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build: a nix shell, because the loader is the one thing gcc cannot build

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make falls back to the GNU toolchain when clang is absent, and that works for
the kernel. It does not work for the UEFI loader: BOOTX64.EFI is PE/COFF and
wants clang plus lld-link, so a machine with only gcc gets a kernel that builds
and a loader that silently stays whatever it already was.

That is not hypothetical -- it is what left a stale boot splash on this tree for
half a day. The mask in include/logo.h was the new mark, the kernel drew it, and
the loader kept drawing one compiled months earlier because nothing could relink
it and nothing said so.

shell.nix declares everything docs/guides/building.md lists, and building.md
gains a NixOS section saying why using it matters rather than just how.

Verified: with clang 21.1.8 and LLD 21.1.8 the loader relinks and the splash
draws Shift in metal, screendumped from the framebuffer rather than inferred.

Pinned to the host's nixpkgs rather than a flake input, deliberately. This is a
convenience for a development machine, not a reproducibility claim; if it ever
needs to be the latter it wants a flake and should say so.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2
This commit is contained in:
Nexoniarz 2026-09-12 18:31:34 +02:00
commit 6c42ab3b1a
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# A development shell with everything docs/guides/building.md asks for.
#
# nix-shell # then: make, make qemu, make iso
#
# Why this exists: the UEFI loader is the one part that genuinely cannot be
# built without clang and lld. `make` falls back to the GNU toolchain for the
# kernel, so a machine with only gcc gets a kernel that builds and a loader that
# silently stays whatever it was -- which is exactly the failure that left a
# stale boot splash on this tree for half a day.
#
# Pinned to whatever nixpkgs the host is on rather than to a flake input, on
# purpose: this is a convenience for one developer's machine, not a
# reproducibility claim. If it ever needs to be the latter it wants a flake.
{ pkgs ? import <nixpkgs> { } }:
pkgs.mkShell {
name = "schos";
nativeBuildInputs = with pkgs; [
# The cross-compiler, and the reason this file exists. clang targets
# bare-metal ELF, 16-bit code and PE/COFF from one install; lld-link is what
# produces BOOTX64.EFI.
clang
lld
llvm # llvm-objcopy, llvm-objdump
# The host compiler, for tools/ -- mkfs, mkschfs, the SchFS suites. Must be
# a real Unix compiler: those need <unistd.h> and <dirent.h>.
gcc
gnumake
perl # generates the interrupt vector table
python3 # boot logo, crash-screen QR, mklogo
qemu # make qemu, and every test harness
OVMF # firmware for the UEFI path; the Makefile searches for it
xorriso # make iso
mtools # ESP staging
curl gnutar # fetching ports
];
shellHook = ''
echo "schos: clang $(clang --version | head -1 | grep -oE '[0-9]+\.[0-9]+\.[0-9]+')," \
"$(qemu-system-x86_64 --version | head -1 | grep -oE '[0-9]+\.[0-9]+\.[0-9]+')"
echo " make | make qemu | make schfs-check | make iso"
'';
}