structlayout: usage hinting

cmd/structlayout/main.go

@@ -1,4 +1,6 @@
-// structlayout displays the layout (field sizes and padding) of structs.
+// Structlayout prints the memory layout of struct types. Structs may contain a
+// combination of fields and padding for memory alignment. Go follows the field
+// order as given in the respective code.
 package main
 
 import (
@@ -18,63 +20,62 @@ import (
 )
 
 var (
-	fJSON    bool
-	fVersion bool
+	jsonFlag    = flag.Bool("json", false, "Format data as JSON")
+	versionFlag = flag.Bool("version", false, "Print version and exit")
 )
 
-func init() {
-	flag.BoolVar(&fJSON, "json", false, "Format data as JSON")
-	flag.BoolVar(&fVersion, "version", false, "Print version and exit")
-}
-
 func main() {
 	log.SetFlags(0)
 	flag.Parse()
 
-	if fVersion {
+	if *versionFlag {
 		version.Print(version.Version, version.MachineVersion)
 		os.Exit(0)
 	}
 
 	if len(flag.Args()) != 2 {
-		flag.Usage()
+		fmt.Fprintln(flag.CommandLine.Output(), "Wrong number of arguments. Need a package and a type.")
+		fmt.Fprintln(flag.CommandLine.Output(), "OPTIONS:")
+		flag.PrintDefaults()
 		os.Exit(1)
 	}
+	matchPkg := flag.Arg(0)
+	matchType := flag.Arg(1)
 
+	// find matching packages
 	cfg := &packages.Config{
 		Mode:  packages.NeedImports | packages.NeedExportFile | packages.NeedTypes | packages.NeedSyntax,
 		Tests: true,
 	}
-	pkgs, err := packages.Load(cfg, flag.Args()[0])
+	pkgs, err := packages.Load(cfg, matchPkg)
 	if err != nil {
 		log.Fatal(err)
 	}
 
+	// find matching types
 	for _, pkg := range pkgs {
-		typName := flag.Args()[1]
-
 		var typ types.Type
-		obj := pkg.Types.Scope().Lookup(typName)
+		obj := pkg.Types.Scope().Lookup(matchType)
 		if obj == nil {
 			continue
 		}
 		typ = obj.Type()
 
 		st, ok := typ.Underlying().(*types.Struct)
 		if !ok {
-			log.Fatal("identifier is not a struct type")
+			log.Fatalf("type %s is not a struct", typ)
 		}
 
 		fields := sizes(st, types.Unalias(typ).(*types.Named).Obj().Name(), 0, nil)
-		if fJSON {
+		if *jsonFlag {
 			emitJSON(fields)
 		} else {
 			emitText(fields)
 		}
 		return
 	}
 
-	log.Fatal("couldn't find type")
+	log.Fatalf("type %s.%s not found", matchPkg, matchType)
 }
 
 func emitJSON(fields []st.Field) {
@@ -89,6 +90,7 @@ func emitText(fields []st.Field) {
 		fmt.Println(field)
 	}
 }
+
 func sizes(typ *types.Struct, prefix string, base int64, out []st.Field) []st.Field {
 	s := gcsizes.ForArch(build.Default.GOARCH)
 	n := typ.NumFields()