使用for循环和匿名函数执行并发性的行为意外 [英] Go concurrency with for loop and anonymous function behaves unexpectedly

问题描述

我已经找到了代码行为的一种方法，但我想知道为什么它的行为如此，以至于我对Go的并发性的理解得到改善。

我正在测试 sync.WaitGroup 以等待一些goroutine完成，因为我打算以这种方式对Amazon S3进行多次上传。

这是我原本的代码：

  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（）
} 
  

我惊讶地发现输出是： 6,6,6,6,6 。

取而代之的是： 2,4,1,5,3 。

由于循环甚至没有达到6，所以这对我没有意义。
我稍后将 i 变量作为参数
传递给匿名函数，然后按照我的预期行事。

为什么会发生这种情况？我不理解它。 解决方案

常见问题解答：以goroutine的形式运行的闭包会发生什么情况？

在这种情况下，没有一个goroutines会被安排到for循环完成。为了使for循环中断 i 不能小于或等于5，因此在那个点上是6。当goroutines运行时，它们每个都打印在闭包中捕获的单变量 i 的值。

当您将 i 作为参数传递给函数时，将当前值复制到一个新变量中，捕获当时的价值。

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.

解决方案

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.

这篇关于使用for循环和匿名函数执行并发性的行为意外的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！