD |
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|---|---|---|---|---|---|
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| `clang`, `ld.lld`, `llvm-objcopy`, `llvm-objdump` | cross-compiles the kernel, the 16/32-bit boot code, and musl userland | LLVM installer | `clang lld llvm` | `brew install llvm` | `pkg install llvm` |
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| `make` | — | Git Bash | `make` | Xcode CLT | `gmake` |
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| `perl` | generates the interrupt vector table | Git Bash | usually present | present | `pkg install perl5` |
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| `python3` | boot logo, crash-screen QR | python.org (`python`) | `python3` | `brew install python` | `pkg install python3` |
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| `qemu-system-x86_64` | running and testing | `scoop install qemu` | `qemu-system-x86` | `brew install qemu` | `pkg install qemu` |
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| `curl`, `tar` | fetching ports | Git Bash | present | present | present |
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| `xorriso` **or** `mkisofs` | `make iso` only | `scoop install cdrtools` | `xorriso` | `brew install xorriso` | `pkg install xorriso` |
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A host C compiler (`cc`) is also needed, for the tools that build the disk
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images. On Windows that must be **mingw-w64's gcc**, not clang: clang defaults
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to an MSVC target there, and `tools/mkfs.c` needs `<unistd.h>` and `<dirent.h>`,
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which the Windows SDK does not have.
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## Why clang rather than a cross-gcc
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clang is a cross-compiler by construction, so one ordinary LLVM install targets
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everything this tree needs — bare-metal ELF, code linked below one megabyte, and
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Linux/musl userland — from any host. A cross binutils+gcc would be a day of
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setup per platform that then rots, and on macOS it means brewing
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`x86_64-elf-gcc` before anything builds at all.
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The Makefile selects clang+lld whenever both are on `PATH`. To force the GNU
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toolchain instead:
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```sh
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make USE_GNU=1 # needs a cross toolchain that emits x86_64 ELF
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make USE_GNU=1 TOOLPREFIX=x86_64-elf-
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```
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## Testing
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```sh
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tools/bootcheck.ps1 # boots, drives the shell, asserts on output
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tools/bootcheck.ps1 -Tty -Ps # plus terminal behaviour and process listing
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make schfs-check # SchFS unit, object and image suites
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```
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`bootcheck` is PowerShell, and PowerShell 7 (`pwsh`) runs on Linux, macOS and
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the BSDs as well as Windows — `brew install powershell`, or your distribution's
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`powershell` package. It finds QEMU on `PATH`.
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Booting to a prompt proves nothing on its own: a filesystem can mount and still
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fail the first real command, which has happened here. `bootcheck` types commands
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at the serial console and greps the transcript for the answers they should
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produce.
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## Ports
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`musl`, `toybox` and `mksh` are built from source into `sysroot/`. **This does
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not work on a Windows host** — `make` prints `ports: skipped` and uses whatever
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is already staged in `sysroot/`. Building them needs a Linux host (or any other
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Unix); do it once and the staged result serves the Windows builds.
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## Known host differences, already handled
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Listed because they are the shape of the next such bug, not because anything is
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outstanding:
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- `<sys/sysmacros.h>` is glibc/musl only. macOS and the BSDs take `major()` and
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`minor()` from `<sys/types.h>`.
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- `sha256sum` exists on Linux and Git Bash; macOS has `shasum -a 256`,
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FreeBSD/NetBSD `sha256`, OpenBSD `cksum -a sha256`. `tools/sha256check.sh`
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finds whichever is present.
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- `nproc` is coreutils, so absent on macOS and BSD, where it is
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`sysctl -n hw.ncpu`.
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- In the PowerShell tools, path literals must use forward slashes:
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`Join-Path $root 'build\x'` produces a *filename containing a backslash* on a
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Unix host.
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## Working on two machines
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Claude Code's project memory is kept in the repo and moved with:
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```sh
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tools/memory-sync.sh status
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tools/memory-sync.sh export # after a session that learned something; then commit
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tools/memory-sync.sh import # after cloning or pulling elsewhere
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```
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