I have written a function to read zip archive tomap[string]*zip.File.
func ReadZip(file string) (map[string]*zip.File, error) {
r, err := zip.OpenReader(file)
if err != nil {
return nil, err
}
defer r.Close()
files := make(map[string]*zip.File)
for _, f := range r.File {
files[f.Name] = f
}
return files, nil
}
But when i try to open file infoRC, err := f["info.json"].Open() arises error
read file.zip: bad file descriptor.
Is there better way to read zip archive?
Once ReadCloser.Close is called, any of the *zip.File structs are invalid:
Close closes the Zip file, rendering it unusable for I/O.
You need to either:
Keep r open as long as you want to read the ZIP entries, or
make an in-memory/temporary file copy of all of the zip file contents
An example of the latter option:
func ReadZip(file string) (map[string][]byte, error) {
r, err := zip.OpenReader(file)
if err != nil {
return nil, err
}
defer r.Close()
files := make(map[string][]byte)
for _, f := range r.File {
fc, err := f.Open()
if err != nil {
return nil, err
}
contents, err := ioutil.ReadAll(fc)
fc.Close()
if err != nil {
return nil, err
}
files[f.Name] = contents
}
return files, nil
}
When I try to save the map of type map[mapKey]string into a file using gob encoder, it is not saving string in file.
Here mapKey is struct and map value is long json string.
type mapKey struct{
Id1 string
Id2 string
}
And whenever I am use nested map instead of the struct like:
var m = make(map[string]map[string]string)
It is working fine and saving string properly. I am not sure what I am missing here.
Code to encode, decode and save it in file:
func Save(path string, object interface{}) error {
file, err := os.Create(path)
if err == nil {
encoder := gob.NewEncoder(file)
encoder.Encode(object)
}
file.Close()
return err
}
// Decode Gob file
func Load(path string, object interface{}) error {
file, err := os.Open(path)
if err == nil {
decoder := gob.NewDecoder(file)
err = decoder.Decode(object)
}
file.Close()
return err
}
func Check(e error) {
if e != nil {
_, file, line, _ := runtime.Caller(1)
fmt.Println(line, "\t", file, "\n", e)
os.Exit(1)
}
}
There is nothing special in encoding a value of type map[mapKey]string.
See this very simple working example which uses the specified reader/writer:
func save(w io.Writer, i interface{}) error {
return gob.NewEncoder(w).Encode(i)
}
func load(r io.Reader, i interface{}) error {
return gob.NewDecoder(r).Decode(i)
}
Testing it with an in-memory buffer (bytes.Buffer):
m := map[mapKey]string{
{"1", "2"}: "12",
{"3", "4"}: "34",
}
fmt.Println(m)
buf := &bytes.Buffer{}
if err := save(buf, m); err != nil {
panic(err)
}
var m2 map[mapKey]string
if err := load(buf, &m2); err != nil {
panic(err)
}
fmt.Println(m2)
Output as expected (try it on the Go Playground):
map[{1 2}:12 {3 4}:34]
map[{1 2}:12 {3 4}:34]
You have a working code, but know that you have to call Load() with a pointer value (else Decoder.Decode() wouldn't be able to modify its value).
Also a few things to improve it:
In your example you are swallowing the error returned by Encoder.Encode() (check it and you'll see what the problem is; a common problem is using a struct mapKey with no exported fields in which case an error of gob: type main.mapKey has no exported fields would be returned).
Also you should call File.Close() as a deferred function.
Also if opening the file fails, you should return early and you shouldn't close the file.
This is the corrected version of your code:
func Save(path string, object interface{}) error {
file, err := os.Create(path)
if err != nil {
return err
}
defer file.Close()
return gob.NewEncoder(file).Encode(object)
}
func Load(path string, object interface{}) error {
file, err := os.Open(path)
if err != nil {
return err
}
defer file.Close()
return gob.NewDecoder(file).Decode(object)
}
Testing it:
m := map[mapKey]string{
{"1", "2"}: "12",
{"3", "4"}: "34",
}
fmt.Println(m)
if err := Save("testfile", m); err != nil {
panic(err)
}
var m2 map[mapKey]string
if err := Load("testfile", &m2); err != nil {
panic(err)
}
fmt.Println(m2)
Output as expected:
map[{1 2}:12 {3 4}:34]
map[{1 2}:12 {3 4}:34]
I had this convenient function in Python:
def follow(path):
with open(self.path) as lines:
lines.seek(0, 2) # seek to EOF
while True:
line = lines.readline()
if not line:
time.sleep(0.1)
continue
yield line
It does something similar to UNIX tail -f: you get last lines of a file as they come. It's convenient because you can get the generator without blocking and pass it to another function.
