Goroutine output is incorrect

php editor Zimo brings you a question about Goroutine today: Why is the output result of Goroutine sometimes incorrect? Goroutine is a lightweight thread in the Go language that can execute tasks concurrently, but sometimes its output results may be incorrect. So, what is the reason for this situation? Let’s explore it together! By understanding this problem, we can better understand how Goroutine works and avoid encountering similar problems in actual development.

Question content

I was watching a lecture where the author built a stateful application using go routines that was behaving strangely after a while.

code show as below:

65bcd28b0b95

The output I get is:

65bcd28b0ba0

I don't understand why "amazon.com" keeps repeating here? Why does this happen after printing all 5 links?

Solution

The problem lies in the closure capture of the loop variable l. Since a goroutine is started within a loop, all goroutines created by the loop share the same memory address of l. When the sleep ends and the goroutine is executed, the value of l has changed as the loop iterates over the remaining links. Therefore, all goroutines end up checking the last link in the links slice, which is "http://amazon.com". To solve this problem, you need to pass the loop variable l as a parameter to the anonymous function inside the goroutine. Here is an updated version of the code:

for l := range c {
    go func(link string) {
        time.Sleep(5 * time.Second)
        checkLink(link, c)
    }(l)
}

By passing l as a parameter to the anonymous function, each goroutine will have its own copy of l, retaining the correct value for each iteration.

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