_README.md

Patch to cross compile to android with net/http support

Installation instructions

We need to patch Go's source code to allow cgo when cross-compiling and make Android-specific changes since it sometimes differs from linux (e.g.: it doesn't use /etc/resolv.conf for DNS config)

1. Clone the go source: hg clone -u release https://code.google.com/p/go
2. Enter the go source directory: cd go
3. Apply the patch in this gist: patch -p1 < /path/to/patch/go_android.patch
4. Enter the go/src directory: cd src
5. Generate binaries for Android cross-compile: GOOS=linux GOARCH=arm ./make.bash
6. Install standard packages for android: GOOS=linux GOARCH=arm CGOENABLED=1 ../bin/go install -tags android -a -v std

Example program for Android and running in the device

Requirements: an android device with remote debugging enabled, adb to build the binary. This doesn't require the device to be rooted.

This example is cross compiling the https://github.com/ernesto-jimenez/emit_urls project.

1. Cross compile the binary: GOARM=5 GOOS=linux GOARCH=arm CGOENABLED=1 ../bin/go build -tags android -o main-arm $GOPATH/src/github.com/ernesto-jimenez/emit_urls/main.go
2. Push binary to the Android device with adb: adb push main-arm /data/local/tmp/
3. Run program in the device: adb shell ./data/local/tmp/main-arm http://rediris.es

Credits

This is based on the following patch from @minux https://codereview.appspot.com/6454055

go_android.patch

diff -r 87dea3f5ebe7 src/cmd/go/build.go
--- a/src/cmd/go/build.go	Fri Nov 29 08:32:31 2013 +1100
+++ b/src/cmd/go/build.go	Thu Dec 19 17:34:24 2013 +0100
@@ -1928,9 +1928,9 @@
 )
 
 func (b *builder) cgo(p *Package, cgoExe, obj string, gccfiles []string, gxxfiles []string) (outGo, outObj []string, err error) {
-	if goos != toolGOOS {
-		return nil, nil, errors.New("cannot use cgo when compiling for a different operating system")
-	}
+	//if goos != toolGOOS {
+		//return nil, nil, errors.New("cannot use cgo when compiling for a different operating system")
+	//}
 
 	cgoCPPFLAGS := stringList(envList("CGO_CPPFLAGS"), p.CgoCPPFLAGS)
 	cgoCFLAGS := stringList(envList("CGO_CFLAGS"), p.CgoCFLAGS)
diff -r 87dea3f5ebe7 src/pkg/net/dnsconfig_android.go
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/pkg/net/dnsconfig_android.go	Thu Dec 19 17:34:24 2013 +0100
@@ -0,0 +1,53 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build android,linux
+
+package net
+
+import (
+	"fmt"
+	"os/exec"
+)
+
+type dnsConfig struct {
+	servers	[]string // servers to use
+	search	 []string // suffixes to append to local name
+	ndots		int			// number of dots in name to trigger absolute lookup
+	timeout	int			// seconds before giving up on packet
+	attempts int			// lost packets before giving up on server
+	rotate	 bool		 // round robin among servers
+}
+
+// Using getprop since Android doesn't have resolv.conf
+func dnsReadConfig() (*dnsConfig, error) {
+	conf := new(dnsConfig)
+	conf.servers = make([]string, 3)[0:0] // small, but the standard limit
+	conf.search = make([]string, 0)
+	conf.ndots = 1
+	conf.timeout = 5
+	conf.attempts = 2
+	conf.rotate = false
+	var ip string
+
+	for i := 1; i <= 4; i++ {
+		out, err := exec.Command("/system/bin/getprop", fmt.Sprintf("net.dns%v", i)).Output()
+		m := len(out)
+		if err != nil {
+			continue
+		}
+		if m < 2 {
+			continue
+		}
+		ip = string(out[:m-1])
+
+		a := conf.servers
+		n := len(a)
+		a = a[0 : n+1]
+		a[n] = ip
+		conf.servers = a
+	}
+
+	return conf, nil
+}
diff -r 87dea3f5ebe7 src/pkg/net/dnsconfig_unix.go
--- a/src/pkg/net/dnsconfig_unix.go	Fri Nov 29 08:32:31 2013 +1100
+++ b/src/pkg/net/dnsconfig_unix.go	Thu Dec 19 17:34:24 2013 +0100
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin dragonfly freebsd linux netbsd openbsd
+// +build darwin dragonfly freebsd !android,linux netbsd openbsd
 
 // Read system DNS config from /etc/resolv.conf
 
diff -r 87dea3f5ebe7 src/pkg/os/file_android.go
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/pkg/os/file_android.go	Thu Dec 19 17:34:24 2013 +0100
@@ -0,0 +1,17 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build android,linux
+
+package os
+
+// TempDir returns the default directory to use for temporary files.
+func TempDir() string {
+	dir := Getenv("TMPDIR")
+	if dir == "" {
+		dir = "/data/local/tmp"
+	}
+	return dir
+}
+
diff -r 87dea3f5ebe7 src/pkg/os/file_non_android.go
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/pkg/os/file_non_android.go	Thu Dec 19 17:34:24 2013 +0100
@@ -0,0 +1,17 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build darwin freebsd !android,linux netbsd openbsd
+
+package os
+
+// TempDir returns the default directory to use for temporary files.
+func TempDir() string {
+	dir := Getenv("TMPDIR")
+	if dir == "" {
+		dir = "/tmp"
+	}
+	return dir
+}
+
diff -r 87dea3f5ebe7 src/pkg/os/file_unix.go
--- a/src/pkg/os/file_unix.go	Fri Nov 29 08:32:31 2013 +1100
+++ b/src/pkg/os/file_unix.go	Thu Dec 19 17:34:24 2013 +0100
@@ -279,11 +279,3 @@
 	return name
 }
 
-// TempDir returns the default directory to use for temporary files.
-func TempDir() string {
-	dir := Getenv("TMPDIR")
-	if dir == "" {
-		dir = "/tmp"
-	}
-	return dir
-}