Source: getwd.go in package os


package os

import (
	"runtime"
	"sync"
	"syscall"
)

var getwdCache struct {
	sync.Mutex
	dir string
}

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() (dir string, err error) {
	if runtime.GOOS == "windows" || runtime.GOOS == "plan9" {
		return syscall.Getwd()
	}

Clumsy but widespread kludge: if $PWD is set and matches ".", use it.

	dot, err := statNolog(".")
	if err != nil {
		return "", err
	}
	dir = Getenv("PWD")
	if len(dir) > 0 && dir[0] == '/' {
		d, err := statNolog(dir)
		if err == nil && SameFile(dot, d) {
			return dir, nil
		}
	}

If the operating system provides a Getwd call, use it. Otherwise, we're trying to find our way back to ".".

	if syscall.ImplementsGetwd {
		var (
			s string
			e error
		)
		for {
			s, e = syscall.Getwd()
			if e != syscall.EINTR {
				break
			}
		}
		return s, NewSyscallError("getwd", e)
	}

Apply same kludge but to cached dir instead of $PWD.

	getwdCache.Lock()
	dir = getwdCache.dir
	getwdCache.Unlock()
	if len(dir) > 0 {
		d, err := statNolog(dir)
		if err == nil && SameFile(dot, d) {
			return dir, nil
		}
	}

Root is a special case because it has no parent and ends in a slash.

	root, err := statNolog("/")

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 dir.

	dir = ""
	for parent := ".."; ; parent = "../" + parent {
		if len(parent) >= 1024 { // Sanity check
			return "", syscall.ENAMETOOLONG
		}
		fd, err := openFileNolog(parent, O_RDONLY, 0)
		if err != nil {
			return "", err
		}

		for {
			names, err := fd.Readdirnames(100)
			if err != nil {
				fd.Close()
				return "", err
			}
			for _, name := range names {
				d, _ := lstatNolog(parent + "/" + name)
				if SameFile(d, dot) {
					dir = "/" + name + dir
					goto Found
				}
			}
		}

	Found:
		pd, err := fd.Stat()
		fd.Close()
		if err != nil {
			return "", err
		}
		if SameFile(pd, root) {
			break

Set up for next round.

		dot = pd
	}

Save answer as hint to avoid the expensive path next time.

	getwdCache.Lock()
	getwdCache.dir = dir
	getwdCache.Unlock()

	return dir, nil