The SDK

Every Darwin component is compiled against an SDK: the headers and stub libraries that describe the system it will run on. Apple’s macOS SDK is proprietary and cannot be redistributed.

PureDarwin used to require a copy of Apple’s MacOSX11.3.sdk.tar.xz, registered in the Nix store by hand. That requirement is gone. The build now assembles its own SDK from open source headers, and a clean machine needs nothing but the repository and Nix.

How It Is Built

nix/pkgs/toolchain/apple-sdk-pinned.nix builds the SDK. It collects the public headers from the in-tree sources (XNU’s bsd, osfmk, iokit and libkern headers, libSystem, the frameworks and so on) and lays them out the way an SDK is laid out:

$out/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk

It is built in two stages. The first stage has the hand-written headers that migcom needs. The second stage adds the <mach/*.h> headers that migcom generates from XNU’s .defs files, so nothing generated is checked into the tree.

The small amount of content that has no open source origin is written by the project and lives in nix/pkgs/toolchain/oss-sdk/, laid out as the SDK tree. It is mostly a few reconstructed headers and text stubs (.tbd files) for frameworks.

The flake wires this SDK in globally, so every target builds against it.

When a Header Is Missing

Almost every gap found so far was a header that already existed somewhere in the tree and was not yet copied into the SDK. Look for it in the tree first. Only write new content in oss-sdk/ when nothing in the tree provides it.

Never copy content out of Apple’s proprietary SDK to fill a gap, even to look at it. The whole point of this SDK is that every file in it has an open source origin.

Other Licensing

The SDK is not the only licensing question in the tree. Security.framework still compiles the old GPL corecrypto, some of Apple’s published source carries no usable license grant, and GPL extras such as fbDOOM are kept outside the tree as an opt-in. See Licensing.