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// Copyright 2013 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 cgo handles cgo preprocessing of files containing `import "C"`.
// The approach taken is to run the cgo processor on the package's
// CgoFiles and parse the output, faking the filenames of the
// resulting ASTs so that the synthetic file containing the C types is
// called "C" (e.g. "~/go/src/net/C") and the preprocessed files
// have their original names (e.g. "~/go/src/net/cgo_unix.go"),
// not the names of the actual temporary files.
// The advantage of this approach is its fidelity to 'go build'. The
// downside is that the token.Position.Offset for each AST node is
// incorrect, being an offset within the temporary file. Line numbers
// should still be correct because of the //line comments.
// The logic of this file is mostly plundered from the 'go build'
// tool, which also invokes the cgo preprocessor.
// An alternative approach that we explored is to extend go/types'
// Importer mechanism to provide the identity of the importing package
// so that each time `import "C"` appears it resolves to a different
// synthetic package containing just the objects needed in that case.
// The loader would invoke cgo but parse only the cgo_types.go file
// defining the package-level objects, discarding the other files
// resulting from preprocessing.
// The benefit of this approach would have been that source-level
// syntax information would correspond exactly to the original cgo
// file, with no preprocessing involved, making source tools like
// godoc, guru, and eg happy. However, the approach was rejected
// due to the additional complexity it would impose on go/types. (It
// made for a beautiful demo, though.)
// cgo files, despite their *.go extension, are not legal Go source
// files per the specification since they may refer to unexported
// members of package "C" such as Also, a function such as
// C.getpwent has in effect two types, one matching its C type and one
// which additionally returns (errno The cgo preprocessor
// uses name mangling to distinguish these two functions in the
// processed code, but go/types would need to duplicate this logic in
// its handling of function calls, analogous to the treatment of map
// lookups in which y=m[k] and y,ok=m[k] are both legal.
package cgo
import (
exec ""
// ProcessFiles invokes the cgo preprocessor on bp.CgoFiles, parses
// the output and returns the resulting ASTs.
func ProcessFiles(bp *build.Package, fset *token.FileSet, DisplayPath func(path string) string, mode parser.Mode) ([]*ast.File, error) {
tmpdir, err := ioutil.TempDir("", strings.Replace(bp.ImportPath, "/", "_", -1)+"_C")
if err != nil {
return nil, err
defer os.RemoveAll(tmpdir)
pkgdir := bp.Dir
if DisplayPath != nil {
pkgdir = DisplayPath(pkgdir)
cgoFiles, cgoDisplayFiles, err := Run(bp, pkgdir, tmpdir, false)
if err != nil {
return nil, err
var files []*ast.File
for i := range cgoFiles {
rd, err := os.Open(cgoFiles[i])
if err != nil {
return nil, err
display := filepath.Join(bp.Dir, cgoDisplayFiles[i])
f, err := parser.ParseFile(fset, display, rd, mode)
if err != nil {
return nil, err
files = append(files, f)
return files, nil
var cgoRe = regexp.MustCompile(`[/\\:]`)
// Run invokes the cgo preprocessor on bp.CgoFiles and returns two
// lists of files: the resulting processed files (in temporary
// directory tmpdir) and the corresponding names of the unprocessed files.
// Run is adapted from (*builder).cgo in
// $GOROOT/src/cmd/go/build.go, but these features are unsupported:
// If useabs is set to true, absolute paths of the bp.CgoFiles will be passed in
// to the cgo preprocessor. This in turn will set the // line comments
// referring to those files to use absolute paths. This is needed for
// go/packages using the legacy go list support so it is able to find
// the original files.
func Run(bp *build.Package, pkgdir, tmpdir string, useabs bool) (files, displayFiles []string, err error) {
cgoCPPFLAGS, _, _, _ := cflags(bp, true)
_, cgoexeCFLAGS, _, _ := cflags(bp, false)
if len(bp.CgoPkgConfig) > 0 {
pcCFLAGS, err := pkgConfigFlags(bp)
if err != nil {
return nil, nil, err
cgoCPPFLAGS = append(cgoCPPFLAGS, pcCFLAGS...)
// Allows including _cgo_export.h from .[ch] files in the package.
cgoCPPFLAGS = append(cgoCPPFLAGS, "-I", tmpdir)
// _cgo_gotypes.go (displayed "C") contains the type definitions.
files = append(files, filepath.Join(tmpdir, "_cgo_gotypes.go"))
displayFiles = append(displayFiles, "C")
for _, fn := range bp.CgoFiles {
// "foo.cgo1.go" (displayed "foo.go") is the processed Go source.
f := cgoRe.ReplaceAllString(fn[:len(fn)-len("go")], "_")
files = append(files, filepath.Join(tmpdir, f+"cgo1.go"))
displayFiles = append(displayFiles, fn)
var cgoflags []string
if bp.Goroot && bp.ImportPath == "runtime/cgo" {
cgoflags = append(cgoflags, "-import_runtime_cgo=false")
if bp.Goroot && bp.ImportPath == "runtime/race" || bp.ImportPath == "runtime/cgo" {
cgoflags = append(cgoflags, "-import_syscall=false")
var cgoFiles []string = bp.CgoFiles
if useabs {
cgoFiles = make([]string, len(bp.CgoFiles))
for i := range cgoFiles {
cgoFiles[i] = filepath.Join(pkgdir, bp.CgoFiles[i])
args := stringList(
"go", "tool", "cgo", "-objdir", tmpdir, cgoflags, "--",
cgoCPPFLAGS, cgoexeCFLAGS, cgoFiles,
if false {
log.Printf("Running cgo for package %q: %s (dir=%s)", bp.ImportPath, args, pkgdir)
cmd := exec.Command(args[0], args[1:]...)
cmd.Dir = pkgdir
cmd.Env = append(os.Environ(), "PWD="+pkgdir)
cmd.Stdout = os.Stderr
cmd.Stderr = os.Stderr
if err := cmd.Run(); err != nil {
return nil, nil, fmt.Errorf("cgo failed: %s: %s", args, err)
return files, displayFiles, nil
// -- unmodified from 'go build' ---------------------------------------
// Return the flags to use when invoking the C or C++ compilers, or cgo.
func cflags(p *build.Package, def bool) (cppflags, cflags, cxxflags, ldflags []string) {
var defaults string
if def {
defaults = "-g -O2"
cppflags = stringList(envList("CGO_CPPFLAGS", ""), p.CgoCPPFLAGS)
cflags = stringList(envList("CGO_CFLAGS", defaults), p.CgoCFLAGS)
cxxflags = stringList(envList("CGO_CXXFLAGS", defaults), p.CgoCXXFLAGS)
ldflags = stringList(envList("CGO_LDFLAGS", defaults), p.CgoLDFLAGS)
// envList returns the value of the given environment variable broken
// into fields, using the default value when the variable is empty.
func envList(key, def string) []string {
v := os.Getenv(key)
if v == "" {
v = def
return strings.Fields(v)
// stringList's arguments should be a sequence of string or []string values.
// stringList flattens them into a single []string.
func stringList(args ...interface{}) []string {
var x []string
for _, arg := range args {
switch arg := arg.(type) {
case []string:
x = append(x, arg...)
case string:
x = append(x, arg)
panic("stringList: invalid argument")
return x