xgo - Go CGO cross compiler
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/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 (not insignificant in size):
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 9995000 Nov 24 16:44 iris-android-16-arm -rwxr-xr-x 1 root root 6776500 Nov 24 16:44 iris-darwin-10.6-386 -rwxr-xr-x 1 root root 8755532 Nov 24 16:44 iris-darwin-10.6-amd64 -rwxr-xr-x 1 root root 7114176 Nov 24 16:45 iris-ios-5.0-arm -rwxr-xr-x 1 root root 10135248 Nov 24 16:44 iris-linux-386 -rwxr-xr-x 1 root root 12598472 Nov 24 16:44 iris-linux-amd64 -rwxr-xr-x 1 root root 10040464 Nov 24 16:44 iris-linux-arm -rwxr-xr-x 1 root root 7516368 Nov 24 16:44 iris-windows-4.0-386.exe -rwxr-xr-x 1 root root 9549416 Nov 24 16:44 iris-windows-4.0-amd64.exe
If the path is not a canonical import path, but rather a local path (starts with
. or a dash
/), xgo will use the local GOPATH contents for the cross
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
-x: prints the build commands as compilation progresses
-race: enables data race detection (supported only on amd64, rest built without)
-tags='tag list': list of build tags to consider satisfied during the build
-ldflags='flag list': arguments to pass on each go tool link invocation
-buildmode=mode: binary type to produce by the compiler
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 1.6.1 github.com/project-iris/iris
Additionally, a few wildcard release strings are also supported:
latestwill use the latest Go release (this is the default)
1.6.xwill use the latest point release of a specific Go version
1.6-developwill use the develop branch of a specific Go version
developwill use the develop branch of the entire Go repository
xgo by default uses the name of the package being cross compiled as the output
file prefix. This can be overridden with the
$ xgo -out iris-v0.3.2 github.com/project-iris/iris ... $ ls -al -rwxr-xr-x 1 root root 9995000 Nov 24 16:44 iris-v0.3.2-android-16-arm -rwxr-xr-x 1 root root 6776500 Nov 24 16:44 iris-v0.3.2-darwin-10.6-386 -rwxr-xr-x 1 root root 8755532 Nov 24 16:44 iris-v0.3.2-darwin-10.6-amd64 -rwxr-xr-x 1 root root 7114176 Nov 24 16:45 iris-v0.3.2-ios-5.0-arm -rwxr-xr-x 1 root root 10135248 Nov 24 16:44 iris-v0.3.2-linux-386 -rwxr-xr-x 1 root root 12598472 Nov 24 16:44 iris-v0.3.2-linux-amd64 -rwxr-xr-x 1 root root 10040464 Nov 24 16:44 iris-v0.3.2-linux-arm -rwxr-xr-x 1 root root 7516368 Nov 24 16:44 iris-v0.3.2-windows-4.0-386.exe -rwxr-xr-x 1 root root 9549416 Nov 24 16:44 iris-v0.3.2-windows-4.0-amd64.exe
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
$ xgo --branch release-branch.go1.4 golang.org/x/tools/cmd/goimports ... $ ls -al -rwxr-xr-x 1 root root 4171248 Nov 24 16:40 goimports-android-16-arm -rwxr-xr-x 1 root root 4139868 Nov 24 16:40 goimports-darwin-10.6-386 -rwxr-xr-x 1 root root 5186720 Nov 24 16:40 goimports-darwin-10.6-amd64 -rwxr-xr-x 1 root root 3202364 Nov 24 16:40 goimports-ios-5.0-arm -rwxr-xr-x 1 root root 4189456 Nov 24 16:40 goimports-linux-386 -rwxr-xr-x 1 root root 5264136 Nov 24 16:40 goimports-linux-amd64 -rwxr-xr-x 1 root root 4209416 Nov 24 16:40 goimports-linux-arm -rwxr-xr-x 1 root root 4348416 Nov 24 16:40 goimports-windows-4.0-386.exe -rwxr-xr-x 1 root root 5415424 Nov 24 16:40 goimports-windows-4.0-amd64.exe
Yet again similarly to
go get, xgo uses the repository remote corresponding to
the import path being built. To switch to a different remote while preserving the
original import path, use the
$ xgo --remote github.com/golang/tools golang.org/x/tools/cmd/goimports ...
