Break out of select loop?

问题

I'm trying to use a select in a loop to receive either a message or a timeout signal. If the timeout signal is received, the loop should abort:

package main
import ("fmt"; "time")
func main() {
    done := time.After(1*time.Millisecond)
    numbers := make(chan int)
    go func() {for n:=0;; {numbers <- n; n++}}()
    for {
        select {
            case <-done:
                break
            case num := <- numbers:
                fmt.Println(num)
        }
    }
}

However, it doesn't seem to be stopping:

$ go run a.go
0
1
2
3
4
5
6
7
8
9
10
11
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13
14
15
16
17
18
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20
21
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[...]
3824
3825
[...]

Why? Am I using time.After wrong?

回答1:

The Go spec says:

A "break" statement terminates execution of the innermost "for", "switch", or "select" statement within the same function.

In your example you're just breaking out of the select statement. If you replace break with a return statement you will see that it's working.

回答2:

The "Go" way for that kind of situations is to use labels and break on the label, for example:

L:
    for {
        select {
            case <-done:
                break L
            case num := <- numbers:
                fmt.Println(num)
        }
    }

Ref:

http://www.goinggo.net/2013/11/label-breaks-in-go.html
http://golang.org/doc/go_spec.html#Break_statements

回答3:

In your example code, a return seems appropriate as Pat says, but for future reference you can use labels:

package main

import (
    "fmt"
    "time"
)

func main() {
    done := time.After(1 * time.Millisecond)
    numbers := make(chan int)

    // Send to channel
    go func() {
        for n := 0; ; {
            numbers <- n
            n++
        }
    }()

readChannel:
    for {
        select {
        case <-done:
            break readChannel
        case num := <-numbers:
            fmt.Println(num)
        }
    }

    // Additional logic...
    fmt.Println("Howdy")
}

回答4:

I have the following solution, by using a anonymous function.

    func() {
    for {
        select {
        case <-time.After(5 * time.Second):
            if token := c.Connect(); token.Wait() && token.Error() != nil {
                fmt.Println("connect err:", token.Error())
            } else {
                fmt.Println("breaking")
                return
            }
        }
    }
    }()

来源：https://stackoverflow.com/questions/25469682/break-out-of-select-loop

标签

select

concurrency

goroutine