前往频道和延迟

Question

I was just experimenting with Go channels on my Ubuntu 64 bit environment and got confused with the output the following program produced.

I got the output: 0 1 2 3 Exit

The output when I uncommented the two commented lines: 0 1 2 3 4 Exit

Please explain the behavior. TIA.

package main

import (
"fmt"
    //"time"
)

func main() {
    ch := make(chan int)
    done := make(chan bool)
    go func() {
        for i := 0; i < 5; i++ {
            ch <- i
        }
                //time.Sleep(1 * time.Second)
        done <- false
    }()
    go func() {
        for {
            select {
            case message := <-ch:
                fmt.Println(message)
            case <-done:
                return
            }
        }
    }()
    <-done
    fmt.Println("Exit")
}

Answer 1

You're not waiting for both goroutines, and only sending a single value over done to 2 receivers, which will deadlock if the second receiver happens to be main.

Using a WaitGroup simplifies the code, and allows you to easily wait for as many goroutines as needed. https://play.golang.org/p/MWknv_9AFKp

ch := make(chan int)
var wg sync.WaitGroup

wg.Add(1)
go func() {
    defer wg.Done()
    defer close(ch)
    for i := 0; i < 5; i++ {
        ch <- i
    }
}()

wg.Add(1)
go func() {
    defer wg.Done()
    for message := range ch {
        fmt.Println(message)
    }

}()

wg.Wait()
fmt.Println("Exit")