spec: range & go: pass a pointer to an element of array leads to a different behavior with/out go

[noxiouz]

I met a strange behavior while passing elements of array by pointer to a function. It looks like a bug.
You can find example here:
http://play.golang.org/p/e0dDpyCYCG

package main

import (
    "fmt"
    "sync"
)

var wg sync.WaitGroup

type Item struct {
    name string
}

func (i *Item) Name() string {
    defer wg.Done()
    return i.name
}

func Go(t *Item) {
    fmt.Println(t.Name())
}

func main() {
    f := make([]Item, 0)
    f = append(f, Item{"1"})
    f = append(f, Item{"2"})
    f = append(f, Item{"3"})
    for _, v := range f {
        wg.Add(1)
        go Go(&v)
    }
    wg.Wait()

This example prints

3
3
3

It doesn't look like an expected behavior.
Moreover this example (http://play.golang.org/p/KY4jEgtgGn) (exactly the same, but without go)

package main

import (
    "fmt"
    "sync"
)

var wg sync.WaitGroup

type Item struct {
    name string
}

func (i *Item) Name() string {
    defer wg.Done()
    return i.name
}

func Go(t *Item) {
    fmt.Println(t.Name())
}

func main() {
    f := make([]Item, 0)
    f = append(f, Item{"1"})
    f = append(f, Item{"2"})
    f = append(f, Item{"3"})
    for _, v := range f {
        wg.Add(1)
        Go(&v)
    }
    wg.Wait()
}

prints

1
2
3

I checked it on 1.3.3, 1.4.1, 1.4.2

[minux]

http://golang.org/doc/faq#closures_and_goroutines

[dvyukov]

Run the program with race detector (go build/test/run -race)

[noxiouz]

What a shame =) Sorry for that. Thanks!

[FrankReh]

You are passing an address to a variable to a goroutine but you let the
value at that address change before the goroutines are potentially run.

The idiomatic way of getting the behavior you want is to create a local
variable within the loop, set it equal to your 'v', and then pass the
address of that local. You will get a new local allocated from the heap
in each iteration this way, and you should get the 1,2,3 output.

On Wed, Mar 11, 2015 at 7:23 AM, Anton Tyurin [email protected]
wrote:

I met a strange behavior while passing elements of array by pointer to a
function. It looks like a bug.
You can find example here:
http://play.golang.org/p/e0dDpyCYCG

package main
import (
"fmt"
"sync"
)
var wg sync.WaitGroup
type Item struct {
name string
}
func (i *Item) Name() string {
defer wg.Done()
return i.name
}
func Go(t *Item) {
fmt.Println(t.Name())
}
func main() {
f := make([]Item, 0)
f = append(f, Item{"1"})
f = append(f, Item{"2"})
f = append(f, Item{"3"})
for _, v := range f {
wg.Add(1)
go Go(&v)
}
wg.Wait()

This example prints

3
3
3

It doesn't look like an expected behavior.
Moreover this example (http://play.golang.org/p/KY4jEgtgGn) (exactly the
same, but without go)

package main
import (
"fmt"
"sync"
)
var wg sync.WaitGroup
type Item struct {
name string
}
func (i *Item) Name() string {
defer wg.Done()
return i.name
}
func Go(t *Item) {
fmt.Println(t.Name())
}
func main() {
f := make([]Item, 0)
f = append(f, Item{"1"})
f = append(f, Item{"2"})
f = append(f, Item{"3"})
for _, v := range f {
wg.Add(1)
Go(&v)
}
wg.Wait()
}

prints

1
2
3

I checked it on 1.3.3, 1.4.1, 1.4.2

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