OpenOSX

An open-source operating system built on Darwin.

OpenOSX is an open-source operating system built on Darwin, the OS foundation underneath macOS, with one uncompromising rule: everything we ship is built from open source. Kernel, drivers, bootloader, userland: no closed Apple binaries anywhere in the boot chain.

The goal is an OS image that boots easily in common VMs (QEMU, VirtualBox, VMware) from a fully open toolchain: the XNU kernel and kernel extensions in this repo, loaded by an open EFI bootloader (xnu-loader), up to a real userland, launchd, a shell, X11/Wayland, and a desktop environment.

Heritage

OpenOSX is a successor to PureDarwin (puredarwin.org), the community project that kept the dream of a usable standalone Darwin alive after OpenDarwin, and before that to Apple's own open-source Darwin releases. Substantial portions of this tree were authored by the PureDarwin developers and by Apple; see PUREDARWIN_LICENSE.txt, APPLE_LICENSE.txt, APPLE_DRIVER_LICENSE.txt, and docs/PUREDARWIN_ATTRIBUTION.md. We are grateful to both.

Building OpenOSX

The supported build is Nix, on Linux or macOS (x86_64 or aarch64):

# One-time: the Apple SDK tarball is proprietary and cannot be fetched by Nix.
# Obtain MacOSX11.3.sdk.tar.xz and register it (Nix verifies its sha256):
nix-store --add-fixed sha256 /path/to/MacOSX11.3.sdk.tar.xz

# Build a bootable disk image (GPT: EFI system partition + root filesystem):
nix build .#image-minimal    # lean image: kernel, launchd, zsh, toybox
nix build .#image            # full image: X11/Wayland, XFCE, and friends

Run the result in QEMU:

nix run .#vm                 # boots result/openosx.img with serial on stdio

A classic CMake build of individual components on macOS also works; see CMakeLists.txt and the CI workflows.

It should be noted that aarch64 support is an extreme work in progress and may break often.

What's in the tree

• src/Kernel/xnu, the XNU kernel (Darwin 20.5 lineage, x86_64 + arm64)
• src/Kernel/Extensions, kernel extensions: ACPI, APIC, PCI, PS/2, HID, IDE/AHCI/NVMe, USB (UHCI/OHCI/EHCI/xHCI), e1000 + VirtIO net, framebuffers (GOP, VirtIO GPU, Intel Gen9), and filesystems (HFS+, ext4, APFS, msdosfs)
• src/Libraries, libSystem, CoreFoundation, dyld, launchd/XPC, objc4, and the rest of the userland library stack
• src/Userspace, userland programs (including fbDOOM, optionally)
• nix/, flake.nix, image.nix, the cross-compilation and image pipeline
• tools, host toolchain (cctools/ld64, mig, xar, kc-tools, xnu-loader)
• docs/, the engineering plans: roadmap, macOS app compatibility, Aqua/UI stack, multi-architecture, and the GUI test corpus

Use cases

OpenOSX is early. It boots, reaches a graphical session, has networking, a shell and sshd, and builds reproducibly from source, but there is no package manager, no installer, and no macOS application compatibility yet. What follows is what it is for, split into what works today and where it is going.

As a desktop OS

The long-term goal is a Darwin desktop that belongs to its users, with macOS's foundations and none of its restrictions: no signing requirements, no notarisation, no telemetry, no hardware lock-in, and every line of it buildable from source on commodity PCs.

Today that means an X11 session with a window manager, a terminal, and the userland in docs/TEST_CORPUS.md's Tier 0. It is usable for exploring the system, not for daily work.

Where it is going, in order: a desktop environment that is OpenOSX's own rather than a borrowed one (see docs/AQUA_UI_PLAN.md), then the ability to run real macOS applications through a clean-room AppKit and Quartz implementation (see docs/MACOS_COMPAT.md). The second is years of work and is described honestly as such in those documents.

As a server OS

This is the nearer-term practical use, because the server-shaped parts of Darwin are the parts that already work: launchd supervises services, sshd accepts logins, networking comes up automatically, and mDNSResponder, configd and notifyd all run.

The distinctive case is building and testing Darwin software without Apple hardware. A CI runner that produces and exercises real Mach-O binaries against a real XNU kernel, on ordinary x86_64 servers or in a VM, is something no Linux box can offer and no Apple licence permits you to rent freely. Related uses that follow from the same property:

• Reproducible Darwin builds. The whole OS is a Nix flake, so a build host is a pinned, hash-verified artifact rather than a hand-maintained machine.
• Kernel and systems research. A real XNU you can patch, instrument and boot in seconds under QEMU, which suits teaching Mach, IOKit and dyld, and security research that would otherwise require Apple hardware and fighting SIP.
• Appliance and embedded-style workloads, where launchd's supervision model and a small, auditable, fully-source-built image matter more than a large package ecosystem.

The honest caveat for both roles: OpenOSX has had no security review, no hardening pass, and no stability guarantees. Run it in a VM, on a test machine, or in a lab. Not on anything you care about, and not exposed to the internet.

End Goal & Author Notes

OpenOSX is designed to be binary compatible with macOS whilst maintaining a pure FOSS design, the end goal of OpenOSX is to be able to run most macOS apps directly on a FOSS operating system without proprietary hardware (and maybe be superior to linux, or never).

License

Code inherited from Apple is under the Apple Public Source License (drivers: APPLE_DRIVER_LICENSE.txt). Code authored by the PureDarwin project is under the PureDarwin license. New OpenOSX code is under the same terms unless noted otherwise.