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// Copyright 2016 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.
// Package gcexportdata provides functions for locating, reading, and
// writing export data files containing type information produced by the
// gc compiler. This package supports go1.7 export data format and all
// later versions.
// Although it might seem convenient for this package to live alongside
// go/types in the standard library, this would cause version skew
// problems for developer tools that use it, since they must be able to
// consume the outputs of the gc compiler both before and after a Go
// update such as from Go 1.7 to Go 1.8. Because this package lives in
//, sites can update their version of this repo some
// time before the Go 1.8 release and rebuild and redeploy their
// developer tools, which will then be able to consume both Go 1.7 and
// Go 1.8 export data files, so they will work before and after the
// Go update. (See discussion at
package gcexportdata // import ""
import (
// Find returns the name of an object (.o) or archive (.a) file
// containing type information for the specified import path,
// using the workspace layout conventions of go/build.
// If no file was found, an empty filename is returned.
// A relative srcDir is interpreted relative to the current working directory.
// Find also returns the package's resolved (canonical) import path,
// reflecting the effects of srcDir and vendoring on importPath.
func Find(importPath, srcDir string) (filename, path string) {
return gcimporter.FindPkg(importPath, srcDir)
// NewReader returns a reader for the export data section of an object
// (.o) or archive (.a) file read from r. The new reader may provide
// additional trailing data beyond the end of the export data.
func NewReader(r io.Reader) (io.Reader, error) {
buf := bufio.NewReader(r)
_, err := gcimporter.FindExportData(buf)
// If we ever switch to a zip-like archive format with the ToC
// at the end, we can return the correct portion of export data,
// but for now we must return the entire rest of the file.
return buf, err
// Read reads export data from in, decodes it, and returns type
// information for the package.
// The package name is specified by path.
// File position information is added to fset.
// Read may inspect and add to the imports map to ensure that references
// within the export data to other packages are consistent. The caller
// must ensure that imports[path] does not exist, or exists but is
// incomplete (see types.Package.Complete), and Read inserts the
// resulting package into this map entry.
// On return, the state of the reader is undefined.
func Read(in io.Reader, fset *token.FileSet, imports map[string]*types.Package, path string) (*types.Package, error) {
data, err := ioutil.ReadAll(in)
if err != nil {
return nil, fmt.Errorf("reading export data for %q: %v", path, err)
if bytes.HasPrefix(data, []byte("!<arch>")) {
return nil, fmt.Errorf("can't read export data for %q directly from an archive file (call gcexportdata.NewReader first to extract export data)", path)
// The App Engine Go runtime v1.6 uses the old export data format.
// TODO(adonovan): delete once v1.7 has been around for a while.
if bytes.HasPrefix(data, []byte("package ")) {
return gcimporter.ImportData(imports, path, path, bytes.NewReader(data))
// The indexed export format starts with an 'i'; the older
// binary export format starts with a 'c', 'd', or 'v'
// (from "version"). Select appropriate importer.
if len(data) > 0 && data[0] == 'i' {
_, pkg, err := gcimporter.IImportData(fset, imports, data[1:], path)
return pkg, err
_, pkg, err := gcimporter.BImportData(fset, imports, data, path)
return pkg, err
// Write writes encoded type information for the specified package to out.
// The FileSet provides file position information for named objects.
func Write(out io.Writer, fset *token.FileSet, pkg *types.Package) error {
b, err := gcimporter.IExportData(fset, pkg)
if err != nil {
return err
_, err = out.Write(b)
return err