To prepare for cross compilation to Darwin, we must install the macOS SDK and a build toolchain that can use it. The easiest way to do this is with the osxcross project. This example shows how to download and install the SDK and tools to build for macOS without using a Macintosh computer. This illustration uses Linux but the process is the same for Windows developers using MSYS2 or Cygwin Command Prompts.

We'll be using clang rather than gcc as it's more portable by design. For this process to work, you'll need to have clang, cmake, and libxml2-dev installed using your package manager:

Next, we need to download the macOS SDK, which is bundled with Xcode. If you don't already have an Apple developer account, you'll need to sign up and agree to their terms and conditions. Using this account, log in to the download site at https://developer.apple.com/download/more/?name=Xcode%207.3 and download XCode.dmg (7.3.1 is recommended for osxcross).

Then, we can install the osxcross tool—start by downloading it with git clone https://github.com/tpoechtrager/osxcross.git and then change into the downloaded directory. Using these tools, we extract the macOS SDK from the downloaded Xcode.dmg file using the package tool provided, ./tools/gen_sdk_package_darling_dmg.sh <path to Xcode.dmg>. The resulting MacOSX10.11.sdk.tar.xz file should be copied into the tarballs/ directory.

Lastly, we build the osxcross compiler extension by executing ./build.sh. Following this, there should be a new directory named target/bin/, which you should add to your PATH environment variable. The compiler can now be used in CGo builds by setting the environment variable, CC=o32-clang. More details about this process and how to adapt it for other platforms are available on the osxcross project website at https://github.com/tpoechtrager/osxcross.