You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: fastwalk_unix.go
    path of file: ./repos/gitkube/vendor/golang.org/x/tools/internal/fastwalk
    
    the code of the file until where you have to start completion: // 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.

// +build linux darwin freebsd openbsd netbsd
// +build !appengine

package fastwalk

import (
	"fmt"
	"os"
	"syscall"
	"unsafe"
)

const blockSize = 8 << 10

// unknownFileMode is a sentinel (and bogus) os.FileMode
// value used to represent a syscall.DT_UNKNOWN Dirent.Type.
const unknownFileMode os.FileMode = os.ModeNamedPipe | os.ModeSocket | os.ModeDevice

func readDir(dirName string, fn func(dirName, entName string, typ os.FileMode) error) error {
	fd, err := syscall.Open(dirName, 0, 0)
	if err != nil {
		return &os.PathError{Op: "open", Path: dirName, Err: err}
	}
	defer s
    
func readDir(dirName string, fn func(dirName, entName string, typ os.FileMode) error) error {
	fd, err := syscall.Open(dirName, 0, 0)
	if err != nil {
		return &os.PathError{Op: "open", Path: dirName, Err: err}
	}
	defer syscall.Close(fd)

	// The buffer must be at least a block long.
	buf := make([]byte, blockSize) // stack-allocated; doesn't escape
	bufp := 0                      // starting read position in buf
	nbuf := 0                      // end valid data in buf
	skipFiles := false
	for {
		if bufp >= nbuf {
			bufp = 0
			nbuf, err = syscall.ReadDirent(fd, buf)
			if err != nil {
				return os.NewSyscallError("readdirent", err)
			}
			if nbuf <= 0 {
				return nil
			}
		}
		consumed, name, typ := parseDirEnt(buf[bufp:nbuf])
		bufp += consumed
		if name == "" || name == "." || name == ".." {
			continue
		}
		// Fallback for filesystems (like old XFS) that don't
		// support Dirent.Type and have DT_UNKNOWN (0) there
		// instead.
		if typ == unknownFileMode {
			fi, err := os.Lstat(dirName + "/" + name)
			if err != nil {
				// It got deleted in the meantime.
				if os.IsNotExist(err) {
					continue
				}
				return err
			}
			typ = fi.Mode() & os.ModeType
		}
		if skipFiles && typ.IsRegular() {
			continue
		}
		if err := fn(dirName, name, typ); err != nil {
			if err == SkipFiles {
				skipFiles = true
				continue
			}
			return err
		}
	}
}