github.com/golangci/go-tools@v0.0.0-20190318060251-af6baa5dc196/cmd/structlayout/main.go (about)

     1  // structlayout displays the layout (field sizes and padding) of structs.
     2  package main
     3  
     4  import (
     5  	"encoding/json"
     6  	"flag"
     7  	"fmt"
     8  	"go/build"
     9  	"go/types"
    10  	"log"
    11  	"os"
    12  
    13  	"github.com/golangci/go-tools/gcsizes"
    14  	st "github.com/golangci/go-tools/structlayout"
    15  	"github.com/golangci/go-tools/version"
    16  
    17  	"golang.org/x/tools/go/loader"
    18  )
    19  
    20  var (
    21  	fJSON    bool
    22  	fVersion bool
    23  )
    24  
    25  func init() {
    26  	flag.BoolVar(&fJSON, "json", false, "Format data as JSON")
    27  	flag.BoolVar(&fVersion, "version", false, "Print version and exit")
    28  }
    29  
    30  func main() {
    31  	log.SetFlags(0)
    32  	flag.Parse()
    33  
    34  	if fVersion {
    35  		version.Print()
    36  		os.Exit(0)
    37  	}
    38  
    39  	if len(flag.Args()) != 2 {
    40  		flag.Usage()
    41  		os.Exit(1)
    42  	}
    43  
    44  	conf := loader.Config{
    45  		Build: &build.Default,
    46  	}
    47  
    48  	var pkg string
    49  	var typName string
    50  	pkg = flag.Args()[0]
    51  	typName = flag.Args()[1]
    52  	conf.Import(pkg)
    53  
    54  	lprog, err := conf.Load()
    55  	if err != nil {
    56  		log.Fatal(err)
    57  	}
    58  	var typ types.Type
    59  	obj := lprog.Package(pkg).Pkg.Scope().Lookup(typName)
    60  	if obj == nil {
    61  		log.Fatal("couldn't find type")
    62  	}
    63  	typ = obj.Type()
    64  
    65  	st, ok := typ.Underlying().(*types.Struct)
    66  	if !ok {
    67  		log.Fatal("identifier is not a struct type")
    68  	}
    69  
    70  	fields := sizes(st, typ.(*types.Named).Obj().Name(), 0, nil)
    71  	if fJSON {
    72  		emitJSON(fields)
    73  	} else {
    74  		emitText(fields)
    75  	}
    76  }
    77  
    78  func emitJSON(fields []st.Field) {
    79  	if fields == nil {
    80  		fields = []st.Field{}
    81  	}
    82  	json.NewEncoder(os.Stdout).Encode(fields)
    83  }
    84  
    85  func emitText(fields []st.Field) {
    86  	for _, field := range fields {
    87  		fmt.Println(field)
    88  	}
    89  }
    90  func sizes(typ *types.Struct, prefix string, base int64, out []st.Field) []st.Field {
    91  	s := gcsizes.ForArch(build.Default.GOARCH)
    92  	n := typ.NumFields()
    93  	var fields []*types.Var
    94  	for i := 0; i < n; i++ {
    95  		fields = append(fields, typ.Field(i))
    96  	}
    97  	offsets := s.Offsetsof(fields)
    98  	for i := range offsets {
    99  		offsets[i] += base
   100  	}
   101  
   102  	pos := base
   103  	for i, field := range fields {
   104  		if offsets[i] > pos {
   105  			padding := offsets[i] - pos
   106  			out = append(out, st.Field{
   107  				IsPadding: true,
   108  				Start:     pos,
   109  				End:       pos + padding,
   110  				Size:      padding,
   111  			})
   112  			pos += padding
   113  		}
   114  		size := s.Sizeof(field.Type())
   115  		if typ2, ok := field.Type().Underlying().(*types.Struct); ok && typ2.NumFields() != 0 {
   116  			out = sizes(typ2, prefix+"."+field.Name(), pos, out)
   117  		} else {
   118  			out = append(out, st.Field{
   119  				Name:  prefix + "." + field.Name(),
   120  				Type:  field.Type().String(),
   121  				Start: offsets[i],
   122  				End:   offsets[i] + size,
   123  				Size:  size,
   124  				Align: s.Alignof(field.Type()),
   125  			})
   126  		}
   127  		pos += size
   128  	}
   129  
   130  	if len(out) == 0 {
   131  		return out
   132  	}
   133  	field := &out[len(out)-1]
   134  	if field.Size == 0 {
   135  		field.Size = 1
   136  		field.End++
   137  	}
   138  	pad := s.Sizeof(typ) - field.End
   139  	if pad > 0 {
   140  		out = append(out, st.Field{
   141  			IsPadding: true,
   142  			Start:     field.End,
   143  			End:       field.End + pad,
   144  			Size:      pad,
   145  		})
   146  	}
   147  
   148  	return out
   149  }