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< ol class = "chapter" > < li class = "chapter-item expanded affix " > < a href = "../index.html" > Go语言圣经< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface.html" > 前言< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1.html" > < strong aria-hidden = "true" > 1.< / strong > 入门< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-01.html" > < strong aria-hidden = "true" > 1.1.< / strong > Hello, World< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-02.html" > < strong aria-hidden = "true" > 1.2.< / strong > 命令行参数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-03.html" > < strong aria-hidden = "true" > 1.3.< / strong > 查找重复的行< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-04.html" > < strong aria-hidden = "true" > 1.4.< / strong > GIF动画< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-05.html" > < strong aria-hidden = "true" > 1.5.< / strong > 获取URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-06.html" > < strong aria-hidden = "true" > 1.6.< / strong > 并发获取多个URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-07.html" > < strong aria-hidden = "true" > 1.7.< / strong > Web服务< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-08.html" > < strong aria-hidden = "true" > 1.8.< / strong > 本章要点< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2.html" > < strong aria-hidden = "true" > 2.< / strong > 程序结构< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-01.html" > < strong aria-hidden = "true" > 2.1.< / strong > 命名< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-02.html" > < strong aria-hidden = "true" > 2.2.< / strong > 声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-03.html" > < strong aria-hidden = "true" > 2.3.< / strong > 变量< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-04.html" > < strong aria-hidden = "true" > 2.4.< / strong > 赋值< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-05.html" > < strong aria-hidden = "true" > 2.5.< / strong > 类型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-06.html" > < strong aria-hidden = "true" > 2.6.< / strong > 包和文件< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-07.html" > < strong aria-hidden = "true" > 2.7.< / strong > 作用域< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3.html" > < strong aria-hidden = "true" > 3.< / strong > 基础数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-01.html" > < strong aria-hidden = "true" > 3.1.< / strong > 整型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-02.html" > < strong aria-hidden = "true" > 3.2.< / strong > 浮点数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-03.html" > < strong aria-hidden = "true" > 3.3.< / strong > 复数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-04.html" > < strong aria-hidden = "true" > 3.4.< / strong > 布尔型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-05.html" > < strong aria-hidden = "true" > 3.5.< / strong > 字符串< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-06.html" > < strong aria-hidden = "true" > 3.6.< / strong > 常量< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4.html" > < strong aria-hidden = "true" > 4.< / strong > 复合数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-01.html" > < strong aria-hidden = "true" > 4.1.< / strong > 数组< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-02.html" > < strong aria-hidden = "true" > 4.2.< / strong > Slice< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-03.html" > < strong aria-hidden = "true" > 4.3.< / strong > Map< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-04.html" > < strong aria-hidden = "true" > 4.4.< / strong > 结构体< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-05.html" > < strong aria-hidden = "true" > 4.5.< / strong > JSON< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-06.html" > < strong aria-hidd
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2022-08-24 10:48:15 +00:00
< li > < em > KusonStack一站式可编程配置技术栈(Go): < a href = "https://github.com/KusionStack/kusion" > https://github.com/KusionStack/kusion< / a > < / em > < / li >
< li > < em > KCL 配置编程语言(Rust): < a href = "https://github.com/KusionStack/KCLVM" > https://github.com/KusionStack/KCLVM< / a > < / em > < / li >
< li > < em > 凹语言™: < a href = "https://github.com/wa-lang/wa" > https://github.com/wa-lang/wa< / a > < / em > < / li >
2022-08-04 06:59:33 +00:00
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< hr >
< h2 id = "85-并发的循环" > < a class = "header" href = "#85-并发的循环" > 8.5. 并发的循环< / a > < / h2 >
< p > 本节中， ， < / p >
< p > < u > < i > gopl.io/ch8/thumbnail< / i > < / u > < / p >
< pre > < code class = "language-go" > package thumbnail
// ImageFile reads an image from infile and writes
// a thumbnail-size version of it in the same directory.
// It returns the generated file name, e.g., " foo.thumb.jpg" .
func ImageFile(infile string) (string, error)
< / code > < / pre >
< p > 下面的程序会循环迭代一些图片文件名，并为每一张图片生成一个缩略图：< / p >
< p > < u > < i > gopl.io/ch8/thumbnail< / i > < / u > < / p >
< pre > < code class = "language-go" > // makeThumbnails makes thumbnails of the specified files.
func makeThumbnails(filenames []string) {
for _, f := range filenames {
if _, err := thumbnail.ImageFile(f); err != nil {
log.Println(err)
}
}
}
< / code > < / pre >
< p > 显然我们处理文件的顺序无关紧要， （ ： ， ） （ ） ， ， < / p >
< p > 下面让我们并行地执行这些操作， ， ， < / p >
< pre > < code class = "language-go" > // NOTE: incorrect!
