Updated: 2017-07-17 17:17:24 CST +08

Golang Goroutines in Loop

Problem

func main() {
    for _, v := range values {
        go func() {
            fmt.Println(v)
        }()
    }
}

each iteration of the loop uses the same instance of the variable v, so each closure shares that single variable. When the closure runs, it prints the value of v at the time fmt.Println is executed, but v may have been modified since the goroutine was launched. - “What happens with closures running as goroutines?”, Golang Frequently Asked Questions (FAQ)

Example

  • Incorrect

    func main() {
    ints := []int{1,2,3}
    for _, i := range ints {
        go func() {
            fmt.Println(i)
        }()
    }
    time.Sleep(3 * time.Second)
    // Output:
    // 3
    // 3
    // 3
}

  • Fixed: pass the variable as an argument to the closure.

    func main() {
    ints := []int{1,2,3}
    for _, i := range ints {
        go func(i int) {
            fmt.Println(i)
        }(i)
    }
    time.Sleep(3 * time.Second)
    // Output:
    // 1
    // 2
    // 3
}

  • Fixed: copy value to a new variable, because of variables declared within the body of a loop are not shared between iterations.

    func main() {
    ints := []int{1,2,3}
    for _, i := range ints {
        i := i
        go func() {
            fmt.Println(i)
        }()
    }
    time.Sleep(3 * time.Second)
    // Output:
    // 1
    // 2
    // 3
}

Example 2: Call Struct Method

  • Incorrect

    type user struct {
    name string
}

func(u *user) Print() {
    fmt.Println(u.name)
}

func main() {
    users := []user{{"dino"}, {"liam"}, {"mei"}}
    for _, user := range users {
            go user.Print()
    }
    time.Sleep(3 * time.Second)
    // Output:
    // mei
    // mei
    // mei
}

  • Fixed: pass by reference

    
type user struct {
    name string
}

func(u *user) Print() {
    fmt.Println(u.name)
}

func main() {
    users := []*user{{"dino"}, {"liam"}, {"mei"}}
    for _, user := range users {
            go user.Print()
    }
    time.Sleep(3 * time.Second)
    // Output:
    // dino
    // liam
    // mei
}

Reference