Golang: file.Seek and file.WriteAt not working as expected

Question

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.

Answer 1

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