// Copyright 2012 The Go Authors. All rights reserved.// Use of this source code is governed by a BSD-style// license that can be found in the LICENSE file. package sync_test import (	"fmt"	"sync") type httpPkg struct{} func (httpPkg) Get(url string) {} var http httpPkg // This example fetches several URLs concurrently,// using a WaitGroup to block until all the fetches are complete.func ExampleWaitGroup() {	var wg sync.WaitGroup	var urls = []string{		"http://www.golang.org/",		"http://www.google.com/",		"http://www.somestupidname.com/",	}	for _, url := range urls {		// Increment the WaitGroup counter.		wg.Add(1)		// Launch a goroutine to fetch the URL.		go func(url string) {			// Decrement the counter when the goroutine completes.			defer wg.Done()			// Fetch the URL.			http.Get(url)		}(url)	}	// Wait for all HTTP fetches to complete.	wg.Wait()} func ExampleOnce() {	var once sync.Once	onceBody := func() {		fmt.Println("Only once")	}	done := make(chan bool)	for i := 0; i < 10; i++ {		go func() {			once.Do(onceBody)			done <- true		}()	}	for i := 0; i < 10; i++ {		<-done	}	// Output:	// Only once}