This is a reproducible Dockerfile for cross compiling for Apple platforms from Linux, usable as base image for CI style setups.
This builds on cctools-port for providing the basic tools for cross compilation, together with xcbuild for building xcode project files, wrapping it up into a easy to use and reproducible Dockerfile.
Not all build tools used by Xcode project files are available; some
are stubbed out (like ibtool), making it usable as a CI tool for
testing compilation, while other Xcode project file features makes
the build fail altogether.
This setup currently uses a vanilla LLVM/Clang release instead of the Apple provided clang sources.
This requires an original Xcode.app bundle (which can't be redistributed freely). It has been tested with and should work with Xcode versions 7, 8, 9 and 10. Older versions of Xcode can be downloaded from Apple.
Using an Xcode 11 or 12 bundle works somewhat; building with generic build systems works fine, but there's a few known issues if building Xcode project files with xcbuild, for other than the simplest project files. (For Xcode 12, xcbuild doesn't seem to work at all.)
The Xcode bundle can be stripped down to more manageable sizes for use with this cross compilation setup, since very little of the bundle actually is used:
cp -a /Applications/Xcode.app .
./strip-xcode.sh Xcode.app
tar -Jcvf xcode.tar.xz Xcode.app
rm -rf Xcode.app
Host the resulting xcode.tar.xz file somewhere, then build the docker image like this:
docker build --build-arg XCODE_URL=http://path/to/your/xcode.tar.xz .
The docker image has got all the necessary tools added to the path, but
at the end of the path, so that e.g. gcc still resolves to the normal
host compiler (for cases when projects need to compile things for both
the host and the target). The cross tools can be used with normal cross
compilation prefixes like x86_64-apple-darwin-gcc (which invokes clang,
not gcc), or less architecturally biased like apple-darwin-gcc (for cases
when the architecture will be specified with an -arch parameter.