Golang select default语句

package main

import (
    "fmt"
    "time"
)

func main() {
    start := time.Now()
    var c1, c2 <-chan int
    select {
    case <-c1:
    case <-c2:
    default:
        fmt.Printf("In default after %v\n\n", time.Since(start))
    }
}