diff --git a/main.go b/main.go
index 5046f39..bb2f60f 100644
--- a/main.go
+++ b/main.go
@@ -5,17 +5,18 @@ import (
 	"compress/gzip"
 	"crypto/sha256"
 	"encoding/json"
+	"flag"
 	"fmt"
-	"github.com/kr/binarydist"
 	"io"
 	"io/ioutil"
 	"os"
 	"path/filepath"
-	//"runtime"
-	//"encoding/base64"
+	"runtime"
+
+	"github.com/kr/binarydist"
 )
 
-var plat, appPath, version, genDir string
+var version, genDir string
 
 type current struct {
 	Version string
@@ -124,34 +125,41 @@ func createUpdate(path string, platform string) {
 }
 
 func printUsage() {
-	fmt.Println("Go-Selfupdate - Enable your Golang applications to self update.\n\n")
-	fmt.Println("Usage:\n")
+	fmt.Println("")
+	fmt.Println("Positional arguments:")
 	fmt.Println("\tSingle platform: go-selfupdate myapp 1.2")
 	fmt.Println("\tCross platform: go-selfupdate /tmp/mybinares/ 1.2")
 }
 
-func isArgsPresent() bool {
-	if len(os.Args) < 2 {
-		return false
-	}
-
-	return true
-}
-
 func createBuildDir() {
 	os.MkdirAll(genDir, 0755)
 }
 
 func main() {
-	if isArgsPresent() == false {
+	outputDirFlag := flag.String("o", "public", "Output directory for writing updates")
+
+	var defaultPlatform string
+	goos := os.Getenv("GOOS")
+	goarch := os.Getenv("GOARCH")
+	if goos != "" && goarch != "" {
+		defaultPlatform = goos + "-" + goarch
+	} else {
+		defaultPlatform = runtime.GOOS + "-" + runtime.GOARCH
+	}
+	platformFlag := flag.String("platform", defaultPlatform,
+		"Target platform in the form OS-ARCH. Defaults to running os/arch or the combination of the environment variables GOOS and GOARCH if both are set.")
+
+	flag.Parse()
+	if flag.NArg() < 2 {
+		flag.Usage()
 		printUsage()
 		os.Exit(0)
 	}
 
-	plat = os.Getenv("GOOS") + "-" + os.Getenv("GOARCH")
-	appPath = os.Args[1]
-	version = os.Args[2]
-	genDir = "public"
+	platform := *platformFlag
+	appPath := flag.Arg(0)
+	version = flag.Arg(1)
+	genDir = *outputDirFlag
 
 	createBuildDir()
 
@@ -164,5 +172,5 @@ func main() {
 		os.Exit(0)
 	}
 
-	createUpdate(appPath, plat)
+	createUpdate(appPath, platform)
 }
diff --git a/selfupdate/selfupdate.go b/selfupdate/selfupdate.go
index f6e4dde..23a8321 100644
--- a/selfupdate/selfupdate.go
+++ b/selfupdate/selfupdate.go
@@ -1,37 +1,4 @@
-package selfupdate
-
-import (
-	"bitbucket.org/kardianos/osext"
-	"bytes"
-	"compress/gzip"
-	"crypto/sha256"
-	"encoding/json"
-	"errors"
-	"fmt"
-	"github.com/inconshreveable/go-update"
-	"github.com/kr/binarydist"
-	"io"
-	"io/ioutil"
-	"log"
-	"math/rand"
-	"net/http"
-	"os"
-	"path/filepath"
-	"runtime"
-	"time"
-)
-
-const (
-	upcktimePath = "cktime"
-	plat         = runtime.GOOS + "-" + runtime.GOARCH
-)
-
-const devValidTime = 7 * 24 * time.Hour
-
-var ErrHashMismatch = errors.New("new file hash mismatch after patch")
-var up = update.New()
-
-// Update protocol.
+// Update protocol:
 //
 //   GET hk.heroku.com/hk/linux-amd64.json
 //
@@ -54,13 +21,66 @@ var up = update.New()
 //
 //   200 ok
 //   [gzipped executable data]
+//
+//
+package selfupdate
+
+import (
+	"bytes"
+	"compress/gzip"
+	"crypto/sha256"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"log"
+	"math/rand"
+	"net/http"
+	"os"
+	"path/filepath"
+	"runtime"
+	"time"
+
+	"bitbucket.org/kardianos/osext"
+	"github.com/inconshreveable/go-update"
+	"github.com/kr/binarydist"
+)
+
+const (
+	upcktimePath = "cktime"
+	plat         = runtime.GOOS + "-" + runtime.GOARCH
+)
+
+const devValidTime = 7 * 24 * time.Hour
+
+var ErrHashMismatch = errors.New("new file hash mismatch after patch")
+var up = update.New()
+
+// Updater is the configuration and runtime data for doing an update.
+//
+// Note that ApiURL, BinURL and DiffURL should have the same value if all files are available at the same location.
+//
+// Example:
+//
+//  updater := &selfupdate.Updater{
+//  	CurrentVersion: version,
+//  	ApiURL:         "http://updates.yourdomain.com/",
+//  	BinURL:         "http://updates.yourdownmain.com/",
+//  	DiffURL:        "http://updates.yourdomain.com/",
+//  	Dir:            "update/",
+//  	CmdName:        "myapp", // app name
+//  }
+//  if updater != nil {
+//  	go updater.BackgroundRun()
+//  }
 type Updater struct {
-	CurrentVersion string
-	ApiURL         string
-	CmdName        string
-	BinURL         string
-	DiffURL        string
-	Dir            string
+	CurrentVersion string // Currently running version.
+	ApiURL         string // Base URL for API requests (json files).
+	CmdName        string // Command name is appended to the ApiURL like http://apiurl/CmdName/. This represents one binary.
+	BinURL         string // Base URL for full binary downloads.
+	DiffURL        string // Base URL for diff downloads.
+	Dir            string // Directory to store selfupdate state.
 	Info           struct {
 		Version string
 		Sha256  []byte
@@ -74,6 +94,7 @@ func (u *Updater) getExecRelativeDir(dir string) string {
 	return path
 }
 
+// BackgroundRun starts the update check and apply cycle.
 func (u *Updater) BackgroundRun() {
 	os.MkdirAll(u.getExecRelativeDir(u.Dir), 0777)
 	if u.wantUpdate() {
diff --git a/selfupdate/selfupdate_test.go b/selfupdate/selfupdate_test.go
index a679db3..84bac00 100644
--- a/selfupdate/selfupdate_test.go
+++ b/selfupdate/selfupdate_test.go
@@ -1,8 +1,7 @@
 package selfupdate
 
-import (
-	"testing"
-)
+import "testing"
+
+func TestUpdater(t *testing.T) {
 
-func TestUpdater() {
 }