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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
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Nexoniarz 2026-09-12 18:31:34 +02:00
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19
docs/guides/building.md
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@ -22,6 +22,25 @@ make qemu # build and boot it
| `curl`, `tar` | fetching ports | Git Bash | present | present | present |
| `xorriso` **or** `mkisofs` | `make iso` only | `scoop install cdrtools` | `xorriso` | `brew install xorriso` | `pkg install xorriso` |
On NixOS, `nix-shell` supplies every row above — see below.
## NixOS
```sh
nix-shell # then: make, make qemu, make iso
```
`shell.nix` in the repository root declares the lot. Worth using rather than
assembling it by hand, for one specific reason: **`make` falls back to the GNU
toolchain for the kernel but the UEFI loader genuinely cannot be built without
clang and `lld-link`.** A machine with only gcc therefore produces a kernel that
builds and a loader that silently stays whatever it already was — which left a
stale boot splash on this tree for half a day before anyone looked at the
screen rather than the build output.
PowerShell is the one thing the shell does not supply, and `tools/windows/`
needs it. See the note at the end of this file.
A host C compiler (`cc`) is also needed, for the tools that build the disk
images. On Windows that must be **mingw-w64's gcc**, not clang: clang defaults
to an MSVC target there, and `tools/mkfs.c` needs `<unistd.h>` and `<dirent.h>`,