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// 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"
"os"
"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.example.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
}
// This example uses OnceValue to perform an "expensive" computation just once,
// even when used concurrently.
func ExampleOnceValue() {
once := sync.OnceValue(func() int {
sum := 0
for i := 0; i < 1000; i++ {
sum += i
}
fmt.Println("Computed once:", sum)
return sum
})
done := make(chan bool)
for i := 0; i < 10; i++ {
go func() {
const want = 499500
got := once()
if got != want {
fmt.Println("want", want, "got", got)
}
done <- true
}()
}
for i := 0; i < 10; i++ {
<-done
}
// Output:
// Computed once: 499500
}
// This example uses OnceValues to read a file just once.
func ExampleOnceValues() {
once := sync.OnceValues(func() ([]byte, error) {
fmt.Println("Reading file once")
return os.ReadFile("example_test.go")
})
done := make(chan bool)
for i := 0; i < 10; i++ {
go func() {
data, err := once()
if err != nil {
fmt.Println("error:", err)
}
_ = data // Ignore the data for this example
done <- true
}()
}
for i := 0; i < 10; i++ {
<-done
}
// Output:
// Reading file once
}
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