Test the effect of sync.Once.

test_once.go

// Test the effect of sync.Once.
// See another example: http://golang.org/pkg/sync/#Once

package main

import (
    "fmt"
    "sync"
    "runtime"
)

var a int
var once sync.Once

func setup() {
	a++
}

func doprint() {
	once.Do(setup)
	fmt.Println(a)
}

func main() {
    runtime.GOMAXPROCS(2)

    // these 3 goroutines runs on the same CPU
	go doprint()
	go doprint()
	go doprint()

    // prevent the process from exit
    for { // infinite loop, consumes 100% CPU
    }
}

// Output:
// 1
// 1
// 1
//