253 lines
12 KiB
Markdown
253 lines
12 KiB
Markdown
# xgo - Go CGO cross compiler
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Although Go strives to be a cross platform language, cross compilation from one
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platform to another is not as simple as it could be, as you need the Go sources
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bootstrapped to each platform and architecture.
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The first step towards cross compiling was Dave Cheney's [golang-crosscompile](https://github.com/davecheney/golang-crosscompile)
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package, which automatically bootstrapped the necessary sources based on your
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existing Go installation. Although this was enough for a lot of cases, certain
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drawbacks became apparent where the official libraries used CGO internally: any
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dependency to third party platform code is unavailable, hence those parts don't
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cross compile nicely (native DNS resolution, system certificate access, etc).
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A step forward in enabling cross compilation was Alan Shreve's [gonative](https://github.com/inconshreveable/gonative)
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package, which instead of bootstrapping the different platforms based on the
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existing Go installation, downloaded the official pre-compiled binaries from the
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golang website and injected those into the local toolchain. Since the pre-built
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binaries already contained the necessary platform specific code, the few missing
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dependencies were resolved, and true cross compilation could commence... of pure
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Go code.
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However, there was still one feature missing: cross compiling Go code that used
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CGO itself, which isn't trivial since you need access to OS specific headers and
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libraries. This becomes very annoying when you need access only to some trivial
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OS specific functionality (e.g. query the CPU load), but need to configure and
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maintain separate build environments to do it.
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## Enter xgo
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My solution to the challenge of cross compiling Go code with embedded C/C++ snippets
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(i.e. CGO_ENABLED=1) is based on the concept of [lightweight Linux containers](http://en.wikipedia.org/wiki/LXC).
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All the necessary Go tool-chains, C cross compilers and platform headers/libraries
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have been assembled into a single Docker container, which can then be called as if
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a single command to compile a Go package to various platforms and architectures.
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## Installation
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Although you could build the container manually, it is available as an automatic
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trusted build from Docker's container registry (not insignificant in size):
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docker pull techknowlogick/xgo:latest
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To prevent having to remember a potentially complex Docker command every time,
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a lightweight Go wrapper was written on top of it.
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go get src.techknowlogick.com/xgo
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## Usage
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Simply specify the import path you want to build, and xgo will do the rest:
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$ xgo github.com/project-iris/iris
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...
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$ ls -al
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-rwxr-xr-x 1 root root 9995000 Nov 24 16:44 iris-android-16-arm
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-rwxr-xr-x 1 root root 6776500 Nov 24 16:44 iris-darwin-10.6-386
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-rwxr-xr-x 1 root root 8755532 Nov 24 16:44 iris-darwin-10.6-amd64
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-rwxr-xr-x 1 root root 7114176 Nov 24 16:45 iris-ios-5.0-arm
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-rwxr-xr-x 1 root root 10135248 Nov 24 16:44 iris-linux-386
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-rwxr-xr-x 1 root root 12598472 Nov 24 16:44 iris-linux-amd64
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-rwxr-xr-x 1 root root 10040464 Nov 24 16:44 iris-linux-arm
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-rwxr-xr-x 1 root root 7516368 Nov 24 16:44 iris-windows-4.0-386.exe
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-rwxr-xr-x 1 root root 9549416 Nov 24 16:44 iris-windows-4.0-amd64.exe
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If the path is not a canonical import path, but rather a local path (starts with
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a dot `.` or a dash `/`), xgo will use the local GOPATH contents for the cross
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compilation.
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### Build flags
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A handful of flags can be passed to `go build`. The currently supported ones are
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- `-v`: prints the names of packages as they are compiled
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- `-x`: prints the build commands as compilation progresses
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- `-race`: enables data race detection (supported only on amd64, rest built without)
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- `-tags='tag list'`: list of build tags to consider satisfied during the build
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- `-ldflags='flag list'`: arguments to pass on each go tool link invocation
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- `-buildmode=mode`: binary type to produce by the compiler
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### Go releases
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As newer versions of the language runtime, libraries and tools get released,
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these will get incorporated into xgo too as extensions layers to the base cross
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compilation image (only Go 1.3 and above will be supported).
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You can select which Go release to work with through the `-go` command line flag
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to xgo and if the specific release was already integrated, it will automatically
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be retrieved and installed.
