The WaitGroup works if wg.Add() is called within the goroutine. The only
purpose for the WaitGroup is to keep main from exiting before the goroutine
has completed.
Moving wg.Add() outside of the goroutine has no effect if the channel is
unbuffered. But if I make the channel buffered and move the wg.Add()
outside the goroutine then the code runs in a loop printing 0 and never
stops. This is another behavior that I don't understand.
Why does making disconnectCh buffered cause the goroutine to never run, or
to run in a loop printing 0?
On Friday, February 11, 2022 at 12:47:09 AM UTC-7 Ian Lance Taylor wrote:
> On Thu, Feb 10, 2022, 9:52 PM Dean Schulze <dean.w....@gmail.com> wrote:
>
>> Here is an exercise using channels and select in a goroutine. If the
>> disconnect channel is changed to a buffered channel the goroutine doesn't
>> run at all.
>>
>> Why does changing from an unbuffered to a buffered channel prevent
>> running the goroutine?
>>
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>> * func SelectDemo(wg *sync.WaitGroup) { messageCh :=
>> make(chan int, 10) disconnectCh := make(chan struct{})
>> // go routine won't run if channel is buffered
>> //disconnectCh := make(chan struct{}, 1) defer
>> close(messageCh) defer close(disconnectCh) go func()
>> { fmt.Println(" goroutine")
>> wg.Add(1)*
>>
>
> In order for a WaitGroup to work, you need to call wg.Add before the go
> statement, not in the goroutine that it starts.
>
> Ian
>
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>> * for { select {
>> case v := <-messageCh:
>> fmt.Println(v) case <-disconnectCh:
>> fmt.Println(" disconnectCh") //
>> empty the message channel before exiting
>> for { select {
>> case v := <-messageCh:
>> fmt.Println(v)
>> default:
>> fmt.Println(" disconnection, return")
>> wg.Done()
>> return }
>> } } }
>> }() fmt.Println("Sending ints") for i := 0; i <
>> 10; i++ { messageCh <- i }
>> fmt.Println("Sending done") disconnectCh <- struct{}{} }*
>> Here's the code to call the function from main. I use the wait group to
>> assure that the goroutine completes before the program exits:
>>
>>
>>
>> *wg := sync.WaitGroup{} ch09.SelectDemo(&wg) wg.Wait()*
>>
>> --
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>>
>> <https://groups.google.com/d/msgid/golang-nuts/533e6da1-e529-4cd3-a18d-1a4ccf09b947n%40googlegroups.com?utm_medium=email&utm_source=footer>
>> .
>>
>
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