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POHow to lock/synchronize access to a variable in Go during concurrent goroutines?
 

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  1. POHow to lock/synchronize access to a variable in Go during concurrent goroutines?
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    <p>In his answer to this question: <a href="https://stackoverflow.com/questions/3918941/golang-for-windows-erratic-behavior">Golang for Windows erratic behavior?</a> user @distributed recommended to lock/synchronize access to a shared variable on concurrent goroutines.</p> <p>How can I do that?</p> <p>More on the issue:</p> <p>I get this code (the returned function with a closure on <code>views</code>) running on several goroutines at the same time:</p> <pre><code>func makeHomeHandler() func(c *http.Conn, r *http.Request) { views := 1 return func(c *http.Conn, r *http.Request) { fmt.Fprintf(c, "Counting %s, %d so far.", r.URL.Path[1:], views) views++ } } </code></pre> <p>It looks like the IO function takes it's time, and as a result I get this kind of output:</p> <pre><code>Counting monkeys, 5 so far. Counting monkeys, 5 so far. Counting monkeys, 5 so far. Counting monkeys, 8 so far. Counting monkeys, 8 so far. Counting monkeys, 8 so far. Counting monkeys, 11 so far. </code></pre> <p>It increments fine, but when it gets printed I can see that the operation printing+incrementing is not atomic at all.</p> <p>If I change it to:</p> <pre><code>func makeHomeHandler() func(c *http.Conn, r *http.Request) { views := 0 return func(c *http.Conn, r *http.Request) { views++ // I can only hope that other goroutine does not increment the counter // at this point, i.e., right after the previous line and before the // next one are executed! views_now := views fmt.Fprintf(c, "Counting %s, %d so far.", r.URL.Path[1:], views_now) } } </code></pre> <p>It seems to work fine, but I'm not completely sure if it will not fail eventually... </p>
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    1. USSebastián Grignoli
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