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$ xgo -go 1.6.1 github.com/project-iris/iris
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Additionally, a few wildcard release strings are also supported:
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- `latest` will use the latest Go release (this is the default)
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- `1.6.x` will use the latest point release of a specific Go version
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- `1.6-develop` will use the develop branch of a specific Go version
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- `develop` will use the develop branch of the entire Go repository
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### Output prefixing
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xgo by default uses the name of the package being cross compiled as the output
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file prefix. This can be overridden with the `-out` flag.
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$ xgo -out iris-v0.3.2 github.com/project-iris/iris
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...
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$ ls -al
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-rwxr-xr-x 1 root root 9995000 Nov 24 16:44 iris-v0.3.2-android-16-arm
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-rwxr-xr-x 1 root root 6776500 Nov 24 16:44 iris-v0.3.2-darwin-10.6-386
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-rwxr-xr-x 1 root root 8755532 Nov 24 16:44 iris-v0.3.2-darwin-10.6-amd64
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-rwxr-xr-x 1 root root 7114176 Nov 24 16:45 iris-v0.3.2-ios-5.0-arm
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-rwxr-xr-x 1 root root 10135248 Nov 24 16:44 iris-v0.3.2-linux-386
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-rwxr-xr-x 1 root root 12598472 Nov 24 16:44 iris-v0.3.2-linux-amd64
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-rwxr-xr-x 1 root root 10040464 Nov 24 16:44 iris-v0.3.2-linux-arm
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-rwxr-xr-x 1 root root 7516368 Nov 24 16:44 iris-v0.3.2-windows-4.0-386.exe
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-rwxr-xr-x 1 root root 9549416 Nov 24 16:44 iris-v0.3.2-windows-4.0-amd64.exe
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### Branch selection
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Similarly to `go get`, xgo also uses the `master` branch of a repository during
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source code retrieval. To switch to a different branch before compilation pass
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the desired branch name through the `--branch` argument.
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$ xgo --branch release-branch.go1.4 golang.org/x/tools/cmd/goimports
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...
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$ ls -al
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-rwxr-xr-x 1 root root 4171248 Nov 24 16:40 goimports-android-16-arm
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-rwxr-xr-x 1 root root 4139868 Nov 24 16:40 goimports-darwin-10.6-386
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-rwxr-xr-x 1 root root 5186720 Nov 24 16:40 goimports-darwin-10.6-amd64
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-rwxr-xr-x 1 root root 3202364 Nov 24 16:40 goimports-ios-5.0-arm
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-rwxr-xr-x 1 root root 4189456 Nov 24 16:40 goimports-linux-386
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-rwxr-xr-x 1 root root 5264136 Nov 24 16:40 goimports-linux-amd64
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-rwxr-xr-x 1 root root 4209416 Nov 24 16:40 goimports-linux-arm
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-rwxr-xr-x 1 root root 4348416 Nov 24 16:40 goimports-windows-4.0-386.exe
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-rwxr-xr-x 1 root root 5415424 Nov 24 16:40 goimports-windows-4.0-amd64.exe
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### Remote selection
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Yet again similarly to `go get`, xgo uses the repository remote corresponding to
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the import path being built. To switch to a different remote while preserving the
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original import path, use the `--remote` argument.
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$ xgo --remote github.com/golang/tools golang.org/x/tools/cmd/goimports
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...
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### Package selection
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If you used the above *branch* or *remote* selection machanisms, it may happen
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that the path you are trying to build is only present in the specific branch and
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not the default repository, causing Go to fail at locating it. To circumvent this,
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you may specify only the repository root for xgo, and use an additional `--pkg`
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parameter to select the exact package within, honoring any prior *branch* and
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*remote* selections.
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$ xgo --pkg cmd/goimports golang.org/x/tools
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...
