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// Copyright 2009 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 os

import (

        "syscall"

)

// Getwd returns a rooted path name corresponding to the

// current directory.  If the current directory can be

// reached via multiple paths (due to symbolic links),

// Getwd may return any one of them.

func Getwd() (pwd string, err error) {

        // If the operating system provides a Getwd call, use it.

        if syscall.ImplementsGetwd {

                s, e := syscall.Getwd()

                return s, NewSyscallError("getwd", e)

        }

        // Otherwise, we're trying to find our way back to ".".

        dot, err := Stat(".")

        if err != nil {

                return "", err

        }

        // Clumsy but widespread kludge:

        // if $PWD is set and matches ".", use it.

        pwd = Getenv("PWD")

        if len(pwd) > 0 && pwd[0] == '/' {

                d, err := Stat(pwd)

                if err == nil && SameFile(dot, d) {

                        return pwd, nil

                }

        }

        // Root is a special case because it has no parent

        // and ends in a slash.

        root, err := Stat("/")

        if err != nil {

                // Can't stat root - no hope.

                return "", err

        }

        if SameFile(root, dot) {

                return "/", nil

        }

        // General algorithm: find name in parent

        // and then find name of parent.  Each iteration

        // adds /name to the beginning of pwd.

        pwd = ""

        for parent := ".."; ; parent = "../" + parent {

                if len(parent) >= 1024 { // Sanity check

                        return "", ENAMETOOLONG

                }

                fd, err := Open(parent)

                if err != nil {

                        return "", err

                }

                for {

                        names, err := fd.Readdirnames(100)

                        if err != nil {

                                fd.Close()

                                return "", err

                        }

                        for _, name := range names {

                                d, _ := Lstat(parent + "/" + name)

                                if SameFile(d, dot) {

                                        pwd = "/" + name + pwd

                                        goto Found

                                }

                        }

                }

                fd.Close()

                return "", ENOENT

        Found:

                pd, err := fd.Stat()

                if err != nil {

                        return "", err

                }

                fd.Close()

                if SameFile(pd, root) {

                        break

                }

                // Set up for next round.

                dot = pd

        }

        return pwd, nil

}