for loop causing deadlock

Question

I have written some code example from GO Concurrency :

func gen(numbers ...int) <-chan int {
    out := make(chan int)

    go func() {
        for _, number := range numbers {
            out <- number
        }
        close(out)
    }()

    return out
}

func sq(in <-chan int) <-chan int {
    out := make(chan int)

    go func() {
        for number := range in {
            out <- number * number
        }
    }()

    return out
}

so I tried to use above code in my main function like this :

func main() {



    result := sq(sq(sq(gen(1, 2, 3, 4))))

    fmt.Println(<-result)
    fmt.Println(<-result)
    fmt.Println(<-result)
    fmt.Println(<-result)

    fmt.Println("-------------------")

    for channelValue := range sq(sq(sq(gen(1, 2, 3, 4)))) {
        fmt.Println(channelValue)
    }

}

I was confused when I run the code I got this message after the loop:

fatal error: all goroutines are asleep - deadlock

Please help me to understand this. From what I understand is calling the fmt.Prinlnt(result) x 4 times is the same as the for loop on for channelValue := range sq(sq(sq(gen(1, 2, 3, 4)))). is this correct?

Could please tell me why I got deadlock after the loop?

user5728991 · Accepted Answer · 2017-02-09 23:16:18Z

2

The range over the channel blocks because the channel is not closed in sq.

func sq(in <-chan int) <-chan int {
  out := make(chan int)

  go func() {
    for number := range in {
        out <- number * number
    }
    close(out)
  }()

  return out
}

A good way to debug deadlocks like this is to send the process a SIGQUIT. The runtime dumps the stacks of all the goroutines when a SIGQUIT is received. The stack dumps will often point to the issue.

answered Feb 9, 2017 at 23:16

user5728991

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Mr_Pink · Accepted Answer · 2017-02-09 23:16:01Z

1

You're not closing the out channel in the sq function

go func() {
    for number := range in {
        out <- number * number
    }
    close(out)
}()

https://play.golang.org/p/kk8-08SfwB

answered Feb 9, 2017 at 23:16

Mr_Pink

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