Although Go strives to be a cross platform language, cross compilation from one platform to another is not as simple as it could be, as you need the Go sources bootstrapped to each platform and architecture.
The first step towards cross compiling was Dave Cheney's golang-crosscompile package, which automatically bootstrapped the necessary sources based on your existing Go installation. Although this was enough for a lot of cases, certain drawbacks became apparent where the official libraries used CGO internally: any dependency to third party platform code is unavailable, hence those parts don't cross compile nicely (native DNS resolution, system certificate access, etc).
A step forward in enabling cross compilation was Alan Shreve's gonative package, which instead of bootstrapping the different platforms based on the existing Go installation, downloaded the official pre-compiled binaries from the golang website and injected those into the local toolchain. Since the pre-built binaries already contained the necessary platform specific code, the few missing dependencies were resolved, and true cross compilation could commence... of pure Go code.
However, there was still one feature missing: cross compiling Go code that used CGO itself, which isn't trivial since you need access to OS specific headers and libraries. This becomes very annoying when you need access only to some trivial OS specific functionality (e.g. query the CPU load), but need to configure and maintain separate build environments to do it.
My solution to the challenge of cross compiling Go code with embedded C snippets (i.e. CGO_ENABLED=1) is based on the concept of lightweight Linux containers. All the necessary Go tool-chains, C cross compilers and platform headers/libraries have been assembled into a single Docker container, which can then be called as if a single command to compile a Go package to various platforms and architectures.
Although you could build the container manually, it is available as an automatic trusted build from Docker's container registry (~530MB):
docker pull karalabe/xgo-latest
To prevent having to remember a potentially complex Docker command every time, a lightweight Go wrapper was written on top of it.
go get github.com/karalabe/xgo
Simply specify the import path you want to build, and xgo will do the rest:
$ xgo github.com/project-iris/iris
...
$ ls -al
-rwxr-xr-x 1 root root 6021828 May 4 10:59 iris-darwin-386
-rwxr-xr-x 1 root root 7664428 May 4 10:59 iris-darwin-amd64
-rwxr-xr-x 1 root root 8292432 May 4 10:59 iris-linux-386
-rwxr-xr-x 1 root root 10252920 May 4 10:59 iris-linux-amd64
-rwxr-xr-x 1 root root 8222976 May 4 10:59 iris-linux-arm
-rwxr-xr-x 1 root root 8373248 May 4 10:59 iris-windows-386.exe
-rwxr-xr-x 1 root root 10331648 May 4 10:59 iris-windows-amd64.exe
A handful of flags can be passed to go build. The currently supported ones are
-v: prints the names of packages as they are compiled-race: enables data race detection (supported only on amd64, rest built without)
As newer versions of the language runtime, libraries and tools get released, these will get incorporated into xgo too as extensions layers to the base cross compilation image (only Go 1.3 and above will be supported).
You can select which Go release to work with through the -go command line flag
to xgo and if the specific release was already integrated, it will automatically
be retrieved and installed.
$ xgo -go 1.4.2 github.com/project-iris/iris
Since xgo depends on not only the official releases, but also on Dave Cheney's ARM packages, there will be a slight delay between official Go updates and the xgo updates.
Additionally, a few wildcard release strings are also supported:
latestwill use the latest Go release1.4.xwill use the latest point release of a specific Go version
xgo by default uses the name of the package being cross compiled as the output
file prefix. This can be overridden with the -out flag.
$ xgo -out iris-v0.3.2 github.com/project-iris/iris
...
