Basic question on Channels

[Aravindhan K]

Hi All,

When I was explaining basics of channels to newcomers, I was using the below example

https://go.dev/play/p/xx2qqU2qqyp

 

I was expecting both Write 5 and Write 3 to be printed. But only Write 5 was printed. I couldn't reason out the behaviour, can somebody point out what I am assuming wrong about.

Thanks,

Aravindhan K

[Brian Candler]

Unless a channel is buffered, it is synchronous.  A send cannot take place until the reader is ready to receive.

You can make it buffered using

    numCh = make(chan int, 1)
    go write()
    go read()

Or you can have two calls to read():

    numCh = make(chan int)
    go write()
    go read()
    go read()

[Brian Candler]

BTW, using time.Sleep() to keep goroutines running is poor practice.

What I suggest is using a sync.WaitGroup.  Call wg.Add(1) for each goroutine you start; call wg.Done() when each goroutine ends; and call wg.Wait() to wait for them.

https://go.dev/play/p/INl7BxG0ZSU

[Aravindhan K]

Thanks Brian, Got it.

"A send cannot take place until the reader is ready to receive." . So an unbuffered channel does not have the same behaviour as a buffered channel of size 1, fair.

I understand part about the waitGroup, I copy pasted code used to tutor the newcomers to programming itself.

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[Brian Candler]

I see. There's a concrete benefit in using the waitgroup though: it would have shown you the second write blocking.

https://go.dev/play/p/WNHRkkUUZUt

[Aravindhan K]

Cool stuff!

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