使用for循环进行并发，匿名函数的行为异常

Question

I have already found out a way for the code to behave as I want, but I would like to understand why it behaves like this so that my understanding of Go concurrency improves.

I was testing out sync.WaitGroup to wait for some goroutines to finish because I plan on doing multiple uploads to Amazon S3 in this way.

This was the code I had originally:

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 output was: 6, 6, 6, 6, 6.

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

Since the loop does not even go to 6, this made no sense to me. I later passed the i variable to the anonymous function as argument and then it behaved as I expected.

Why does this happen? I don't understand it.

Answer 1

This is covered in the faq: What happens with closures running as goroutines?

In this case, none of the goroutines get scheduled until the for loop completes. In order for the for loop to break i must not be less than or equal to 5, therefore it is 6 at that point. When the goroutines run, they each print the value of the single variable i which is captured in the closures.

When you pass i as an argument to the function, you copy the current value to a new variable, capturing the value at that moment.