$ ls -al
-rwxr-xr-x 1 root root 6021828 May 4 11:00 iris-v0.3.2-darwin-386
-rwxr-xr-x 1 root root 7664428 May 4 11:00 iris-v0.3.2-darwin-amd64
-rwxr-xr-x 1 root root 8292432 May 4 11:00 iris-v0.3.2-linux-386
-rwxr-xr-x 1 root root 10252920 May 4 11:00 iris-v0.3.2-linux-amd64
-rwxr-xr-x 1 root root 8222976 May 4 11:00 iris-v0.3.2-linux-arm
-rwxr-xr-x 1 root root 8373248 May 4 11:00 iris-v0.3.2-windows-386.exe
-rwxr-xr-x 1 root root 10331648 May 4 11:00 iris-v0.3.2-windows-amd64.exe
If the project you are cross compiling is not a single executable, but rather a
larger project containing multiple commands, you can select the specific sub-
package to build via the --pkg flag.
$ xgo --pkg cmd/goimports golang.org/x/tools
...
$ ls -al
-rwxr-xr-x 1 root root 3824276 May 4 11:13 goimports-darwin-386
-rwxr-xr-x 1 root root 4947056 May 4 11:13 goimports-darwin-amd64
-rwxr-xr-x 1 root root 3867592 May 4 11:13 goimports-linux-386
-rwxr-xr-x 1 root root 4992584 May 4 11:13 goimports-linux-amd64
-rwxr-xr-x 1 root root 3880544 May 4 11:13 goimports-linux-arm
-rwxr-xr-x 1 root root 4005376 May 4 11:13 goimports-windows-386.exe
-rwxr-xr-x 1 root root 5145600 May 4 11:13 goimports-windows-amd64.exe
This argument may at some point be merged into the import path itself, but for now it exists as an independent build parameter.
Similarly to go get, xgo also uses the master branch of a repository during
source code retrieval. To switch to a different branch before compilation pass
the desired branch name through the --branch argument.
$ xgo --pkg cmd/goimports --branch release-branch.go1.4 golang.org/x/tools
...
$ ls -al
-rwxr-xr-x 1 root root 3828396 May 4 11:33 goimports-darwin-386
-rwxr-xr-x 1 root root 4959376 May 4 11:33 goimports-darwin-amd64
-rwxr-xr-x 1 root root 3872736 May 4 11:33 goimports-linux-386
-rwxr-xr-x 1 root root 4997976 May 4 11:33 goimports-linux-amd64
-rwxr-xr-x 1 root root 3885552 May 4 11:33 goimports-linux-arm
-rwxr-xr-x 1 root root 4012032 May 4 11:33 goimports-windows-386.exe
-rwxr-xr-x 1 root root 5153280 May 4 11:33 goimports-windows-amd64.exe
The main differentiator of xgo versus other cross compilers is support for basic embedded C code and target-platform specific OS SDK availability. The current xgo release introduces an experimental CGO dependency cross compilation, enabling building Go programs that require external C libraries.
It is assumed that the dependent C library is configure/make based, was properly
prepared for cross compilation and is available as a tarball download (.tar,
.tar.gz or .tar.bz2).
Such dependencies can be added via the --deps CLI argument. A complex sample
for such a scenario is building the Ethereum CLI node, which has the GNU Multiple
Precision Arithmetic Library as it's dependency.
$ xgo --pkg=cmd/geth --branch=develop --deps=https://gmplib.org/download/gmp/gmp-6.0.0a.tar.bz2 github.com/ethereum/go-ethereum
...
$ ls -al
-rwxr-xr-x 1 root root 12605252 May 4 11:32 geth-darwin-386
-rwxr-xr-x 1 root root 14989860 May 4 11:32 geth-darwin-amd64
-rwxr-xr-x 1 root root 17137020 May 4 11:32 geth-linux-386
-rwxr-xr-x 1 root root 20212335 May 4 11:32 geth-linux-amd64
-rwxr-xr-x 1 root root 16475468 May 4 11:32 geth-linux-arm
-rwxr-xr-x 1 root root 16928256 May 4 11:32 geth-windows-386.exe
-rwxr-xr-x 1 root root 19760640 May 4 11:32 geth-windows-amd64.exe
Note, that since xgo needs to cross compile the dependencies for each platform and architecture separately, build time can increase significantly.