If you used the above branch or remote selection machanisms, it may happen
that the path you are trying to build is only present in the specific branch and
not the default repository, causing Go to fail at locating it. To circumvent this,
you may specify only the repository root for xgo, and use an additional
parameter to select the exact package within, honoring any prior branch and
$ xgo --pkg cmd/goimports golang.org/x/tools ... $ ls -al -rwxr-xr-x 1 root root 4194956 Nov 24 16:38 goimports-android-16-arm -rwxr-xr-x 1 root root 4164448 Nov 24 16:38 goimports-darwin-10.6-386 -rwxr-xr-x 1 root root 5223584 Nov 24 16:38 goimports-darwin-10.6-amd64 -rwxr-xr-x 1 root root 3222848 Nov 24 16:39 goimports-ios-5.0-arm -rwxr-xr-x 1 root root 4217184 Nov 24 16:38 goimports-linux-386 -rwxr-xr-x 1 root root 5295768 Nov 24 16:38 goimports-linux-amd64 -rwxr-xr-x 1 root root 4233120 Nov 24 16:38 goimports-linux-arm -rwxr-xr-x 1 root root 4373504 Nov 24 16:38 goimports-windows-4.0-386.exe -rwxr-xr-x 1 root root 5450240 Nov 24 16:38 goimports-windows-4.0-amd64.exe
This argument may at some point be integrated into the import path itself, but for now it exists as an independent build parameter. Also, there is not possibility for now to build mulitple commands in one go.
Limit build targets
xgo will try and build the specified package to all platforms and
architectures supported by the underlying Go runtime. If you wish to restrict
the build to only a few target systems, use the comma separated
--targets=linux/arm: builds only the ARMv5 Linux binaries (
--targets=windows/*,darwin/*: builds all Windows and OSX binaries
--targets=*/arm: builds ARM binaries for all platforms
--targets=*/*: builds all suppoted targets (default)
The supported targets are:
xgo tries to cross compile to the lowest possible versions of every
supported platform, in order to produce binaries that are portable among various
versions of the same operating system. This however can lead to issues if a used
dependency is only supported by more recent systems. As such,
xgo supports the
selection of specific platform versions by appending them to the OS target string.
--targets=ios-8.1/*: cross compile to iOS 8.1
--targets=android-16/*: cross compile to Android Jelly Bean
--targets=darwin-10.9/*: cross compile to Mac OS X Mavericks
--targets=windows-6.0/*: cross compile to Windows Vista
The supported platforms are:
- All Android APIs up to Android Lollipop 5.0 (API level ids)
- All Windows APIs up to Windows 8.1 limited by
mingw-w64(API level ids)
- OSX APIs in the range of 10.6 - 10.11
- All iOS APIs up to iOS 9.3
Apart from the usual runnable binaries,
xgo also supports building library
archives for Android (
android/aar) and iOS (
ios/framework). Opposed to
xgo does not derive library APIs from the Go code, so
proper CGO C external methods must be defined within the package.
In the case of Android archives, all architectures will be bundled that are
supported by the requested Android platform version. For iOS frameworks
will bundle armv7 and arm64 by default, and also the x86_64 simulator builds
if the iPhoneSimulator.sdk was injected by the user:
- Create a new docker image based on xgo:
- Inject the simulator SDK:
ADD iPhoneSimulator9.3.sdk.tar.xz /iPhoneSimulator9.3.sdk.tar.xz
- Bootstrap the simulator SDK:
The main differentiator of xgo versus other cross compilers is support for basic embedded C/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/C++ libraries.
It is assumed that the dependent C/C++ library is
configure/make based, was
properly prepared for cross compilation and is available as a tarball download
.tar.bz2). Further plans include extending this to cmake
based projects, if need arises (please open an issue if it's important to you).
Such dependencies can be added via the
--deps argument. They will be retrieved
prior to starting the cross compilation and the packages cached to save bandwidth
on subsequent calls.
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 --deps=https://gmplib.org/download/gmp/gmp-6.1.0.tar.bz2 \ --targets=windows/* github.com/ethereum/go-ethereum/cmd/geth ... $ ls -al -rwxr-xr-x 1 root root 16315679 Nov 24 16:39 geth-windows-4.0-386.exe -rwxr-xr-x 1 root root 19452036 Nov 24 16:38 geth-windows-4.0-amd64.exe
Some trivial arguments may be passed to the dependencies' configure script via
$ xgo --deps=https://gmplib.org/download/gmp/gmp-6.1.0.tar.bz2 \ --targets=ios/* --depsargs=--disable-assembly \ github.com/ethereum/go-ethereum/cmd/geth ... $ ls -al -rwxr-xr-x 1 root root 14804160 Nov 24 16:32 geth-ios-5.0-arm
Note, that since xgo needs to cross compile the dependencies for each platform and architecture separately, build time can increase significantly.