how to seek in file using go

[hui zhang]

I am using encoding/binary to read/write (struct)data to/from file.

Some times , I need to seek in the file while reading .

how to do this in go. 

Check the code below

package main

import (
   _ "bytes"
   "fmt"
   _ "math"

   "bufio"
   "encoding/binary"
   "os"
)

func main() {
   var pi float64
   //b := []byte{0x18, 0x2d, 0x44, 0x54, 0xfb, 0x21, 0x09, 0x40,
   //         0x78, 0x2d, 0x44, 0x54, 0xfb, 0x21, 0x09, 0x40}
   //buf := bytes.NewReader(b)
   //err := binary.Read(buf, binary.LittleEndian, &pi)
   //if err != nil {
   // fmt.Println("binary.Read failed:", err)
   //}
   //fmt.Println(pi)
   //// Output: 3.141592653589793
   //err = binary.Read(buf, binary.LittleEndian, &pi)
   //if err != nil {
   // fmt.Println("binary.Read failed:", err)
   //}
   //fmt.Println(pi)
//------------------------------------------------------------
   // open output file
   fo, err := os.Create("./output.bin")
   if err != nil {
      panic(err)
   }

   // make a write buffer
   var fl1 float64 = 3.13
   var fl2 float64 = 3.14
   w := bufio.NewWriter(fo)
   err = binary.Write(w, binary.LittleEndian, fl1)
   if err != nil {
      fmt.Println("binary.Write failed:", err)
   }
   err = binary.Write(w, binary.LittleEndian, fl2)
   if err != nil {
      fmt.Println("binary.Write failed:", err)
   }
   w.Flush()
   // close fo on exit and check for its returned error
   if err := fo.Close(); err != nil {
      panic(err)
   }

   //------------------------------------------------------------
   fi, err := os.Open("./output.bin")
   if err != nil {
      panic(err)
   }
   // close fi on exit and check for its returned error
   defer func() {
      if err := fi.Close(); err != nil {
         panic(err)
      }
   }()

   rd := bufio.NewReader(fi)
   err = binary.Read(rd, binary.LittleEndian, &pi)
   if err != nil {
      fmt.Println("binary.Read failed:", err)
   }
   fmt.Println(pi)

   fi.seek()////?????

   err = binary.Read(rd, binary.LittleEndian, &pi)
   if err != nil {
      fmt.Println("binary.Read failed:", err)
   }
   fmt.Println(pi)

}

[Ayan George]

On 01/19/2017 01:22 AM, hui zhang wrote:
> I am using encoding/binary to read/write (struct)data to/from file.
> Some times , I need to seek in the file while reading .
> how to do this in go.
> Check the code below

[snip!]

>
>
> fi, err := os.Open("./output.bin")
> if err != nil {
> panic(err)
> }
> // close fi on exit and check for its returned error
> defer func() {
> if err := fi.Close(); err != nil {
> panic(err)
> }
> }()
>
> rd := bufio.NewReader(fi)
> err = binary.Read(rd, binary.LittleEndian, &pi)
> if err != nil {
> fmt.Println("binary.Read failed:", err)
> }
> fmt.Println(pi)
>
> fi.seek()////?????
>

Is File.Seek() what you're looking for or do you need something else?

https://golang.org/pkg/os/#File.Seek

-ayan

[Konstantin Khomoutov]

On Wed, 18 Jan 2017 22:22:48 -0800 (PST)
hui zhang <fastf...@gmail.com> wrote:

[...]

> fi, err := os.Open("./output.bin")

[...]
> fi.seek()////?????

The usual drill is:

1. Get the documentation on os.Open():

$ go doc os.Open

Notice what it returns in its 1st return value has the
type *File. Since it has no "pkg." qualifier it means
that name is defined in the same package.

2. So we now read the docs on os.File:

$ go doc os.File

...and see all of its methods there, inclusing Seek().

3. So we read the docs on it:

$ go doc os.File.Seek

[hui zhang]

thanks all,   I use ide go to definition,  it go to the file_unix.go

in this file seek  is a private method.

