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Question

package main

import (
    "fmt"
)

func main() {
    in := make(chan int, 10)
    out := make(chan int)

    for i := 0; i < 10; i++ {
        in <- i
    }

    close(in)

    go func() {
        for {
            i, ok := <-in
            if !ok {
                close(out)
                break
            }

            out <- i * 2 //THIS LINE
        }
    }()

    for v := range out { // AND HERE
        fmt.Println(v)
    }
}

I am a bit new to Go and learned the difference between buffered and unbuffered channels, but I am not sure how this code works. Since the out channel is not buffered should not it create deadlock? What am I missing?

Answer 1

out channel is unbuffered so if you write to it without another goroutine waiting to read from it, it will block. Same for read: it will block until another goroutine writes to it. So, the goroutine blocks on write to out until the for loop in main reads from it. The main goroutine will read from out until it is closed, so when the goroutine finally closes out, the for loop terminates.

Answer 2

Following how it is working:

in:= make(chan int, 10) out:= make(chan int)

Above you are creating two channels. in is buffered can hold 10 values. out is unbuffered can hold only 1 value

for i:= 0; i < 10; i++ { in <- i }

Above you are adding 10 values [0-9] to your in channel

close(in)

Above you are closing write on in channel. Keep in mind read on in is still open.

go func() {

You are creating a go routine to read values of in channel. (Best practice really is to pass the in to the func but that's not of focus here.

 for { i, ok:= <-in

Above in your loop you read value till in is empty ( ok will be false when it's empty)

 if !ok { close(out) break }

Above you would close out and break when you are done reading values from in . Till then below line executes.

 out <- i * 2 //THIS LINE

Above you write to channel out and block this goroutine till it's value is read. The cycle would continue 10 times.

for v:= range out { // AND HERE fmt.Println(v) }

Above, back in main goroutine, you would read value from out , block and pass control to the other goroutine. This continues till for loop breaks and out is closed.