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<header>Updated: 2017-07-17 17:17:24 CST +08</header>
<main>
<h1 id="golang-goroutines-in-loop">Golang Goroutines in Loop</h1>
<h2 id="problem">Problem</h2>
<pre><code class="language-go">func main() {
for _, v := range values {
go func() {
fmt.Println(v)
}()
}
}
</code></pre>
<blockquote>
<p>each iteration of the loop uses the same instance of the variable v, so each closure shares that single variable. When the closure runs, it prints the value of v at the time fmt.Println is executed, but v may have been modified since the goroutine was launched. - “What happens with closures running as goroutines?”, Golang Frequently Asked Questions (FAQ)</p>
</blockquote>
<h2 id="example">Example</h2>
<ul>
<li><p>Incorrect</p>
<pre><code class="language-go">func main() {
ints := []int{1,2,3}
for _, i := range ints {
go func() {
fmt.Println(i)
}()
}
time.Sleep(3 * time.Second)
// Output:
// 3
// 3
// 3
}
</code></pre></li>
<li><p>Fixed: pass the variable as an argument to the closure.</p>
<pre><code class="language-go">func main() {
ints := []int{1,2,3}
for _, i := range ints {
go func(i int) {
fmt.Println(i)
}(i)
}
time.Sleep(3 * time.Second)
// Output:
// 1
// 2
// 3
}
</code></pre></li>
<li><p>Fixed: copy value to a new variable, because of variables declared within the body of a loop are not shared between iterations.</p>
<pre><code class="language-go">func main() {
ints := []int{1,2,3}
for _, i := range ints {
i := i
go func() {
fmt.Println(i)
}()
}
time.Sleep(3 * time.Second)
// Output:
// 1
// 2
// 3
}
</code></pre></li>
</ul>
<h2 id="example-2-call-struct-method">Example 2: Call Struct Method</h2>
<ul>
<li><p>Incorrect</p>
<pre><code class="language-go">type user struct {
name string
}
func(u *user) Print() {
fmt.Println(u.name)
}
func main() {
users := []user{{"dino"}, {"liam"}, {"mei"}}
for _, user := range users {
go user.Print()
}
time.Sleep(3 * time.Second)
// Output:
// mei
// mei
// mei
}
</code></pre></li>
<li><p>Fixed: pass by reference</p>
<pre><code class="language-go">
type user struct {
name string
}
func(u *user) Print() {
fmt.Println(u.name)
}
func main() {
users := []*user{{"dino"}, {"liam"}, {"mei"}}
for _, user := range users {
go user.Print()
}
time.Sleep(3 * time.Second)
// Output:
// dino
// liam
// mei
}
</code></pre></li>
</ul>
<h2 id="reference">Reference</h2>
<ul>
<li><a href="http://devs.cloudimmunity.com/gotchas-and-common-mistakes-in-go-golang/index.html#closure_for_it_vars">Iteration Variables and Closures in “for” Statements@50 Shades of Go: Traps, Gotchas, and Common Mistakes for New Golang Devs</a></li>
<li><a href="https://github.com/golang/go/wiki/CommonMistakes">CommonMistakes@golang github wiki</a></li>
<li><a href="https://golang.org/doc/faq#closures_and_goroutines">What happens with closures running as goroutines?@golang doc faq</a></li>
</ul>
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