Go concurrency with loop and anonymous function behaves unexpectedly

Question

I already figured out how the code will behave the way I want, but I would like to understand why it behaves in a way that improves my understanding of Go concurrency.

I tested sync.WaitGroupto wait for some goroutines to complete, because I plan to make several downloads on Amazon S3 this way.

This was the source code:

func main() {

    var wg sync.WaitGroup

    for i := 1; i <= 5; i++ {
        wg.Add(1)
        go func() {
            fmt.Println(i)
            time.Sleep(time.Second * 1)
            wg.Done()
        }()
    }

    wg.Wait()
}

I was surprised to see that the result of: 6, 6, 6, 6, 6.

Instead of something like: 2, 4, 1, 5, 3.

Since the loop doesn't even go to 6, it made no sense to me. I later passed the variable ito an anonymous function as an argument, and then it behaved as I expected.

Why is this happening? I do not understand this.

Answer 1

faq: , goroutines?

goroutines , for . , break i 5, 6. goroutines, i, .

i , , .

Answer 2

, i func, i.

, :

func main() {
    var wg sync.WaitGroup

    for i := 1; i <= 5; i++ {
        wg.Add(1)
        go func(i int) {
            fmt.Println(i)
            time.Sleep(time.Second * 1)
            wg.Done()
        }(i)
    }

    wg.Wait()
}

, :

import "sync"

// Parallelize parallelizes the function calls
func Parallelize(functions ...func()) {
    var waitGroup sync.WaitGroup
    waitGroup.Add(len(functions))

    defer waitGroup.Wait()

    for _, function := range functions {
        go func(copy func()) {
            defer waitGroup.Done()
            copy()
        }(function)
    }
}

,

func main() {
    functions := []func(){}
    for i := 1; i <= 5; i++ {
            function := func(i int) func() {
                return func() {
                        fmt.Println(i)
                }
            }(i)

        functions = append(functions, function)
    }

    Parallelize(functions...)

    fmt.Println("Done")
}

Parallelize, https://github.com/shomali11/util