Incorrect output by goroutines

July 2, 2023

I am watching a lecture in which author build a status app using go routines which is behaving weirdly after sometime.

Here’s the code:

func main() {
    links := []string{
        "http://google.com",
        "http://facebook.com",
        "http://stackoverflow.com",
        "http://amazon.com",
        "http://golang.org",
    }

    c := make(chan string)

    for _, link := range links {
        go checkLink(link, c)
    }

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

func checkLink(link string, c chan string) {
    _, err := http.Get(link)

    if err != nil {
        fmt.Println(link, "might be down!")
        c <- link
        return
    }
    fmt.Println(link, "is up!")
    c <- link
}

The output I get is:

http://stackoverflow.com is up!

http://google.com is up!

http://facebook.com is up!

http://golang.org is up!

http://amazon.com is up!

http://amazon.com is up!

http://amazon.com is up!

http://amazon.com is up!

http://amazon.com is up!

http://amazon.com is up!

I don’t understand why the "amazon.com" keeps on repeating here?? Why is this happening after all the 5 links has been printed?

>Solution :

The issue lies in the closure capturing of the loop variable l. Since the goroutine is started inside a loop, all goroutines created by the loop share the same memory address for l. By the time the sleep is over and the goroutine is executed, the value of l has changed due to the loop iterating over the remaining links. Consequently, all goroutines end up checking the last link in the links slice, which is "http://amazon.com".
To fix this, you need to pass the loop variable l as an argument to the anonymous function inside the goroutine. Here’s 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 an argument to the anonymous function, each goroutine will have its own copy of l, preserving the correct value for each iteration.