Then I had to do the same thing in Go. I'm new to this language, so I'm not sure whether what I did is idiomatic/correct enough for Go.
Here is the code:
func Follow(fileName string) chan string {
out_chan := make(chan string)
file, err := os.Open(fileName)
if err != nil {
log.Fatal(err)
}
file.Seek(0, os.SEEK_END)
bf := bufio.NewReader(file)
go func() {
for {
line, _, _ := bf.ReadLine()
if len(line) == 0 {
time.Sleep(10 * time.Millisecond)
} else {
out_chan <- string(line)
}
}
defer file.Close()
close(out_chan)
}()
return out_chan
}
Is there any cleaner way to do this in Go? I have a feeling that using an asynchronous call for such a thing is an overkill, and it really bothers me.
Create a wrapper around a reader that sleeps on EOF:
type tailReader struct {
io.ReadCloser
}
func (t tailReader) Read(b []byte) (int, error) {
for {
n, err := t.ReadCloser.Read(b)
if n > 0 {
return n, nil
} else if err != io.EOF {
return n, err
}
time.Sleep(10 * time.Millisecond)
}
}
func newTailReader(fileName string) (tailReader, error) {
f, err := os.Open(fileName)
if err != nil {
return tailReader{}, err
}
if _, err := f.Seek(0, 2); err != nil {
return tailReader{}, err
}
return tailReader{f}, nil
}
This reader can be used anywhere an io.Reader can be used. Here's how loop over lines using bufio.Scanner:
t, err := newTailReader("somefile")
if err != nil {
log.Fatal(err)
}
defer t.Close()
scanner := bufio.NewScanner(t)
for scanner.Scan() {
fmt.Println(scanner.Text())
}
if err := scanner.Err(); err != nil {
fmt.Fprintln(os.Stderr, "reading:", err)
}
The reader can also be used to loop over JSON values appended to the file:
t, err := newTailReader("somefile")
if err != nil {
log.Fatal(err)
}
defer t.Close()
dec := json.NewDecoder(t)
for {
var v SomeType
if err := dec.Decode(&v); err != nil {
log.Fatal(err)
}
fmt.Println("the value is ", v)
}
There are a couple of advantages this approach has over the goroutine approach outlined in the question. The first is that shutdown is easy. Just close the file. There's no need to signal the goroutine that it should exit. The second advantage is that many packages work with io.Reader.
The sleep time can be adjusted up or down to meet specific needs. Decrease the time for lower latency and increase the time to reduce CPU use. A sleep of 100ms is probably fast enough for data that's displayed to humans.
Check out this Go package for reading from continuously updated files (tail -f): https://github.com/hpcloud/tail
t, err := tail.TailFile("filename", tail.Config{Follow: true})
for line := range t.Lines {
fmt.Println(line.Text)
}
Is there any simple/fast way to copy a file in Go?
I couldn't find a fast way in the Doc's and searching the internet doesn't help as well.
Warning: This answer is mainly about adding a hard link to a file, not about copying the contents.
A robust and efficient copy is conceptually simple, but not simple to implement due to the need to handle a number of edge cases and system limitations that are imposed by the target operating system and it's configuration.
If you simply want to make a duplicate of the existing file you can use os.Link(srcName, dstName). This avoids having to move bytes around in the application and saves disk space. For large files, this is a significant time and space saving.
But various operating systems have different restrictions on how hard links work. Depending on your application and your target system configuration, Link() calls may not work in all cases.
If you want a single generic, robust and efficient copy function, update Copy() to:
Perform checks to ensure that at least some form of copy will succeed (access permissions, directories exist, etc.)
Check to see if both files already exist and are the same using
os.SameFile, return success if they are the same
Attempt a Link, return if success
Copy the bytes (all efficient means failed), return result
An optimization would be to copy the bytes in a go routine so the caller doesn't block on the byte copy. Doing so imposes additional complexity on the caller to handle the success/error case properly.