So  I am confused 

在 2017年1月19日星期四 UTC+8下午2:45:16，Konstantin Khomoutov写道：

[hui zhang]

fi.Seek(0, os.SEEK_SET )////?????

I set this in the code  and I expected to print 3.13 twice , but this code print 3.13 3.14

why?

   fo, err := os.Create("./output.bin")

   if err != nil {
      panic(err)
   }

   // make a write buffer
   var fl1 float64 = 3.13
   var fl2 float64 = 3.14
   w := bufio.NewWriter(fo)
   err = binary.Write(w, binary.LittleEndian, fl1)

   if err != nil {

      fmt.Println("binary.Write failed:", err)
   }
   err = binary.Write(w, binary.LittleEndian, fl2)

   if err != nil {

      fmt.Println("binary.Write failed:", err)
   }
   w.Flush()
   // close fo on exit and check for its returned error
   if err := fo.Close(); err != nil {
      panic(err)
   }

   //------------------------------------------------------------

   fi, err := os.Open("./output.bin")
   if err != nil {
      panic(err)
   }
   // close fi on exit and check for its returned error
   defer func() {
      if err := fi.Close(); err != nil {
         panic(err)
      }
   }()

   rd := bufio.NewReader(fi)
   err = binary.Read(rd, binary.LittleEndian, &pi)
   if err != nil {
      fmt.Println("binary.Read failed:", err)
   }
   fmt.Println(pi)

   fi.Seek(0, os.SEEK_SET )////?????

   err = binary.Read(rd, binary.LittleEndian, &pi)
   if err != nil {
      fmt.Println("binary.Read failed:", err)
   }
   fmt.Println(pi)

}

在 2017年1月19日星期四 UTC+8下午2:43:31，Ayan George写道：

[hui zhang]

so the file seek does not work in bufio.Read  ?

在 2017年1月19日星期四 UTC+8下午2:59:39，hui zhang写道：

[Konstantin Khomoutov]

On Wed, 18 Jan 2017 22:54:31 -0800 (PST)

hui zhang <fastf...@gmail.com> wrote:

[...]

> > 2. So we now read the docs on os.File:
> >
> > $ go doc os.File
> >
> > ...and see all of its methods there, inclusing Seek().
> >
> > 3. So we read the docs on it:
> >
> > $ go doc os.File.Seek
> >

> thanks all, I use ide go to definition, it go to the file_unix.go
> in this file seek is a private method.
> So I am confused

That's the problem with your IDE.
Consider reporting this bug to its developers.

The `go doc` command only deals with exported symbols, so when you use
it, you're on the safe side.

[Konstantin Khomoutov]

On Wed, 18 Jan 2017 23:00:33 -0800 (PST)
hui zhang <fastf...@gmail.com> wrote:

> so the file seek does not work in bufio.Read ?

You might need to use the Reset() method of your bufio.Reader value
before you reposition the pointer in the underlying opened file.

Please see `go doc bufio.Reader.Reset`.

But this poses the question of whether you really need buffered I/O.
May be just use io.ReadFull() helper to read data in whole chunks from
the file directly.

[Ayan George]

On 01/19/2017 02:00 AM, hui zhang wrote:
> so the file seek does not work in bufio.Read ?
>

It looks like this has been answered before:

https://groups.google.com/forum/#!topic/golang-nuts/mOvX0bmJoeI

You could also create another bufio.Reader after you seek like:

rd := bufio.NewReader(fi)
err = binary.Read(rd, binary.LittleEndian, &pi)

fmt.Println(pi)


fi.Seek(0, os.SEEK_SET )////?????

rd = bufio.NewReader(fi)
err = binary.Read(rd, binary.LittleEndian, &pi)

fmt.Println(pi)

If you look at the bufio.Reader struct, it doesn't appear to have a
notion of where it is in the file -- it just contains a buffer, read and
write positions within that buffer, etc.

-ayan

[C Banning]

Why rd := bufio.NewReader(fi)?  'fi' implements io.Reader.  Try: err = binary.Read(fi, binary.LittleEndian, &pi) instead.  Then fi.Seek(...) will  manipulate your io.Reader.  [An example: https://github.com/clbanning/rfile/blob/master/reverse.go#L57.]