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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-zh.html" > 译者序< / 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< /
2022-08-04 06:59:33 +00:00
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2024-06-12 07:20:07 +00:00
< li > < em > Go语言圣经读书笔记(挖坑中...): < a href = "https://github.com/chai2010/gopl-notes-zh" > https://github.com/chai2010/gopl-notes-zh< / a > < / em > < / li >
< li > < em > 凹语言(Go实现, 面向WASM设计): < a href = "https://github.com/wa-lang/wa" > https://github.com/wa-lang/wa< / a > < / em > < / li >
2023-06-26 17:46:27 +00:00
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2022-08-04 06:59:33 +00:00
< hr >
< h2 id = "91-竞争条件" > < a class = "header" href = "#91-竞争条件" > 9.1. 竞争条件< / a > < / h2 >
< p > 在一个线性（ ） ， ， ， （ ） ， ， ， ， ， < / p >
< p > 考虑一下，一个函数在线性程序中可以正确地工作。如果在并发的情况下，这个函数依然可以正确地工作的话，那么我们就说这个函数是并发安全的，并发安全的函数不需要额外的同步工作。我们可以把这个概念概括为一个特定类型的一些方法和操作函数，对于某个类型来说，如果其所有可访问的方法和操作都是并发安全的话，那么该类型便是并发安全的。< / p >
< p > 在一个程序中有非并发安全的类型的情况下， ， ， ， ， ， ， ， < / p >
< p > 相反， ， < / p >
< p > 一个函数在并发调用时没法工作的原因太多了， （ ） （ ） （ ） ， < / p >
< p > 竞争条件指的是程序在多个goroutine交叉执行操作时， ， ， ， ， < / p >
< p > 传统上经常用经济损失来为竞争条件做比喻，所以我们来看一个简单的银行账户程序。< / p >
< pre > < code class = "language-go" > // Package bank implements a bank with only one account.
package bank
var balance int
func Deposit(amount int) { balance = balance + amount }
func Balance() int { return balance }
< / code > < / pre >
< p > (当然我们也可以把Deposit存款函数写成balance += amount， ， < / p >
< p > 对于这个简单的程序而言， ， ， ， ， ， ： < / p >
< pre > < code class = "language-go" > // Alice:
go func() {
bank.Deposit(200) // A1
fmt.Println(" =" , bank.Balance()) // A2
}()
// Bob:
go bank.Deposit(100) // B
< / code > < / pre >
< p > Alice存了$200， ， ， ， ： ， < / p >
< pre > < code > Alice first Bob first Alice/Bob/Alice
0 0 0
A1 200 B 100 A1 200
A2 " = 200" A1 300 B 300
B 300 A2 " = 300" A2 " = 300"
< / code > < / pre >
< p > 所有情况下最终的余额都是$300。唯一的变数是Alice的余额单是否包含了Bob交易， < / p >
< p > 但是事实是上面的直觉推断是错误的。第四种可能的结果是事实存在的， ， （ ） ， （ ） ， ， ； ： < / p >
< pre > < code > Data race
0
A1r 0 ... = balance + amount
B 100
A1w 200 balance = ...
A2 " = 200"
< / code > < / pre >
< p > 在A1r之后， ， ， （ ： ， ） < / p >
< p > 这个程序包含了一个特定的竞争条件， ， ， < / p >
< p > 如果数据竞争的对象是一个比一个机器字（ ： ） ， ， ， ： < / p >
< pre > < code class = "language-go" > var x []int
go func() { x = make([]int, 10) }()
go func() { x = make([]int, 1000000) }()
x[999999] = 1 // NOTE: undefined behavior; memory corruption possible!
< / code > < / pre >
< p > 最后一个语句中的x的值是未定义的； ， ， ： （ ） （ ） （ ） ， ， ， ， ， ， ； < / p >
< p > 尽管并发程序的概念让我们知道并发并不是简单的语句交叉执行。我们将会在9.4节中看到， ， ， ： ， ， ， ， ？ < / p >
< p > 我们来重复一下数据竞争的定义， ： ， ： < / p >
< p > 第一种方法是不要去写变量。考虑一下下面的map， ， ， ， ， < / p >
< pre > < code class = "language-go" > var icons = make(map[string]image.Image)
func loadIcon(name string) image.Image
// NOTE: not concurrency-safe!
func Icon(name string) image.Image {
icon, ok := icons[name]
if !ok {
icon = loadIcon(name)
icons[name] = icon
}
return icon
}
< / code > < / pre >
< p > 反之， ， ， ， < / p >
< pre > < code class = "language-go" > var icons = map[string]image.Image{
" spades.png" : loadIcon(" spades.png" ),
" hearts.png" : loadIcon(" hearts.png" ),
" diamonds.png" : loadIcon(" diamonds.png" ),
" clubs.png" : loadIcon(" clubs.png" ),
}
// Concurrency-safe.
func Icon(name string) image.Image { return icons[name] }
< / code > < / pre >
< p > 上面的例子里icons变量在包初始化阶段就已经被赋值了， ， ， ， ， ， < / p >
< p > 第二种避免数据竞争的方法是， （ ） ， （ ） < / p >
< p > 由于其它的goroutine不能够直接访问变量， ； （ ） < / p >
< p > 下面是一个重写了的银行的例子， ， ： < / p >
< p > < u > < i > gopl.io/ch9/bank1< / i > < / u > < / p >
< pre > < code class = "language-go" > // Package bank provides a concurrency-safe bank with one account.
package bank
var deposits = make(chan int) // send amount to deposit
var balances = make(chan int) // receive balance
func Deposit(amount int) { deposits < - amount }
func Balance() int { return < -balances }
func teller() {
var balance int // balance is confined to teller goroutine
for {
select {
case amount := < -deposits:
balance += amount
case balances < - balance:
}
}
}
func init() {
go teller() // start the monitor goroutine
}
< / code > < / pre >
< p > 即使当一个变量无法在其整个生命周期内被绑定到一个独立的goroutine， ， ， ， ， < / p >
< p > 下面的例子中， ， ， ： < / p >
< pre > < code class = "language-go" > type Cake struct{ state string }
func baker(cooked chan< - *Cake) {
for {
cake := new(Cake)
cake.state = " cooked"
cooked < - cake // baker never touches this cake again
}
}
func icer(iced chan< - *Cake, cooked < -chan *Cake) {
for cake := range cooked {
cake.state = " iced"
iced < - cake // icer never touches this cake again
}
}
< / code > < / pre >
< p > 第三种避免数据竞争的方法是允许很多goroutine去访问变量， ， < / p >
< p > < strong > 练习 9.1： < / strong > 给gopl.io/ch9/bank1程序添加一个Withdraw(amount int)取款函数。其返回结果应该要表明事务是成功了还是因为没有足够资金失败了。这条消息会被发送给monitor的goroutine， ， < / p >
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