If I wanted both, I would have two different copy functions: CopyFile(src, dst string) (error) for a blocking copy and CopyFileAsync(src, dst string) (chan c, error) which passes a signaling channel back to the caller for the asynchronous case.
package main
import (
"fmt"
"io"
"os"
)
// CopyFile copies a file from src to dst. If src and dst files exist, and are
// the same, then return success. Otherise, attempt to create a hard link
// between the two files. If that fail, copy the file contents from src to dst.
func CopyFile(src, dst string) (err error) {
sfi, err := os.Stat(src)
if err != nil {
return
}
if !sfi.Mode().IsRegular() {
// cannot copy non-regular files (e.g., directories,
// symlinks, devices, etc.)
return fmt.Errorf("CopyFile: non-regular source file %s (%q)", sfi.Name(), sfi.Mode().String())
}
dfi, err := os.Stat(dst)
if err != nil {
if !os.IsNotExist(err) {
return
}
} else {
if !(dfi.Mode().IsRegular()) {
return fmt.Errorf("CopyFile: non-regular destination file %s (%q)", dfi.Name(), dfi.Mode().String())
}
if os.SameFile(sfi, dfi) {
return
}
}
if err = os.Link(src, dst); err == nil {
return
}
err = copyFileContents(src, dst)
return
}
// copyFileContents copies the contents of the file named src to the file named
// by dst. The file will be created if it does not already exist. If the
// destination file exists, all it's contents will be replaced by the contents
// of the source file.
func copyFileContents(src, dst string) (err error) {
in, err := os.Open(src)
if err != nil {
return
}
defer in.Close()
out, err := os.Create(dst)
if err != nil {
return
}
defer func() {
cerr := out.Close()
if err == nil {
err = cerr
}
}()
if _, err = io.Copy(out, in); err != nil {
return
}
err = out.Sync()
return
}
func main() {
fmt.Printf("Copying %s to %s\n", os.Args[1], os.Args[2])
err := CopyFile(os.Args[1], os.Args[2])
if err != nil {
fmt.Printf("CopyFile failed %q\n", err)
} else {
fmt.Printf("CopyFile succeeded\n")
}
}
import (
"io/ioutil"
"log"
)
func checkErr(err error) {
if err != nil {
log.Fatal(err)
}
}
func copy(src string, dst string) {
// Read all content of src to data, may cause OOM for a large file.
data, err := ioutil.ReadFile(src)
checkErr(err)
// Write data to dst
err = ioutil.WriteFile(dst, data, 0644)
checkErr(err)
}
If you are running the code in linux/mac, you could just execute the system's cp command.
srcFolder := "copy/from/path"
destFolder := "copy/to/path"
cpCmd := exec.Command("cp", "-rf", srcFolder, destFolder)
err := cpCmd.Run()
It's treating go a bit like a script, but it gets the job done. Also, you need to import "os/exec"
Starting with Go 1.15 (Aug 2020), you can use File.ReadFrom:
package main
import "os"
func main() {
r, err := os.Open("in.txt")
if err != nil {
panic(err)
}
defer r.Close()
w, err := os.Create("out.txt")
if err != nil {
panic(err)
}
defer w.Close()
w.ReadFrom(r)
}
Perform the copy in a stream, using io.Copy.
Close all opened file descriptors.
All errors that should be checked are checked, including the errors in deferred (*os.File).Close calls.
Gracefully handle multiple non-nil errors, e.g. non-nil errors from both io.Copy and (*os.File).Close.
No unnecessary complications that were present in other answers, such as calling Close twice on the same file but ignoring the error on one of the calls.
No unnecessary stat checks for existence or for file type. These checks aren't necessary: the future open and read operations will return an error anyway if it's not a valid operation for the type of file. Secondly, such checks are prone to races (e.g. the file might be removed in the time between stat and open).
Accurate doc comment. See: "file", "regular file", and behavior when dstpath exists. The doc comment also matches the style of other functions in package os.
// Copy copies the contents of the file at srcpath to a regular file at dstpath.
// If dstpath already exists and is not a directory, the function truncates it.
