# 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](https://github.com/davecheney/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](https://github.com/inconshreveable/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.

## Enter xgo

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](http://en.wikipedia.org/wiki/LXC).
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.

## Installation

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

## Usage

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  10899488 Sep 14 18:05 iris-android-21-arm
    -rwxr-xr-x  1 root  root   6442188 Sep 14 18:05 iris-darwin-386
    -rwxr-xr-x  1 root  root   8228756 Sep 14 18:05 iris-darwin-amd64
    -rwxr-xr-x  1 root  root   9532568 Sep 14 18:05 iris-linux-386
    -rwxr-xr-x  1 root  root  11776368 Sep 14 18:05 iris-linux-amd64
    -rwxr-xr-x  1 root  root   9408928 Sep 14 18:05 iris-linux-arm
    -rwxr-xr-x  1 root  root   7131477 Sep 14 18:05 iris-windows-386.exe
    -rwxr-xr-x  1 root  root   8963900 Sep 14 18:05 iris-windows-amd64.exe

If the path is not a canonical import path, but rather a local path (starts with
a dot `.` or a dash `/`), xgo will use the local GOPATH contents for the cross
compilation.


### Build flags

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)


### Go releases

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 1.5.1 github.com/project-iris/iris

Additionally, a few wildcard release strings are also supported:

  - `latest` will use the latest Go release
  - `1.5.x` will use the latest point release of a specific Go version

### Output prefixing

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  10899488 Sep 14 18:08 iris-v0.3.2-android-21-arm
    -rwxr-xr-x  1 root  root   6442188 Sep 14 18:08 iris-v0.3.2-darwin-386
    -rwxr-xr-x  1 root  root   8228756 Sep 14 18:08 iris-v0.3.2-darwin-amd64
    -rwxr-xr-x  1 root  root   9532568 Sep 14 18:08 iris-v0.3.2-linux-386
    -rwxr-xr-x  1 root  root  11776368 Sep 14 18:08 iris-v0.3.2-linux-amd64
    -rwxr-xr-x  1 root  root   9408928 Sep 14 18:08 iris-v0.3.2-linux-arm
    -rwxr-xr-x  1 root  root   7131477 Sep 14 18:08 iris-v0.3.2-windows-386.exe
    -rwxr-xr-x  1 root  root   8963900 Sep 14 18:08 iris-v0.3.2-windows-amd64.exe

### Branch selection

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 --branch release-branch.go1.4 golang.org/x/tools/cmd/goimports
    ...

    $ ls -al
    -rwxr-xr-x  1 root  root   4928992 Sep 14 18:10 goimports-android-21-arm
    -rwxr-xr-x  1 root  root   4139868 Sep 14 18:10 goimports-darwin-386
    -rwxr-xr-x  1 root  root   5186720 Sep 14 18:10 goimports-darwin-amd64
    -rwxr-xr-x  1 root  root   4189448 Sep 14 18:10 goimports-linux-386
    -rwxr-xr-x  1 root  root   5264120 Sep 14 18:10 goimports-linux-amd64
    -rwxr-xr-x  1 root  root   4209400 Sep 14 18:10 goimports-linux-arm
    -rwxr-xr-x  1 root  root   4348416 Sep 14 18:10 goimports-windows-386.exe
    -rwxr-xr-x  1 root  root   5415424 Sep 14 18:10 goimports-windows-amd64.exe

### Remote selection

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 `--remote` argument.

    $ xgo --remote github.com/golang/tools golang.org/x/tools/cmd/goimports
    ...

### Package selection

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 respoitory, causing Go to fail at locating it. To circumvent this,
you may specify only the repository root for xgo, and use an additional `--pkg`
parameter to select the exact package within, honoring any prior *branch* and
*remote* selections.

    $ xgo --pkg cmd/goimports golang.org/x/tools
    ...

    $ ls -al
    -rwxr-xr-x  1 root  root   4924036 Sep 14 18:09 goimports-android-21-arm
    -rwxr-xr-x  1 root  root   4135776 Sep 14 18:09 goimports-darwin-386
    -rwxr-xr-x  1 root  root   5182624 Sep 14 18:09 goimports-darwin-amd64
    -rwxr-xr-x  1 root  root   4184416 Sep 14 18:09 goimports-linux-386
    -rwxr-xr-x  1 root  root   5254800 Sep 14 18:09 goimports-linux-amd64
    -rwxr-xr-x  1 root  root   4204440 Sep 14 18:09 goimports-linux-arm
    -rwxr-xr-x  1 root  root   4343296 Sep 14 18:09 goimports-windows-386.exe
    -rwxr-xr-x  1 root  root   5409280 Sep 14 18:09 goimports-windows-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

By default `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` CLI
argument:

  * `--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 Android platform is handled a bit differently currently due to the multitude
of available platform versions (23 as of writing, some obsolted). As it is mostly
pointless to build for all possible versions, `xgo` by default builds only against
the latest release, controllable via a numerical argument after the platform:

  * `--targets=android-16/*`: build all supported architectures for Jelly Bean
  * `--targets=android-16/arm,android-21/arm`: build for Jelly Bean and Lollipop

Note, `xgo` honors the Android's position independent executables (PIE) security
requirement, builing all binaries equal and above to Jelly Bean with PIE enabled.

    $ readelf -h iris-android-15-arm | grep Type
      Type:                              EXEC (Executable file)
    $ readelf -h iris-android-21-arm | grep Type
      Type:                              DYN (Shared object file)


### CGO dependencies

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`, `.tar.gz` or `.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-6.0.0a.tar.bz2 \
        --branch=develop github.com/ethereum/go-ethereum/cmd/geth
    ...

    $ ls -al
    -rwxr-xr-x  1 root  root  23213372 Sep 14 17:59 geth-android-21-arm
    -rwxr-xr-x  1 root  root  14373980 Sep 14 17:59 geth-darwin-386
    -rwxr-xr-x  1 root  root  17373676 Sep 14 17:59 geth-darwin-amd64
    -rwxr-xr-x  1 root  root  21098910 Sep 14 17:59 geth-linux-386
    -rwxr-xr-x  1 root  root  25049693 Sep 14 17:59 geth-linux-amd64
    -rwxr-xr-x  1 root  root  20578535 Sep 14 17:59 geth-linux-arm
    -rwxr-xr-x  1 root  root  16351260 Sep 14 17:59 geth-windows-386.exe
    -rwxr-xr-x  1 root  root  19418071 Sep 14 17:59 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.