Golang：file.Seek和file.WriteAt不能按预期工作

I am trying to make a program which writes at provided offsets in the file, like i can start writing from 20th offset etc.

here is one of sample code i was using as reference

package main

import (
    "fmt"
    "io/ioutil"
    "os"
)

const (
    filename   = "sample.txt"
    start_data = "12345"
)

func printContents() {
    data, err := ioutil.ReadFile(filename)
    if err != nil {
        panic(err)
    }
    fmt.Println("CONTENTS:", string(data))
}

func main() {
    err := ioutil.WriteFile(filename, []byte(start_data), 0644)
    if err != nil {
        panic(err)
    }

    printContents()

    f, err := os.OpenFile(filename, os.O_RDWR, 0644)
    if err != nil {
        panic(err)
    }
    defer f.Close()

    if _, err := f.Seek(20, 0); err != nil {
        panic(err)
    }

    if _, err := f.WriteAt([]byte("A"), 15); err != nil {
        panic(err)
    }

    printContents()
}

But i am always getting the same file content which is beginning from start like

12345A

I tried changing the seek values to (0,0) and (20,0) and (10,1) randomly which results in same output

Also i tried changing WriteAt offset to other offset like 10, 20 but this also resulted in same.

I want to get a solution so that i can write at any specified position in file, suggest me what is wrong in this code.

It works as expected.

After running your code, your "sample.txt" file content is (16 bytes):

[49 50 51 52 53 0 0 0 0 0 0 0 0 0 0 65]

try:

package main

import (
    "fmt"
    "io/ioutil"
)

const (
    filename   = "sample.txt"
    start_data = "12345"
)

func printContents() {
    data, err := ioutil.ReadFile(filename)
    if err != nil {
        panic(err)
    }
    fmt.Println(data)
}

func main() {
    printContents()
}

you need to write enough bytes first, the use WriteAt offset:
e.g. edit :

start_data = "1234567890123456789012345678901234567890"

then test your code:

package main

import (
    "fmt"
    "io/ioutil"
    "os"
)

const (
    filename   = "sample.txt"
    start_data = "1234567890123456789012345678901234567890"
)

func printContents() {
    data, err := ioutil.ReadFile(filename)
    if err != nil {
        panic(err)
    }
    fmt.Println(string(data))
}

func main() {
    err := ioutil.WriteFile(filename, []byte(start_data), 0644)
    if err != nil {
        panic(err)
    }

    printContents()

    f, err := os.OpenFile(filename, os.O_RDWR, 0644)
    if err != nil {
        panic(err)
    }
    defer f.Close()

    if _, err := f.Seek(20, 0); err != nil {
        panic(err)
    }

    if _, err := f.WriteAt([]byte("A"), 15); err != nil {
        panic(err)
    }

    printContents()
}

output:

1234567890123456789012345678901234567890
123456789012345A789012345678901234567890