# Rust: Cross compiling from Ubuntu to OS X

A few months ago, I saw this post by Chris Krycho on Hackernews, which points out just how easy it can be to share Rust binaries with friends. The use case really spoke to me — I spend a lot of time building prototypes and I often need to share my prototypes with a few non-technical people. The typical sharing workflow looks like this:

1. I package my prototype as a Python script with a bunch of dependencies into a simple command line tool.
2. I send the package over to a collaborator.
3. Collaborator is missing roughly half of the dependencies.
4. I debate teaching collaborator about virtual environments or anaconda. Meanwhile, collaborator is busy typing things like
sudo pip install randompackage

into their terminal.

5. Collaborator says, “it’s still not working”. Turns out collaborator only has Python 2.7 but the script is meant for Python 3.
6. The process repeats until we get the script running.

After reading Chris’s post, I started looking into what would be involved in cross compiling from Ubuntu (my preferred OS) to OS X (the preferred OS of most of my collaborators). I was discouraged–it looked complicated and I wasn’t sure it could be done without having a running version of OS X with Xcode. Dismayed, I dropped the idea entirely. But the burden of sharing my work continues to weigh on me while my love for the Rust langauge grows ever stronger. Finally, this weekend I decided enough was enough and I was going to attempt to figure out how to cross compile from Ubuntu to OS X.

And it turns out that it’s not nearly as hard as I expected! Most of the key ideas have already been fully explained by Brian Pearce in this post. Brian cross compiles from a Vagrant Ubuntu build environment running on an OS X host to an OS X binary. But Brian’s post isn’t quite enough for the devoted Linux user — it still assumes you’re running a copy of OS X somewhere (namely on the Vagrant host).

Below, I give a complete account of how to cross compile a Rust library from Ubuntu 14.04 for an OS X target. I emphasize again that most of the ideas are not mine but come from Brian’s post as well as from the work of the osxcross project. I’ve just added a few details to complete the build without having to run OS X anywhere.

## Prerequisites

Make sure you’ve got the following dependencies installed directly from apt-get:

sudo apt-get update
sudo apt-get install clang autotools-dev automake cmake libfuse-dev

Note, a difference from Brian’s post is that we require libfuse. This allows osxcross to work directly with the necessary Xcode disk image so that we need only have the Xcode image and not a full OS X system to build the OS X SDK. Next, install rustup:

curl https://sh.rustup.rs -sSf | sh

You’ll then want to run (if this is the first time using rustup):

source .cargo/env

Next clone the osxcross project:

git clone https://github.com/tpoechtrager/osxcross

Finally, download version 7.3 of Xcode. For this you’ll need to signin in with any valid Apple ID. You want to obtain a .dmg file, e.g. Xcode_7.3.1.dmg.

## Bootstrapping the OS X cross compile toolchain

First, we’ll use osxcross to build an OS X SDK package for cross compilation. Change into the osxcross root directory and run:

./tools/gen_sdk_package_p7zip.sh Xcode_7.3.1.dmg

providing the full path to your Xcode .dmg file. If all goes well, you osxcross will have built a packaged OS X SDK into the root of the osxcross project. Move this package into the tarballs directory:

mv MacOSX10.11.sdk.tar.xz tarballs/

Now we build osxcross:

OSX_VERSION_MIN=10.7 ./build.sh

This creates the full OS X cross compiler tool chain, allowing us to compile Rust targets just like we would on a native OS X system. To make the toolchain available, you need to add it to your path. For this, you might consider edit your ~/.profile from:

# this line was automatically added to .profile by rustup:
export PATH="$HOME/.cargo/bin:$PATH"

to:

export PATH="$HOME/.cargo/bin:$HOME/projects/osxcross/target/bin:\$PATH"

## Cross compiling

Now we are almost ready to cross compile. Change into your Rust project directory and use Rustup to configure an OS X target:

rustup target add x86_64-apple-darwin

Finally, we need to tell rustc where to find the linker. Add or edit a .cargo/config file to the root of your Rust project to contain the following line:

[target.x86_64-apple-darwin]
linker = "x86_64-apple-darwin15-clang"

We can then cross compile our Rust code with:

cargo build --release --target=x86_64-apple-darwin

Your shiny new binary can be found under target/x86_64-apple-darwin/release.

Hooray and happy sharing!