func makeThumbnails2(filenames []string) {
for _, f := range filenames {
go thumbnail.ImageFile(f) // NOTE: ignoring errors
}
}
< / code > < / pre >
< p > 这个版本运行的实在有点太快， ， ， ， ， ？ ， ， < / p >
< p > 没有什么直接的办法能够等待goroutine完成， ， ， < / p >
< pre > < code class = "language-go" > // makeThumbnails3 makes thumbnails of the specified files in parallel.
func makeThumbnails3(filenames []string) {
ch := make(chan struct{})
for _, f := range filenames {
go func(f string) {
thumbnail.ImageFile(f) // NOTE: ignoring errors
ch < - struct{}{}
}(f)
}
// Wait for goroutines to complete.
for range filenames {
< -ch
}
}
< / code > < / pre >
< p > 注意我们将f的值作为一个显式的变量传给了函数， ： < / p >
< pre > < code class = "language-go" > for _, f := range filenames {
go func() {
thumbnail.ImageFile(f) // NOTE: incorrect!
// ...
}()
}
< / code > < / pre >
< p > 回忆一下之前在5.6.1节中， ， ， （ ） ， ， ， < / p >
< p > 如果我们想要从每一个worker goroutine往主goroutine中返回值时该怎么办呢？ ， ： < / p >
< pre > < code class = "language-go" > // makeThumbnails4 makes thumbnails for the specified files in parallel.
// It returns an error if any step failed.
func makeThumbnails4(filenames []string) error {
errors := make(chan error)
for _, f := range filenames {
go func(f string) {
_, err := thumbnail.ImageFile(f)
errors < - err
}(f)
}
for range filenames {
if err := < -errors; err != nil {
return err // NOTE: incorrect: goroutine leak!
}
}
return nil
}
< / code > < / pre >
< p > 这个程序有一个微妙的bug。当它遇到第一个非nil的error时会直接将error返回到调用方， ， ， （ ） ， < / p >
< p > 最简单的解决办法就是用一个具有合适大小的buffered channel， （ ， ） < / p >
< p > 下一个版本的makeThumbnails使用了一个buffered channel来返回生成的图片文件的名字， < / p >
< pre > < code class = "language-go" > // makeThumbnails5 makes thumbnails for the specified files in parallel.
// It returns the generated file names in an arbitrary order,
// or an error if any step failed.
func makeThumbnails5(filenames []string) (thumbfiles []string, err error) {
type item struct {
thumbfile string
err error
}
ch := make(chan item, len(filenames))
for _, f := range filenames {
go func(f string) {
var it item
it.thumbfile, it.err = thumbnail.ImageFile(f)
ch < - it
}(f)
}
for range filenames {
it := < -ch
if it.err != nil {
return nil, it.err
}
thumbfiles = append(thumbfiles, it.thumbfile)
}
return thumbfiles, nil
}
< / code > < / pre >
< p > 我们最后一个版本的makeThumbnails返回了新文件们的大小总计数（ ） ， ， < / p >
< p > 为了知道最后一个goroutine什么时候结束（ ） ， ， ， ， ， ： < / p >
< pre > < code class = "language-go" > // makeThumbnails6 makes thumbnails for each file received from the channel.
// It returns the number of bytes occupied by the files it creates.
func makeThumbnails6(filenames < -chan string) int64 {
sizes := make(chan int64)
var wg sync.WaitGroup // number of working goroutines
for f := range filenames {
wg.Add(1)
// worker
go func(f string) {
defer wg.Done()
thumb, err := thumbnail.ImageFile(f)
if err != nil {
log.Println(err)
return
}
info, _ := os.Stat(thumb) // OK to ignore error
sizes < - info.Size()
}(f)
}
// closer
go func() {
wg.Wait()
close(sizes)
}()
var total int64
for size := range sizes {
total += size
}
return total
}
< / code > < / pre >
< p > 注意Add和Done方法的不对称。Add是为计数器加一， ， ； " closer" goroutine调用Wait之前被调用。并且Add还有一个参数， ； ， < / p >
< p > sizes channel携带了每一个文件的大小到main goroutine， ， ： ， ： ， ， ， ， ， ， < / p >
< p > 图8.5 表明了makethumbnails6函数中事件的序列。纵列表示goroutine。窄线段代表sleep， ， < / p >
< p > < img src = "../images/ch8-05.png" alt = "" / > < / p >
< p > < strong > 练习 8.4： < / strong > 修改reverb2服务器， ， ， ， < / p >
< p > < strong > 练习 8.5： < / strong > 使用一个已有的CPU绑定的顺序程序， ， ， ？ ？ < / p >
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