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$ ls -al
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-rwxr-xr-x 1 root root 4194956 Nov 24 16:38 goimports-android-16-arm
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-rwxr-xr-x 1 root root 4164448 Nov 24 16:38 goimports-darwin-10.6-386
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-rwxr-xr-x 1 root root 5223584 Nov 24 16:38 goimports-darwin-10.6-amd64
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-rwxr-xr-x 1 root root 3222848 Nov 24 16:39 goimports-ios-5.0-arm
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-rwxr-xr-x 1 root root 4217184 Nov 24 16:38 goimports-linux-386
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-rwxr-xr-x 1 root root 5295768 Nov 24 16:38 goimports-linux-amd64
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-rwxr-xr-x 1 root root 4233120 Nov 24 16:38 goimports-linux-arm
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-rwxr-xr-x 1 root root 4373504 Nov 24 16:38 goimports-windows-4.0-386.exe
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-rwxr-xr-x 1 root root 5450240 Nov 24 16:38 goimports-windows-4.0-amd64.exe
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This argument may at some point be integrated into the import path itself, but for
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now it exists as an independent build parameter. Also, there is not possibility
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for now to build mulitple commands in one go.
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### Limit build targets
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By default `xgo` will try and build the specified package to all platforms and
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architectures supported by the underlying Go runtime. If you wish to restrict
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the build to only a few target systems, use the comma separated `--targets` CLI
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argument:
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* `--targets=linux/arm`: builds only the ARMv5 Linux binaries (`arm-6`/`arm-7` allowed)
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* `--targets=windows/*,darwin/*`: builds all Windows and OSX binaries
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* `--targets=*/arm`: builds ARM binaries for all platforms
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* `--targets=*/*`: builds all suppoted targets (default)
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The supported targets are:
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* Platforms: `android`, `darwin`, `ios`, `linux`, `windows`
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* Achitectures: `386`, `amd64`, `arm-5`, `arm-6`, `arm-7`, `arm64`, `mips`, `mipsle`, `mips64`, `mips64le`
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### Platform versions
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By default `xgo` tries to cross compile to the lowest possible versions of every
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supported platform, in order to produce binaries that are portable among various
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versions of the same operating system. This however can lead to issues if a used
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dependency is only supported by more recent systems. As such, `xgo` supports the
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selection of specific platform versions by appending them to the OS target string.
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* `--targets=darwin-10.9/*`: cross compile to Mac OS X Mavericks
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* `--targets=windows-6.0/*`: cross compile to Windows Vista
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The supported platforms are:
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* All Android APIs up to Android Lollipop 5.0 ([API level ids](https://source.android.com/source/build-numbers.html))
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* All Windows APIs up to Windows 8.1 limited by `mingw-w64` ([API level ids](https://en.wikipedia.org/wiki/Windows_NT#Releases))
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* OSX APIs in the range of 10.6 - 10.14
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### CGO dependencies
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The main differentiator of xgo versus other cross compilers is support for basic
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embedded C/C++ code and target-platform specific OS SDK availability. The current
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xgo release introduces an experimental CGO *dependency* cross compilation, enabling
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building Go programs that require external C/C++ libraries.
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It is assumed that the dependent C/C++ library is `configure/make` based, was
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properly prepared for cross compilation and is available as a tarball download
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(`.tar`, `.tar.gz` or `.tar.bz2`). Further plans include extending this to cmake
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based projects, if need arises (please open an issue if it's important to you).
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Such dependencies can be added via the `--deps` argument. They will be retrieved
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prior to starting the cross compilation and the packages cached to save bandwidth
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on subsequent calls.
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A complex sample for such a scenario is building the Ethereum CLI node, which has
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the GNU Multiple Precision Arithmetic Library as it's dependency.
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$ xgo --deps=https://gmplib.org/download/gmp/gmp-6.1.0.tar.bz2 \
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--targets=windows/* github.com/ethereum/go-ethereum/cmd/geth
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...
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$ ls -al
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-rwxr-xr-x 1 root root 16315679 Nov 24 16:39 geth-windows-4.0-386.exe
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-rwxr-xr-x 1 root root 19452036 Nov 24 16:38 geth-windows-4.0-amd64.exe
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Some trivial arguments may be passed to the dependencies' configure script via
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`--depsargs`.
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$ xgo --deps=https://gmplib.org/download/gmp/gmp-6.1.0.tar.bz2 \
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--targets=ios/* --depsargs=--disable-assembly \
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github.com/ethereum/go-ethereum/cmd/geth
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...
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$ ls -al
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-rwxr-xr-x 1 root root 14804160 Nov 24 16:32 geth-ios-5.0-arm
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Note, that since xgo needs to cross compile the dependencies for each platform
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and architecture separately, build time can increase significantly.
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