Go cross compilation

Tue, Sep 8, 2015

Note: This article extends Dave Cheney’s Go 1.5 cross compilers post.

Cross compilers got easier with Go 1.5. You don’t have to bootstrap the standard library and toolchain as you used to do prior to 1.5.

If cgo is not required

The go tool won’t require any bootstrapping if cgo is not required. That allows you to target the following program to any GOOS/GOARCH without requiring you to do any additional work. Invoke go build.

$ cat main.go
package main
import "fmt"
func main() {
    fmt.Println("hello world")
}

In order to target android/arm, run the following command.

$ GOOS=android GOARCH=arm GOARM=7 go build .

The produced binary is targeting ARMv7 processors that runs Android. All possible GOOS and GOARCH values are listed on the environment docs.

If cgo is required

If you need to have cgo enabled, the go tool allows you to provide custom C and C++ compilers via CC and CXX environment variables.

$ CGO_ENABLED=1 CC=android-armeabi-gcc CXX=android-armeabi-g++ \
    GOOS=android GOARCH=arm GOARM=7 go build .

The toolchain will invoke android-armeabi-gcc and android-armeabi-g++ if it is required to compile any part of the package with a C or C++ compiler. Consider the following program with a slightly different main function. Rather than outputting “hello world” to the standard I/O, it will use Android system libraries to write “hello world” to the system log.

$ cat main.go
// +build android

package main

// #cgo LDFLAGS: -llog
//
// #include <android/log.h>
//
// void hello() {
//   __android_log_print(
//     ANDROID_LOG_INFO, "MyProgram", "hello world");
// }
import "C"
func main() {
    C.hello()
}

If you build the program with the command above and examine the build with -x, you can observe that cgo is delegating the C compilation to arm-linux-androideabi-gcc.

$ CGO_ENABLED=1 \
CC=arm-linux-androideabi-gcc \
CXX=arm-linux-androideabi-g++ \
GOOS=android GOARCH=arm GOARM=7 go build -x .
...
CGO_LDFLAGS=”-g” “-O2” “-llog” /Users/jbd/go/pkg/tool/darwin_amd64/cgo -objdir $WORK/github.com/rakyll/hello/_obj/ -importpath github.com/rakyll/hello — -I $WORK/github.com/rakyll/hello/_obj/ main.go
arm-linux-androideabi-gcc -I . -fPIC -marm -pthread -fmessage-length=0 -print-libgcc-file-name
arm-linux-androideabi-gcc -I . -fPIC -marm -pthread -fmessage-length=0 -I $WORK/github.com/rakyll/hello/_obj/ -g -O2 -o $WORK/github.com/rakyll/hello/_obj/_cgo_main.o -c $WORK/github.com/rakyll/hello/_obj/_cgo_main.c
...

Pre-building the standard library

The go tool also provides a utility if you would like to pre-build the standard library, targeting a specific GOOS and GOARCH.

$ CGO_ENABLED=1 \
    CC=arm-linux-androideabi-gcc \
    CXX=arm-linux-androideabi-g++ \
    GOOS=android GOARCH=arm GOARM=7 go install std

The standard library targeting android/armv7 will be available at $GOROOT/pkg/android_arm.

$ ls $GOROOT/pkg/android_arm
archive    fmt.a      math       runtime.a
bufio.a    go         math.a     sort.a
bytes.a    hash       mime       strconv.a
compress   hash.a     mime.a     strings.a
container  html       net        sync
crypto     html.a     net.a      sync.a
crypto.a   image      os         syscall.a
database   image.a    os.a       testing
debug      index      path       testing.a
encoding   internal   path.a     text
encoding.a io         reflect.a  time.a
errors.a   io.a       regexp     unicode
expvar.a   log        regexp.a   unicode.a
flag.a     log.a      runtime

If you prefer not to pre-build and install the standard library to the GOROOT, required libraries will be built while building user packages. But, the standard libraries builds are not preserved for future use at this stage and they will be rebuilt each time you run go build.