Goroutine called in a loop on function with pointer overwrites value?

Question

I have just started playing with Go and stumbled upon problem how the pointers are handled in this language.

I have such code and can`t figure out how to fix it to get the correct response.

Changing func (me *Str) Start() to func (me Str) Start() fixes the problem, but I need this as reference to write this structure (it will have channel in it).

Sample code (https://play.golang.org/p/EsPejyCrX7):

package main

import (
    "fmt"
    "sync"
)

var wg1 sync.WaitGroup

type Str struct {
    id int
}

func (me *Str) Start() {
    wg1.Add(1)
    fmt.Println("F1 ", me)
    go me.startFn()
}

func (me *Str) startFn() {
    defer wg1.Done()
    fmt.Println("F2 ", me)
}

func main() {
    Fl2 := []Str{
        {1},
        {2},
        {3},
    }

    for _, fl := range Fl2 {
        fl.Start()
    }
    wg1.Wait()
}

Response:

F1  &{1}
F1  &{2}
F1  &{3}
F2  &{3}
F2  &{3}
F2  &{3}

Expected response (F2 can be random):

F1  &{1}
F1  &{2}
F1  &{3}
F2  &{3}
F2  &{2}
F2  &{1}

janos · Accepted Answer · 2017-07-01 22:01:31Z

3

Using goroutines on loop iterator variables is a common mistake.

Call .Start on the element of Fl2 by the range index, instead of the range value (playground):

for i := range Fl2 {
    Fl2[i].Start()
}

However, the output might still not become exactly as you expect, due to the scheduling of the goroutines, for example it might be like this:

F1  &{1}
F1  &{2}
F1  &{3}
F2  &{3}
F2  &{1}
F2  &{2}

edited Jul 1, 2017 at 22:01

answered Jul 1, 2017 at 21:54

janos

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1 Comment

luki Over a year ago

Thanks, that fixes it. Are you able to explain why changing to (&fl).Start() does not fix it? I though in Go it will return me the underlying structure.

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