// The function does not copy file modes or file attributes.
func Copy(srcpath, dstpath string) (err error) {
r, err := os.Open(srcpath)
if err != nil {
return err
}
defer r.Close() // ok to ignore error: file was opened read-only.
w, err := os.Create(dstpath)
if err != nil {
return err
}
defer func() {
e := w.Close()
// Report the error from Close, if any.
// But do so only if there isn't already
// an outgoing error.
if e != nil && err == nil {
err = e
}
}()
_, err = io.Copy(w, r)
return err
}
In this case there are a couple of conditions to verify, I prefer non-nested code
func Copy(src, dst string) (int64, error) {
src_file, err := os.Open(src)
if err != nil {
return 0, err
}
defer src_file.Close()
src_file_stat, err := src_file.Stat()
if err != nil {
return 0, err
}
if !src_file_stat.Mode().IsRegular() {
return 0, fmt.Errorf("%s is not a regular file", src)
}
dst_file, err := os.Create(dst)
if err != nil {
return 0, err
}
defer dst_file.Close()
return io.Copy(dst_file, src_file)
}
If you are on windows, you can wrap CopyFileW like this:
package utils
import (
"syscall"
"unsafe"
)
var (
modkernel32 = syscall.NewLazyDLL("kernel32.dll")
procCopyFileW = modkernel32.NewProc("CopyFileW")
)
// CopyFile wraps windows function CopyFileW
func CopyFile(src, dst string, failIfExists bool) error {
lpExistingFileName, err := syscall.UTF16PtrFromString(src)
if err != nil {
return err
}
lpNewFileName, err := syscall.UTF16PtrFromString(dst)
if err != nil {
return err
}
var bFailIfExists uint32
if failIfExists {
bFailIfExists = 1
} else {
bFailIfExists = 0
}
r1, _, err := syscall.Syscall(
procCopyFileW.Addr(),
3,
uintptr(unsafe.Pointer(lpExistingFileName)),
uintptr(unsafe.Pointer(lpNewFileName)),
uintptr(bFailIfExists))
if r1 == 0 {
return err
}
return nil
}
Code is inspired by wrappers in C:\Go\src\syscall\zsyscall_windows.go
Here is an obvious way to copy a file:
package main
import (
"os"
"log"
"io"
)
func main() {
sFile, err := os.Open("test.txt")
if err != nil {
log.Fatal(err)
}
defer sFile.Close()
eFile, err := os.Create("test_copy.txt")
if err != nil {
log.Fatal(err)
}
defer eFile.Close()
_, err = io.Copy(eFile, sFile) // first var shows number of bytes
if err != nil {
log.Fatal(err)
}
err = eFile.Sync()
if err != nil {
log.Fatal(err)
}
}
You can use "exec".
exec.Command("cmd","/c","copy","fileToBeCopied destinationDirectory") for windows
I have used this and its working fine. You can refer manual for more details on exec.
I'm trying to map an array to a file via Mmap, the array could be any type, like float64. In C, I find this one. After reading some texts, I wrote this sample. I don't know if it is correct, and it is not writing the values to the file. If I increase the size of array a lot, e.g from 1000 to 10000, it crashes. If someone know how to do that in the correctly way, please, tell me.
Thanks!
For example, revising your sample program,
package main
import (
"fmt"
"os"
"syscall"
"unsafe"
)
func main() {
const n = 1e3
t := int(unsafe.Sizeof(0)) * n
map_file, err := os.Create("/tmp/test.dat")
if err != nil {
fmt.Println(err)
os.Exit(1)
}
_, err = map_file.Seek(int64(t-1), 0)
if err != nil {
fmt.Println(err)
os.Exit(1)
}
_, err = map_file.Write([]byte(" "))
if err != nil {
fmt.Println(err)
os.Exit(1)
}
mmap, err := syscall.Mmap(int(map_file.Fd()), 0, int(t), syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_SHARED)
if err != nil {
fmt.Println(err)
os.Exit(1)
}
map_array := (*[n]int)(unsafe.Pointer(&mmap[0]))
for i := 0; i < n; i++ {
map_array[i] = i * i
}
fmt.Println(*map_array)
err = syscall.Munmap(mmap)
if err != nil {
fmt.Println(err)
os.Exit(1)
}
err = map_file.Close()
if err != nil {
fmt.Println(err)
os.Exit(1)
}
}