Break out of select loop?

Question

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 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 [...] 3824 3825 [...] 

Why? Am I using time.After wrong?

Answer 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.

Answer 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